Alteration in heart rate response to hemorrhage in conscious dogs with volume overload

The heart rate response to hemorrhage was studied in conscious dogs before and up to 2 mo after the establishment of volume overload due to systemic arteriovenous (a-v) fistulas. Before a-v fistula, heart rate increased markedly during hemorrhage. When hemorrhage was preceded by dextran infusion, bleeding resulted in a gradual reduction in heart rate. The a-v fistula caused marked increases in resting heart rate, central venous pressure, pulse pressure, and blood volume. During hemorrhage, heart rate initially remained constant, but then declined abruptly from the resting value of 121 +/- 3.7 beats/min to a nadir of 89 +/- 6.5 beats/min (P less than 0.01). Although mean arterial pressure decreased markedly, there was no significant change in pulse pressure, and central venous pressure tended to stabilize with the heart rate decline. The abrupt heart rate decline was prevented by atropine but unaltered by propranolol. The response was observed as early as 5 days after a-v fistula. We conclude that an alteration in the heart rate response to hemorrhage appears early during volume overload. This alteration appears to be reflex in nature and to be mediated by the parasympathetic nervous system.