Changes in the vulnerable period of the rat myocardium during hypoxia, hyperventilation and heart failure
Kujaník, S.; Wilk, P.; Tomcová, D.
Physiologia Bohemoslovaca 33(5): 470-480
1984
ISSN/ISBN: 0369-9463 PMID: 6438669 Document Number: 224723
Changes in the duration and size of the vulnerable period of the myocardium in the presence of respiratory changes were studied in acute experiments on rats. The limits of the vulnerable period were determined by directly stimulating the heart during ventilation via the enlarged respiratory dead space, during hyperventilation and during heart failure. In the control group (normal ventilation without enlargement of the dead space), the vulnerable period lasted 5.7 .+-. 0.76 ms. During ventilation via the enlarged dead space, hypercapnic hypoxemia developed and the vulnerable period was markedly prolonged (18.55 .+-. 5.29 ms) by a shift of its inner limit to the left. Hyperventilation caused normoxic to hyperoxic hypocapnia and markedly reduced the duration of the vulnerable period (8.17 .+-. 2.21 and 9.31 .+-. 2.38 ms, respectively). The vulnerable period lengthened the most in heart failure (25.46 .+-. 3.93), mainly as a result of a shift of its outer limit. In all the experimental groups there was a shift of the vulnerable period to the right, which was fastest in hypercapnic hypoxemia and slowest in hyperoxic hypocapnia. The administration of Inderal (3 mg/kg i.p.) or Arfonad (50 mg/kg i.p.) markedly shortened the vulnerable period during hypercapnic hypoxemia (9.87 .+-. 2.78 and 9.32 .+-. 2.16 ms, respectively), but did not blocks the shift. Lengthening of the vulnerable period during hypercapnic hypoxemia was probably due to activation of sympathetic nerves via .beta.-adrenergic receptors.