The airway occlusion pressure at 100 ms (
P0.1) is a readily accessible and noninvasive measurement that reflects output of the respiratory centers. The
P0.1 is the static pressure generated by all inspiratory muscles against an occluded airway at 0.1 s after the onset of inspiration. The
P0.1 was described over 40 years ago as an indirect measurement of drive that increases proportionally to an increase in inspiratory CO
2 and directly depends on neural stimulus (i.e., diaphragm electromyography or phrenic nerve activity) [
36]. Advantages of
P0.1 are that short and unexpected occlusions are performed at irregular intervals such that there is no unconscious reaction (normal reaction time is >0.15 s) [
36]. Second, the maneuver itself is relatively independent of respiratory mechanics, for the following reasons: (1)
P0.1 starts from end-expiratory lung volume, meaning that the drop in airway pressure is independent of the recoil pressures of the lung or chest wall; (2) since there is no flow during the maneuver,
P0.1 is not affected by flow resistance; and (3) lung volume during an occlusion does not change (with the exception of a small change due to gas decompression), which makes it unlikely that vagal volume-related reflexes or force-velocity relations of the respiratory muscles influence the measured pressure [
7,
36]. In addition, the maneuver remains reliable in patients with respiratory muscle weakness [
37], and in patients with various levels of intrinsic PEEP and dynamic hyperinflation [
38]. Although the latter patient category shows an important delay between the onset of inspiratory activity at the alveolar level (estimated by esophageal pressure [
Pes]) and the drop in airway pressure during an end-expiratory occlusion, Conti et al. proved good correlation and clinically acceptable agreement between
P0.1 measured at the mouth and the drop in
Pes at the first 0.1 s of the inspiratory effort (
r = 0.92, bias 0.3 ± 0.5 cmH
2O) [
38]. The
P0.1 can therefore be considered as a valuable index for the estimation of respiratory drive.
Reference Values
During tidal breathing in healthy subjects,
P0.1 varies between 0.5 and 1.5 cmH
2O with an intrasubject breath-to-breath variability of 50%. Due to this variation, it is recommended to use an average of three or four
P0.1 measures for a reliable estimation of respiratory drive. In stable, non-intubated patients with COPD,
P0.1 values between 2.4 and 5 cmH
2O have been reported [
7], and from 3 to 6 cmH
2O in patients with acute respiratory distress syndrome (ARDS) receiving mechanical ventilation [
39]. An optimal upper threshold for
P0.1 was 3.5 cmH
2O in mechanically ventilated patients; a
P0.1 above this level is associated with increased respiratory muscle effort (i.e., esophageal pressure-time product [PTP] > 200 cmH
2O∙s/min [
40]).
Limitations
Although the
P0.1 is readily available on most modern mechanical ventilators, each ventilator type has a different algorithm to calculate
P0.1; some require manual activation of the maneuver, others continuously display an estimated value based on the ventilator trigger phase (i.e., the measured pressure decrease before the ventilator is triggered, extrapolated to 0.1 s), whether or not averaged over a few consecutive breaths. Considering that the trigger phase is often shorter than 0.05 s,
P0.1 is likely to underestimate true respiratory drive, especially in patients with high drive [
39]. The accuracy of the different calculation methods remains to be investigated.
In addition, extra caution is required when interpreting the
P0.1 in patients with expiratory muscle activity; since recruitment of expiratory muscles results in an end-expiratory lung volume that may fall below functional residual capacity, the initial decrease in
P0.1 during the next inspiration may not reflect inspiratory muscle activity solely, but comprises the relaxation of the expiratory muscles and recoil of the chest wall as well [
7].