P Imax Measured at the Ventilator May Underestimate Patient Effort

Abstract

Background:

The measurement of maximal inspiratory pressure during airway occlusion (P_Imax) is recommended for extubation readiness testing by the Pediatric Acute Lung Injury and Sepsis Investigators (PALISI) Network. The gold standard for performing P_Imax is to place a pressure manometer at the patient y-piece and perform a sustained end expiratory occlusion. P_Imax is also available as a built-in ventilator maneuver for newer ventilators, but the circuit remains connected to the patient and the pressure is measured at the ventilator instead of proximal to the patient. We sought to compare the accuracy of ventilator P_Imax to P_Imax gold standard across a variety of simulated conditions.

Methods:

A ventilator (NKV-550) with P_Imax ventilator capabilities was connected to a lung simulator (ASL 5000) via an age-appropriate ETT. P_Imax ventilator was obtained by performing the built-in maneuver on the ventilator during an expiratory hold, and P_Imax gold standard was obtained by disconnecting the circuit at the patient y-piece and placing a pressure adapter connected to a standalone pressure manometer and manually occluding the airway with a gloved hand. A range of simulated patient inspiratory muscle effort (P_mus) was tested across various adult and neonatal models with varying compliance and resistance to simulate normal conditions, ARDS, and asthma of various severities. Each experimental condition was repeated in triplicate.

Results:

300 experiments were performed and the difference between P_Imax ventilator and P_Imax gold standard stratified by P_mus and model type are reported in the Figure. P_Imax ventilator was lower than P_Imax gold standard for almost every condition. Simulating normal conditions for adult and neonatal models resulted in a low mean difference, 3.4 ± 1.2 and 0.7 ± 0.8 cm H₂O respectively. Differences increased when simulating ARDS (6.8 ± 3.8 and 5.7 ± 2.6) and asthma (7.8 ± 3.6 and 5.2 ± 2.2). In ARDS and asthma models the P_Imax ventilator underestimated P_Imax gold standard in increasing amounts as a function of increasing P_mus.

Conclusions:

The P_Imax ventilator is simple to perform as it does not require circuit disconnection however it results in a lower value than P_Imax gold standard and may be significantly lower when there is derangement in respiratory system compliance and resistance. If the P_Imax ventilator results in a low value that affects clinical decision-making it may be valuable to perform P_Imax gold standard to verify the result.