Baroreceptor control of systemic haemodynamic at incremental halothane levels in the dog

The open-loop carotid sinus reflex control of systemic haemodynamics was studied in an acute dog preparation with isolated perfused carotid sinuses at three end-tidal halothane levels of 0.66 +/- 0.02% (HI); 0.88 +/- 0.02% (H2); and 1.17 +/- 0.02% (H3), in order to investigate the dose dependency of the carotid sinus reflex operating characteristics for halothane. Corresponding to the above halothane levels were reflex operating point pressures (set point pressures) of 104.9 +/- 2.9, 103.3 +/- 4.1 and 76.0 +/- 3.8 mmHg, respectively. Carotid sinus reflex gain decreased significantly with progressively increasing halothane levels (1.4. +/- 0.18; 0.84 +/- 0.12; 0.48 +/- 0.09), as did the range of reflex changes in systemic arterial pressure for equal overall changes in carotid sinus pressure (87.1 +/- 7.9; 64.1 +/- 7.4; 33.6 +/- 5.1 mmHg; P less than 0.01). For halothane levels below approximately 0.9%, this depression was not dependent upon changes in mean systemic arterial pressure. The relationship between reflex gain and halothane concentration could be described by a first-order exponential which suggested virtual ablation of the carotid sinus reflex control of mean arterial pressure at an end-tidal halothane concentration exceeding about 1.6%.