Hemodynamic Monitoring: Principles to Practice – M. L. Cheatham ...

Hemodynamic Monitoring: Principles to Practice – M. L. Cheatham, MD, FACS, FCCMSUMMARYHEMODYNAMIC MONITORING:FROM PRINCIPLES TO PRACTICEMichael L. Cheatham, MD, FACS, FCCMDirector, Surgical Intensive Care UnitsOrlando Regional Medical CenterOrlando, Florida• Fluid-filled catheters are commonly utilized in theICU to measure a variety of physiologic parameters– Systemic blood pressures– Pulmonary blood pressures– Intra-abdominal abdominal pressure– Intracranial pressure• To appropriately assess and utilize the dataprovided by such monitoring devices, certain basicconcepts must be understoodINTRODUCTION• Invasive pressure monitoring in the critically illprovides valuable information– Most accurate method for determining bloodpressure– Allows continuous physiologic monitoring– Identifies physiologic abnormalities– Can be used to guide appropriate resuscitativetherapies– Waveform interpretation provides valuableinformation on the patient’s cardiac contractilityand heart valve competencyINTRODUCTION• To improve patient outcome, accurate measurementof physiologic variables requires:– Constant vigilance to ensure accuracy– Thorough understanding of monitoringprinciples p and pitfalls– Appropriate application of information gained• WARNING: Physics lies up ahead! If you developedPTSD following college physics, you may want toskip the remainder of this lecture (relax, it’s actuallypretty cool and explains why you needed to takephysics after all)G-I-G-O• “Garbage In…Garbage Out”– Erroneous physiologic measurements can resultin inappropriate patient therapy– You should always ask “Is this data valid?”– Example» Diebel et al. found that 52% of PAOPmeasurements in surgical patients areinaccurate and misleading as a result ofmonitoring artifacts» Reliance upon erroneous PAOP and CVP valuesto resuscitate critically ill patients may lead tounder-resuscitation resuscitation and inappropriate therapyMEASURING PRESSURE VARIABLES• Each fluid-based pressure monitoring system hasthe following components– Intravascular catheter– Connecting tubing and stopcocksHydraulic– Pressure transducersystem– Continuous flush device– Amplifier– Oscilloscope / Digital displayElectronic– Processorsystem– RecorderRevised 01/13/20091

Hemodynamic Monitoring: Principles to Practice – M. L. Cheatham, MD, FACS, FCCMMEASURING PRESSURE VARIABLES• The hydraulic system is much more subject topotential errors and artifacts than is the electronicsystem– Learning to troubleshoot the hydraulic portion of ainvasive pressure monitoring system is essential» Confirms the validity of the data» Helps avoid inappropriate therapeuticinterventions based upon erroneous dataHYDRAULIC SYSTEM COMPONENTS• Intravascular catheter– Tubing to access the desired blood vessel orcompartment– Detects the pressure waves generated by cardiaccontraction• Connecting tubing and stopcocks– Transmits pressure wave from patient tomeasuring device– Allows control of blood vessel to avoidhemorrhage, introduction of air, etc…PRESSURE TRANSDUCER• Converts the mechanical impulse of a pressure waveinto an electrical signal through movement of a “straingauge”– Modern transducers utilize a displaceable siliconsensing diaphragm and a “Wheatstone Bridge”Transducer DomeSensing DiaphragmWHEATSTONE BRIDGE• An electrical circuit forcomparison of resistances• Consists of a power source andfour resistors, three of whichhave a known resistance• To determine the unknownresistance, the resistance of theother three are balanced until thecurrent passing through bothsides of the parallel “bridge”decreases to zeroPressure TubingPressure WaveWHEATSTONE BRIDGE• The Wheatstone Bridge is used to measure theresistance change in a strain gauge– The resistance change is proportional to thechanging physiologic variable (i.e. pressure) ormechanical strain applied to the transducer– “Zeroing” a transducer is simply determining thevalue of the unknown resistance at rest» “Balancing the bridge”– As the physiologic variable changes, theresistance varies proportionally, a current isinduced across the bridge, and the voltmetervalue is converted to a pressure measurementHYDRAULIC SYSTEM COMPONENTS• Continuous flush device– Flushes the tubing with fluidat a rate of 1-3 mL/hr andhelps prevent blood fromclotting off the catheter– The “fast flush” featureincreases the flow to 30mL/min and can be used totest the system’s complianceRevised 01/13/20092

Hemodynamic Monitoring: Principles to Practice – M. L. Cheatham, MD, FACS, FCCMELECTRONIC SYSTEM COMPONENTSMEASURING PRESSURE VARIABLES• Amplifier– Increases the low voltage signal from the pressuretransducer to a signal that can be displayed– Most include electronic filters to remove unwantedphysiologic “noise”SIGNALPRESSURETRANSDUCERAMPLIFIER & SIGNALCONDITIONINGANALOG TO DIGITALCONVERTER• Oscilloscope / Digital display– Used to display waveforms and numerical dataSTRIP RECORDERBEDSIDE MONITORMICROPROCESSOR• Processor– Used to calculate various hemodynamic parameters• Recorder– A printer, strip chart recorder, or other deviceTYPICAL RADIAL ARTERY PRESSUREMONITORING SYSTEMCLINICAL CORRELATION• What do these measurements mean clinically?– Systolic pressure» The pressure exerted on the artery walls due toleft ventricular contraction (i.e., contractility)– Diastolic pressure» The pressure exerted during left ventricularrelaxation (i.e., vascular resistance)– Pulse pressure» The difference between peak systolic anddiastolic pressures (i.e., perfusion)CLINICAL CORRELATION• What do these measurements mean clinically?– Dicrotic notch pressure» The pressure thatreflects aortic valveclosure– Heart rate» The frequency ofcontraction measuredin beats per minute– Ejection time» The time of leftventricular ejectionPHYSICS OF PRESSURE MONITORING• The typical catheter-transducer transducer system in the ICU isconsidered to be a “second-order order dynamic system”– The pressure waveform dampens over time– Determined by two factors» “Natural frequency”– The number of oscillations per unit time thatoccurs without any damping» “Damping coefficient”– The time taken to dampen the waveformRevised 01/13/20093

Hemodynamic Monitoring: Principles to Practice – M. L. Cheatham, MD, FACS, FCCMPHYSICS OF PRESSURE MONITORING• For example, when dropped onto a hard floor, a ballbounces several times before coming to rest– With each successive bounce, it does not rise ashigh as on the previous bounce– Each bounce has a characteristic frequency (thenumber of oscillations per unit time) anddamping coefficient (time that it takes the ball tocome to a rest)Damping CoefficientFrequencyPHYSICS OF PRESSURE MONITORING• However, if the same ball is dropped onto soft earth,the ball will not bounce as high, resulting in adecreased frequency, and will come to rest sooner,reflecting an increased damping coefficient– This can be expressed mathematically BUT youdon’t need to know how to calculate it ☺P2=P12f2− Fn1f+ 2 jξF n+ 1P 1 , P 2 are output and input signals of the pressure transducerrespectively, f is an arbitrary frequency, F n is the natural frequency,ξ is the damping coefficient, and j is the complex numberPHYSICS OF PRESSURE MONITORING• The accuracy of a second-orderorder system is subject tothree mechanical factors1. Compliance• The stiffness of the fluid-filledfilled system (tubing)2. Fluid inertia• The pressure required to move fluid (blood)through the system3. Fluid resistance• The viscosity of the fluid moving through thesystem (resistance due to friction)DETERMINING THE NATURAL FREQUENCYAND DAMPING COEFFICIENT• This can also be expressed mathematically BUTyou don’t need to know how to calculate it ☺F n1=RCξ =2πIC 2 IF n is the natural frequency, ξ is the damping coefficient,C = compliance, I = inertia, and R = resistancePHYSICS OF PRESSURE MONITORING• The complex pressure wave generated with eachbeat of the heart is not unlike the bouncing ball– A pressure waveform is propagated at a givenfrequency (beats per minute)– The vascular resistance acts as the dampingcoefficient anddiminishesthewaveform’senergy and magnitude over time• The resulting arterial sine wave, occurring at therateof thepatient’spulse, iscalledthefirstharmonic or fundamentalPHYSICS OF PRESSURE MONITORING• If the waveform is reflected off atransduceror otherobstruction within the catheter-tubingtubing system, awave reflection or “harmonic” is generated– These harmonic waveforms are additive and canintroduce error into pressure measurementsRevised 01/13/20094

Hemodynamic Monitoring: Principles to Practice – M. L. Cheatham, MD, FACS, FCCMPHYSICS OF PRESSURE MONITORING• Without some degree of damping within a system,pressure waves reverberate within the catheter andtubing leading to the formation of harmonics andoverestimation of true blood pressure– An “underdamped”” system• With too much damping, the frictional forces impedethe arterial waveform such that it loses energyleading to underestimation of blood pressure– An “overdamped”” systemUNDERDAMPED WAVEFORM• Note the characteristicnarrow, peaked waveform– Overestimates systolicand underestimatesdiastolic blood pressure– Mean arterial pressureremains unchanged!• Causes– Long stiff tubing,increased vascularresistanceOVERDAMPED WAVEFORM• Note the characteristic widenedand slurred waveform– Underestimates systolicand overestimates diastolicblood pressure– Mean arterial pressureremains unchanged!• Causes– Air bubbles, complianttubing, catheter kinks,blood clots / fibrin,stopcocks, no fluid or lowflush bag pressureOPTIMAL DAMPING• Some damping is essentialto avoid harmonics– The “optimal” amount ofdamping is crucial toaccurate measurementof physiologic pressures• A catheter-transducertransducersystem accurately measurespressure only if its naturalfrequency and dampingcoefficient are appropriateFREQUENCY RESPONSE• A pressure monitoring system should be able todetect changes quickly (known as the “frequencyresponse”)• Damping will tend to decrease frequency response– Important if changes are occurring rapidly suchas with tachycardia or a hyperdynamic heart• The ideal monitoring system has ahigh“natural” or“undamped” frequency– The frequency that would occur in the absence ofany frictional forces or damping– Allows accurate measurement of fast heart ratesDYNAMIC RESPONSE ARTIFACTS• Underdamped and overdamped waveforms areencountered in the ICU on a daily basis– Look for them at the bedside!• The ability to recognize when these potentialsources of error or “dynamic response artifacts” arepresent is essential to the…– Appropriate use of hemodynamic measurements– Prevention of inappropriate therapy based uponerroneous dataRevised 01/13/20095

Hemodynamic Monitoring: Principles to Practice – M. L. Cheatham, MD, FACS, FCCMDYNAMIC RESPONSE ARTIFACTS• Because dynamic response artifacts are commonlyencountered during patient monitoring, titration ofmedications should ALWAYS be based upon meanarterial pressure (MAP)– This variable is less subject to measurementerror due to under- or overdamping• Systolic and diastolic blood pressure should NOTbe used to titrate therapy!TROUBLESHOOTING• OK, now let’s get practical…• The simpler the pressure monitoring system, thehigher its fidelity, the less it is subject to dynamicresponse artifacts, and the less likely it will be toproduce erroneous data• There are a number of steps that should beundertaken whenever setting up or troubleshootinga catheter-transducer transducer systemTROUBLESHOOTING• Remove multiple stopcocks, multiple injection ports,and long lengths of tubing whenever possible– The optimal length of pressure tubing is 4 feet» Longer lengths of tubing promote harmonicamplification and underdamping– Ensure that arterial pressure tubing is being used» Overly compliant tubing leads to overdamping– Avoid large diameter tubing» Prevents achievement of optimal dampingTROUBLESHOOTING• Remove all air bubbles from the system– Perhaps the single most important step inoptimizing dynamic response» Air acts as a “shock absorber”– Bubbles as small as 1 mm in diameter cancause substantial waveform distortion» Leads to overdamping and flattened waveforms– Ensure that all connections are tight andperiodically flush all tubing and stopcocks toremove air bubblesTROUBLESHOOTING• Whenever you are evaluating a patient’s changinghemodynamics– Check all transducers, stopcocks, tubing, andinjection ports for air– Gently tap the tubing and stopcocks as thecontinuous flush valve is opened to dislodge anybubbles» This will usually clear the system and restoremeasurement accuracy» Flushing a few small bubbles through thecatheter is OK; if more air is present, aspirate itfrom the tubingTROUBLESHOOTING• Zero the transducer– The accuracy of invasive pressure measurementsis dependent upon a proper reference point» The “midaxillary line” or “phlebostatic axis” iscommonly utilized– Each transducer should be zeroed at least onceeach day and whenever data is considered suspectRevised 01/13/20096

Hemodynamic Monitoring: Principles to Practice – M. L. Cheatham, MD, FACS, FCCMTROUBLESHOOTING– Transducers may be attached to the patient or to apole at the head of the bed– Changes in bed positioning generally require re-zeroing the pressure transducer» If the transducer is below the phlebostatic axis,the resulting arterial pressure will beerroneously high» If the transducer is above the phlebostatic axis,the resulting arterial pressure will beerroneously lowTROUBLESHOOTING• The “fast-flush” flush” or “square wave” test– Performed by opening the continuous flush valvefor several seconds creating a square wave» A system with appropriate dynamic responsecharacteristics will return to the baselinewaveform within one to two oscillations» If dynamic response characteristics areinadequate, troubleshoot the system untilacceptable dynamic response is achievedCONCLUSIONS• Direct pressure monitoring is essential fordetermining immediate changes in blood pressure• The arterial waveforms provide valuable diagnosticand treatment information• If not accurate, the erroneous data within thesewaveforms can potentially lead to detrimentaltreatment• Look at both the bedside monitor waveforms ANDthe numerical data– The waveforms can tell you a great dealCONCLUSIONS• All hemodynamic data should be considerederroneous until you are satisfied that the dynamicresponse characteristics of the monitoring systemare appropriate• Learn to troubleshoot each monitoring system sothat you can ensure the accuracy of your patient’sdata• Physics is important after all ☺Revised 01/13/20097