Breath by Breath Variability

Breath by breath variability in Vt, timing, flow rate and Ve is a prominent feature of breathing in healthy alert subjects [61, 62]. Its magnitude varies with level of vigilance, being less pronounced during sleep and minimal under anesthesia [63]. Breath by breath variability decreases in patients with weak muscles and abnormal mechanics [64, 65]. When the abnormal mechanics are offset by PAV, breath by breath variability becomes evident again. Typically, the coefficient of variation in ventilated patients on PAV is in the range of 10-40%, depending on their level of vigilance (personal observations). This is in the same range observed in normal subjects and indicates that the decrease in variability in patients with respiratory disease in not because they are unhealthy, per se, but is due to the effect of abnormal mechanics and weak muscles attenuating the ventilatory expression of breath by breath changes in inspiratory activity.

Clinical Implications

1. When, in a healthy subject, one measures average values of Vt, Ti and flow, then places the subject on VCV at the same settings as the average spontaneous values, the settings are not tolerated [12]. The subject requests more volume and flow. We believe that this is related to the fact that the fixed settings do not allow a ventilatory expression of the spontaneous breath by breath changes in demand. Given that patients do display breath by breath changes in demand, it is possible that this phenomenon results in Vt and flow at which the patient feels comfortable being higher than the average values that would be tolerated if the patient were able to change Vt and flow at will. This needs to be confirmed, however.

2. There is considerable interest at present in the potential benefits to gas exchange of having breath by breath differences in Vt, and there is evidence to support this idea during controlled ventilation in animals [66,67]. If these observations are confirmed in ventilator dependent patients on assisted ventilation, then current modes of assisted ventilation, which result in fairly monotonous Vt, may not be advantageous to gas exchange.

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