Ventricular arrhythmias caused by repeat exposure to hypoxia are dependent on duration of reoxygenation

The three objectives of this study were: 1) to determine whether an initial exposure to systemic hypoxia could affect the ventricular ectopy caused by a second period of hypoxia, 2) to see whether the duration of the intervening period of reoxygenation was important, and 3) to determine whether propranolol could produce results similar to those caused by an inadequate period of reoxygenation. Anesthetized instrumented beagles of either sex weighing 10.2 +/- 0.4 kg (n = 25) were exposed to dual periods of hypoxia (4 min each, PO2 approximately 15 +/- 3 mmHg). Dogs were divided into four groups according to the duration of reoxygenation and propranolol treatment: group 1 (n = 7), 20 min of reoxygenation; group 2 (n = 7), 40 min of reoxygenation; group 3 (n = 6), 60 min of reoxygenation; group 4 (n = 5), 50 min of reoxygenation plus propranolol. Dogs in groups 1 and 4 experienced a significant reduction in percent ectopy during their second exposure to hypoxia [group 1; 47 +/- 9% vs. 11 +/- 6%, group 4; 50 +/- 1% vs. 1 +/- 2%, (P < 0.05)]. There were no significant differences in percent ectopy between the two periods of hypoxia in either group 2 or group 3 dogs (e.g., group 2; 50 +/- 1% vs. 51 +/- 11%). The results show that a first exposure to hypoxia can precondition the myocardium against arrhythmogenic effects of a second period of hypoxia.