Hemodynamic effects of blood loss during a passive response to a stressor in the conscious rabbit

In the conscious rabbit, exposure to an air jet stressor increases arterial pressure, heart rate, and cardiac output. During hemorrhage, air jet exposure extends the blood loss necessary to produce hypotension. It is possible that this enhanced defense of arterial pressure is a general characteristic of stressors. However, some stressors such as oscillation (OSC), although they increase arterial pressure, do not change heart rate or cardiac output. The cardiovascular changes during OSC resemble those seen during freezing behavior. In the present study, our hypothesis was that, unlike air jet, OSC would not affect defense of arterial blood pressure during blood loss. Male New Zealand White rabbits were chronically prepared with arterial and venous catheters and Doppler flow probes. We removed venous blood until mean arterial pressure decreased to 40 mmHg. We repeated the experiment in each rabbit on separate days in the presence and absence (SHAM) of OSC. Compared with SHAM, OSC increased arterial pressure 14 ± 1 mmHg, central venous pressure 3.3 ± 0.4 mmHg, and hindquarter blood flow 34 ± 4% while decreasing mesenteric conductance 32 ± 3% and not changing heart rate or cardiac output. During normotensive hemorrhage, OSC enhanced hindquarter and renal vasoconstriction. Contrary to our hypothesis, OSC (23.5 ± 0.6 ml/kg) increased the blood loss necessary to produce hypotension compared with SHAM (16.8 ± 0.6 ml/kg). In nine rabbits, OSC prevented hypotension even after a blood loss of 27 ml/kg. Thus a stressful stimulus that resulted in cardiovascular changes similar to those seen during freezing behavior enhanced defense of arterial pressure during hemorrhage.