The baroreflex loop consists of a fast neural arc and a slow mechanical arc. We hypothesized that the neural baroreflex arc compensates the slow mechanical response and thus improves the quality of blood pressure regulation. We estimated the open-loop transfer characteristics of the neural baroreflex arc (HP), i.e., from carotid sinus pressure to sympathetic nerve activity (SNA), and that of the effective peripheral baroreflex arc (Hp), i.e., from SNA to arterial pressure, in anesthetized rabbits. The gain of Hn was constant below 0.12 +/- 0.057 Hz and increased with a slope of 6.1 +/- 0.06 dB/octave above its frequency up to 1 Hz. In contrast, the gain of Hp was constant below 0.071 +/- 0.03 Hz and decreased with a slope of -11.0 +/- 1.48 dB/octave above the frequency. These data indicate that Hn accelerates slow peripheral responses in the frequency range of 0.1-1 Hz. Although too much acceleration in the high-frequency range could result in instability of the system, numerical analysis of the closed-loop baroreflex response indicated that the neural arc optimized arterial pressure regulation in achieving both stability and quickness.

In the circulatory system, a change in blood pressure operates through the baroreflex to alter sympathetic efferent nerve activity, which in turn affects blood pressure. Existence of this closed feedback loop makes it difficult to identify the baroreflex open-loop transfer characteristics by means of conventional frequency domain approaches. Although several investigators have demonstrated the advantages of the time domain approach using parametric models such as the autoregressive moving average model, specification of the model structure critically affects their results. Thus we investigated the applicability of a nonparametric closed-loop identification technique to the carotid sinus baroreflex system by using an exogenous perturbation according to a binary white-noise sequence. To validate the identification method, we compared the transfer functions estimated by the closed-loop identification with those estimated by open-loop identification. The transfer functions determined by the two identification methods did not differ statistically in their fitted parameters. We conclude that exogenous perturbation to the baroreflex system enables us to estimate the open-loop baroreflex transfer characteristics under closed-loop conditions.

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a positive response. The probability of no response Summary to stress was analysed using logistic regression to We have investigated the cardiovascular and plasma obtain the probability of no response vs. end-tidal noradrenaline response to surgical incision under sesevoflurane concentration and the best-fit curve from voflurane anaesthesia and determined the end-tidal the maximum likelihood estimators of the model paraconcentration of sevoflurane that blocks the admeters. MAC BAR (mean±SE) was 8.0±0.2%, MAC BCR renergic response or responses to surgical incision was 7.9±0.2%. However, such high doses of se-(MAC BAR ) and changes in mean arterial pressure (MAP) voflurane cannot be used clinically because of their high toxicity. It may be preferable to combine sein response to surgical incision (MAC BCR ) in 50% of voflurane with other anaesthetics to reduce haemowomen. We randomly assigned 64 female patients, dynamic responses to strong stimulation. aged 20-49 years, to eight groups according to endtidal sevoflurane concentration: 5.0%, 5.5%, 6.0%,

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