Effect of Anesthetic Depth on Spontaneous Minute Ventilation

Despite the variable changes in respiratory pattern and rate as anesthesia deepens, overall spontaneous V̇E progressively decreases. The normal awake response to breathing CO₂ (the x axis in Fig. 17-31 shows an increasing end-tidal concentration of CO₂ ) causes a linear increase in V̇E (see the y axis in Fig. 17-31 ). In Figure 17-31 the slope of the line relating V̇E to the end-tidal CO₂ concentration in awake individuals is 2 L/min/mm Hg. (In healthy individuals, the variation in the slope of this response is large.) Figure 17-31 also shows that increasing halothane concentration displaces the end-tidal CO₂ ventilation-response curve progressively to the right (meaning that at any CO₂ concentration, ventilation is less than before), decreases the slope of the curve, and shifts the apneic threshold to a higher end-tidal CO₂ concentration level.^[128] Similar alterations are observed with narcotics and other halogenated anesthetics.^[129] Figure 17-22 Figure 17-23 Figure 17-24 show that decreases in V̇E cause increases in PaCO₂ and decreases in PaO₂ . The relative increases in PaCO₂ caused by depression of V̇E (<1.24 MAC) by halogenated anesthetics is enflurane > desflurane = isoflurane > sevoflurane > halothane. At higher concentrations, desflurane causes increasing ventilatory depression and becomes similar to enflurane, and sevoflurane becomes similar to isoflurane (see Chapter 6 for additional further study).

Figure 17-31 In conscious controls (heavy solid line), increasing end-expiratory PCO₂ increases pulmonary minute volume. The dashed line is an extrapolation of the CO₂ response curve to zero ventilation and represents the apneic threshold. An increase in anesthetic (halothane) concentration (end-expiratory concentration) progressively diminishes the slope of the CO₂ response curve and shifts the apneic threshold to a higher PCO₂ . The heavy line interrupted by dots shows the decrease in minute ventilation and the increase in PCO₂ that occur with increasing depth of anesthesia. (Redrawn with modification from Munson ES, Larson CP Jr, Babad AA, et al: The effects of halothane, fluroxene and cyclopropane on ventilation: A comparative study in man. Anesthesiology 27:716, 1966.)