Minute Ventilation: It Takes Time to Get It Right

The search for ideal predictors of successful extubation or “liberation” from mechanical ventilation has been a subject of constant investigation for over half a century. Initial investigations were spurred by the need to determine adequate reversal of paralytic agents after general anesthesia, sufficient to allow sustained spontaneous ventilation and permit extubation.1,2 It is notable that these and other investigators felt that restoration of adequate minute ventilation (V̇E), in and of itself, was not sufficient to allow safe extubation. They sought another measurement of ventilatory reserve, which led to measurement of the peak inspiratory pressure (PImax, or negative inspiratory pressure against an occluded airway). They found that the amount of negative pressure generated increased with the duration of time the airway was occluded and as the paralytic effect dissipated or was reversed. A threshold value of 20 cm H2O identified suitability for extubation.