Factors confounding blockade of cardiac afferents by intrapericardial procaine in conscious rabbits

Abstract

Intrapericardial procaine has been used by several groups to block cardiac afferent nerves to study effects of cardiogenic reflexes. In eight conscious rabbits, procaine (17-113 mg ipc; median 32) blocked cardiac efferents. Procaine (17-113 mg ipc; median 39) abolished the reflex depressor effects of the cardiac C-fiber excitant 1-phenylbiguanide (PBG), and in four of eight rabbits prevented the hypotensive phase 2 of acute central hypovolemia, which has been attributed to a signal from the heart. However, in three of the rabbits respiratory incoordination and blood gas abnormalities developed. In another study of four rabbits, procaine (165-335 mg ipc; median 235) invariably caused phrenic nerve blockade and underventilation. In three rabbits, after intrapericardial (250 mg) or subcutaneous (50 mg) procaine, plasma procaine levels rose to 9.4 and 4.8 micrograms/ml, respectively. During intravenous infusion of procaine, the PBG chemoreflex was abolished at plasma levels > 3.1 micrograms/ml, and phase 2 of acute hypovolemia at levels > or = 4.3 micrograms/ml. There is a narrow margin between a dose of intrapericardial procaine that blocks cardiac nerves and one that can produce confounding effects from phrenic nerve blockade or absorption into the bloodstream.

Interactions Between the Circulatory Effects of Central Hypovolaemia and Arterial Hypoxia in Conscious Rabbits

“…When our rabbits breathed room air (Table l), the mean Pao2 of 95 mmHg was similar to that in other reports, but the mean P~co, of 39 mmHg was higher by 5-1 1 mmHg (Korner et al 1969;Evans et al 1993Evans et al , 1994. The latter finding may merely reflect the insulation of our rabbits from the environment by the tank in which they were placed.…”

“…The procedures used have been described in detail previously (Ludbrook et al 1988;Evans et al 1992Evans et al , 1993. They were performed under halothane anaes-thesia after induction with intravenous thiopentone sodium (25 mg/kg) and endotracheal intubation.…”