inaccuracies in manometric central venous pressure measurement

8/9/2019 Inaccuracies in Manometric Central Venous Pressure Measurement

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Rwwcitution 16 w2S) 221-220Elsevier Scientific Publishers I reland Ltd.

221

INACCURACIES IN MANOM ETRIC CENTRAL VENOUS PRESSUREME SUREMENT

D.G. CLAYT ON*

Intonsivs Care Unit, Royal A&hide Hoapita4 North Termce Adekde S. Awtndia SO00lAwtrald

(Received J uly 20th. 1287)(Revision received November 1st. 1987)(Accepted November 12th, 1987)

SUMMARY

Manom etric m easurement of CV P was compared to electronic measurem entin ten patients. Manom etric measurem entwas found to give readings of up to 5cm& O greater than electronic, with a mean difference of 2.4 cmH ,O. This wasshown to he due to two factors. Firstly, a r@ eniscus effec t caused an error of 1.07cmH ,O in the manom eters used. Secondly, an error attributed to the way meanvalues of CV P are commonly read from manom eters caused a further over esti-

mate of 1.33 cmH ,O. In intensive care units, where it is important to recogniseand treat sm all changes in CV P, the use of electronic transducers to measureCVP is recommended.

Key words: Clinical measurem ent - Intensive care - Central venous pressure

INTRODUCTION

Manom etric measurem ent of central venous pressure (CVP l continues to heused widely. In a critically ill patient, the choice of treatm ent may depend onthe CV P measurem ent. It is thus important that measurem ents should be asaccurate as possible. When changing from manom etric to electronic measure-ment, a clinical impression was formed that values obtained with the manome-ter have been higher than those found electronically. The relative inaccuracy ofmanom eters has been previously noted [l-3]. It was therefore decided to con-firm the presence of this discrepancy between manom etric and electronic mea-surement of CV P, and to elucidate the causative factors.

02004672/@ 02.50 0 1928 Elsevier Scientific Publishers Ireland Ltd.Printed and Published in I reland


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MATERIALS AND METHODS

The investigation was conducted in three parts. The first part studiedwhether a meniscus effect could be demonstrated in CV P manom eters. The

second part was designed to eliminate any meniscus effect and determinewhether any further discrepancy between electronic and manom etric C VPexisted. The third part was to demonstrate whether electronic or manom etricmeasurem ent gave the most accurate mean readings.

To assess whether a meniscus effect existed with the CV P manom eters usedin this unit (Tuta laboratories), 11 man ome ters from eight d ifferent batcheswere examined. Each set was primed with 5 dextrose, and care taken toexclude air bubbles. The end of the measuring set which would normally beconnected to the central venous cannula w as left open to air, and held consecu-tively at the four positions on the m anom eter column (Fig. 11. In all case s an

approximately hemispherical meniscus formed at the open end of the measur-ing set. Thus surface tension forces at the orifice should have been constant,and no effect due to a hanging drop was a llowed to occur. The level at which themeniscus in the manom eter settled was noted and the height differencebetween the meniscus and the end of the measuring set recorded. In total, 44observations were made .

E

Fig. 1. Method of demonstrating meniscus effect.


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To confirm that a meniscus effect was present comm ercial detergent wasadded to the 5 dextrose to form a 0.05 solution. T he experiment was thenrepeated, again with care being taken to standardise the shape of drop formedat the orifice open to air. The internal diameter of the manom eter column w asdetermined by injecting 0.8 ml of 5 dextrose into the manom eter column,mea suring the height of the column of liquid form ed, and calculating a mea ndiameter assuming the column w as a uniform cylinder. Finally, once the diame-ter of the manometer was known, this value was used to calculate the predictedmeniscus effect. The expected meniscus effect is calculated using the formulabelow, quoted in standard textbooks on physics, which relates size of me niscuseffec t for a given solution to the sur face tension of that solution, and the dimen-sions of its container.

4 x ST x case& =

x w

whe re: h is the height of men iscus effect; ST is the surfac e tension of 5 dex-trose; 8 is the wetting angle - assumed to be 25O; i s the diameter of thecolumn; w is the specific weight of 6 dextrose.

To determine if the meniscus effect completely explained the discrepancybetween manom etric and electronic CV P readings which had previously beennoted, ten patients with subclavian central venous cannulae in situ we re stud-ied. The cannulae were all 16 gauge and either Angioguides or Intracaths. Thetip position wa s in either low superior vena ca va, or right atrium and wasconfirm ed rad iologically. Kinking of the cannula or positioning against a vesse lwall wa s also excluded . B oth spontaneously breathing and mec hanically venti-lated patients were included. N o patient was receiving PE EP or CP AP. Thereference point for readings was the mid-axillary line. A three-way tap andshort connection we re conn ected to the cannula hub. One port of the three-waytap was connected to a Tuta C VP measuring set. The second port was con-nected to a Gould Pressure Transducer and on to a Siemens pressure module,Sirecust 404, with frequency response 0 - 20 Hz (Fig. 21. Care was taken in zero-ing the transduc er against the manom eter to eliminate the error due to themeniscus effect. This was done by filling the manom eter up to 20 cm of water

and allowing it to vent to air through a three-way tap in line with thetransducer. When the manometer level had settled, the tap was turned so thatthe manom eter was connected to the transducer, and the transducer set tozero. When the transducer was zeroed it was connected only to the manom etercolumn and no other taps w ere op en, thus eliminating any error s due to me nis-cus effects at open taps, or in the manometer itself. The CV P manom eter wasthen u sed to confirm linearity and gain of the transdu cer by filling it to 10.20and 30 cmH ,O, and checking the values recorded by the transducer. As thetransducer was calibrated against the manom eter, any inaccuracies due topatient position, an d refer ence point on the body we re eliminated. W hen


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4

30

20

10

0

10

Fig. 2. Method used for obtaining paired readinga of manometric and electronic CVP.

readings were being m ade any other infusions were stopped and the lineflushed with 6 dextrose. Paired readings of CV P were then made as follows.The patients were laid flat, and measurem ents taken from the mid-axillary line.For the m anometric CV P a swing of at least 1 cm was required to confirmcatheter patency before readings were taken. With only the manometer opento the patient a me an value of CV P was estimated without stopping ventilation,from the maximum and minimum values recorded over 1 min. This is believedto reflect clinical practice. T he manom eter was then sw itched off and the trans-

ducer opened to the patient and an electronic mean value for CV P was read offthe press ure modu le, again with ventilation continuing. Each reading w astaken tw ice and an average value obtained. A total of 31 pairs of readings weremade. Value measurem ents in mm Hg from the pressure transducer were con-verted to cmH ,O by multiplying by 1.36.

To demonstrate which of the two methods of measuring CV P was the moreaccurate, a third study was conducted. A range of pressure wave forms gener-ated by a Bio-Tek Model 6OlA blood pressure systems monitor w ere used. Themean values of these wave form s were m easured using the method employed inthe second part of this study. Each reading was taken three times and an aver-


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age value obtained. The wave forms used were: a square wave, a sine wave, aright ventricular wave, two pulmonary artery waves, and a pulmonary arterywedge pressure wave. In addition to measuring the mean value of these waveswith the manometer and Siemens pressure module, a third, more sophisticatedmeasuring system was employed to obtain a more accurate response, andderive a reference mean. This comprised a Hewlett-Packard pressure amplifiermodel 8805 with filter set at 450 Hz, connected to a Honeywell 1508 ultravioletpaper recorder, with a frequency response of 800 Hz. Having recorded the gen-erated pressure wave forms using this third system, a true arithmetical meanwas determined by measuring the area under the waveform using 2 mmsquared graph paper.

Statistical analysis of the manometrically and electronically determinedCVPs was by paired t-testing, and of differences in error at low, normal, andhigh CVP by one-way analysis of variance.

RESULTS

The results of the investigation to detect any meniscus effect are shown inTable I. With the manometer primed with 5 dextrose and open to air, themean of 44 manometer readings was 1.07 f 0.43 cmH,O. With detergent addedto the dextrose the manometer reading was 0 + 0 cmH,O. A meniscus effectwas thus clearly demonstrated. The mean internal diameter of the manometercolumn was found to be 1.70 f 0.02 mm in the 11 sets used. Calculation of theexpected meniscus error in a manometer column of diameter 1.7 mm gave aresult of 1.53 cm.

In the second investigation with the meniscus effect eliminated, the resultsof paired CVP readings taken with the manometer and with an electronic trans-ducer are shown in Tables II, III and IV. The range of CVP readings was from- 1 to +20 cmH,O and range of difference in readings from - 0.8 to + 3.4cmH,O. The manometer gave values greater than the transducer in 29 of the 31cases (93.5 ). Breakdown of these results showed the error to be significantlygreater in ventilated than spontaneously breathing patients, and greatest atlow CVPs (Tables III and IV). Therefore a second error affected by mode of

TABLE I

DEMONSTRATION OF MENISCUS EFFECT WITH 5 DEXTROSE AND ITS ABSENCEWHEN DETERGENT WAS ADDED TO THE PRIMING SOLU TION

Priming solution

5 Dextrose0.06 detergent

in 6 Dextrose

No. observations

4444

Mean man omete r reading Statisticalopen to air f S.D. cmH,O significance

1.07 f 0.43 t = 109;o+o P 0.0001


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TABLE N

COMPARISON FDIFFERENCE ETWEEN ANOMETRIC ND ELECTRONIC EADINGSAT LOW < 6 cm&O); NORMAL S- 12 cmH,O), ND HIGH > 12 cmH,O) CVP, BY ONE-WAYANALYSIS OF VARIANCE

*Difference in low CVP group statistically greater than normal CVP group, but not high CVP

group.

CVP cmH,O No. of observations Mean difference Statisticalf S.D. cmH,O significance

12 13 1.42 f 0.93

ventilation was demonstrated to be causing over-reading of the manometer.The largest total error recorded, due to meniscus effect plus this second effect,was 6 cmH,O, and the least error 0.5 cmH,O.

The results of the third study to determine a reference mean value asaccurately as possible and compare it to the manometric and electronic meansare shown in Table V. It is seen that for the simple wave forms of square waveand sine wave the three systems were in close agreement. However, with themore complex wave forms, of the type found in patients, marked differenceswere found. In these four cases the reference mean was closer to the electroni-cally determined mean than the manometrically determined mean. In each casethe electronic mean was within 1 cmH,O of the reference mean, while the mano-

meter mean was more than 1 cmH,O different, and always greater than the ref-erence mean.

DISCUSSION

This investigation shows that a manometric determination of CVP with acommercially available set of 1.7 mm diameter may differ by an average 2.4

TABLE V

COMPARISON OF MANOM ETRIC. ELECTRONIC AND,REFERENCE MEAN VALUES FORSIX GENERATED WAVE FO RMS AT 1 Hz

Waveform Manometric Electronic Referencemean cm 0 mean cmH O mean cmH,O

Square waveSine waveRight ventricularPulmonary arteryPulmonary rtery

with catheter whippulmonary artery

wedge with v wave

28.513.56.09.09.0

7.0

28.5 28 013.6 14.1

4.1 4.18.2 7.88.2 7.3

5.4 5.9


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cmH,O from an electronic determination. The error range for any given patientvaried by as much as 5 cmH,O, could not be predicted, and varied at differenttimes in the same patient. Manometric readings were always higher than elec-tronic readings when the meniscus effect was taken into account. The error was

due to at least two factors. Firstly, a meniscus or surface tension effect of 1.07cmH,O was found in the manometers tested. Secondly, another factor, or fac-tors led to a further over-estimate of 1.33 cmH,O.

The method of demonstrating the meniscus effect may be criticized becauseany meniscus effect existing at the manometer orifice held open to air was nottaken into account. The measured value for meniscus effect was less than, butclose to, the calculated value. Thus pressure effects at this orifice tended toreduce the observed meniscus effect, and did not introduce a major error. Thesame criticism may be used when considering the method of zeroing the trans-ducer. Again, the error was not large and tended to reduce errors from themanometer, not increase them.

The second error of 1.33 cmH,O could have been due to either of twopossibilities. Firstly, that the manometer was over-reading, or secondly thatthe meaning process in the electronic system led to under-reading. The thirdmethod used to determine a reference mean using more accurate equipment,and a mathematically calculated mean suggested that the manometer was inerror. It thus remains to be explained why the Tuta manometers used in thisstudy should over-estimate mean CVP. This has been ascribed to resonancewave fronts, bends in the catheter, and the low frequency response rate of thefluid manometer by Mann et al. [2]. These factors, however, would not beexpected to affect the mean pressure. A possible explanation is that the method

of reading by eye, a mean CVP from a manometer column, fluctuating with bothpulse and respiration, caused this error. The CVP waveform is a complex phasicsignal, and as with arterial waveforms, the true mean value is closer to thelowest point of the waveform than the highest. Thus reading a middle valuebetween highest and lowest swings on the manometer, as is common clinicalpractice, will cause over-reading of manometric CVP. This argument issupported by the findings using generated waveforms. There was goodagreement between the two systems, and also with the mathematically deter-mined mean with the sine and square waves. This suggests that there was nointrinsic fault in the experimental set-up. These two signals are symmetrical.The phasic physiological signals used however caused marked differences be-tween the measuring systems. Here the systolic diastolic ratio is less than 1 : 1,and thus the short systolic spike would cause an over-reading of mean pressure,where the mean pressure is read as the half way point between highestand lowest oscillation of the manometer. Further support for this argumentcomes from the comparison of results for spontaneously breathing, andventilated patients. In ventilated patients with an inspiratory to expiratoryratio of 1 : 2, as used in this investigation, the short inspiration causes a shortpeak in the CVP, followed by a longer trough during expiration. Having ashorter peak time than trough time causes over-reading of the mean value, in


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the same way as discussed above for phasic pressure waves. During sponta-neous ventilation this effect is reversed, and as would be predicted, smallererrors in the CVP readings were found.

The greatest error in CV P readings occurred in patients with CV Ps of


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ter also dem onstrated. In intensive care units, wher e it is important to recog-nise and treat even small changes in CV P, the use of electronic transducers isrecommended.

ACKNOWLEDGEMENTS

The author would lik e to thank Dr . W .J. Ru ssell for help with the protocol,statistics and manuscript, M r. G. Elsegood of the Biomedical EngineeringDepartmen t for help in setting up the experiments, Dr. T.R. W illiams of W est-minster Hospital, London for helpful criticisms and Miss A. Roberts and M issP. King for typing the m anuscript.

REFERENCES

1 J.M. Civetta. Pulmonary artery pressure determination: Electronic Sup erior to Manom etric,N. Engl. J. Med., 285 (1970) 1146 1140.

2 RL. Mann, G.C. Carlon and A.D. Turnbull. Comparison of electronic and manometric centralvenous pressures, Crit. Care Med., 9 (2) (1991) 99- 100.

2 J. Werweij, A. Kester, W. Stroes and L.G. Thijs. Comparison of three methods for measuringcentral venous pressure, Crit. Care Med., 14 (4) (1996) 288 - 290.

inaccuracies in manometric central venous pressure measurement

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