Heart rate responses to deep breathing and 4-seconds of exercise before and after pharmacological blockade with atropine and propranolol.

Two autonomic tests which evaluate cardiac vagal activity, the respiratory sinus arrhythmia and the newer 4-second exercise test, have been compared. From electrocardiograph tracings, respiratory sinus arrhythmia was quantified by the ratio between the longest R-R interval during expiration and the shortest one during inspiration (E/I ratio), and the 4-second exercise test by the ratio between the last R-R interval before and the shortest one during exercise (B/C ratio). In 29 healthy subjects there was a correlation (R = 0.60, p less than 0.05) between the responses to the two tests. In a group of six healthy subjects the same tests were performed after autonomic blockade with intravenous atropine and/or propranolol. The heart rate rise during the 4-second exercise test was nearly abolished by atropine (mean +/- SD) (B/C: control = 1.53/0.33; after atropine = 1.04/0.03), whereas RSA was diminished to a lesser extent (E/I: control = 1.59/0.24; after atropine = 1.13/0.07). beta-adrenoceptor blockade did not affect the test ratios (after propranolol: B/C = 1.51/0.33 and E/I = 1.45/0.14). Successive tests during the following hour after atropine infusion showed a somewhat faster recovery of the respiratory sinus arrhythmia than the heart rate acceleration induced by the 4-second exercise test (p less than 0.05). We conclude that there may be some difference in the mechanisms which contribute to the heart rate changes in these two autonomic cardiovascular tests; these remain to be clarified. The 4-second exercise test may be an alternative to the respiratory sinus arrhythmia test in the non-invasive evaluation of cardiac parasympathetic activity.