Inhalation anaesthetics decrease heart rate in isolated hearts but mostly increase heart rate in the intact organism, although most inhibit sympathetic drive. Differences in the degree of increase in heart rate between agents may be related to differences in their vagolytic action. To test this hypothesis, we studied the effects of halothane (H), isoflurane (I), enflurane (E), sevoflurane (S) and desflurane (D) [1-3 MAC (minimum alveolar concentration)] on heart rate and heart rate variability (HRV) as a measure of cardiac vagal activity in seven dogs. HRV was analysed in the time domain as the standard deviation of the RR interval (SDNN) and in the frequency domain as power in the high-frequency (HF, 0.15-0.5 Hz) and low-frequency (LF, 0.04-0.15 Hz) ranges. Heart rate increased with anaesthetic concentration and there were corresponding decreases in SDNN, HF power and LF power. Heart rate increased most with D (+40 beats min(-1)), least with H (+8 beats min(-1)) and to an intermediate extent with S, I and E. SDNN and HF power, as measures of vagal activity, changed in the opposite direction and decreased in the same order as heart rate increased. However, SDNN and HF power correlated significantly with heart rate [r=-0.81 (0.04) and -0.81 (0.03) respectively] and were independent of the anaesthetic and its concentration (P<0.05). Consistent with our hypothesis, these results suggest that differences between agents in the degree of increase in heart rate are explained by differences in their vagolytic action.

The effects of methohexitone, Althesin, ketamine and etomidate on single fibre discharge of cardiac vagal efferents and on heart rate were studied in cats. Cardiac vagal efferents were inhibited markedly and regularly for equihypnotic doses of methohexitone (2.0 mg kg-1), Althesin (0.1 ml kg-1) and ketamine (5.0 mg kg-1), but not of etomidate (0.8 mg kg-1). These inhibitory effects were independent of arterial pressure and mirrored the increases of heart rate elicited by the first three agents. Etomidate did not consistently affect cardiac vagal discharge or heart rate. Thus methohexitone. Althesin and ketamine inhibit efferent cardiac vagal drive by their central action independently of baroreflex function. This central vagolysis is probably the cause of their positive chronotropic effects.

Inhalation anaesthesia is characterized by a uniform Q/VO2 relation with an almost linear course at an anaesthetic concentration up to 2 MAC, regardless of the anaesthetic. Metabolic regulation of blood flow apparently operates also during inhalation anaesthesia up to 2 MAC so that the decrease in VO2 determines Q. This implies that cardiac output alone provides little information on the function of the circulation during inhalation anaesthesia unless related to metabolic demands, i.e. to VO2.

Multiple regional skin temperatures were measured continuously before and after complete sympathetic nerve block by extradural anaesthesia in conscious dogs in environmental temperatures slightly below (22.4 (SEM 0.4) degrees C) and slightly above (27.2 (0.4) degrees C) the thermoneutral range. In the cold environment, skin temperature increased in the distal (+2.13 degrees C, P less than 0.001) and proximal (+1.1 degrees C, P less than 0.02) front limbs and distal (+3.25 degrees C, P less than 0.001) and proximal (+1.72 degrees C, P less than 0.001) hind limbs, but decreased on the chest (-0.83 degrees C, P less than 0.01) and abdomen (-0.67 degrees C, P less than 0.002). Similar changes (all significant) were seen in the warm environment, although the effects were smaller. Rectal temperature did not change during the experiments. Arterial pressure decreased in both groups after denervation, but was the same in the denervated state regardless of ambient temperature. Our results are incompatible with increased sympathetic discharge from unblocked regions being a dominant factor in the decrease in trunk skin temperature above and within the analgesic areas, and demonstrate a substantial contribution of the sympathetic nervous system to the maintenance of the normal skin temperature gradient along the long axis of the body.

Both methods underestimate blood volume by about the same extent compared with BVEB, probably because slowly perfused compartments are not detected during the short measurement period of 4 min. In the case of the transit time approach, rather short transit times result and in the case of the mass conservation principle, back-extra-polation yields rather high plasma concentrations of ICG at the time of injection. Accordingly, the two methods seem to be equivalent for measuring blood volume rapidly, although the absolute volume is underestimated by about 40%.