The Paradox of Occlusion Pressure at 0.1 s (P_0.1) Measurement Without Airway Occlusion

To the Editor:

Almost 50 years after its birth in respiratory physiology,^1,2 we are seeing a growing interest in the airway-occlusion pressure at 0.1 s (P_0.1) parameter as an expression of the mechanical output of the respiratory drive and, therefore, of the inspiratory effort made by the patient. The assessment of P_0.1 in patients with acute hypoxemic respiratory failure may help in decision making regarding mechanical ventilation and sedation, with the aim to protect the patient from self-induced lung and diaphragm injury.^3,4 The measurement of P_0.1 is offered by a few mechanical ventilators according to different technologies: either a short patient-triggered occlusion performed on demand by closure of the ventilator valves or a continuous breath-by-breath analysis of airway pressure during the short interval preceding inspiratory trigger activation.

In turn, continuous P_0.1 monitoring is based on two different methods. The mini-occlusion method was designed in the 1990s and is based on the measurement and extrapolation to 100 ms of the maximum slope of the airway-pressure drop during the short—but full—occlusion associated with pressure-triggering.^5,6 It was shown to provide accurate P_0.1 assessments even on trigger intervals < 100 ms.⁶ The non-occlusive method was developed later for the pneumatics of modern ventilators with flow-triggering associated with a substantial, adaptive flow-by during expiration and no occlusion during the trigger interval.

Continuous P_0.1 monitoring provides obvious advantages over performing punctual measurements, especially if we consider the breath-by-breath variability of spontaneous breathing and thus of P_0.1. However, 3 studies found relevant accuracy limitations for continuous P_0.1 monitoring, contrarily to P_0.1 measurements from occlusions on demand. The non-occlusive continuous method resulted in significant underestimation of P_0.1 both on the bench⁴ and in a clinical study.⁷ These results have been recently confirmed by the bench study of Katayama et al,⁸ who found a relevant underestimation for the non-occlusive method and also an overestimation for the mini-occlusion method, both proportional to the magnitude of the P_0.1 value.

Wondering whether the attractive concept of continuous P_0.1 monitoring be destined to fatally die as affected by insufficient accuracy, we performed additional bench tests. We analyzed two different ventilators: a Hamilton-G5 for the mini-occlusion method and a Hamilton-C6 for the non-occlusive method (Hamilton Medical, Bonaduz, Switzerland). Of note, the P_0.1 measurement algorithm of Hamilton-C6 has been recently modified (software v1.2.3). Using an ASL 5000 simulator (IngMar Medical, Pittsburgh, Pennsylvania) and the ventilator settings as published by Katayama et al,⁸ we reproduced the same 42 scenarios (normal model and obstructive model, with sinusoidal wave pattern of muscular pressure).

In the comparison with reference P_0.1 measurements obtained by formal end-expiratory occlusion maneuvers, the bias of continuous P_0.1 was 0.20 cm H₂O with 95% limits of agreement (as an expression of precision) of ± 0.86 cm H₂O as performed by Hamilton-G5 with the mini-occlusion method (Fig. 1 panel A) and −0.04 cm H₂O with 95% limits of agreement of ± 2.35 cm H₂O as performed by Hamilton-C6 with the non-occlusive method (Fig. 1 panel B). Among the different simulation scenarios, the plots show no relevant dependence of the bias on the magnitude of P_0.1. The mini-occlusion method maintains a good precision within the entire explored range, while the non-occlusive method shows a moderate decrease in precision with increasing P_0.1 values. This is primarily caused by a tendency toward an overestimation with the normal simulation model and a slight underestimation with the obstructive model.

Fig. 1.

Bland-Altman plots comparing ventilator continuous airway-occlusion pressure at 0.1 s (P_0.1) and reference P_0.1 by end-expiratory occlusion for the Hamilton-G5 with software v2.91 (panel A) and Hamilton-C6 with software v1.2.3 (panel B). The solid line represents bias, and the dashed lines represent 95% limits of agreement. The white dots refer to the normal simulation model, the black dots to the obstructive model.

Continuous P_0.1 measurement by mini-occlusion was designed when pressure-trigger was the only inspiratory synchronization technology available. Our results show that the subsequent evolution of ventilator pneumatics did not impede to maintain the good accuracy that was originally reported.⁶ However, the applicability of the mini-occlusion method is restricted to pressure-triggering, while flow-triggering is probably the most common choice in current clinical practice. Continuous measurement of P_0.1 in the pneumatic condition of flow-triggering is obviously quite challenging, the ultimate goal being to obtain an occlusion pressure measurement while the ventilator circuit is open. Our results show that, although a true occlusion is lacking, the trigger interval can provide the information for a P_0.1 calculation of sufficient accuracy to give the clinician an unequivocal understanding of the magnitude of inspiratory effort performed by the patient.

In conclusion, the accuracy of continuous P_0.1 measurement is good with the mini-occlusion method and sufficient for clinical use with the non-occlusive method. Whereas the paradox of measuring an occlusion pressure without occlusion maneuver has been technically overcome, a semantic issue probably remains open. For sake of clarity, perhaps a different name should be used for this non-occlusive equivalent of P_0.1.