Influence of age on the cardiovascular response during graded haemorrhage in anaesthetized rats

The present study was designed to follow the cardiovascular response to graded haemorrhage in two groups of adult rats of different ages. Group A (n = 10) had a mean body weight (bwt) of 655 g (50 weeks old) and group B (n = 10) had a mean bwt of 250 g (12 weeks old). After induction of anaesthesia, the carotid artery was cannulated for continuous intra-arterial blood pressure monitoring and for controlled haemorrhage. Five per cent of the total blood volume (TBV) was removed every 15 min until a maximum of 50% was reached. Mean arterial pressure (MAP) fell by 4-9 mm Hg each time 5% TBV was removed in both groups, and at the end of the study the MAP was comparable in the two groups (34 +/- 10 mm Hg in group A and 28 +/- 8 mm Hg in group B). There were no significant changes in heart rate (HR) in the young animals during haemorrhage, while there was a significant drop in heart rate in the older animals when blood loss exceeded 15% of TBV (P < 0.05). A significant difference was also observed in HR between the two groups of animals with blood loss in excess of 20% TBV (P < 0.05). It was concluded that although the hypotension induced by graded haemorrhage was similar in young and old rats, the older animals were less able to maintain their HR, probably as a consequence of age- and anaesthetic-related changes in sympathetic activity.