Anesthesia Management |
|Chapter 30:||Cardiovascular Monitoring|
Frequency Content of the Arterial Pressure Waveform
The arterial blood pressure waveform is a periodic complex wave, which can be reproduced by Fourier analysis, a technique that recreates the original complex pressure wave by summing a series of simpler sine waves of various amplitudes and frequencies. 126, 131 The original pressure wave has a characteristic periodicity termed the fundamental frequency, which is equal to the pulse rate. Note that the pulse rate is reported in beats per minute, whereas the fundamental frequency is reported in cycles per second or Hertz (Hz). For example, a pulse rate of 60 beats/min equals 1 beat/s or 1 cycle/s or 1 Hz.
The sine waves that sum to produce the complex wave have frequencies that are multiples or harmonics of the fundamental frequency. A crude arterial waveform, which displays a systolic upstroke, systolic peak, dicrotic notch, and so forth, can be reconstructed with reasonable accuracy from two sine waves, the fundamental frequency and the second harmonic (Fig. 3010). If the original arterial pressure waveform contains high-frequency components such as a steep systolic upstroke or other sharp details, higher frequency sine waves (and more harmonics) are needed to provide a faithful reconstruction of the original pressure waveform. As a general rule, six to ten harmonics are required to provide adequate reproduction of most arterial pressure waveforms, 131, 132 Hence, accurate blood pressure measurement in a patient with a pulse rate of 120 beats/min (2 cycles/s or 2 Hz) requires a monitoring system dynamic response of 12 to 20 Hz. Clearly, the faster the heart rate and the steeper the systolic pressure upstroke, the greater will be the dynamic response that is required from the monitoring system. As a corollary, venous pressure waveforms, which generally do not have steep waves or high frequency components, do not require monitoring systems with such high frequency responses.
|FIGURE 3010 Arterial blood pressure waveform produced by summation of sine waves. The fundamental wave (top) added to 63% of the second harmonic wave (middle) results in a pressure wave (bottom) that resembles an arterial blood pressure waveform (box). See text for greater detail. (From Mark JB: Atlas of Cardiovascular Monitoring. New York, Churchill Livingstone, 1998: Fig. 91.)|
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