Simulation of acute haemorrhage in unanaesthetized rabbits.

1. We have shown that it is feasible to match the linear rate of fall of cardiac output that occurs during haemorrhage at 2.7 ml/kg per min in unanaesthetized rabbits by constricting the thoracic inferior vena cava so as to decrease venous return. 2. The changes in systemic vascular resistance, arterial pressure and heart rate that occurred during haemorrhage were mimicked by simulated haemorrhage. They were reproducible when simulated haemorrhage was performed three times at 90 min intervals, and when it was repeated four times over 12 days. 3. Simulated haemorrhage caused rises in plasma renin activity (PRA) and plasma arginine vasopressin concentration (AVP) that were similar to those reported after haemorrhage. The response of PRA was unaffected by repeated simulated haemorrhage, but the response of AVP was less on the third occasion. 4. When the shed blood was re-infused after haemorrhage, cardiac output tended to remain low and systemic vascular resistance high. After simulated haemorrhage, all haemodynamic variables returned to normal within 2 min of releasing the caval cuff. 5. Haematocrit fell during haemorrhage, and remained low for at least 5 days after replacement of the shed blood. Haematocrit was unaffected by simulated haemorrhage. 6. Venous pressure below the inflatable cuff rose by 6 mmHg in the course of simulated haemorrhage. 7. We conclude that the central haemodynamic effects of haemorrhage can be closely and repeatedly simulated by inflating a cuff on the inferior vena cava. This provides a useful technique for repeatedly studying the effects of acute reduction of central blood volume in conscious animals.