Describe the potential causes, and effects, of resonance and damping on an invasive arterial blood pressure trace. 

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College Answer

For a good answer candidates were expected to mention that the arterial pressure 
waveform is made up of many different sine waves (as determined by Fourier Analysis) with 
each sine wave having a specific frequency. Every system has its own natural oscillatory 
frequency, or resonant frequency. The pressure measuring system has a resonant frequency 
at which oscillations occur, and if this is less than 40 Hz, it falls within the range of 
frequencies present in the blood pressure waveform and oscillations may produce a sine 
wave which is superimposed on the blood pressure wave form. The resonant frequency can 
be increased by using a short, wide, stiff catheter. In respect to damping, some damping is 
inherent in any system and acts to slow down the rate of change of signal between the 
patient and pressure transducer. Mention of causes of damping and the optimal damping 
coefficient (0.677) were expected. An under-damped system is one whereby resonance 
occurs causing the signal to oscillate and overshoot (damping factor <0.7) and an overdamped is one whereby the signal takes a long time to reach equilibrium but will not 
overshoot. It may not reach equilibrium in time for a true reading to be given (damping factor >1.0). Both resonance and damping can alter the measured systolic and diastolic values but the mean pressure is not affected