Drugs that affect the autonomic nervous system can influence the Q-T interval directly or by changing the heart rate. Bazett's formula to correct for rate may be misleading after certain drug interventions. This hypothesis was tested in 20 patients receiving both propranolol (0.15 mg/kg intravenously) and atropine (0.03 mg/kg intravenously). Six patients received propranolol first, 7 patients received atropine first, and 7 patients received atropine plus propranolol simultaneously. During control and after drug intervention, the Q-T interval was measured directly in sinus rhythm and during a fixed atrial paced rate, and was calculated using Bazett's formula. The ventricular effective refractory period was also determined in 6 patients after administration of atropine plus propranolol. The sinus cycle length (836 ± 156 to 648 ± 84 ms, mean ± standard deviation), measured Q-T interval (367 ± 26 to 329 ± 26 ms), and atrially paced Q-T interval (330 ± 28 to 315 ±27 ms) shortened after atropine plus propranolol (p <0.001), but the corrected Q-T interval with use of Bazett's formula did not change (402 ± 33 to 412 ± 24 ms). The ventricular effective refractory period also shortened from 241 to 20 to 218 ± 21 ms after atropine plus propranolol (p <0.02). The sinus cycle length increased after propranolol (750 ± 97 to 907 ± 108 ms, p <0.001), but no change occurred in the measured Q-T interval or atrial paced Q-T interval although the corrected Q-T interval using Bazett's formula was greatly shortened (428 ± 15 to 391 ±22 ms, p <0.001). The sinus cycle length, measured Q-T interval, and atrially paced Q-T interval decreased after atropine (p <0.01), but the corrected Q-T interval lengthened (375 ± 29 to 418 ± 28 ms, p <0.01). In summary, atropine and atropine plus propranolol shorten the Q-T interval independent of rate, demonstrating a direct vagal effect on the Q-T interval. Bazett's formula inaccurately predicts the Q-T interval after administration of atropine, propranolol, and atropine plus propranolol.
ASJC Scopus subject areas
- Cardiology and Cardiovascular Medicine