Ventilation and thoracoabdominal asynchrony during halothane anesthesia in infants.

To evaluate the ventilatory consequences of high chest wall compliance during anesthesia in infants, we assessed the effects of halothane at different fractions of minimal alveolar concentration (0.75, 1.0, and 1.5 MAC) on ventilation and movements of the rib cage and abdomen in infants < or = 12 mo of age (group I) and children (group II) > or = 12 mo of age. Minute ventilation decreased in group I, (20.6%, 0.75 to 1.5 MAC), but the change in group II did not reach the level of statistical significance. Tidal volume decreased with halothane level between 0.75 and 1.5 MAC, and its fall was greater in group I (32.7 +/- 11.2 vs. 22.6 +/- 9.3% in group II, P < 0.05). Duty cycle, or ratio of inspiratory to total time (TI/TT), increased in group II with halothane level but did not change in group I, resulting in a decreased TI in group I at higher halothane levels. Thoracic paradox increased with halothane level in group I but not group II. The increase in thoracic paradox in association with the fall in tidal volume between 0.75 and 1.5 MAC was greater in group I than group II (P < 0.05). We conclude that smaller infants depend more on inspiratory intercostal muscle activity to stabilize the thorax, leading to a greater degree of depression of ventilation during halothane depression of inspiratory intercostal activity.