Tidal volume changes due to the interaction of anesthesia machine and anesthesia ventilator

Tidal volume (VT) delivered by mechanical ventilation during anesthesia may be influenced by factors related not only to the patient and the breathing circuit, but also to the interaction between the anesthesia machine and the anesthesia ventilator. To characterize this interaction, we studied in a test lung the effect of fresh-gas-flow (FGF) (0.25, 2.5, 5, and 10 L/min), inspiratory-to-expiratory time ratio (I:E) (1:1, 1:2, and 1:3), and ventilatory frequency (8, 12, and 16 breaths/min) at fixed ventilator bellows excursions of 300, 600, and 900 ml. The influence of these variables was also estimated mathematically for a pediatric situation: a bellows excursion of 50 ml at 20 and 30 breaths/min. Each variable studied was associated with an increase, sometimes dramatic, in the delivered VT compared with that which was set. The VT augmentation was greatest at the highest FGF rate, largest I:E ratio, and slowest respiratory rate. Because the magnitude of the augmentation is independent of the VT setting, the percent increase is much larger for pediatric settings. For example, with VT set at 50 ml, delivered VT ranged from 71 ml (FGF 2.5 L/min, I:E 1:3, and 30 breaths/min) to 300 ml (FGF 10 L/min, I:E 1:1, and 20 breaths/min). Thus it is possible in the pediatric situation to increase the delivered VT by sixfold without changing the ventilator bellows excursion. The magnitude of the changes was slightly larger for the VT settings for adult patients because of the slower respiratory rate. This VT augmentation can be predicted by the product of FGF (ml/s) and inspiratory time (seconds).(ABSTRACT TRUNCATED AT 250 WORDS)