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

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