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http://ccn.aacnjournals.orgPublished online © 2002 American Association of Critical-Care NursesCrit Care Nurse. 2002;22: 60-79 Beate H. McGhee and Elizabeth J. Bridges Monitoring Arterial Blood Pressure: What You May Not Know

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60 CRITICAL CARE NURSE Vol 22, No. 2, APRIL 2002

and mismanagement. The resultsof a recent pilot study1 at 2 univer-sity-affiliated hospitals suggesteda knowledge deficit in arterialpressure monitoring and some ofthe most basic aspects of hemo-dynamic monitoring. A total of391 critical care nurses practicingin various critical care specialtieswere invited to participate in thestudy. The response rate was17.4% (n = 68). Most of the partici-pants were between the ages of 30and 39 years (56.1%) and had abaccalaureate degree as theirbasic (61.8%) and highest degreein nursing (58.8%). Most partici-pants had more than 4 years ofnursing experience (94.1%) andcritical care experience (83.9%),and most did direct ABP monitor-ing at least once or twice eachweek (97.1%). The participantswere asked to complete an 18-item,criterion-referenced question-naire on ABP physiology, techni-cal aspects of ABP monitoring,and ABP waveform interpretationin selected pathophysiologicalconditions. The mean score inthis pilot study was 36.7% (SD,11.8%). Total scores ranged from11.1% to 61.1%.

Literature on nurses’ knowl-edge of hemodynamic monitoringis limited, but several studies,2-5

Beate H. McGhee is an adult carenurse practitioner and a recentgraduate of the master’s program inthe School of Nursing, University ofWashington, Seattle, Wash. MajElizabeth J. Bridges has a doctoraldegree from the School of Nursing,University of Washington, Seattle.

published and unpublished, indi-cate a general knowledge deficit inpulmonary artery pressure moni-toring. Because of these researchfindings, in this article, we focuson areas of particular knowledgedeficit related to essential princi-ples of hemodynamic monitoringand ABP monitoring. We discussthe physiology of ABP, physiologi-cal and pathophysiological factorsthat affect ABP, and the arterialpressure waveform and its inter-pretation in clinical situationscommon in critical care patients.

ABP PHYSIOLOGYThe cardiovascular system has

3 types of pressures6,7: hemody-namic, kinetic energy, and hydro-static. Hemodynamic pressure isthe energy imparted to the bloodby contraction of the left ventricle.This type of pressure is preservedby the elastic properties of thearterial system. Kinetic energy isthe energy associated with motionand affects the pressure measuredduring direct ABP monitoring.Fluid density and gravity con-tribute to hydrostatic pressure,which is the pressure a column offluid exerts on the container wall.For example, in a column of fluid,the pressure at a given level in thecontainer is proportional to the

Monitoring ArterialBlood Pressure:What You May Not KnowBeate H. McGhee, BSN, MN, APNMaj Elizabeth J. Bridges, USAF, NC

r t e r i a lblood pres-sure (ABP)is a basich e m o d y -n a m i cindex oftenutilized to

guide therapeutic interventions,especially in critically ill patients.Inaccurate ABP measuring cre-ates a potential for misdiagnosis

ATo purchase reprints, contact The InnoVisionGroup, 101 Columbia, Aliso Viejo, CA 92656.Phone, (800) 809-2273 or (949) 362-2050 (ext 532); fax, (949) 362-2049; e-mail,[email protected].

To receive CEcredit for thisarticle, visit theAmerican Asso-ciation of Critical-C a r e N u r s e s ’(AACN) Web siteat http://www.aacn.org, click on

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CRITICAL CARE NURSE Vol 22, No. 2, APRIL 2002 61

height of the fluid column abovethat level. The pressure is highestat the bottom of the column. Inthe vascular system, hydrostaticpressure is proportional to theheight of the column of bloodbetween the heart and the periph-eral vasculature. In a standingperson, the pressure in the leg ishigher than the pressure in thearm by the difference in hydro-static pressure. In summary, arte-rial blood pressure represents theforce exerted by the blood perunit area on the arterial wall6-8 andis the sum of hemodynamic,kinetic, and hydrostatic pressure.

The arterial tree starts with theaorta and the major branches ofthis vessel. The aorta and itsbranches stretch to receive bloodfrom the left ventricle and recoilto distribute the blood and tomaintain arterial pressure. Art-eries and arterioles control bloodpressure through vasoconstric-tion or vasodilation.6 Arteriolesare the primary sites that con-tribute to systemic vascular resis-tance (SVR).6,9-11 In addition,adrenergic control of the arteri-oles is a major determinant ofblood flow into the capillaries. Inthe skin, for instance, blood canbe shunted from the capillarybeds to flow directly from arteri-oles into the venous system.9

Arteriovenous shunting occurs inshock states and helps to redirectblood flow to vital organs. Arterio-venous shunting is one reasonmeasurements of blood pressurealone are not a good indicator ofperipheral tissue perfusion.

Arterial pressure is measured atits peak, which is the systolic bloodpressure (SBP), and at its trough,which is the diastolic blood pres-sure (DBP). The SBP is determinedby the stroke volume, the velocityof left ventricular ejection (an indi-

rect indicator of left ventricularcontractile force), systemic arterialresistance, the distensibility of theaortic and arterial walls, the vis-cosity of blood, and the left ven-tricular preload (end-diastolicvolume).11-13 The blood pressure inthe aorta during systole is a clinicalindicator of afterload (the sum ofthe forces the left ventricle mustovercome to eject blood).14,15 Thediastolic pressure is affected byblood viscosity, arterial distensibil-ity, systemic resistance, and thelength of the cardiac cycle.11,16

Pulse pressure is the differencebetween systolic and diastolicpressure. A normal pulse pressurein the brachial artery is approxi-mately 40 mm Hg. An increasedpulse pressure may be the result ofincreased stroke volume or ejec-tion velocity and is common dur-ing fever, exercise, anemia, andhyperthyroidism.11 Other causes ofincreased pulse pressure includebradycardia (increased stroke vol-ume), aortic regurgitation, andarterial stiffening, which is mostnoticeable after the age of 50 to 60years.11,17-19 An acute decrease inpulse pressure may indicate anincrease in vascular resistance,decreased stroke volume, ordecreased intravascular volume.11-13

Systemic mean arterial pres-sure (MAP) is defined as the meanperfusion pressure throughout thecardiac cycle. MAP is sensed bybaroreceptors located in thecarotid sinuses and the arch of theaorta. These receptors control arte-rial pressure mainly by adjustingheart rate and arteriolar vesselradius. MAP is also the basis forautoregulation by some organ sys-tems such as the kidney, heart, andbrain. Autoregulation is the auto-matic adaptation of the radius ofan arteriolar vessel in an organ tomaintain constant blood flow over

a wide range of mean pressures(60-150 mm Hg) to protect func-tioning of the organ.7,20 MAP is theproduct of SVR and cardiac output(MAP=SVR×cardiac output).7,10

As indicated previously, a maindeterminant of SVR is the radius ofarterial, and particularly arteriolar,vessels. Changes in cardiac outputare related to heart rate and strokevolume. Stroke volume, in turn, isdetermined by several factors,including heart rate, preload, after-load, cardiac contractility, and syn-ergy of cardiac contraction (relatedto ventricular dilatation, abnormal-ities in ventricular wall motion, andventricular arrhythmias).10,11 MAP isgenerally closer to diastolic pres-sure because diastole representsabout two thirds of the cardiaccycle when the mean heart rate isclose to 60/min. This relationshipis expressed in the well-known for-mulas MAP = DBP + (SBP -DBP)/3and MAP = [SBP + (DBP x 2)]/3.

However, the proportion ofdiastole in the cardiac cyclechanges with changes in heartrate. In calculations of MAP for amanually obtained ABP, these for-mulas must be used with caution,because they provide a good esti-mate of MAP only when the heartrate is close to 60/min.10 Fortu-nately, MAP is provided by mostautomatic ABP measuring devicesand direct ABP monitoring sys-tems, each of which uses a system-specific method to directlydetermine MAP.

In summary, because of themultiplicity of factors that con-tribute to ABP and the complexityof their interrelationships, inter-preting changes in arterial pres-sure and its components (SBP,DBP, MAP, and pulse pressure) asindicative of any single factor maylead to an erroneous assessmentof a patient’s condition. When SBP




and DBP are measured using dif-ferent (oscillometric or direct)monitoring methods, the valuescan differ significantly. When MAPis measured using different moni-toring methods, however, the val-ues are very similar, because MAPis little affected by the phenom-enon of wave reflection.21-25 Wavereflection and other factors thataffect measurement of SBP andDBP are discussed later.

PRINCIPLES OFHEMODYNAMICMONITORINGThree Conventions ofCardiovascular PressureMeasurement

For measurement of cardio-vascular pressures, 3 conventionsare observed6,7,20: (1) Cardio-vascular pressures are expressedin millimeters of mercury, withthe exception of central venouspressure, which may be measuredin millimeters of mercury or incentimeters of water. For convert-ing values in centimeters of waterto values in millimeters of mer-cury, the value given in centime-ters of water is divided by thefactor 1.36. (2) Most cardiovascu-lar pressures, such as ABP, centralvenous pressure, and pulmonaryartery pressure, are referenced tothe heart or, more specifically, tothe atria, to eliminate hydrostaticpressure. (3) All cardiovascularpressure monitoring devices arezeroed to ambient atmosphericpressure, so that the actual pres-sure measured reflects the pres-sure above atmospheric pressure.

The HemodynamicMonitoring System

A hemodynamic monitoringsystem contains 2 compartments:the electronic system and thefluid-filled tubing system. Al-

though clinicians have little con-trol over the electronic compo-nents such as the monitor, correctsetup and maintenance of thetubing system and the pressuretransducer are absolutely crucialto avoid error. With an improperlyprepared or inadequately func-tioning monitoring system, notonly the actively measured hemo-dynamic indices but also anyderived variables will be errone-ous,26 potentially invalidating apatient’s entire hemodynamic pro-file. Three procedural steps shouldbe followed to prepare the moni-toring tubing system and ensure itscontinued accuracy: priming of thepressure tubing, leveling and zero-ing, and dynamic response testing.

Priming of the Pressure Tubing.The generation and recording of allarterial waveforms (systemic arteri-al pressure and pulmonary arterypressure) are based on the samebasic principles. The invasivecatheter provides access to thearterial system being monitoredand is designed to pick up the pres-sure waves generated in the arterialsystem by cardiac contractions.The catheter is connected to thefluid-filled tubing of the monitor-ing system. The fluid column in thetubing system carries the mechan-ical signal created by the pressurewave to the diaphragm of the elec-trical pressure transducer. Thetransducer creates the link betweenthe fluid-filled tubing system andthe electronic system, and convertsthe mechanical signal into an elec-trical signal. The electrical signal istransmitted to the monitor andthen is amplified and displayed asan analog waveform and digitaloutput.

Air as a medium transmitsmechanical impulses much dif-ferently than does fluid. Air bub-bles in the tubing system are one

of the most frequent and impor-tant sources of error in hemody-namic monitoring. Air bubblesmost often blunt or damp propa-gation of the mechanical signal,causing a damped analog wave-form and erroneous pressurereadings.27 Even tiny air bubblesonly 1 mm in diameter can causeserious waveform distortion.28

Therefore, air-free priming of theentire tubing system is one of themost important steps to avoidtechnical error.

Air-free priming starts withremoval of all air from the flushsolution to prevent air from goinginto the solution as a result of thepressure applied by the externalpressure cuff. Then, the entiretubing system should be flushed.Stopcocks, Luer-Lok interconnec-tions, and the transducer are com-mon locations of air entrapment27

and deserve special attentionthroughout priming and use of thecatheter system. In order to main-tain the air-free status after theinitial setup of the system, the fol-lowing measures are important:

■ After opening the system forblood sampling or zeroing, brieflyfast-flush the tubing system,including the proximal accessstopcock or the air-fluid interfacestopcock.

■ Tighten all connections, andensure that the stopcocks areclosed to air.

■ Avoid adding stopcocks andline extensions.

■ Keep the flush solution bagadequately filled, and keep theexternal pressure cuff at 300 mm Hg.

■ Periodically flick the tubingsystem and flush the tubing sys-tem and stopcocks to eliminatetiny air bubbles escaping theflushing solution.

Leveling and Zeroing. Thearterial pressure monitoring sys-



tem must be referenced to heartlevel, technically the level of theleft atrium, and set at atmospher-ic pressure as the zero referencepoint. These criteria can be metthrough leveling and zeroing.Leveling or referencing of thecatheter system is accomplishedby aligning the air-fluid interfaceof the monitoring system (eg, thestopcock on top of the transduc-er) with the external referencepoint of the heart. The externalreference point of the heart is thephlebostatic axis, which can belocated by finding the junction of2 lines: a vertical line drawn outfrom the fourth intercostal spaceat the sternum and a horizontalline drawn through the midpointof a line going from the anterior tothe posterior side of the chest.29,30

Where to level the air-fluidinterface is a matter of discussion.The answer depends on whichvascular bed is to be monitored.In most clinical situations, the

central arterial pressure is thepressure of interest, because it is a key determinant in cardiac andcerebral perfusion. In order tomonitor central arterial pressure,the monitoring system must beleveled to the heart by using thephlebostatic axis. Research indi-cates that the phlebostatic axismost accurately reflects the levelof the atrium in both supine andupright patients.29-31 The midaxil-lary line, which has also beenused as an external referencepoint for the heart, is not accuratein all chest configurations andthus is not recommended.31 Whenthe monitoring system is refer-enced to the tip of the catheter,then the transmural pressure of aparticular point in the arterial treeis monitored and not central arte-rial pressure. Peripherally mea-sured transmural pressure ismarkedly increased by hydrostat-ic pressure unless the patient issupine.32

Zeroing consists of 2 steps.First, the air-fluid interface isopened to atmospheric pressure.Then the monitor’s zeroing func-tion key or button is pressed.Zeroing in this fashion has severalpurposes. Zeroing electronicallyestablishes for the monitor atmo-spheric pressure as the atmospher-ic zero reference point. Zeroingestablishes the interface level asthe hydrostatic zero referencepoint. Zeroing also eliminates zero-drift. Zero-drift is the potential, butusually minimal, transducer offsetor distortion occurring over time.33

Two key practice objectives arerelated to referencing and zeroing:accuracy and consistency. In orderto ensure accuracy, leveling andzeroing must be done wheneverthe relationship between the air-fluid interface and the referencepoint is changed. The reason ishydrostatic pressure. For every 1cm the air-fluid interface is aboveor below the actual level of the left




atrium, 0.74 mm Hg of hydrostaticpressure is subtracted or added tothe measured pressure. If the air-fluid interface is placed 10 cmbelow the phlebostatic axis, themeasured pressure will be an over-estimation of the actual hemody-namic pressure by 7.4 mm Hg. Thisexample illustrates that thoughaccurate referencing and zeroingare important for all hemodynamicmonitoring, they become evenmore crucial when small hemody-namic pressures, such as pul-monary artery wedge pressure(6-12 mm Hg) and central venouspressure (2-6 mm Hg) are beingmonitored. In these instances,small offsets from the phlebostaticaxis and the zero reference pointcan cause large errors34 and mayprompt inappropriate treatment.

In order to ensure consistency,once the external reference pointhas been selected, it should bemarked on the patient for easyidentification. Consistency allows

correct determination of trends ina patient’s hemodynamic status.Whether the same body positionmust be used for consecutivemeasurements may be debatable.Although several studies on pul-monary artery pressure monitor-ing indicated no significantchanges in pulmonary arterypressures related to certain bedpositions, according to a recentresearch abstract,35 ABP valuesobtained with various bed posi-tions differed even when the phle-bostatic axis was used as theexternal reference point. The sta-tistical significance of differencesin ABP values for monitoring sys-tems referenced to the phlebo-static axis was not addressed inthe abstract.

Dynamic Response Testing. Inorder to determine if a hemody-namic monitoring system canadequately reproduce a patient’scardiovascular pressures, thedynamic response characteristics

of the catheter system must betested. Only when system accura-cy, also termed fidelity, has beenconfirmed, can the analog wave-form be accepted as an accuratereflection of a patient’s status.

The dynamic response of ahemodynamic monitoring systemis defined by its natural frequencyand the damping coefficient. Thenatural frequency indicates howfast the pressure monitoring sys-tem vibrates when shock excited bya signal such as the arterial pres-sure pulse or the pressure signalcaused by a fast-flush test. Thedamping coefficient of a monitor-ing system is a measure of howquickly the oscillations of a shock-excited system dampen and even-tually come to rest.23,26,36

Dynamic response testing is a3-step procedure: determiningnatural frequency, determiningthe amplitude ratio of 2 consecu-tive fast-flush oscillations (as anindirect way of determining the




damping coefficient), and deter-mining the dynamic responsecharacteristics of the monitoringsystem (eg, optimal, adequate,underdamped, overdamped, orunacceptable). These steps aresummarized in Figure 1.

Dynamic response testing mayseem complicated at first. Clinicalexperience has shown, however,that with proper training, dynam-ic response testing can be per-formed in less than 2 minutes. Afew simple observations providealmost as much information andmay serve as a close estimate. (1)If the period between 2 oscilla-tions of the fast-flush test is lessthan 1.2 mm, then natural fre-quency will be at least 21, and thesystem will most likely be ade-quate. If the period between 2oscillations is 1 mm or less, thenthe natural frequency is 25 orhigher, and the system will almostalways function properly with anydegree of damping. As a rule-of-thumb, the higher the natural fre-quency (or the smaller theperiod), the better is the dynamicresponse of the monitoring sys-tem. (2) The system is over-damped if the fast-flush producessluggish or no oscillations. If thesquare wave shows undulationsor if ringing occurs after therelease of the fast-flush device, thesystem is most likely under-damped. Overdamping and under-damping both indicate that thedynamic response characteristicsof the monitoring system areunsatisfactory.

Simple visual evaluation of thearterial waveform, the squarewaveform, and oscillations gener-ated by the fast-flush has beensuggested as a suitable methodfor determining the dynamic response characteristics of amonitoring system. However, the

square waveform and fast-flushoscillations may look adequatewhen the natural frequency andamplitude ratio of the systemactually make the system inade-quate (underdamped, over-damped, or unacceptable). Or, thearterial waveform may look dis-torted because of physiologicalvariations, yet the dynamic re-sponse characteristics of the sys-tem assure a faithful recording ofthe arterial pressure.36 When accu-racy is needed for clinical decisionmaking, the ideal method of deter-mining if the measured ABP, orany other directly monitoredhemodynamic parameter, is trueis to determine the dynamic re-sponse characteristics of the moni-toring system.26,36 Examples of theimportance of formally assessingthe response characteristics arediscussed in more detail later.

COMPONENTS OFTHE NORMAL ABPWAVEFORM

Left ventricular contractioncreates a pressure pulse or pulsewave. It is the pressure pulse thata clinician feels when determin-ing a patient’s pulse by palpation.The pressure pulse is also what issensed by the intra-arterialcatheter.34,35 The normal arterialpressure waveform is shown inFigure 2. The systolic upstroke oranacrotic limb mainly reflects thepressure pulse produced by leftventricular contraction. The pres-sure pulse is followed slightly later by the flow wave caused bythe actual displacement of bloodvolume. The anacrotic shoulder,that is, the rounded part at the topof the waveform, reflects primari-ly volume displacement.22,37 Thesystolic pressure is measured atthe peak of the waveform. Thedicrotic (or downward) limb is

demarcated by the dicrotic notch,representing closure of the aorticvalve and subsequent retrogradeflow. The location of the dicroticnotch varies according to the tim-ing of aortic closure in the cardiaccycle. For example, aortic closureis delayed in patients with hypov-olemia. Consequently, the dicrot-ic notch occurs farther down onthe dicrotic limb in hypovolemicpatients. The dicrotic notch alsoappears lower on the dicroticlimb when arterial pressure ismeasured at more distal sites inthe arterial tree (Figure 3). Theshape and proportion of the dias-tolic runoff wave that follows thedicrotic notch changes with arte-rial compliance and heart rate.Diastolic pressure is measuredjust before the beginning of thenext systolic upstroke.

The analog waveform visibleon the monitor or recorded on astrip chart is not only caused bythe forward pressure pulse but isalso a result of a phenomenonknown as wave reflection. Wavereflection is related to the im-pedance of blood flow by the nar-rowing and bifurcation of thearterial vessels; the impedanceleads to backward or retrogradereflection of the pressure wave.23,38-40

In a manner similar to waves onthe beach, the forward or ante-grade pressure waves and thereflected waves collide. The com-bination of the 2 types of wavesincreasingly augments the SBPthe farther down the blood pres-sure is measured in the arterialcircuit. In other words, the contri-bution of reflected waves to themeasured systolic pressure occursearlier in the periphery, particu-larly in the radial and dorsalispedis arteries (Figure 3), wherethe measured SBP may be 20 to 25mm Hg higher than central aortic




1. Determining natural frequency (Fn) At the end of a waveforma. Perform the fast-flush maneuver (square waveform test): pull and release the pigtail or compress and release

the button of the fast-flush device of the monitoring system.

b. Record the resulting square waveform and subsequent oscillations on calibrated strip chart paper.

c. Measure the distance (period, t, of 1 cycle) in millimeters between 2 oscillations. (One small box on the calibrated strip chart paper equals 1 mm.)

d. Calculate Fn by using the formulaFn = paper speed (mm/s)/t of 1 cycle (mm)(The standard paper speed is 25 mm/s.)Example: paper speed = 25 mm/s; t = 1 mm; Fn = (25 mm/s)/1 mm = 25 Hz

2. Determine the amplitude ratioa. Measure the amplitude (A) in millimeters of 2 successive oscillations (A1, A2). b. Calculate the amplitude ratio: divide A2 by A1 (A2/A1).

3. Determine the dynamic response characteristicsOn the Fn-versus-amplitude ratio graph

a. Plot Fn along the x-axis (result of step 1)b. Plot the amplitude ratio along the y-axis on the right (result of step 2)c. Find the intersection of the 2 lines. Where the 2 lines intersect on the Fn-versus-amplitude ratio graph determines

if the system is able to correctly reproduce the hemodynamic waveform (adequate, optimal) or if the system is notfunctioning at a desirable level (overdamped, underdamped, unacceptable).

Figure 1 The 3 steps of dynamic response testing. A, The fast-flush test. B, The frequency-versus-damping coefficientgraph.

A, Adapted from Bridges and Middleton,36 with permission. B, Reprinted from Gardner and Hollingsworth,28 with permission; adapted from Bridges and Middleton,36

with permission.

A

BB

Period (t) = 1.2 mm

A2 =3 mm

A1 = 7 mm

Fast flush



pressure.22,23 Under normal condi-tions, 80% of the original wave isthought to be reflected.22

Clinically, wave reflection playsan important role in left ventricu-lar workload and cardiac oxygenconsumption. In young adultswith elastic arteries, the reflectedwave returns to the heart duringthe diastolic phase of the cardiaccycle and thus augments coronaryartery perfusion. In elderly patientsor patients with stiff, atheroscle-rotic vessels, the reflected wavereturns to the heart during systoleand thus increases systolic pres-sure and left ventricular afterload.39

Pharmacological vasoconstrictioncan similarly increase wave reflec-tion and cardiac workload.23,39

The contribution of reflectedwaves to the measured systolicpressure is diminished duringhypovolemia, hypotension, theValsalva maneuver, and vasodi-latation.23 In pharmacologicallyinduced vasodilation, such asoccurs with nitroglycerin forinstance, peripherally measuredsystolic pressure may not changein proportion to the actual degreeof reduction in central aortic pres-sure.39,40 The effect of nitroglycerinmay be visible in the appearanceof reflected waves after the systolicpeak (Figure 4), but reduced aorticpressure, afterload, and cardiacwork load may be more evidentthrough clinical improvement ofthe patient. During shock withvasoconstriction, wave reflectioncan lead to the overestimation ofcentral aortic pressure, becauseperipheral SBP may be 20 mm Hghigher than aortic pressure.36,39

Peripherally measured SBP couldin this situation provide a falsesense of security that the patient ismaintaining adequate perfusionpressures. A slower systolic up-stroke and a prominent diastolicwaveform with reflected wavesmay be visual indicators of shockwith vasoconstriction39 (Figure 5).

SOURCES OFARTIFACT ANDCHANGES INWAVEFORMMORPHOLOGY:RECOMMENDATIONSFOR CLINICALPRACTICEDamping, Overdamping, andUnderdamping

A common scenario whenworking with arterial catheters iscomparing the blood pressurerecorded by the arterial catheterwith the blood pressure obtainedmanually or with an oscilloscope.If a discrepancy occurs, the arterialcatheter is often said to be some-how “damped,” with the underly-ing understanding that pressurereadings obtained with the cath-eter cannot be trusted. Such aninterpretation may be prematureand probably disregards severalimportant facts. Differences existbetween (1) damping, overdamp-ing, and underdamping; (2) over-damped and underdamped pres-sure waveforms caused by over-damped or underdamped moni-toring systems and overdamped-or underdamped-appearing waveforms that reflect the truephysiological status of a patient;and (3) blood pressures obtainedvia direct versus indirect monitor-ing methods.

All hemodynamic monitoringsystems are damped. Damping isdesired, because without damp-ing the vibrations of the system’sfluid column caused by the arteri-al pressure pulse would go onindefinitely and no accuratewaveform could be recorded.Conversely, both overdampedand underdamped systems exist,and their recordings are indeederroneous. With an overdampedsystem, the waveform loses itscharacteristic landmarks and


Figure 2 The normal arterial pressure waveform.

MAP indicates mean arterial pressure. Reprinted from Darovic,10 with permission.

Figure 3 Changes of the arterialpressure waveform configurationthroughout the arterial tree. Notethe increasing steepness andamplitude of the systolic upstrokeand the changing location of thedicrotic notch.

Reprinted from Gorny,8 with permission.

Centralaorta

Brachial

Radial

Femoral

Dorsalispedis

Anacrotic limb Dicrotic limbSystolic=115 mm Hg

Pulse pressure=35 mm Hg

Diastolic=80 mm Hg

MAP=Area + Base=97 mm Hg

Dicrotic notch

BASE

120

9780

40

0



appears unnaturally smooth, witha diminished or absent dicroticnotch. Overdamping results infalsely low systolic and falsely highdiastolic pressure readings26

(Figures 6 and 7). An overdamped-appearing (often simply calleddamped) waveform can also be theresult of aortic stenosis, vasodi-latation, or low cardiac outputstates such as cardiogenic shock,sepsis, or severe hypovolemia(Figure 5). In order to determine ifthe waveform is a result of an over-damped system or is an accuratereflection of a patient’s status, thedynamic response characteristicsmust be tested.

Today’s commercially avail-able catheter-transducer systems

tend to have a low natural fre-quency and consequently areoften underdamped.22,36 Anec-dotal reports of manufacturers ofan improved natural frequencyoften do not hold up in clinicalpractice. Typically, the naturalfrequency of hemodynamic mon-itoring systems currently on themarket is less than 25 Hz. Anunderdamped system will recordfalsely high systolic pressures (15-30 mm Hg) and falsely lowdiastolic pressures26 (Figures 8-10).Underdamping, most often inthe form of systolic overshoot(the artificial exaggeration of sys-tolic pressure), must be suspect-ed in patients with hypertension,atherosclerosis, vasoconstriction,

aortic regurgitation, or hyperdy-namic states such as fever.23 Inthese conditions, typically a rapidrise in the systolic slope occurs,frequently exceeding the dynam-ic response characteristics of themonitoring system. A heart rategreater than 150/min may alsocause systolic overshoot, becauseof the rapid succession ofimpulses.26

The variable natural frequencyof available monitoring systemsexplains why with different moni-toring systems, different hemody-namic pressures can be measuredeven though the patient’s condi-tion has not changed.22 The oftenobserved discrepancies betweendirectly and indirectly obtainedSBP and DBP readings are alsopartly caused by underdamping.22

Clinically, recognition of pos-sibly overdamped and under-damped waveforms and anawareness of physiological situa-tions when each type of wave-form may occur are important.Then the question to beanswered is whether the cause ofthe waveform is the patient’scondition or the monitoring sys-tem. Practically, the interventionfor both overdamped and under-damped systems is the same:maximizing natural frequency.

Techniques for MaximizingNatural Frequency

As mentioned earlier, air bub-bles are a main factor in wave-form blunting or overdamping. Inaddition to air bubbles, other fac-tors may alter the natural fre-quency of a monitoring systemand distort the recorded signal.These factors include narrow,compliant, and long tubing; pres-ence of additional stopcocks; andloose connections. Translated intopractice, the monitoring system


Figure 4 Example of a waveform common in patients with hypertension(arterial blood pressure, 192/84 mm Hg; pulse pressure, 108 mm Hg). Notethe narrow systolic tip, the position of the dicrotic notch (D), and thereflected waves (R). A reflected wave like the first reflected wave after thesystolic upstroke may appear with the initiation of nitroglycerin therapy.40

Overall, the arterial waveform appears underdamped, but the dynamicresponse characteristic is adequate, close to optimal. Natural frequency, 25/1 = 25 Hz; amplitude ratio, 3.5/8 = 0.44.

Figure 5 Waveform in a patient in shock with vasoconstriction. Note slowsystolic upstroke and relatively high diastolic wave with reflection waves (R).The waveform appears overdamped.

D indicates dicrotic notch.



should (1) be primed air-free, (2)consist of wide-bore, high-pres-sure tubing with its length limitedto 122 cm (48 in), (3) not beextended with tubing or addedstopcocks, and (4) have tightlysecured connections.27,36 In addi-tion, the continuous flush bagshould be cleared of any air andbe maintained adequately filled,and the external pressure cuff sur-rounding the flush solution bagshould be maintained at a pres-sure of 300 mm Hg. This practicewill not only prevent air fromgoing into the solution but alsohelp prevent catheter clotting.

Catheter clotting is a rare, butpossibly serious, complication ofintra-arterial monitoring. One ofthe first indications of clotting maybe a waveform that looks over-damped. Whenever waveformoverdamping is observed, the

patient should be assessedfor signs and symptoms of hypo-tension or low cardiac output.Then, the potential clot shouldbe removed by aspirating bloodfrom the distal stopcock beforeperforming any flushing maneuverfor the dynamic response test.37

Fluids with viscosities higher thanthe viscosity of normal isotonicsodium chloride solution also leadto overdamping of the hemody-namic waveform.23 For example,blood in the arterial catheter (dueto blood backup or insufficientflushing) should be cleared fromthe system. As a general rule, thecatheter-tubing system and stop-cocks should be flushed before anyhemodynamic measurement isperformed, especially when clini-cal decisions are to be made. TheTable summarizes the mostimportant recommendations for

optimizing the natural frequencyof the monitoring system anddynamic response testing.

In general, overdamping andunderdamping affect mostly SBPand DBP. MAP is less sensitive tothese sources of waveform distor-tion and is therefore less depen-dent on the dynamic responsecharacteristics of the catheter sys-tem.26 When all steps have beentaken to maximize the natural frequency of a system, yet thedynamic response test indicatesoverdamping or underdamping,then either MAP should be followed or an alternative method of monitoring (eg, oscillometricblood pressure monitoring)should be used.

End-Hole ArtifactThe arterial catheter of the

ABP monitoring system pointsupstream. The forward-flowingblood contains kinetic energy.When the flowing blood is sud-denly stopped by the tip of thecatheter, the kinetic energy of theblood is partially converted intopressure. This converted pressuremay add 2 to 10 mm Hg to thesystolic pressure measured by anintra-arterial monitoring system.23

The artificial augmentation ofdirectly monitored systolic pres-


Figure 8 Example of anunderdamped waveform. Naturalfrequency, 25/1.5 = 16.7 Hz;amplitude ratio, 7/14 = 0.5.

Figure 6 Overdamped waveform due to an overdamped monitoring system.Note the absence of fast-flush oscillations.

Figure 7 Example of an overdamped-appearing waveform. The dynamicresponse characteristic of the system gives an answer to the question, is thewaveform overdamped because of the monitoring system or is it a correctreflection of the patient’s condition?




sure by converted kinetic energyis referred to as the end-hole arti-fact or the end-pressure product.

Movement ArtifactMotion of the tubing system

enhances the fluid oscillations ofthe system. Although the clinicalsignificance of movement artifactis not known, it is recommendedthat extrinsic movement of thetubing system be kept at an abso-lute minimum.10

Monitor ArtifactThe differences (>5 mm Hg)

between the digital pressure out-put displayed on the monitor andpressures directly read from analogor strip chart recordings are poten-tially important.43 These differencescan lead to erroneous data collec-tion and are especially notable inpatients with hypotension, hyper-

tension, dysrhythmias, or pulsusparadoxus.43 The monitoring sys-tem cannot discriminate betweenpressure readings during zeroing,obtaining blood samples, andfast-flushing and real arterialpressure readings. Consequently,all readings are incorporated intopressure trends.43 Monitoring arti-fact (ie, monitoring noise causedby movement of the patient anddisturbances in a monitoring sys-tem due to, for example, electricalheating or cooling blankets) mayalso be superimposed on thepatient’s pressure waveform.Experienced clinicians can recog-nize and eliminate some of theseerror sources and use a represen-tative set of waveform tracings ona calibrated strip-chart recordingto obtain the most valid assess-ment of a patient’s hemodynamicstatus.43 Thus, the optimal method

for hemodynamic pressure mea-surement is to use the analogstrip-chart recording.43,44

Respiratory VariationNormal breathing leads to

changes in intrathoracic pressure,which affect cardiac output andsystemic pressure. With sponta-neous inspiration, intrathoracicpressure decreases. The de-creased pressure is recognizableon the ABP tracing as a downwarddisplacement of the waveformbaseline. Concurrently, during in-spiration, venous return to theright side of the heart is increased,augmenting right ventricularstroke volume. The increase inright ventricular stroke volume isoffset by increased pulmonary vas-cular compliance and blood pool-ing during inspiratory thoracicexpansion. Consequently, left ven-tricular stroke volume is decreased.Heart rate and SVR both increaseas a compensatory reflex. The netresult of this chain of reflexes is thephenomenon known as physiolog-ical pulsus paradoxus (a decreasein ABP of usually <10 mm Hg dur-ing spontaneous, unassisted inspi-ration).45 An inspiratory decrease inABP greater than 10 mm Hg (pul-sus paradoxus) may indicate car-diac tamponade or restrictivepericarditis. Pulsus paradoxus alsooccurs in patients with obstructivelung disease, pulmonary em-bolism, and severe heart failureand can be induced by mechanicalventilation.46,47

During positive-pressure venti-lation, the inspiratory increase inintrathoracic pressure can be rec-ognized in the upward displace-ment of the baseline of the arterialpressure waveform. If a patientreceiving mechanical ventilation ishypovolemic, the increase in

Figure 9 Example of an underdamped waveform. Natural frequency, 25/2 =12.5 Hz; amplitude ratio, 4/11 = 0.36. Note the normal-appearing fast-flushoscillations.

Figure 10 Example of an overdamped-appearing waveform. However, thedynamic response test places the system on the border betweenunderdamped and adequate. Natural frequency, 25/1.8 = 13.9 Hz; amplituderatio, 9/19 = 0.47.




intrathoracic pressure may lead toartificial augmentation of directlymonitored SBP.

Current monitoring systemsdetermine the mean of pressurereadings at predetermined inter-vals. Respiratory artifact could leadto erroneous digital output data.The unpredictable effect of respira-tory variation on arterial pressureprovides a strong argument forreading and recording arterial pres-sure, like any other hemodynamicindex, at the end of expiration byusing a freeze-frame picture or,best, a strip-chart recording.48,49

Hypertension andAtherosclerosis

Hypertension is due to age-related arterial stiffening, athero-sclerotic narrowing, or renin-related vasoconstriction, all ofwhich increase the magnitude ofreflected waves. In these physiolog-ical conditions, reflected wavesfuse with the systolic upstroke,

resulting in a high pulse pressure

and late high systolic peak,39 often

manifested as a narrow systolic

peak in the peripheral ABP wave-

form tracing.10,38 In addition, the

diastolic wave may be reduced or

disappear.39 Figures 4 and 11 show

peripheral ABP tracings typical in

patients with hypertension or

atherosclerosis. In each instance,

the small and narrow tip of the

waveform may be an overesti-

mation of systolic pressure and

thereby central aortic pressure. As

explained earlier, hypertension

and atherosclerosis place high

demands on the monitoring sys-

tem. The arterial waveforms

shown in Figures 4 and 11 could

also be the result of systolic over-

shoot due to inadequate dynamic

response characteristics of the

Figure 11 Example of a waveform common in patients with hypertension(arterial blood pressure, 150/45 mm Hg). Note steep systolic upstroke,narrow systolic peak, diminished diastolic run-off wave, and relative decreasein the diastolic proportion of the waveform due to a heart rate of 100/min.The waveform appears underdamped.

Recommendations for optimizing the natural frequency of the arterial pressure monitoring system and dynamic responsetesting

Natural frequency of the monitoring system23,27,41

Dynamic response test27,41

Feature

System requirementsUse wide-bore, high-pressure tubing no longer than 122 cm (48 in)Avoid tubing extensions and minimize stopcocksEnsure that all connections are tightenedEliminate air from the flush fluid and air bubbles from the tubing systemKeep continuous flush bag filled and keep external pressure cuff at 300 mm Hg

pressureClear access catheter and tubing system of any fluid other than isotonic sodium

chloride solutionPrevention of catheter clotting

Maintain continuous flush device as describedUse heparinized flush solution42

Prevention of catheter kinkingKeep cannulated extremity in a neutral or slightly extended position

Implementation of testWhenever the waveform seems overdamped or underdampedWhenever physiological changes of the patient (increased heart rate,

vasoconstriction) place higher demands on the monitoring systemAfter opening the systemBefore implementing interventions or changes of interventionsWhenever the accuracy of the arterial blood pressure measurement is in doubtAt least every 8-12 hours

Recommendations



monitoring system. The result ofthe dynamic response test providesreassurance that the recorded ABPtracing is correct.

MAP is measured as the areaunder the pressure curve dividedby the width of the base of thepressure curve (the time intervalof a single cardiac cycle)10,37

(Figure 2). A small and narrowsystolic tip such as the ones inFigures 4 and 11 adds relativelylittle to the total area under thepressure curve, whereas it canadd significantly to measuredSBP. Consequently, the measuredMAP is less affected by wavereflection and the response char-acteristics of the monitoring sys-tem than is measured SBP. TheMAP remains relatively constantwhen measured at different sitesthroughout the arterial circuit,whereas measured SBP and DBPmay differ.10 In general, MAP is amore stable hemodynamic para-meter and provides a more accu-rate interpretation of a patient’shemodynamic status.10,34,36,38

INDIRECT VERSUSDIRECT ABPMONITORING

A discussion of the variousmethods of indirect ABP mea-surement and how they comparewith direct ABP monitoring isbeyond the scope of this article.However, several key principlesshould be emphasized in thiscontext. Direct ABP monitoringmeasures pressure pulse, whereasall indirect methods of ABP mea-surement are related to bloodflow. No absolute relationshipexists between these 2 phenome-na because they follow differentlaws of physics and physiology. Inlow-flow conditions, such ashypotension and vasoconstrictedstates, indirect methods yield

lower pressure readings than doesdirect ABP monitoring.22,36 Con-versely, if SVR is low, in patientswith sepsis for instance, the rela-tively high flow results in an indi-rect ABP reading that is higherthan the directly measuredABP.22,36 Factors such as under-damping, end-hole artifact, andwave reflection contribute todirect systolic pressure readingsthat are often higher than indi-rectly obtained pressure values.In other words, when obtainingABP readings by using differentmeasuring methods, differentresults should be expected. Morespecifically, a “good” correlationbetween the oscilloscope and thearterial pressure monitoring sys-tem is not a gauge for the properfunctioning of the pressure moni-toring system. MAP, on the con-trary, is closely approximatedwhen oscillometric and directmeasurements are compared.21-25,36

Although both the oscillomet-ric and the direct ABP monitoringmethods are generally accurate inclinical practice, direct ABP moni-toring has a distinct advantage.Direct ABP monitoring is the onlyscientifically and clinically vali-dated method that allows real-time and continuous monitoringof a patient’s ABP. With strip-chart recording, beat-to-beatanalysis of a patient’s ABP is pos-sible. This feature may be of clini-cal relevance when evaluating theeffect of positive end-expiratorypressure during mechanical ven-tilation or the effect of changingstroke volume in atrial fibrillationor other arrhythmias.

SUMMARYHemodynamic monitoring is a

costly procedure, both materiallyand with regard to nursing time

involved to ensure proper func-tioning of the monitoring systemand correct interpretation of thedata obtained. Dynamic responsetesting is the ideal method of con-firming the ability of a monitoringsystem to accurately reproducehemodynamic waveforms. MAP isa stable hemodynamic parameter,because it is least affected by moni-toring method, catheter insertionsite, the dynamic response charac-teristics of the catheter system, andwave reflection. MAP provides thebest estimate of central aortic pres-sure and is the main hemodynamicparameter monitored by the neu-rohormonal system to controlblood pressure. The superior infor-mational value of MAP providesstrong support for its preferred usein clinical practice, especially whenuse of vasoactive drugs is started orthe dosages of these drugs aretitrated.10,21 However, numericallysatisfactory ABP or MAP values arenot necessarily related to adequateperipheral tissue perfusion andorgan system function. For optimalmanagement of patients, dataobtained from assessment toolssuch as hemodynamic monitoringdevices must be integrated withinformation gained from clinicalassessment of patients’ status. ✙

AcknowledgmentsI (B.H.M.) am grateful to Maj Bridges forsharing her expertise and for her help andsupport in preparing this article.

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