Hypoxic ventilatory response in cats lightly anesthetized with ketamine: effects of halothane and sevoflurane in low concentrations

Abstract

The effect of low concentration sevoflurane and halothane on the ventilatory response to isocapnic hypoxia was studied in sixteen cats. The cats were divided into two groups, sevoflurane group and halothane group, of eight subjects each. As parameters of the hypoxic ventilatory response, A value [the slope of the hyperbolic curve,$\dot V_E = \dot V_0 + A/(Pa_{O_2 } - 32)]$ and ratio of$\dot V_{50}$ (the minute volume obtained from the hyperbolic equation when Pa_{O 2}=50 mmHg) to$\dot V_0$ were studied. These two parameters were examined at three states, sedative state with ketamine as the control, ketamine plus 0.1 MAC inhalation anesthetic, and ketamine plus 0.5 MAC inhalation anesthetic. In the sevoflurane group, the A values were 4789±1518, 2187±1214, 1730±880 (mean ± SE. ml·min⁻¹·mmHg) at the control state, 0.1 MAC and 0.5 MAC, respectively. In the halothane group, the A values were 6411±2368, 2529±842 and 2372±545, respectively. The ratios of$\dot V_{50}$ to$\dot V_0$ were 1.32±0.09, 1.22±0.09, 1.25±0.08 in the sevoflurane group, 1.47±0.18, 1.32±0.11, 1.54±0.18 in the halothane group, respectively. The A value at 0.1 MAC of the halothane group was less than the control value significantly. This proved that even low concentration halothane depressed the hypoxic ventilatory responses. The depression of hypoxic ventilatory response could cause postanesthetic hypoventilation. On the other hand, we could not find significant depression on the hypoxic ventilatory response in the sevoflurane group, but we should notice that variances of the hypoxic ventilatory response were large.