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

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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
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