General Ventilatory Effects of Anesthetics

In general, all volatile anesthetics decrease tidal volume. This resultant depression of minute ventilation may be partially offset by a concomitant increase in respiratory rate ( Fig. 6-13 ).^{[223] [224]} Early studies demonstrated that isoflurane produced more profound respiratory depression than did halothane, but in contrast to halothane, isoflurane did not progressively increase respiratory rate.^{[225] [226]} Like halothane, enflurane reduces tidal volume and minute ventilation concomitant with compensatory tachypnea in a dose-related manner.^[227] Prolonged administration (3 to 7 hours) of halothane or enflurane decreased the degree of respiratory depression as indicated by resting arterial carbon dioxide tension.^{[227] [228]} Halothane also exerts differential effects on ventilatory timing parameters, including the ratio of occluded-to-unoccluded inspiratory time, in children^[229] and infants younger than 2 months of age, depending on postconceptual age.^[230]

Desflurane and sevoflurane also exhibit dose-related ventilatory depression, primarily by reducing tidal volume.^{[231] [232] [233] [234]} Unlike isoflurane, sevoflurane and desflurane cause dose-related increases in respiratory rate. Desflurane does not significantly reduce minute ventilation at concentrations less than 1.6 MAC. The reduction of respiratory rate produced by desflurane at higher concentrations was less than that caused by halothane. The relative increases in arterial carbon dioxide tension as an index of respiratory depression with volatile anesthetics (<1.24 MAC) are caused by, in order, enflurane, desflurane or isoflurane, and sevoflurane or halothane. Halothane and sevoflurane also cause similar respiratory changes in spontaneously ventilating children.^{[235] [236]} Nevertheless, potentially important differences between these two agents have been identified and include reduced minute ventilation and respiratory rate, a delay in the timing of peak inspiratory flow, and an earlier occurrence of peak expiratory flow during sevoflurane compared with halothane anesthesia. ^[236] The ventilatory depression produced by desflurane and sevoflurane is similar to that produced by enflurane and isoflurane, respectively, at higher concentrations.^{[224] [225] [231] [232] [233] [234]} In spontaneously ventilating dogs, respiratory effects of 1 to 2 MAC sevoflurane are similar to those of isoflurane, greater than those of halothane at 2 MAC, and less than those of enflurane at 1.5 and 2 MAC.^[237] Isoflurane-induced respiratory depression is reduced by surgical stimulation ( Fig. 6-14 ).^[238] Respiratory depression observed during desflurane or sevoflurane anesthesia is reduced in the presence of nitrous oxide at equi-MAC values.^[239] The respiratory depressant effects of xenon were first described in 1955.^[240] Unlike other inhaled anesthetics, xenon decreases the respiratory rate and increases tidal volume, resulting in a reduction^[241] or an increase in minute ventilation.^[242]