Pitfalls in Anesthesia Monitoring

8/22/2019 Pitfalls in Anesthesia Monitoring

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Dr.Anupam Goswami

Professor & Head

Department of Cardiac AnesthesiologyI.P.G.M.E&R KOLKATA

Dr.Sandeep Kumar Kar

Rmo-cum clinical tutor Department of Cardiac

Anesthesiology I.P.G.M.E&R KOLKATA

When Numbers Are Wrong!(Pitfalls In Anesthethesia Monitoring)
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INTRODUCTION

Heisenbergs uncertainty principle The very act of measuring a physiologic

variable may affect the value of that

variable. It is therefore appropriate to the use in

practice of imperfect measures of the

status of our patients with humility . Be vigilant for potentially misleading

information.


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Uncertainty principle and ourmeasurement systems

A more formal inequality relatingthe standard deviationof position x andthe standard deviation of momentum

p was derived by Heisenberg in 1927x*p >= h/4

where x is the uncertainty as to the

particle's position andp is theuncertainty as to its momentum (andthus its velocity).
http://en.wikipedia.org/wiki/Standard_deviationhttp://en.wikipedia.org/wiki/Standard_deviation


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What We Infer?

Historically, the uncertainty principle hasbeen confused with a somewhat similareffect in physics, called the observereffect, which notes that measurementsof certain systems cannot be madewithout affecting the systems.

Uncertainty principle is inherent in the

properties of allwave-like systems

How can our Haemodynamic waveformwhether IBP, CVP , ECG escape from it?
http://en.wikipedia.org/wiki/Physicshttp://en.wikipedia.org/wiki/Observer_effect_(physics)http://en.wikipedia.org/wiki/Observer_effect_(physics)http://en.wikipedia.org/wiki/Wavehttp://en.wikipedia.org/wiki/Wavehttp://en.wikipedia.org/wiki/Wavehttp://en.wikipedia.org/wiki/Wavehttp://en.wikipedia.org/wiki/Wavehttp://en.wikipedia.org/wiki/Observer_effect_(physics)http://en.wikipedia.org/wiki/Observer_effect_(physics)http://en.wikipedia.org/wiki/Physics


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The ideal Clinician:

The practitioner must be :

1)Aware of potential sources of error inmeasurement and interpretation.

2)Interpret data in context

3)Consider the entire picture of the

patient. 4)Should not view the data as a

collection of independent variables.


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Invasive Blood Pressure Monitoring

It is not uncommon for discrepancies tooccur between invasive and noninvasive

measurements of blood pressureresulting in confusion at the bedsideover which number to use in directing

therapy.


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Invasive Blood Pressure Monitoring

Physical properties of the system beingmeasured have significant influence onthe potential errors.

The vascular system is complex, withperiodic variations in pressure followingeach heart beat over a wide range offrequency.

The high heart rate of infants interferedin responsiveness and ideality of thetransduced waveforms early indevelopment of monitoring devices

T R P I A


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T e Rea Pro em In AccurateMonitoring

Transmission of the pressure waveformdown a progressively smaller, branchingvascular tree with variable elastance andresistance depending on humoral andneural regulation of vascular tone.

In clinical situations associated withincreased resistance (hypovolemia, earlycompensated shock), reflection of kineticenergy back from the vascular tree to the

end-hole vascular cannula can giveSBP


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Damping Damping of the pulse waveform is

associated with falsely low systolicpressures and falsely high diastolicpressures.

Most common cause :

1.vasospasm of the vessel in which thecannula lies.

2. Air bubble in the fluid-filled tubingleading from the vascular cannula to theelectronic transducer.

3. Loose connection in the tubing


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How to Reduce Damping?

Make tubing bubble-free.

Ensure there are no loose connections.

Adding papaverine to the arterial lineinfusate is often helpful in reducingvasospasm at the site of the lineinsertion, which may result in a betterwaveform and more reliable pressuremeasurement.

Use MABP in guiding therapy


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Why MABP?

It is a function of the area under thearterial pulse waveform curve and isless affected by damping of thesignal.


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The Problem of Resonance

Resonance refers to the interactionbetween the natural frequency of a

physical monitoring system and thefrequency of the physiologic parameterbeing measured.

This causes erroneous waveforms andpressures to be displayed.


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How To Reduce Resonance? The natural frequency of a

tubing/transducer system is notdissimilar to that of a xylophone, so thatthe monitoring system itself may, with itsintrinsic physical properties, influence thedata generated.

Using stiff, noncompliant, narrow gauge,and short tubing.

It is common to seeflingelevation of theSBP visible as a needle point at the peakof the pulse waveform as the system are

set up to reduce damping of the signal


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Catheter whip due to movement

Catheter whip due to movement withinthe vessel in a hyperdynamic circulatorystate may impart kinetic energy to theend hole of the cannula, resulting in ahigher pressure measurement.

This is more in catheters placed in larger

central arteries or the aorta orpulmonary artery.


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Strive for Excellence!


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Noninvasive Blood Pressure Monitoring

Two methods:

1) Auscultation of Korotkoff sounds.

2) Oscillometric Method( Automated)

Both the methods depend on flow being

present in the extremity being subjectedto assessment.

Unreliable in low flow states

The effect of kinetic energy componentin IBP monitoring makes intraarterialIBP 8-16 Torr higher than NIBP

normally and 25-30 Torr higher in Sepsis.

Pitf ll i i i bl d


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Pitfalls in noninvasive bloodpressure monitoring

Data acquisition is intermittent ratherthan continuous making it more difficultto assess response to therapy with

titration. Severe vasoconstriction may make

noninvasive blood pressure

measurements unobtainable. Cuff size must be appropriate to

acquire accurate measurements.


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DETERMINANTS OF CARDIACOUTPUT

Cardiac output is the product ofstroke volume and heart rate.Determinants of stroke volume

include

Preload,

Contractility

Afterload.


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Easy question?

What is the most directphysiological measure ofPreload?


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The Answer comes

Measurement of fiber length in thesarcomere at end-diastole

This is not clinically feasible

Wh t i th l ti ?


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What is the solution?

Extrapolation from measurementsthat are clinically available is necessary.

But.

With each step away from directmeasurement there is more potentialerror in measurement andinterpretation.


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Contractility

Contractility is also an extrapolatedassessment in clinical practice

Either derived from hemodynamic

calculations or echocardiographic visualevidence involving measurementsthemselves subject to error.


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Afterload?

Clinically recognized by physicalexamination.

Qualitatively assessed if anatomic

abnormalities or abnormal flow patternsare present by echocardiography.

Quantitatively calculated if pulmonary

artery catheterization is performed.All methods are subject to errors in

interpretation.

Why?


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We need to be Realistic

Despite the difficulties inmeasurement and interpretation,such data can be successfully

incorporated into goal-directedtherapeutic algorithms with a holisticclinical approach leavened by

knowledge of the pitfalls.


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And Remember this person

x*p>= h/4


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Preload Assessment(CVP)

Being utilized to assess a remote left-sided volume As a right-sided pressuremeasurement being utilized to assess a

remote left sided volume parameter, leftventricular end diastolic volume(LVEDV),numerous factors may result in a

misleading value. In pediatric patients isolated LV

dysfunction is not reflected in high CVP

measurements but must be kept in mind.


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Preload Assessment(CVP) Myocardial infarction related to

anomalous left coronary artery arisingfrom the pulmonary (ALCAPA)Kawasaki disease with coronary

aneurysms. Acute myocarditis or cardiomyopathy

related to various metabolic disorders

may also present with significant leftventricular (LV) dysfunction notnecessarily reflected by a high CVP ifright ventricular (RV) compliance is

normal.


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Ventricular ComplianceA stiff ventricle, either on the right or the

left side, may result in a high pressureeven if the left ventricle is under filled

pericardial effusion with tamponade,

constrictive pericarditis, or highpericardial pressure secondary to highventilatory pressures may produce highfilling pressures with low end-diastolic

volume

Obstructions to left ventricular emptyingsuch as aortic valvular stenosis produce a

high end-diastolic pressure.


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Ventricular ComplianceAny anatomic or pathophysiologic cause of

increased resistance to left ventricular fillingsituated between the tip of the catheter andthe left ventricular chamber may result inmisleading high pressure readings suggestingadequate preload is present.

On left side:MV disease, Pulmonary VenousObstructionOn The right side:PulmonaryHypertension, PVD,PE, TVD, R-L shunts,allthese obfuscate the true volume status of LVby producing high pressure measurement


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How Much We Rely on CVP

Observation of the response to volumeexpansion with continuous monitoringof the CVP can help identify adequate

volume loading and prompt addition ofinotropic support.

Echocardiography comes to our rescue

in case we get dubious results. The knowledge of abnormal wave

forms in CVP


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Pulmonary artery occlusionpressure (PAOP)

The assumption may be made thatpulmonary artery occlusion pressureaccurately reflects intravascular volume

status and left ventricular preload.

PAOP may be high in LV dysfunction andMitral valve disease with decreased

compliance, prompting the potentiallyincorrect conclusion that luidresuscitation hasbeen adequate or

excessive.


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Airway Pressure & PAOPAirway pressure is another very important

factor in interpretation of the PAOP in patientsreceiving positive pressure ventilatorysupport.

placement of the catheter in West Zone III toachieve a continuous fluid column betweenthe catheter tip and the LV

Placement in upper lobes or anteriorsegments with relatively more inflation ofalveoli than pulmonary blood flow, WestZone I, will exaggerate the effect of PPV on the

measurement causing misleading high value.


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The Cause of erroneous PAOP

If the respiratory cycle alveolar airwaypressure exceeds pulmonary venous pressure,thereby interrupting pulmonary blood lowthrough vascular compression

High levels of positive ventilatory pressure mayconvert the monitored lung site from optimalWest Zone III to Zone II (lower low to

ventilation ratio) despite proper initialplacement of the catheter tip in a dependentlower lobe.


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Avoiding this Pitfall

Careful measurement of PAOP at end-expiration as judged by observation ofrespiratory variation in the pressuretracing

Aggressive diuresis resulting inhypovolemia may cause the conversionof Zone III lung physiology to that of

Zone II, causing misleadingly high PAOPin volume-depleted patients on highlevels of PEEP.


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Contractility Assessment

Contractility is defined as the velocity of

myocardial fiber shortening.

Measuring approximating strokevolume as a marker of contractility are

subject to the influence of afterload andvalvular regurgitation.

High afterload secondary to valvular

stenosis or increased systemic vascularresistance will decrease stroke volumeeven with normal contractility.


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Contractility Assessment Pharmacologic vasodilation can increase

stroke volume even with poor contractility.Valvular regurgitation may effectively reduce

afterload allowing a large stroke volume,

partially in the wrong direction with a net resultof inadequate systemic low.

Estimations of contractility via nuclear medicine

scans or calculations based on data derivedfrom thermodilution pulmonary arterycatheters are subject to the same confounding

factors


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Afterload AssessmentPitfalls

Pitfall in interpretation of a high PAP isthat high flow due to L-R shunt mayproduce a high pressure even in thepresence of normal pulmonary vascularresistance.

Echocardiographic data estimatingresistance via flow patterns is availableonly intermittently and is subject to error.


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Cardiac Output Assessment

Operator variation, computational errors,

Based on the assumption that there isno intracardiac shunt through septaldefects.

Other confounding conditions which mayinvalidate the measurement include

tricuspid regurgitation, atrialarrhythmias, and variation in timing ofthe respiratory cycle with indicator

injection.


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Echocardiography

Low estimation depends on thecross-sectional area of the vesselbeing evaluated; a high flow across

a narrow vessel may not representadequate output.

Measurements of ejection fraction and

shortening fraction may suggest ahigher systemic output than exists dueto mitral valve regurgitation or aortic

insufficiency.


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Capnography Pitfalls in interpretation are revealed when

arterial blood gases are performedshowing a gap between paCO2 andETCO2 ,and include:

1) severe airway or parenchymallung disease causing significant ventilation-perfusion mismatch, resulting in dead-space

ventilation and a false low ETCO2 2) false low value because of failure to

obtain a true alveolar sample due to

expiratory obstruction.


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Pulse- oximetry limitations

Carbon Monoxide

Carbon monoxide molecules, even in a smallamount, can attach to the patient'shemoglobin replacing oxygen molecules. Apulse oximeter cannot distinguish thedifferences and the reading will show the total

saturation level of oxygen and carbonmonoxide. If 15% of hemoglobin has carbonmonoxide and 80% has oxygen, the reading

would be 95%.


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Blood Volume Deficiency

Conditions, such as hypovolemia,hypotension, and hypothermia, mayhave adequate oxygen saturation, butlow oxygen carrying capacity.

Due to the reduction in blood flow, the

sensor may not be able to pick upadequately the pulsatile waveformresulting in no signal or loss of

accuracy.


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Skin Pigmentation Dark skin pigmentation can give over-

estimated SpO2 readings when it is

below 80%. Find a place where the skin color is

lighter.

Intravenous Dyes Intravenous dyes (such as methylene

blue, indigo carmine, and indocyanine

green) can cause inaccurate readings.


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External Interference

Exposure to strong external lightwhile taking measurement mayresult in inaccurate readings.

Shield the sensors from bright lights.


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Pulse -oximetry andMethemoglobenemia

Methemoglobin is a form of hemoglobinthat does not carry oxygen.

It is normal to have 1-2% of

haemoglobin in this form. A high level of methaemoglobin would

cause a pulse oximter to have a reading

of around 85% regardless of the actualoxygen saturation level.

A i Pitf ll Mi l di N b !


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A serious Pitfall. Misleading Numbers!Alveolar hypoventilation is the main

form of respiratory failurepostoperatively results fromcombinations of central respiratorydepression,muscular weakness, andupper airways obstruction.

As arterial carbondioxide tension risesso does alveolar carbon dioxide tension

(Pco2); alveolar Po2 falls, leading toarterial hypoxaemia.

This is diagnosed very late if the patient

in on supplemental 02 or Anesthesia


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Electrocardiography Artifacts


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REFERENCES 1. Swedlow DB, Cohen DE. Invasive

assessment of the failing circulation. Clinics inCritical Care

2. Clark JA, Lieh-Lai MW, Sarnaik A, MattooTK. Discrepancies between direct and indirectblood pressure measurements usingvariousrecommendations for arm cuff selection.

Pediatrics. 2002;110(5):920-923. 3. Pinsky MR. Clinical signfiicance

ofpulmonary artery occlusion pressure.

Intensive Care Med. 2003;29:175-178.


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REFERENCES 4.SM, Murdoch IA. Monitoring cardiac function

in intensive care. Arch Dis Child. 2003;88:46-52.

5.Piehl, MD, Manning J, McCurdy SL, et al.

Comparison of pulse contour analysis withpulmonary artery thermodilution in 2004Pediatric Critical Care.

6. Egan JR, Festa M, Cole AD, et al. Clinicalassessment of cardiac performance in infantsand children following cardiac surgery.

Intensive Care Med. 2005;31:568-573.

Are we living in the world of


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Are we living in the world ofApproximations?

THINK A


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THANK YOU

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Pitfalls in Anesthesia Monitoring