Heart Rate Monitoring
The simplest and least invasive form of cardiac monitoring remains
measurement of the heart rate. As a circulatory vital sign, the heart rate provides
an important guide to the patient's baseline condition and the influence of anesthetics
and surgical stimuli. The ability to estimate the heart rate quickly with a "finger
on the pulse" is a skill as important as this expression is common. However, under
most circumstances in modern anesthesia practice, electronic monitoring devices are
used to provide a continuous, numeric display of the heart rate.
Although any monitor that senses the period of the cardiac cycle
can be used to determine the heart rate, the most common technique applied in the
operating room is the ECG. Current monitors use multiple ECG leads to sense cardiac
electrical activity.[18]
This redundancy aids in
detection of the R wave and improves the accuracy of heart rate measurement. As
a result, a single noisy lead, or one in which the R waves are of very low amplitude,
will not prevent the monitor from calculating the heart rate accurately.
ECG measurement of the heart rate begins with accurate detection
of the R wave and measurement of the R-R interval. The digital value displayed for
the heart rate is generated from an algorithm designed to count and average a certain
number of beats and then display a number that is updated every 5 to 15 seconds.
[19]
As a result, transient changes in heart rate
may have little impact on the displayed digital value. For example, consider an
episode of complete heart block that interrupts slow sinus rhythm. Depending on
the algorithm used by the monitor, the digital value for heart rate may show only
a slight reduction from the baseline value. In this instance, the heart rate of
49 beats/min displayed by the monitor fails to alert the clinician to the dangerous
transient bradyarrhythmia ( Fig. 32-1
).
Although the monitor could update the digital value for the heart rate after each
beat calculated from the R-R interval, numbers would then be flashing on the display
screen, changing with each heartbeat, because of the normal physiologic beat-to-beat
variability in heart rate. Instead, the displayed digital value
Figure 32-1
Digital heart rate (HR) displays may fail to warn of
dangerous bradyarrhythmias. Direct observation of the electrocardiogram (ECG) and
the arterial blood pressure traces reveals complete heart block and a 4-second period
of asystole, whereas the digital display reports an HR of 49 beats/min. Note that
the ECG filter (arrow) corrects the baseline drift
so that the trace remains on the recording screen. (From Mark JB: Atlas
of Cardiovascular Monitoring. New York, Churchill Livingstone, 1998, Fig. 13-2.)
for the heart rate is derived by the monitor by using a moving average filter that
gives greater weight to the most recent values in deriving the average value for
the display.
Occasionally, the clinician must check electrode attachment, increase
ECG signal gain, or select alternative ECG leads to facilitate heart rate monitoring.
Some monitors allow manual adjustment of the threshold or sensitivity for R-wave
detection. Despite these measures, the monitor may display inaccurate heart rates
when patient movement or other electrical interference distorts the ECG trace. In
the operating room, such interference frequently occurs when the electrosurgical
unit is in use. The ECG trace should always be visually inspected to confirm the
numeric value for the heart rate displayed on the monitor. Spurious values are recognized
quickly, and the true heart rate can be estimated from the ECG waveform tracing.
In addition, the arterial pressure waveform or the pulse oximeter plethysmograph
should be used to confirm the pulse rate in these instances.
Electrical interference in the ECG trace may arise from sources
other than the electrosurgical unit. Power line noise appears as a 60-Hz artifact
and may be eliminated by selecting narrower-bandpass ECG filters, including a 60-Hz
notch filter. Other artifacts result from muscle twitching and fasciculations, as
well as various medical devices, including lithotripsy machines, cardiopulmonary
bypass equipment, and fluid warmers.[20]
Paced
rhythms often produce problems with ECG measurement of the heart rate. When tall
pacing spikes are present, the monitor may misinterpret these high-amplitude signals
as R waves and miscalculate the heart rate. Tall T waves may produce the same artifact
when the monitor mistakenly counts these T waves as R waves. These problems may
be lessened by decreasing ECG gain, adjusting R-wave detection sensitivity, or changing
the ECG lead to one with a smaller pacing spike or T-wave amplitude.
