In addition, the heart rate, after rising initially in response to upright standing posture (not with the tilt table), slows after about 15 beats to reach a stable rate by the thirtieth beat. The ratio of R-R intervals in the electrocardiogram (ECG), corresponding to the 30th and 15th beats (the 30:15 ratio), is an even more sensitive measure of the integrity of vagal inhibition of the sinus node. A ratio in adults under age 60 of less than 1.07 is usually abnormal, indicating a loss of vagal tone and the normal ratio is progressively higher for younger ages, for example, it is usually above 1.12 at age 30 and 1.1 at age 40.
Another simple procedure for quantitating purely vagal function consists of measuring the variation in heart rate during deep breathing (respiratory sinus arrhythmia). The ECG is recorded while the patient breathes at a regular rate of 6 breaths per minute. Normally, the heart rate varies by as many as 10 beats per minute or even more between expiration and inspiration; differences of less than 7 beats per minute for ages 60 to 69 and 9 for ages 50 to 59 may be abnormal.
A yet more accurate test of vagal function is the measurement of the ratio of the longest R-R interval during forceful slow expiration (standardized as constant blowing at a pressure of 40 mm Hg for 10 s) to the shortest R-R interval during inspiration, which allows the derivation of an expiration–inspiration (E:I) ratio. This is the best validated of all the heart-rate measurements, particularly as computerized methods can be used to display the spectrum of beat-to-beat ECG intervals during breathing. The results of these tests must always be compared with those obtained in normal individuals of the same age. Up to age 40 years, E:I ratios of less than 1.2 (signifying a variation of 20 percent) are abnormal. The ratio decreases with age, and markedly so beyond age 60 years (at which time it approaches 1.04 or less), as it does also in the presence of even mild diabetic neuropathy. Thus the test results must be interpreted cautiously in the elderly or in diabetic individuals. Similar ratios have been developed for heart-rate change during the Valsalva maneuver; the Valsalva ratio.
Computerized methods of power spectral analysis may be used to express the variance in heart rate as a function of the beat-to-beat interval. Several power peaks are appreciated: one related to the respiratory sinus arrhythmia and others that reflect baroreceptor and cardiac sympathetic activity. All of these tests of heart-rate variation are usually combined with measurement of heart rate and blood pressure during the Valsalva maneuver, as described below, and with the tilt-table test, as described in Chap. 17.
In the Valsalva maneuver, the subject exhales into a manometer or against a closed glottis for 10 s, creating a markedly positive intrathoracic pressure. The sharp reduction in venous return to the heart causes a drop in cardiac output and in blood pressure; the response on baroreceptors is to cause a reflex tachycardia and, to a lesser extent, peripheral vasoconstriction. With release of intrathoracic pressure, the venous return, stroke volume, and blood pressure rise to higher-than-normal levels; reflex parasympathetic influence then predominates and a bradycardia results (see Fig. 25-5). Failure of the heart rate to increase during the positive intrathoracic pressure phase of the Valsalva maneuver points to sympathetic dysfunction, and failure of the rate to slow during the period of blood pressure overshoot points to a parasympathetic disturbance. In patients with autonomic failure, the fall in blood pressure is not aborted during the last few seconds of increased intrathoracic pressure, and there is no overshoot of blood pressure when the breath is released. The Valsalva ratio, referring to the maximum heart rate generated by the maneuver to the lowest heart rate within 30 s of that peak, is another often-used measure in comprehensive autonomic testing.