Effects of barbiturate anesthesia on functional residual capacity and ribcage/diaphragm contributions to ventilation

The effect of iv methohexital infusion anesthesia on functional residual capacity (FRC) (helium dilution) in 14 surgical patients (age 23 to 59 years) was determined. Eight subjects were studied wearing an inflatable mask, sealed with surgical lubricant. They showed a mean +/- SD 3.5 +/- 6.4% FRC decrease (no significance). Six subjects studied via mouthpiece awake and via endotracheal tube during anesthesia showed a mean 22 +/- 19% reduction in FRC, significantly greater than face mask studies (P less than 0.05). The greatest FRC decrease occurred in subjects with repetitive or protracted coughing after intubation. The serum methohexital level was 6.6 +/- 3.6 micrograms/ml for intubated patients, and 6.0 +/- 1.1 micrograms/ml in those with face mask (no significance). The depth of anesthesia was sufficient to produce a 50% reduction in ventilatory response to CO2 rebreathing, from 15.8 to 8.7 l/min/% CO2. Respitrace plethysmography indicated a 38 +/- 12% ribcage contribution to tidal volume during quiet breathing, which increased to 47 +/- 14% with CO2 breathing (end-tidal FCO2 9-10%). There was no dimunition of ribcage contribution during anesthesia in either group, irrespective of CO2 concentration. The authors interpret their findings to indicate that iv methohexital anesthesia does not produce FRC reduction, in contrast to an inhaled anesthetic such as halothane. It is proposed that this difference may be related to maintenance of coordinated ribcage/diaphragm muscle activity, because ribcage activity is markedly suppressed by halothane. In addition, it is proposed that FRC reduction in intubated subjects was the result of a confounding variable, namely coughing in response to the endotracheal tube.