Holter monitor recordings can be used to assess heart-rate variability as a measurement of the standard deviation of the sinus rhythm cycle length (or fluctuation around the mean RR interval). Heart-rate variability reflects the parasympathetic and sympathetic balance of the autonomic nervous system and therefore offers insight into the risk for sudden cardiac death.
What is the clinical significance of heart-rate variability?
Recent studies show that analysis of heart-rate variability is important in evaluation of postinfarction and diabetic patients. In both groups decreased heart-rate variability is associated with an increased risk of sudden cardiac death. Lower heart-rate variability is also recorded with acute MI. Here a predominance of sympathetic activity and reduction in parasympathetic cardiac control result in increased sympathetic activity, which decreases the fibrillation threshold and predisposes to ventricular fibrillation. In addition, anterior-wall MI results in a more profound reduction in heart-rate variability than inferior-wall infarction.
What is signal-averaged electrocardiography (SAECG)?
SAECG is a method of recording the ECG in which amplifiers and filters record cardiac signals with amplitudes of only a few microvolts. Electrical potentials corresponding to delayed and fragmental conduction in the ventricle are recorded in microvolts and waveforms continuous with the QRS complex. Three criteria are of importance: (1) QRS duration, (2) low-voltage signals in the last 40 msec of the QRS, and (3) low-frequency waveforms lasting 30 msec after terminal QRS complex.
Positive SAECG with late potentials highlighted are the end of the QRS complex between 150 and 190 ms. The low-amplitude signals (25 mV) exceed the voltage for noise at the terminal 40 ms. The QRS duration is normal (114 ms). QRS 114 ms (normal 120 ms) LA S – 51 ms (normal 40 ms). RMS 40 12.5 mV (normal 25 mV).