the control of the acid-base balance during open-heart surgery
TRANSCRIPT
THE CONTROL OF THE ACID-BASE BALANCE DURING OPEN-HEART SURCIERY ~
WAL'Ixl~ ZINGG, ~ D
THE ~am~'qANCV. Of the acid-base balance during r of mcIeasmg nnportance as the scope of cardm c sm prolonged periods of bypass A metabohc ac~dosas by:pass and commonly dunng hypothermia Various m to deal with flus problem It Is the purpose of this carned out an the years 1961-6~ leading to a method
,xtra~rporeal ctrculatmn gory is extended requm
occurs frequently dun ~thods have been descnl~
I
paper to descnbe stud whrch is now employed
all cases of extracorporeal clrculaaon including those At the beginning the effects of hypothernua were expermaentally The pnnclpl~s evolved an tlns study corporeal clrculatmn with and w~thout hypothermm
Many, of the physlologmal dasturbgnees causer I by not been mvesUgated adequately, although r
of several hours' duraU, ,tudred both clmmally a ~r later applied to ext
reduced hypothenma h~ lg procedures have be gory and to neurosurge
i s
ng ng ed t e s
m ) n ,
nd t ' a -
w e
en
1T m -
~ly o r
accepted as valuable adNncts to carchpvascula~ sm General hypothermla affects every organ and man 3 of the biochemical cc ponents of the rmheu mtermur The slgmficance of these alterations is larg unknown, and often at is even unknown whether a c e ~tam change is beneficial detrimental to the orgamsm' under reduced hypothelmla Observataons made m the laboratory are slowly rmprovmg our understandlrtg, but the persons who are responstble for the chmeal use of induced hypothermla still frequently find them-
I selves m a dilemma In the planning of an operatloaand dunng the procedure they have to make certain declsmns whmh are base~t on previous observatmns and experience but whmh are not based on a thoroug~ understanding or on sound scmntafic knowledge
Thas sltuataon pertains to the acid-base balance Changes m temperature influ- ence the buffer systems an various ways and somelames m opposite dlrectaons/In the absence of hard and fast rules at nvas decaded to ,,et up a working hypoth~,sls based on theoretmal cons~deratmns and on certain expernnental observatmns The hypothesis conmsts of three parts
1 E~tracorporeal carculaUon, especmlly during hypotherrnm, is not as effective as the normal c~rculatmn Therefore, at as desrrable to reduce vasoddatatmn, p~r- t~cu]ar]y m the brain since th~s organ is most lrkely to surlier damage by hypoyaa assocmted wath msufl~clent perfus16n
2 Under hypotherrma and after anoxrc cardmc re]Test, ventncular fibrillation may occur Therefore, towards the end of the cardmpulmonary bypass,
*Presented, m part, at the Annual Meeting, The Royal Collelreg, of Phymcmns and Surgeons of Canada, Edmonton, Alberta, January 17, 1963 / ,
From the Department of Surgery, The Umversl~ of Mamtobb~,, Wmmpeg Children s Hospl~al, ~md Wmmpeg General Hospatal Wodc supported by the Ma~toba Heart Foundataon
{MechcaI College, Wmmpeg 3, Mamtoba
123
Can Anaes Soe ] , voI 11, no 9., March, 1964
12,4 .srmms sl jov XL
conchtmns must be created ~luch arol favdurabl~ for defibnllataon A temporary~ lowering of the pH is thought to be m&cated at tins tune
3 The normal acid-base balance i~ n~,iLintamed wlttnn a narrow range and devmtaons are not well tolerated Therdo~e; dunng cardmpulmbnary bypass fluctuatmns of the pH should be avoided except the temporary' ]~wenng men- honed above
The practmal apphcatmn includes th~ following components 1 Rou~.e anaesthesm, but before bWas,!, exc,~sswe hv~erventalatmn rs avoided
Anaesthetic agents hawng a vasodalah~g effect,, such as~ ~halothane. [are preferred dunng bypass
g The pCO2 xs kept wathm the nomaal shghtly towards the end of bypass ff veOtnc
3 After bypass the pCO2 is reduced ]by c 4 Interference wath the metabohc sxde
Ammomum chloride, wluch is saxd ~ofa Sodmm bmarbonate as admamstered only ff
range dunrig bypass ar, d at is rinsed Mar flbrfllahon occurs 3ntrolled hy[mrventflatao a os the acid-base balance is avoided
,clhtate deflbnllahon, 1~ never given mdmated
MATERIAL AND ME1~-IODS
Seventy-two conseeuhve paiaents undergoing open-heart surgery were siuched, of these undex hypothermm (25 ~ C )
In adults and older children an arterial artery and m these pahents the acid-base rang os the anaesthesia through the opera the recovery room Dunng the extracor r taken from the oxygenator In the smallei inserted Thus, only the samples taken fro' plemented by one or two blood samples shortly before and after bypass Hepanmz
needle was placed m hhe left radaal balance was followed from the begin- 1on and dunng the first few hours m oreal bypass samples of blood were cbaldren, the arterial needle was not
the oxygenator were available, sup- :aken by the surgeon from the aorta. d syringes were used throughout and
suitable precautmns were observed to avolcl admassmn of mr rote the syringe The speennens were then analysed without delay on Astrup's a~,paratus ~ with
the mmroelectrode at a temperature of 37 ~ ,C Tim pH was corrected for tempera- o . . / ture using Rosenthal s factor- and the pC02 was corrected for temperature using
Sevennghaus' nomogram 3 All operatmns were done by means of la Kay-Cross rotating ~h
and roller pumps ~ The size of the oxygenator was chosen ace weight of the pahent A perfuslon rate eli 2 3--2 4 L / m ~/rmn wz most pahents In some the venous return ~vas not adequate and t~ be decreased accordingly The roller pumps were eahbrated and f was checked by measuring the venous ret!urn ,at intervals dunng At a blood temperature of 25 ~ C the peifuslon rate was decreas, m ately 75 per cent of the predetermined flow Since at that temper oxygen reqmrements are less tlhan 50 per cent of normal, 4 the high was maintained during hypothermm The temperatures of the pah4 blood m the oxygenator and 1~ the heat exchanger were reeorder 401 thernnstors and te|ethermometers.~
*Pemco Inc, Cleveland, Ohm {Yeltow Springs Instrument Co, Yeliow Spnngs~ Ohm
~k oxygenator 3rdmg to the s achieved 111 Le flow had to ae actual flow
t
he procedurte L
to approxa- tture the total flow principle �9 nt and of ~ e
by means of
t ) ~ I I I zmae, w ~. A c ~ , ~ s ~ . ~ c ~ vtrn~c O~,rs-rW.~ str~c~aY 1~,5
The presentalaqn of bmchemmal data in a forn~ wl~ chmcally acceptable as not easy a In tins prese~tatac and the dmgram introduced by Saggaard Anders(~n 7 v that m the latter dmgram the pH ~s plotted on ahnea reversed loganthmm scale m mm Hg Excesses of ba,
ch ~'i accurate, smaple, and n ' the pH/pCO~ dlagrmn ~ ,ere used It ~s to be not ,~
sca]e and the pCOe on ,in are expressed as positive
TABLE I C~m~c~n DATx
Dmgnosls Number
Atrial septal defect P-lmonary stenos~s Ventneular septal defect Congemtal aome stenos~s Tetralogy of FaUot Acqmred heart chsease Pulmonary atresm
Total
r' , , , ,
Ago Mortahty
6 1 2o ~.~8 yr a 11 3--1~6 yr 0 7 z~-~48 yr 9. 6 23-t[f0 yr 5 1 8 mo 1
72 12
7 7
7 6
7 5
E
~ 12 YRS ~ TETRALOGY OF FALLOT a. ~ (PREVIOUS POTTS' PRODECL~RE)
20~ 34 25 26 36 37 38 TEMP ~
~ , ~ 10 mEq NaHCO 3
, o , oo - _
7 3, 50 -5,~ - - - - - - pCO 2
~--~,.~ pCO 2 (conrecJted} 60
pH
-I0, -'-- ~ pH (corrected) 7 "~ 70 . . . . BASE EXCESS
~CARDIAC ARREST " - - ~ TOTAL
] 2 3 4 5 6
7 O, TIM1 (HOURS)
Fmv~ 1 Acud-base balance of a patient undergoing open-heart surgery for correction o~ a tetralogy with extraeorporeal circulation and hypothermm After cooling to 25* oesophageal temperature there was a 9-minute period of eirdt~tatoxy arrest, followed by hypothemue bypass with cardiac arrest During a sea)nd penod o~ normothemUe bypass the nght pulmon~ay outflow t] act was enlarged The co~eetedpU and pCO, values o[btamed during the hypotheraua are included
126 c a ~ ~ , ~ AN~s~l~m~sws" soctm~ jomm~a,
' I values and deficats as negatave valu6s, m m E q / L on a logagthmlc scale with a senum blcarb0nate of 22 9 mEq/E l ~ zero. The normal rahge ~s mdw~ted by the two horizontal hnes
The dmgnosls and age of the lqatle~ The bmehemlcal data obtained m
~ts are gwen an Table I al patmnt undergoing op ~n-heart surgery s r
complete eorreetaon of a tetmlggy t m Fagure 1 ,The patxont, a girl, was born w~.th a Itetn Ports procedure at the age of five years At age 15 de )ns was performe d During anaesthesm and before bypa ;s, the changes assocmte6 with hyperventflO_-
hermm cx~mmenced an the bas~ deficit
)f Fallot are summarize logy of Fanot and had a ~mtlve repatr of the lesi,
tton were found As soon as the part t~tl bypass and the hypo! there was an increase m pCO2,~ drgp m pH, and an mcreas~
At a temperature os 25 ~ complete ]circulatory arrest was m: latuted for a pen0d of rune minutes, whde the PottS" an,~stomos~s was closed Then w~th the I pataellt, on complete cardIopuknonary bypas,~!, the oulmonary stenosxs and the ventncuhLr septal defect were repaared, the latter under cardiac arrest produced by clamp~r g the aorta at an oesophageal ten~pera/ture of 25 ~ At point A , Fig 1) thelflow qff gas into the oxygenator was decreased to less than one htre per minute becau~;e the pCOz was found to be levelhng bff at too low a level Fcllowmg the clrcuh- tory arrest a metabohc acidosis bee~me evadent and when ~ base defie~t of ][0 m E q / L had been reached 101 mEIq of sodmm baearbona~e was gwen intra- venously The~patlent was then ~-ewa~-mecl After 20 minutes the temperature h~d returned to normal, the heart resumed a rhythrmeal aetmn spontaneously, and
E E
o u
100
4 80 =
60
50
40
30 =
20
[3 %
'1 ~/I zg @
0', PRE PUMP
~ AFTER FIRST RUN
ON PUMP 1
ON PUMP 2
~ POST PUMP
O
7 0 71 7 2 7 3 7 4 7 5 7 6 7 7 pH
Fmmw. 2 Same pataent as m Figure 1 Uncorrected vaklLes plotted pH/pCO~ chagram
ZINGG, W ACID-BASE BALANCE DURING OPEI~T-HEt!kl~T SURGERY 1 2 7
E E
o,. 1~ 0,.
7 7 . 15,
7 6, :20,
25
7 5,
30,
7 4,,
40,
7 3, 50,
7 2~ 70
7 1 " too
7 0~
E
x W
41 u r A - v rtsruta (BRAI~)
34' 28 21 ~ 35 rEMI~ '~C
15,
10,
5,
0,
~5,
8 O , ,J
o O
pCO 2 | "'~
BASE EXCESS o pH
/
- - ~ - - pCO 2 o - ~ pCO2 (corrected)
pH pH (corrected)
. . . . BASE EXCESS
" " -C',,
-10,
-15,
r~-~q ~ ~ ~ BYPASS
0 1 2 3 4 5 6 7 8 9 10
TIME (HOURS)
FIGURE 3 Acid-base balance of a patient undergoing neurosurgCry with extracorporeal carcu~ latlon and hypothermla The lowest oesophageal temperature fwas 21 ~ There were thre~ periods of circulatory arrest totalhng 36 minutes
the bypass was discontinued Pressures taken at that tlllne revealed an excesslv~ Kradlent between right ventncular and pulmonary art,~ry pressures and it wa~ ~Leclded to enlarge the nght ventncular outflow tract I by mseltlng a patch o I pericardium Accordingly, cardlopulmonary bypass Was-agam instituted bul: �9 ~ without hypothermla The pCO2 increased again and Sl~Ce a further drop m bas~ wax anticipated, another 10 mEq of sodium bicarbonate was gwen, After ter- mination of the bypass the anaesthetist hyperventllateid the patient and at th6 completion of the operation the pH and base were ,vlthm the normal rang~, whereas the pCO2 was shghtly lower than normal "Ihe pH/pCO2 dmgram 1~ reproduced an Figure 2 The stralght-hne l elatlonshlp is not as clear as in lessl- complicated cases, indicating some disturbance of the actd-base balance
The second case was operated upon shortly after the, termination of the series and as presented as another example of a comphcated ~procedure The patient, a 41-year-old white male, had undergone several prewous operations attempting tb Lmprove an mtracramal artenovenous fistula Finally It wax decided to approac~ the lesion again under circulatory a~Test and hypotl~ermia, using the routine
128 XaZSaarmr soberer jota
procedures of earchopul_m__onary 1 the total period of arrest bemg 3 presented m Figures 3 and 4 Se ent evidence of hypervent_datao~ pCO~. at the beginning of bypas levelled off, the flow of the gas htres per minute and, following levels The base defie~t was not and no blrearbonate was gwen " pH/pCO2 diagram, eonflrmmg-t
These two patients were eho, trate the changes produced in ff~ of cardlopulmonary bypass was pH were less pronounced
tlon was arreste a-operative ael( tth the last pard en fall of pH a
At the tmae rgenator was d t the pH reach ,*r complete cm .ationsbap iS ma ~ked metabohr ~sentation beea
In other pataq ~rease m pCOg.
d th ree hinges, -base status as mt are appa~- ad mcrease ~f hen the pCO~ ~ereased to ~6 ,~1 the desrr6xl ulatory arrest, ntamed m the acidosis lse they allt~s- nts the pengd md the fall m
Figure 5 is a composste graph of ~ll thte:uneorrected data obtained The dots
100
90 @
80-
70.
60
50
4O
pCO 2
mmHg
30-
20-
10
pH
FmtraE 4
@
Q
@
@
& O
O
O
Pre Pump O
On Pump �9
Post Pump A
i ' a i , 1 ,,,, i i i
71 72 , 73 7 4 7 5 76 7 7
Same pattent as m Flg0re 3 Uncorrected values pl~,tted m pH/pCO2 diagram
ZINGG, W ACID-BASE B~.LA.NCE DUBING OPEN-~HEABT SUBGEBY 129
E
0
IO0 m
90~
80"
70'~
0 O0 0
0
@ |
60 ~
50~
80~
20~
O 0 O
|
o ~ | |
O 0 |
0| r e %r
o~r162 o|
O
O | o
o
O
o
O O
o 0
0
O
O O _ @ | O @
O
O O
O
0
O0 @
o 0
$0 | $
|
o @
|
@
~
~| Q O
O o ~
_ O O e ~ 0 0
~ o
@ o o 0 ~
| 0 |
ID |
~ Q
Q
| @
@
7 0 71 7 2 73 7 4 7 5 7 6 7 7 pH
F m ~ 5 Composite pH/pC02 &agmm incorporating all data obtained m the course of the study
are x esults ob ta ined in survxvors, the c~cles are dl &ed on the day of the operatxon It can be seen throughout the whole range but are relatxvely mor, the centre This 1Hustrates the well-known fact th~ monary bypass is often assoemted with a metabohe or more biochemical determmataons we~ e pers successful cases only 3 or 4 were made This explau m the diagram The circles m the left upper come w~th acqmred heart disease affecting both mltral
tta obtained m patients �9 at the ctrcles are scatt
frequent on the left sic death fell| cardlc
acid| In these patron l, whereas m many short Is the large number of cl r we,re obtained m a pa and aortae valves The
who e r e d
[e of pul- ~s 12 and
rcles taent 1cart
130 CAN ADIAN ANAESTHETISTS[ SOCIETY JOURNAL
TABLE II MEANS AND STANDARD DEVIA']~ION80~ pH, pCO,, Am) BAsE DEFICIT
t I IN DONOR BLOOD AND DUPJ~G CA',Pd0IOI~Lma~ONARY BXrPASS
(F, gures m parentheses indicate th,,!~ }lumber of observalaons) I
pH
Donor blood Before anaesthesm One hour of anaesthesm Before bypass End of bypass
Normothenma Corrected Hypothemua
729-+-005 (63) 741___003 (7) 7 48• 07 (35) 7 45___0 09 (39)
7 28• 08 (46) 7 39,+__o 08 (27) 7 20+__0 07 (24)
1 ,CO2 (ram Hg) Base deft,'lt ( m E q / L )
66+_ 73 (59) 61 +_ 4 6 + 41 (7) 07 • 7 3 • 79 (35) 14 __ .80+ 71 (38) 30 _
,8 7___13 8 (44) 6 ~7 9• 0 (27) ;1 2• 4 (23)
3 +
9,3 (57) 27 (7) 14 (35) 3 1 (3s)
9,7 (r
Corrected End of operatmn
*Hypothernua cases
7 33___0 08 (24) 15 7 36• 11 (35) ;1
5 8• 9. (11) Normo~hern~a
r 52 26 (23) S _ l l 3 (23) 0__.136 (36) 50 +~:3fi ~ (33)
cases 4 2-+-3 2 (22)
resumed a regular Ihythm after defib: allatLon but the patient dmc haemorrhage In Figure 5, as well as m the rable II, all the data obt: the course of tins study are mcorporate~
Since the period of bypass was dfl~ere~,~t m each patient it wa obtain mean values of comparable states Ii. Table II tins was attem sentmg the mean*obtained at different sta~ es (1) the donor blood
I mr, after it had been circulated through the pump oxygenator, (2) before anaesthesm, (3) arterial blood after reduction of anaesthesia blood immediately before bypass, (5) blogd taken from the arten~ oxygenator towards the end of the bypass Iwhen the pH had reach value, dlwded into a hypothermm and a ngrmothermm group (in ax m the latter group the temperature had dropped 2-3 , in these and thermm cases the con ected values are also/gwen), (6) arterial blo~ of the operation Values for 2, 3, and 6 co~Md only be obtained m had an indwelling arterial needle
The donor blood was one day old and bypass the effects of hyperventdatlon were pCO2 and a small base deficit At the end a metabohe acidosis, more pronounced in operated upon at normal body temperatur~ pineally m F,gure 6
'1 I later, from ~med during
shghtly acidotic Immed ~vldent, as shown by a hi ,f the operation there wa the hypothermm eases l
The same data are rep
difficult to )ted by pre'- exposed to
~enal blood (4) arterial
1 end of the ~d its lowest ,any patients m all hypo-
d at the end mtmnts who
lately before ~h pH, a low evidence of
aa~ m those resented gra-
DIsCUssi,o~
Before discussing the working hypothells m~d the results obtained, a short description of the method used for the ar~tra-operatxve momtormg of the areal,- base balance and an outline of the dlreet e~ffect of hypothermla on the aod-base balance is presented
1 The Intra-operat~ve Momtol mg of the Acld-Base Balance Addmonal mtederence with the acid-base balance demands addlhonal safe-
guards The extent of the changes should be known at all hines Continuously
ZINGG, W ACID-BASE BAT,A.NCE DURING 0PEN~HEA.RT SURGERY 131
" w
ca
7 6
7 5 ,
74~
7 3
7 2
7 1
:E
E
O L~ E t~
15~ " ' x
,r
20~
25~ 15,
10, 30~ -,- / ' pC 02
40~ O,
. . . . . . . . . I ,~II IN, li I I pH
so, -s, ]',- "
60, Z O
70, r~
100!
-10,
-15,
p ~
,. =, ,~,~ BASE EXCESS
, BYPACIIS ANAESTHESIA ~ ~
0 1 2 3 4 5 TIME (HC)UR')
Fmvru~ 6 Means and standard dewatmns os the pH, pCO,,land base excess m donor b]~d, m arterial blood before anaesthesm, after mductmn-of anaest~lesm , lmmechately before byl~ass, at the end of bypass, and at the end of the anaesthesm The tared relatJtonslups are approx~mata0ns
recording instruments present a number of mherr chfficultaes and are Inot really reqmred On the other hand, routane labora!ory methods are too time- consuming and do not lend themselves to rapad senal]determmataons Nomograms are 0s hmlted value, since the eondltmns for which these are calculated may not preva'tl during open-heart surgery
The adeal method, which as yet to be found, should by eha,nges other than those ~currmg m the ae~d-I be rap~d, easy to perform, and dependable The l ecent]y described by Astrup and co-workers In C these reqmrements s It has been shown by Brewm e sample, as equflabrated at var3nng C02 tensions, and
be completely umnfluen )ase status, and should method and the apparl openhagen fulfills man) t al 6 that ff the same bl then the pH values and
corresponding pCO2 values are plotted on semdogarlthmrc paper, the dots faY. a straaght hne The hne as shifted to the left m acldoms and to the right m a losas Based on three pH determinations (the actual! pH and two done after blood sample has been equlhbrated with known COe tensmns), the actual the pCO2, and the base excess or deficit 9 can be obtmned Smee a sepa pH/pCO2 hue as drawn for each sample, the results are independent of char
ted dso Ltus
of 9od the o n
[ka- the )H,
ate ges
m the eoneentrataon of haemoglobin and also stud to be independent of temperatur~
Astrup's method has been designed for
CANADIAN .~N.M~STI-IETISTS" 8OC'I~ Pi" JOUKN/tL
plasma proteins. The pH/pCO~ hne ~s ~6
use ,~ normothermm, and at ~he present tame at as trnpossable to evaluate the meth~d when l he blood samples are taken at a lower temperature and warmed m the apparatus It ~s tnghly probable that the
are not accurate even ~]': correction factors are used I However, results obtamed experience has shown that, ff a base deficat ~s found dunng hypoth6rmm, ~t ~s
/
stall present after the pataent ~s rewarrne~l, and wee', versa The same ~bservatmn has been made m regard to the pCO2 clmng,t!~s an~d ~t as thought that [the values obt~uned, although perhaps anaceurate, are stdl helpful m the assessment of the pataents
2 The Changes ~n the Acid-Base Balance Procluced by Hypotherm~a It is not easy to gave a r~sum~ of the present ,,tate of knowledge, lone of the
mare dd~eultles being the terminology q he tierm pH was introduced m 1909 by Sorensen and defined as the negatwe lo I ;ant~ an of the hydrogen mn/eoncentra- taon The hydrogen mn eoncentrataon itself s an unwieldy figure wJ~tth several decimal points and Sorensen's concept samphfied matters greatly The pH umts are small figures and have been accepted umt ormly, but ~t is eoneelva]~le that, an the hght of newer knowledge, other umts ma ,~ht be introduced lo Today accord- mg to both U S and Bntmh standards, the p]-I values are purely arbatJrary umts, a certain value being assigned to a certain tc st solutxon Strictly spea ~mg, then, at is not adn~sslble to define the blood pH ~ ~" means of the Henderson-Hassel- baleh equatton However, everybody as famfl ar with thas equation, it is used m every treatise on the subject, and at serves well to explain some of the changes m the acid-base balance occurring when the t~:,mperature of the blood 1~ changed With these hmltatmns m mind the equatmn ~i.s used m the followmg daseuss~on
( a ) Phys~cochemwaZ Conmdera~wns In dmeusslng the Henderson-Hasselbaleh equataon, E Nell n stated that "other
people (non-experts) are rushing m wher~ angels fear to tread N~vertheless, before attempting to describe the effect of temperature changes on th~ aead-base balance, a partial review of the theoretical background appears mandatory Only a cursory desenptlon of the concepts which ~!Lre needed to exp|aln the tempera- ture effects wdl be gaven
The sltuatmn as not as clear-cut as one might wish and, it seems a, dvlsable to began with a defimtmn of the term, s Wh, at do ~ve mean by aeld and base ?
According to Clark TM the word qgase' has ~t least five dtfferent met nmgs For Lavolsaer a base was the prmeapal mgredlentl of a compound The de ~mttons of G N Lewis are difficult to understand for t~.ose untrained m advanc~'d ehemas- try In between, are the lomzatlon theory and Bronsted's theory, ~rhaeh are thought to be most useful m the present eont~;xt According to the fort ler, a base as a substance that lomzes m water to gave hyctroxy'l runs (an acad gaves hydrogen aons) and the mteraehons between base and ~.eld are assoemted with ~L eombma- tmn of the hydroxyl and the hydrogen 1on ~ N Bronsted's concept is more general An acid as a proton donor, a base as, a proton aeceptor In the formula
~IRIGG, Vr ACID-BASE BAI,ANCE DURING OP~N-I-I~ART 1 SURGERY 133
HR -~ R - q- H+, HR xs the acid and R - its qonlu the base may be a cataon, an amon, or an unch~r always be more negatwe (or less pomtwe) than H R
In tins paper, the term qgase wall be used m th~ H enderson s usage, whach as often employed cljmca meaning cataons Thus profem as a base, since]at cc conjugate acad HP, and the bacarbonate ion HC08 bomc acad H2COs ~
g~te base. Both the acad and ged molecule, but R - must 3
s sense Tins is m contrast to fly (tMed bases, buffer b0se), rnbmes wath H+ to form the -- as a base conjugate to car-
aclchtyF~ the quanlatatwe assessmentumts of the acid-base ] b a l a n c e T he ,4~xact- measurement ( or of the ofdegr~e of or alkalamty) pH are employed th~ pH
is chfl~cult, requmng a hydrogen electrode Mo~t labora|ory pH meters make use of a glass electrode la In these, the test solution as separated from a refelence solutaon by a glass membrane An electrode is m cqntact with the reference solu- taon, another electrode is m contact wath the ~est ~olutlton by means of a liquxd ]unetlLon The interstices in the specml glass u~ed ~or the membrane permit the exchange of protons (H+) and accordingly a yertmn electromotive force I, pro- duced across the glass, winch after suitable amphfieahon can be read off a n teter The pH meter as first cahbrated with a solutaon of known pH and then th, test soluhon as introduced
The value obtained with a pH meter does/not Conform strictly to Sorensen's defi~thon of the pU The achvaty rather than the concentrataon of the hydrogen ions as measured in and, when defined thermodynamically, the pH as an expr~smon of the avadablhty of protons, rather than of the hy]rogen 1on concentrahonl lz In modern blochermstry pH umts are arbitrary and the pH of a test solution is related to the pH of a standard solution In the det~mtlon, the difference betlween the pH of the test solutaon and the pH of the standard soluhon as proportional to the d~fference m electromotave force produced acioss the glass electrode ~y the two solutions ~o
If this dafference m E M F is known, the pH value can be calculated theo- rehcally using a certain mathematical formula whllch includes the absolute I tem- perature It follows that the pH values measured by the pH meter change wath temperature The pH of the buffer solutions used for the standardlzahon Olso as temperature-dependent 17 Thus when reporting pHI values obtained during hypo- thea~a it as essentml to state at which temperatur~ the pH was measured
In the following paragraph the discussion will b e restricted to an outline of acid base changes occurring an pataents under general anaesthesm wath ar|~ficml resp~ratlon, coo|ed by blood stTeam coohng 18 As a ifurther slmphficataon onl buffer systems (blcarbonate--carbomc acid and prcttems ) wxll be conmdered pH can therefore be defined according to HendersOn and Hasselbalch as fo]
p H - - p K l ' + log[ (HCOs-) / (H2COs)] -----pK~' + log(prot - /Hprot )
If two (or more) buffer pa~rs coexast an the s~ equahbnum with the same hydrogen 1on concen clple) pK' is the symbol for the negatwe logarithm constant of the acid Each buffer system has ats nary circumstances remains constant It must be r
r t w o
The lOWS
line solubon, they must be m trataon (pH) (asohydncl pnn- of K p, the apparent dlssoclataon own pK', wlach under ordl-
ememl~ered, however, that the
I~M CANADIAN ANAESTHE'r] STS' t,O(~I~-y jOURNAL
K' is temperature-dependent and chant,es ~mder hypothermm These changes have been studied by a number of mve:~lagaiors , and Severmghaus 3 )roduced a nomogram for the calculataon of the car home lacad pK' at varying tel aperatures
Rosenthal 2 stuched the effect of temperatm~ on theblood pH He ~letermmed the pH of the same blood sample at varying lemperatures and found an pH during coohng The mean change wa~ --00147 umts/~ C fo r thas figure is wadely quoted as "Rosenth~l's cbrrectmn factor" Man} c of the acad-base balance are temperature-dependent, e g the dassoc stants, and the fact that accoldmg to physical Ilaws more CO2 as dlsso plasma at a lower temperature than at norrnothenma The mmzatlon c amons as reduced at the lower temper~turel TM This leads to a deer4 number of avmlab]e hydrogen aons ang th~s to an increase an pH. dtfferent processes influence the pH dmmg ternperature changes a: change an pH would be the sum of the dlffc rent forces that may act cally or antagomstacally
linear rise blood, and omponents ration con- lved m the f all buffer ,~ase an the Evadently
ad the net synerglsta-
(b) Physwlogwal Cons~deratwns t
The results obtained by m v~t~ o studm I ca~ orgamsm under hypotherrma ColTectmn/fact~ measuring a certa~m variable anaerobmally cable only under these condltaons They are 1 blood sample as taken anaerobmally at a low t apparatus to body temperature before th~ me~:
In a hying anarnal under hypothermm the 1 bm condltmns the total CO2 content does nc crated wath a decrease m pCO2 In the lung re-eqmhbrates with a samdar alveolar carbo dunng the cooling, the CO2 content increase! of CO9 It was first stressed by Brewm et al expermaents cannot be apphed to the hypothe
not be apphed dtrectly t o the hwng )rs wbach have been detl~rmmed by t d~?erent temperatures are apph- ndaspensable an the laboratory The emperature and then walmed m the surement as made ~9 dood re-eqmhbrates Under anaero- t change, therefore coohng as asso-
(or m the oxygenator) the blood n dmxlde tensmn and accordingly, because of the mcreas~:l solubahty that conclusmns based on m vztro
role a m m a l
Conversely under hypotherrma the same k lood CO,_, content as m wath a lower alveolar tensaon and consequently less CO2 wall be excret the lungs However, m the tissues, less CO211s being produced unde cumstances Other changes m the respiratory system, such as the dead space at a lower temperature, 7 also may lrdhlence the acad-base ]
The reasons for the acadosas which is common]Ly assoemted with h are not entlrely clear Evadence has been pre cannot increase the excretmn of hydrogen 1or
In conclusion it can be stated that the eft( balance is rather complex The aonlzatmn of
sented that the hypother Ls an the presence of acid ~ct otF hypotherrnla on th all t.he buffer systems 1,
,qmhbrmm ed through r t h e s e c l r -
i n c r e a s e i n
)a]ance ~pothermm mm kldney OS1S 21
e acid-base decreased
when the temperature is lowered and the e~Fect ,of these changes is an mcrease
1;a:d?l]maob~: b:c~robn~ daTh ; : :ho:,:;tPe:pa:~:/:12:dOt~:h:cr~
ZINC, G, W ACID-BASE BALANCE DURING O p E N - ~ 1 SURGERY
m a relataHe preponderance of the latter buffer pair The total buffer capacity the b ood is thus reduced, the b~carbonate-carborr e aetd system~ being compar ttvely inefficient m the physiological pH range wr:h of approxmaately 6 3 A ecordangly, under hypothermla alterations m ~ae ~CO2, are relatively mot effective 1 h changing the pH, and the small ehan~es lr~ pCO2 produce consld& able pH shifts is
All the variables of the Henderson-Hasselbaleh e dependent The determination of the acid-base balanc C thus presents a number of difficulties since cor~c et~ must be used winch may not give precise results m ~ is further compheated by the observation that du~ ng are temperature gradients between various parts 9t tt various areas of the same organ 18,2a-26 Fmally eve~a
It. acid-base balance under hypothermm were posslb!e 1t?, ful since the optimal values for pH and blood gasOs at u n k n o w n 20
8 P l
q uaeton are temperatur~- e m the laboratory at 37 ~ m factors and nomogran s tyen patient The situattcn the eoohng process, the 'e v body and even betwe ,n an exact assessment of t' te meaning would be doubt- I any given temperature a "e
3 D~scusmon or the Working Hypothesm During extraeorporeal ctrculatlon and hypothermm,
to the vital organs usually can be maintained Howevc as e~eient as the normal heart and maintenance i of tO prnnary concern while the bypass is m progress ~Fow, the state of the myocardmm assumes an nnport~nce since condlhons must be created wlueh are favoUrabl the subsequent function of the heart These two aspec~
an adequate blood sup]~ x 4 the artrficlal heart is r ie cerebral etreulataon is trds the end of the byp / equal to that of the bra e foI defibrillation and f :s will now be discussed. 3xlmately 2 per cent of 1 ~ut and consumes aim ;1i known that the cereb
Ly )t a
~S
!n Dr
l e
,st In adult men, the brain, the weight of which is ~ppi
body weight, receives 15 per cent of the cardia~ ou 20 er cent of the total o en re mrement 2r It s w~ c~rPulatlon ~s not governed%~ the qame factors as ~ crrculatlon 2s Provided the systemic blood pressur~ o s level of 60-70 mm Hg, 29 the cerebral cLrculatxon l~md, constricting and vasoddatmg effects 30 The mare facto eirculalaon are the blood gas concentrations with a
,stemle and the pldmon." s not drop below a cntr
~pendent of the usual va~s
al ry
O-
,r~ influencing the cere]: al ~gh pC02 and a low r :)2
producing cerebral vasoddatatlon and a low pCO2 and a high pO~ producing cerebral vasoconstriction sl It is not clear whetherJthehigh pCO2 causes the va,'.o- drlatatlon directly or whether the lowering of the pH is the responsible faet,~r. Some mvesttgators have presented evidence for the ~drmer, 8x,32 whereas othl~;rs
led, it would appear to be favor the latter theory as Since tlns question is unset preferable to reduce both a ]ugh pCO2 and a low pH a high cerebral blood flow is desired A simda~ va: carbon dioxide and said to be independent of pH ha, temlc crrculataon a4 and m the coronary circulation. 35
mstead of a low pH onl, oddatmg effect caused" been described m the
[
If by 7S-
Under the conditions of extracorporeal crreulahon rand bypass, a high oxygen tension is desirable It has been shown that this alone 'Will not lead to an excessive
36,37 "1[ 1 E e I n decrease of cerebral blood flow To raise the pCO2 ander th se condlta~ s may appear contrary to common practice slfice hyperventllatton has bden
136
recommended dunng anaesthesm as a earher d~scussmns centred mamly arc taon, which ~s one of the hm~tmg f~ shown experimentally that a rapad t~ ventrmular fibnllataon ~o Others attnl
CANADIAN ANAESTI-~TIS~ S" SOCIETY JOURNAL I
nd ~specmlly during hypothermm 39 The ,und / the prevent'ion of ventrtcular fibrdla- :tors! of ,surface hypotherm~,. It has been dl f,~0m a lugh pCO2 level may produce ~utec the increased myocardml ~rratabdlty
to the acados~s produced by the haghlpC( contractahty was found to be assocmted been x ecommended that increases m pCO latmn dunng the period of hypotherrnta ~4 umversally accepted 46.~ Under condttaon dassocmtaon of oxygen from haemoglot,m ~
~e z9,41 ~e Depressmn of tlae myocardial w~th a low pH a3 Accordingly, ~t has , be avoided altogether l,y hyperventa- ,o Tlns concept, however i, has not been s of hypothermm and hypocapma the t the tassues may be amp~red ~0 Hyper-
ventilation also produces several changes n the blood chemistry m addition to a respiratory alkalosls In a dlseussll~)n c,n the maintenance of the acid-base balance the increased excretaon of sodmm and potassium bmarbon~tte is of parta- cular importance 52 It has been well documented that acute resplrttory alkalosls leads to a decrease m blood bmarbonate imd to an increased bicarbonate excre- tion m the unne 5z,59 An increase m blood lactic acid has also been observed ~4
Conversely, m respiratory aeadosls the renal tubular reabsorptlon ~f bmarbonate is increased, thus decreasing the bacarbon,~.te e~cretaon m the unne A high pCO2 has been shown to be the mare stamulu~ affecting the tubular eabsorptaon 55 Conservataon of b~ase is of maportance dl nng any surgical operat 1on, but espe- cmlly under condataons of hypothermm sm ~e the hypothermlc kadney as unable to produce an acid urine 2~ 56 It would seem the1 efore, that excesslv~ hyperventfla- taon before bypass should be avoaded and that a moderate mereas 9/m pCO2 and a moderate decrease an pH dunng byp, tss are of benefit sance/these changes cause a vasodflatatlon m the cerebral carculataon
If hypothermm is produced by means c,f an extracorporeal clrctllatmn with an oxygenator an the clrcmt, the occurrence of ventncular fibrdlatlon is of no slgnfl~- canoe At the low temperatures commonly used wath thas system and after cardmc arrest, the lncadence of ventncular ~brdlatlon is increased unles~ certain drugs are gwen 57 The important tinng, therefore, 1,, not to prevent fib~ dlatlon but to create condataons whmh are favourable fcr defibnllataon at the en ] of bypass It has been suggested that defibnllatmn as aq;comphshed most easdy m the presence of an acadosls, 58 although the evadence presented as rather meagre. Based on t.has same evadence, it has been the practme n certain centres to prod~me an acadosas by the mr_ravenous admmastrahon of amr omum chloride, whmh later as neutra- hzed with soehum bicarbonate Since the occurrence of a m e t a ~ h c acldosas ~ as one of the major comphchtJons followm~ hypothermm, 6,59 thas rc~utlne may not be wathout danger A resparatory acidosis can easdy be corrected by subsequent hyperventfiatlon and, whale present, it actually preserves base 60
The vasodflatmg effect of carbon dmxade has already been ~mentloned It appears to be partacularly mlportant under hypothermm when t~e vascoslty o~ the blood is increased 61 Further, the carclllatmg blood volume has been reported to be increased dunng hypothermm ~2 ~z l~h~s may be due to poohng as a result of a loss m vasomotor tone ~ although the total peripheral resastance has been reported to be increased under hypothelmm '~ A Ingh blood flow and the pCe-
ZINGG, W ACre-BASE BALANCE DIYRING OPEN~I-IEA-RT SURGERY 137
ventaon of a fall m blood pressure appear to] be ~)f specml rmportance raider hypothermm, since it has been reported that at lo~er temperatures the cn acal closing pressure m certain areas of the crrcul@aonlmaY be higher than no:Lanal
6 6 ~ because of the increased v~coslty If the bloo d pressure drops below the critical dosing pressure tins pamcdlar area would not be Derfused at all The rmportance
I of mmntammg a state of vasod~latataon not only ~v the cerebral carculatlon but also m the systemm clrculatmn by preventangi hyp0eapma under these cm'um- stances ~s evident
At tlns point, reference may be made to the] exp,l~nmental work on coohrtg to lower temperatures than those employed m tins s~ndy, on freezing and sttper- coohng Under these crrcumstances, as first shown ]by (:hala, the surwval rate as increased ff the annnals are m a state of hypoxm an,] hypercapnm This worl has been rewewed recently 67 It appears that t h e effeq,,~s of hypercapma are n am- tamed at low temperatures Experimental ewdqnce ~at crrculatory reacUons to a ingh CO2 are not abohshed at 20 ~ C has been p~ese :lted 0s
During extracorporeal crrculataon when the gas exchange takes place ar the oxygenator, the pCO2 can, wlttun l~mlts, be altered by the compomtmn anc the flow aate of the gas to winch the blood as expbsed m t]he oxygenator The I, :om- monly used high flow rates of several htres of th( gas m~xture (approx~ml,tely 97 5 per cent O2 and 2 5 per cent CO2) per minute provide an excess of ox?gen It was found m prehmmary studms that flow rate,, of o n e htre per rmnutc and less provade adequate oxygenatton of the arterial k lood, this quanmy of o~ ygen fully covering the oxygen a equrrements of the patmnt At these flow rates the blood does not get nd of the CO2 and the pCO2 rises A h~gh flow of th, gas leads to a lowering of the pCQ (a mtuat~on descnll)ed by others 6~) but th~s type of "hyperventrlatmn" is by no means as effectwe as :he gas exchange m the lmgs
Beiore the plan was finally adopted m patients, he hterature was searched for evadence of possible sade-effeets of a temporary res~',~ratory amdosas Although the problem as touched upon by several authors, no d,i~mte evidence of deleterious effects could be found H~gh CO2 tensmns were well tolerated by experimental ar~lmals 826~ and by patients 7o Following hyp,:rventflatmn wath oxygen m normal sublects ~t was found that dunng breath h( ldmg the pCO2 may rea,~'h 90 mm Hg 7~ After the Astrup apparatus was set up, t [e acld-base balance of several pataents was determined during open-heart surgery, using the perfusaon mel~hods whach were an routine use at that tame It was found that a change towards a h~gher pC02 and a lower pH took place dunng nOrmothermm and hypotherrmc bypass and that a comparatively small further mcr,~ase m the pCO2 would Iower the pH to the levels desrred at the end of the cardm]
The acid-base balance may be drfferent an the body The measurement of the mtracellular pH r wall help an the understanding of these problems I
mlmonary bypass various compartments (ff the aay produce new data whmh )etermmatmn of the acad-base
ba l~ce of the cerebrospmal fired may be of considerable ,mportance since 1t may reftect certain metabohc aspects of the functions of the central nervous system Only few data are avmlable 72 It has recently beer I shown that httle bmarb~nate penetrates into the cerebrospma| iquld and that th~ pH is mainly de te rmm~ by the pCO2 72 These findings mdlcate that changes, n the pH of the blood may not
CANADIAN ANAESTItEq
retteet pH changes m the cerebrospmal bonate rote the blood stream may not fired The significance of tins chscrepanc2,
Hence ~t was postulated that durra ingh normal range would be desirable it
~TS'
i ad C 31"I
ho exl
w a
end of a hypothemne bypass proeedur~ a s: prowde #t foil m pH winch would factt tate mg tecbrnque for the control of the ac ld-b the value of the addmon of CO,. to the gas ~t xs only since the mtroductmn of a sah ,fact berate attempt at increasing the carbor the undertaken. Carbon dmxlde has also b, ~en level by a group using the Edmark tec mlq as the temperature falls 74
Although prlmanly designed for use lJ concept was thought also to be of value fo temperature General and cerebral vqsodlJ adchtlonal acidosis at the end of the bypa fibnllataon orchnanly does not occur Agcorc the pCO2 to the rrormal range durmgbyp , of the operation were handled as descnl~ed fl
' Z jotn~xn I
t the adrmmstral: an nemesis m the o
~ e v e r , i s u n k n o w n . I zacorporeal bypass, a
s further postulated that nail further increase m defibnllalaon An adeqt
e ]balance was avadab m~lxtare had been recogl 9~y momtonng techmqu xide tensmn of the patu~ ~sed to lower the pH to at, 1 e dehberately lowe
: pahents under hypo I r bypass procedures do1 atataon again is tmport ~S ~s not necessary smc I lmgly, an attempt was rl .ss but not higher The >r the hypothermm cases
on of bmar- ;rebrospmal
I
CO~ m the towards the ~CO2 would ~te momtor- le Whereas azed before, that a deh-
nt has been! the des:red
nng the pH
hermm, the. ,e at normal ant but th~
ventncula~ ade to ra/s~ )ther phases,
4 D~scusswn of Results
In th~_s chmeal study the temperature o~ the Astrup apparatus Could not be altered since the equipment had to be aval]lable at the same tame for the routine laboratory work of the hospital Further, I under condmons of fiarly rapldl) reduced hypotherm,a there are consxderabl~ temperature gradmnts between van. ous parts of the body and the body temperature as such cannot be defined For: these reasons all the determmahons w~re done at a temperature o~ 37 ~ C an~ correction factors were apphed The values were corrected to the tetnperature ot the blood m the oxygenator This teml~erature is close to the temperature of th~ mixed venous blood of the patient sm~e th~ heat exchanger was incorporated m the artenal hne | / The eorrectaon factor --00147/~ C des( abed by Rosenthal e is sl--mllar to the ones reported by others r5-77 Consldenng the old definmon of the pH as aft expression of the hydrogen 1on concentrah, tahng into account the recreate m ~ e dl increase m temperature and dlsregardli~g a slgmfies neutral reactions, according to the c
(H~) ----- ( O H -
With increasing temperature the dlssocuah~ 37 ~ C a neutral reaction is lndmated by a I 0 1 9 / 1 2 - - 0 0158 per degree temperature n
When the pataent's blood was warmed t, mined, erroneously high results were obtaJ the nomogram of Sevennghaus ~
)n, a smadar factor can be calculatex~ sSOClatuon constant of winter with [1 other changes At 25 ~ C a pH of~ [d formula
-- 10-r
n constant of water mcleases and a 1 ~H of 6 81 Accordingly, lhe pH drops ~e 78
37 ~ C, before the pCO~. was detel- ned and these were corrected using
ZINGG~ W ACID-BASE BALANCE DURINGI OPE~-HEART SURGERY 139
Wxth regard to base excess or to deficit, data ~n tetmperature changes have]not been ~ound m the hterature Small changes may t]ake place and th~s orol~lem needs ~?urther mvest~galaon In tlus work the tmeor~ected values obtam~ed ~v~th Astrup's method were reported
Th ere are two major difficulties in assessing t method In tins series all the hearts wl~ch fibraUated bnllated with one electrmal shbck But It is a c!lmc, ~t is m~poss~ble to state defimtely that the low pH re~ |atmn It is equally dlflacult to assess the cerebral cl
he possible benefits of the (except one) could be ea-
.1 serms without controls and dly chd facdxtate the defibnl- rculatmn quantatatavely ze~th ephalogram was reeorde ] on the evadence avadable Unfortunately the electrpenc
an oscilloscope, and no permanent records haye ]zeen kept m the majority of cases It is our trnpresslon that at 25 ~ C body tefaperature, the E E G s[.ows more fast wave acttvlty and a higher amphtude wilton the pCO2 1s dehber~Ltely increased
The other dl~culty of assessment stems fro m tile fact that the method iwas mtraduced gradually As outhne~ previously, a mlxlure of O2 and CO2 was ~ased m all our pump cases After the introduction of an a,~lequate monltonng systepa, ~t was found that a certain degree qf CO2 retentmtl took place dunng extracorp0real bypass with and without hypothermm Based on |he theorelacal conslderahons mentaoned above, the CO2 retention was then' de]Jiberately increased This/was done gradually, trying to obtain higher and h lghe r~ - p c o z s as tame went on uon- sequently there is no sharp dwldmg hne betweenl two groups whmh coul d be ,compared by means of the conventmnal stahstmal methods
i
The mean values obtained an the patmnts' arterml]blood before bypass, to~'ards the end of the bypass, and at the end of the operaimn are given m Table ]I In seven adult patmnts the first arterml blood sample ~ould be taken before a aaes- �9 thesm The samples labelled "after anaesthesm" w~'re taken 30--60 minutes after the start of the anaesthesm The samples labelledl / "before bypass" were aken from the patmnt either through an arterial needle m the radml artery or frora the aorta By that tame the patmnt had been under anaesthesm for two hours or longer The effects of moderate hyperventdatmn a~e readdy apparent Th~ pro- gresswe base deficat has been observed by others u~der ,,nndar circumstance ~ 79;so In ordel to assess the base deficit, the same figure lobtamed m aM arterml }dood sample taken before the patmnt is gwen the preanaesthetm medication should be available for comparison However, m the pataent's interest it was decided r Lot to take this samp|e, whmh would have necessitated an / arterial puncture at 6 30 a m on the day of surgery The zero vaJue used by Astmp s corresponds to a standard bmarbonate concentratmn of 22 9 m E q / L m the serum It cannot be assumed that all our patmnts started out with this concentr~hon, although the mean~ and standard dewatmns mdmate that some were m the ~ero range at the begmmlng of anaesthesm and also after one to two hours of anaesthesl,t
During bypass b]c~t samples were taken every110--20 minutes at the arterml end of the oxygenator For the purpose of companson the data obtained m the sample wath the lowest pH for each run were anal)tsed tfurther and the mea~ls are reported an the table Since a progresswe acidosis developed wtule the bypass was m paogress, th~ usually was the last sample taken/during bypass In the pahents who,oe temperature dropped at ]east two degrees, c0rrectmn factors were apphed,
140 C~AD~a~ ,~a~.~_.STrmaTsxs" s ~ jo t raN~
and the means of both corrected and uncx corrected pH values were 7 32 m the non thermm group values winch are cons pCO2 values (corrected for temperature the normal range durmg the bypass, but
)rrL:~
,lOth~
dere ,hen the i
ted values are reported nua group and 7 33 m
to be shghtly acad ~ecessary) were found cans do not gwe the c
ture A gradual mcrease of pCO2 values ) th~ normal range and shgh than normal occurred dunng bypass Durflag sl~ort hans, the normal ran~ reached, hence the values obtamed towards 61~ end of bypass m the s as a rule were below the mean, wherea~ th(~se cbtamed m longer c above the mean
Oceasmnally a rapid mcrease of the pCO2 ~zas observed as soon as pulmonary bypass was started This may ~e d ae to the fact that the p blood mcreased as its temperature mcrea :ed If blood ~s warmed fron body temperature the pCO~. almost double san, i CCh may be gaven off ! under these ctreumstances 81 In accordmme V~lth thls theory the pi adopted an hypothermm cases of exposmg t~6 blood m the oxygen mstead of the O~CO~. mixture Another reasofi for tins practace is the of oxygen bubble formahon ff the cold blood~ is exposed to a high o~ centrahon s2
The mcrease m base deficat dunng bypgss w~s not unexpected Large of bank blood had ]Seen added to the patmnts' bloo:l Although a large as well tolerated by a pahent m the absence Of shock, s~ dunng bypa~
I
account for part of the amdosxs Furthermore I m the course of prolon dures, a base deficat develops even when the two cases w~th circulatory arrest it wa: although other mveshgators have not cot stances was the base deficit allowed to d:
hlgl
a~ 2 op 1
trary hmat was reached, sodium bicarbonate w At the end of the operatmn the pH ha, ns~
had fallen shghtly and there was stall a b ~e ( of the bypass The deficit was shghtly ess operated upon under normothermm that m d~fference may be accounted for by the fact prises the more dxfflcult cases wath longer by r possabahty that a metabohc acxdosls always o(
perfusmn flows are usq [cxpated that a deficat wo ~d t~as findmg 69 Under ~elow 10 m E q / L Whel as gwen mtravenously (i m to low normal values leficlt although less than pronounced in pattents/ those who were cooled I that the hypothermm
,ass periods On the othe ;curs followmg hypother
I
['he mean the hypo- )he The o be near erect pm- tly h, gh r e was not lort cases ases were
he carcho- 302 of the 25 ~ C to
) the body t c h c e w a s
tot to mr ~ossabflaty
ygen con-
quanlataes r a n s f u s l o n
tins may ed proce-
d 83 and m dd occur s~ 1o clrcum- that arb~- patients ) The pCO2 at the end who were The small roup corn-
hand, the laa cannot
be discounted 18 59 The causes of thts te~ tden zy towards acidosis are aot readdy apparent It was concewable that under t ypot hernna the avallabthty c oxygen to the ceils decreases more than the oxygen req~ l~rements of the cells, re ulhng in a relahve hypoxm There is some expermael ttal ,wldence that tins may b( the case 85
The mcrease m base deficit during the first period of the procedure, before the bypass was started, was found to be similar ~ magmtude to the further mcrease winch occurred during and following bypassl This second period was considera- bly longer than the first one and furth~rmole, during the second p~'nod, large amounts of the more acxdotm blood were gwen as priming volume 0t" the pump oxygenatoi" and as transfusmn The further m(trease m acidosis may mdmate, how- ever, that the perfusmn was not as eflaclent as the normal blood clrculatmn,
ZINGG, W ACID-BASE BALANCE DURIN G O P E N + ~ T SURGERY 141
resulhng m inadequate tassue perfusmn wbach ~r~ tm el may, cause anaerobm r~ bohsm and acxdos~s s0 Following bypass, the conl.pev tory mechamsms deere~ the base deficxt again and no pataents recewed ~.ddal hal base during this pe~
In order to produce an ac~doms at the end of th, ~ardmpulmonary bypas: postulated an the workang hypothes~s, at was ~ot ~ e~es,,ary to gave mtrave~ ammomum chloride A base deficat (or excess ~cld
e t a -
~sed lod. ~, a s
lOLlS
the ~nge the
thon tell- tally
was already present If pCO2 was increased from the low levels found ~efo~e bypass to the normal r~ or somewhat bagher, the pH ch-opped to the d~slred ilevel Following bypas'., pCO2 was lowered again by controlled resptrahon ~a~ tile base def ic i t w a s p. eorrected by the body s regulatory mechamsmsj Th~ ; at the end of the oper ihe acld-bas'e balance'was cl~ ose to the normal r~nge By paying metaeulou~s hon to the acid-base balance throughout the yehol ~ procedure tins was us~ actueved w~thout addltaonal infusion of sodmm Ibmarbonate
Over-breat/nng may occur with controlled respmLtaon under anaesthesia ~ nd a progresswe loss of base may follow _Indeed, lt~has been shown that the acl tosls associated with thoracm operahons for a variety of reasons is smallar to that observed following open-heart surgery 79.86 The r?sult,~ obtmned m this : ~ d y confirm this findmg since they suggest that a part oflthe ae,d slnft occurred b.~fore
I the bypass was started The productmn of ~ res~lratory alkalosas as corILmon anaesthetm practace 3s In tl~s series vagorous hyp~enh lahon was avoided and the results show that the resparatory alkalosls v~as o ! a rmld degree After cor aple- hon of the extracorooreal c~rculataon hvoerventdatLon was employed m order to increase the carbon'dmxlde excretmn bu~t no atter~ipt was made to mduce~ res- piratory alkalosas Again wgorous hyperventdahon ~vas avoided as before bypass
"I~he pH thus underwent a cychcal change After an lmtml nse during bypass there was a progresswe fall an pH associated wjlth a return of the pC0.~ to normal levels or shghtly higher After termmatmn I of the bypass the pCO~ was reduced by controlled resplrahon At the end 6t? the operatmn the pH returned to a normal level m the majority of the pataents In some cases a shght met~Lbohc acidosis persisted at that tame, whmh always was corrected spontaneously jwlth- out further treatment This would appear to be a physmloglcal way to handle the acid-base balance dunng extracorporeal clrcUlatl~n and hypothermm avoxdmg the add~tmn of ac~dlfymg substances and avoad~hg the admm~strahon of base unless it is specifically required
At every stage of the procedure large shifts of the pH m either dlrechor~ were avoided The importance of preventing extensWe cllanges m pH has been ou}hned by several mveshgators 87 88 It is interesting to note that the preventaon I of an excesswely low pH during extraeorporeal clrculalmn and hypothermm w~s one of the mare arguments of those who recommend l~yperventdahon and/or t~ae use of pure oxygen an the oxygenator 89 It is quest:onable, however, whether all authors included the necessary correchon faclors n the calculahon of the actual pH of the pataents Furthermore, according to [m working hypothesis, ~ tem- poaary lowenng of the pH at the end of the byp~Lss was thought to be detarable and no attempt was made to prevent the fall, Im f~.ct, 11 was dehberately reduced In addatmn to faellltatang ventncular defibnllataoiL and[ the other effects o~tthned in the d~scussmn, the h~gh C�9 conserves base by d~mmlshmg the renal exeretmn
142 CANADmN ANAES~TS" soama'r JO~mNAL
of bicarbonate and the recovery period bohc acid-base balance ~176 and by adopting tae management describe can have the benefits of a lngh CO2 CLunngJ long peduslons and th with hypothernna, without being exposed L0 the risks of excesswe i blood pH The pnnclples of ~ e method, flaerefore, are dose to recently by Theye et al 9: e g 'mlmmal Lr/:erference w~th and ma~
m so doing tt r~ay a~lp prevent a metabolic acidosis m By carefully rnomton ag both the respiratory md thq metla -
1, the patients 3se eombmqld :hanges in tile 'hose outhned real preserve-
txon of" the various body functions It should be remembered, however, that
ber of assumptions, some of which may The problems associated with the m~asu: at low temperatures are being actwr 1 quite possible that some of our ideas Will e
~he whole concept is bqsed on a num- tot be found acceptable Im the future
, I ement of pH, pC02, a~d bicarbonate ~yeshgated in many cegtres and it is lange
SUMMARY ~ND
Many aspects of the acid-base balanc during hypothemna are still problematic ground is reviewed which led to t h e ad~ thesis
~0NC%USlONS
~d during extracorporeal ClrculaUon an Jk In this paper the theoretical bae -
ptaon of the following Worhng hyl~o-
The acid-base balance has a dire@ mflgence on two nnportant aspects of cgr- dlopulmonary bypass Dunng bypas~ a ~lgh carbon dioxide tens on wdl ensqre an adequate cerebral blood flow and at ~he end Of the bypass a low pH is s~ud to facilitate defibrillation Both these cgndmons can be create t by avoiding excessive hyperventdahon of the p~taent before bypass and by.lmcreasmg die pCO2 during bypass The arterial p, .C02 was controlled by varying the voiu~e of the O2/CO2 gas mixture delivered to the oxygenator ~ |
P'' I ~ [ The method has been employed m ~2 consecutive patients T h e acld-base balance was monitored using Astrup's atlparatus Large pH shfft~ were avoided
I and cltmcally the pahents appeared ~o d!l well under this type of managementl The rationale of the concept is discussed and it is concluded tha~ it has bee~ of
benefit for the patients upon whom it was used
Plusleurs aspects de l'6qmhbre acide-l:ase au eours de la circulation extrac:or- porelle et de l'hypothermle demeurdnt encore probldmataques Dans cet artr21e, nous revoyons la thdor:e de base qua eoncLmt at l'adoptlon de l'hypgth~se smvm Lte
L'dqmhbre acade-base exeree une influt;nce dtrecte sur deux asl~ects imports nts de la ctrculahon cardlo-pulmonalreJ extiacoqzorelle Au tours d e l a circulation extracorporelle, un taux 61evd de ghz carbomque va assurer ufl debit sangmn
�9 t �9 i {:~, cerdbral adequat et, ~ la fin de la clrcul~ttaon, un pH bas est suppos6 faclhte: la
p ^ I defibrillation Ces de~x 6tats peuven~ etr~ r6ahs6s en 6vltant une ~yperventdalaon excesswe du malade avant la elrcUlatt0n extraeorporelle et en au~rnentant la I, I o presslon de CO2 au eours de cette cl~eulataon La pCO2 artdnelle ~a dt6 mamtehue en variant le volume du m6lange gazeux 02/CO2 lntr0dmt dans l'oxlygdnateur
ZINC, G, 'W ACID-BASE B A L A N C E DURING O P E N - H E A R T SURGERY 143
Nous avons utfl~s6 cette m6thode chez 72 malades cons6cutafs La courbe de " ' ' ^ " ' ! ' I " ' l eqmhbre aclde-base a 6t6 vasuahsee grace a 1 apparell Astrup Nous avons eviI6
des chan~ements marqu6s du DH et, ehmcluement, ~!~et~e facon de oroe6derla _ --, .~ ~ . t / I ~ ~" ~.
sembl6 avantageuse ~ nos malades Nous avons e~pos~i, la log~que de ce po,mt de rue, et nous en venons ~ la conclusmn que, chdz le,, malades ott nous 1 avofis utahs6e, cette m6thode a rendu serwce
A C K N O W L E D G M E N T S
The author is indebted to the members oL the car:hac surgery teams of tile Wmn~_peg Chfldrens Hospital and of the Winnipeg ~tJeneral Hospital for their eo-operatmn in obtaining the data presented in thin papex, and m particular :o D~ Colin C Ferguson, Professor and Head of th~ Departrrient of Surgery and to Da H D Sanders, Department of Pharmacology and Therapeutms, for their help m the preparatmn of th~s manuscript
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