Anesthesia Management |
|Chapter 30:||Cardiovascular Monitoring|
Pulse Rate Monitoring
Although the preceding discussion has focused on methods to estimate heart rate from the ECG trace, it might be argued that monitoring pulse rate is more important than monitoring heart rate, in terms of perioperative hemodynamic assessment. By definition, the distinction centers on whether a given electrical depolarization and systolic contraction of the heart (heart rate) generates a palpable peripheral arterial pulsation (pulse rate). Pulse deficit describes the extent to which the pulse rate is less than the heart rate. This is typically seen in patients with atrial fibrillation, in which short R-R intervals compromise cardiac filling during diastole, resulting in reduced stroke volume and imperceptible arterial pulse. The most extreme example of a pulse deficit is electrical-mechanical dissociation or pulseless electrical activity, seen in patients with cardiac tamponade, extreme hypovolemia, and other conditions in which cardiac contraction does not generate a palpable peripheral pulse.
Most monitors report heart rate and pulse rate separately. The former is measured from the ECG trace, and the latter is determined by a pulse source, which is generally selectable by the user. For example, the pulse oximeter plethysmograph trace will provide a suitable pulse measurement source for most patients except those with severe arterial occlusive disease or those with marked peripheral vasoconstriction. Automatic noninvasive blood pressure devices determine the pulse rate by counting oscillations in pressure sensed by the surrounding cuff. When direct arterial pressure measurement is in place, the pressure waveform provides a reliable pulse source. As already emphasized, this is particularly useful in distinguishing artifactual ECG signals and erroneous heart rates from important real cardiac events. Beware, however, that even the arterial pressure trace may be misleading when nonsystolic arterial pulsations are detected by the monitor and counted separately. In patients treated with intraaortic balloon counterpulsation, the pressure pulse resulting from balloon inflation during diastole may be detected and produce a factitiously high pulse rate. 19 Other arterial pressure waveforms that have a bisferiens, double-peaked morphology, such as those arising in patients with aortic valve regurgitation, may produce a similar artifactually increased pulse rate measurement. In contrast, patients with pulsus alternans may have inappropriately low pulse rates measured, owing to the diminished magnitude of every other arterial pulsation. In these cases, selection of an alternative pulse source, such as the pulmonary artery pressure waveform, can provide a reliable pulse rate.
In summary, pulse rate monitoring and heart rate monitoring complement one another. Even though monitoring both pulse rate and heart rate may seem redundant in many cases, such intentional redundancy is being applied to modern computerized monitoring algorithms. One example is termed robust fusion sensor technology, which fuses heart rate and pulse rate data from multiple sourcesECG, pulse oximeter plethysmogram, and arterial pressure waveform. 20 This approach improves the accuracy of heart and pulse rate monitoring and reduces the frequency of distracting false alarms. 21 In the end, however, as with all numeric information displayed on the bedside monitor, it is the clinicians ultimate responsibility to scrutinize the analog ECG and pressure waveforms to ensure the veracity of all digital values displayed by bedside monitors.
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