Accuracy of P0.1 measurements performed by ICU ventilators: a bench study

Abstract

Background: Occlusion pressure at 100 ms (P0.1), defined as the negative pressure measured 100 ms after the initiation of an inspiratory effort performed against a closed respiratory circuit, has been shown to be well correlated with central respiratory drive and respiratory effort. Automated P0.1 measurement is available on modern ventilators. However, the reliability of this measurement has never been studied. This bench study aimed at assessing the accuracy of P0.1 measurements automatically performed by different ICU ventilators.

Methods: Five ventilators set in pressure support mode were tested using a two-chamber test lung model simulating spontaneous breathing. P0.1 automatically displayed on the ventilator screen (P0.1vent) was recorded at three levels of simulated inspiratory effort corresponding to P0.1 of 2.5, 5 and 10 cm H2O measured directly at the test lung and considered as the reference values of P0.1 (P0.1ref). The pressure drop after 100 ms was measured offline on the airway pressure-time curves recorded during the automated P0.1 measurements (P0.1aw). P0.1vent was compared to P0.1ref and to P0.1aw. To assess the potential impact of the circuit length, P0.1 were also measured with circuits of different lengths (P0.1circuit).

Results: Variations of P0.1vent correlated well with variations of P0.1ref. Overall, P0.1vent underestimated P0.1ref except for the Löwenstein® ventilator at P0.1ref 2.5 cm H2O and for the Getinge group® ventilator at P0.1ref 10 cm H2O. The agreement between P0.1vent and P0.1ref assessed with the Bland-Altman method gave a mean bias of - 1.3 cm H2O (limits of agreement: 1 and - 3.7 cm H2O). Analysis of airway pressure-time and flow-time curves showed that all the tested ventilators except the Getinge group® ventilator performed an occlusion of at least 100 ms to measure P0.1. The agreement between P0.1vent and P0.1aw assessed with the Bland-Altman method gave a mean bias of 0.5 cm H2O (limits of agreement: 2.4 and - 1.4 cm H2O). The circuit's length impacted P0.1 measurements' values. A longer circuit was associated with lower P0.1circuit values.

Conclusion: P0.1vent relative changes are well correlated to P0.1ref changes in all the tested ventilators. Accuracy of absolute values of P0.1vent varies according to the ventilator model. Overall, P0.1vent underestimates P0.1ref. The length of the circuit may partially explain P0.1vent underestimation.

Keywords: Inspiratory effort; Mechanical ventilation; Occlusion pressure; Respiratory drive; Respiratory failure.

Conflict of interest statement

FB reports personal fees from Löwenstein Medical and research support from Covidien, GE Healthcare and Getinge Group, outside this work. LP reports research support from Getinge Group, Dräger and Hamilton, outside this work. PYO reports personal fees from Air Liquid Medical Systems, outside this work. AM reports personal fees from Faron Pharmaceuticals, Air Liquid Medical Systems, Pfizer, Resmed and Draeger and grants and personal fees from Fisher and Paykel and Covidien, outside this work. JCMR reports personal fees from Air Liquide Medical Systems and Vygon and grants from Creative Air Liquide, outside this work. AV and EY have no competing interests to declare.

Figures

Fig. 1

Experimental setup. A chamber of a two-chamber test lung was connected to a driving ventilator set in volume-controlled ventilation mode. The second chamber (2) was connected to the tested ventilator by a double limb circuit with an active humidifier. The two chambers of the test lung were linked by a rigid metal piece, so that the positive pressure insufflated by the driving ventilator into the first chamber (1) generates a negative pressure in the second chamber, recognized as an inspiratory effort by the tested ventilator. Data were acquired via an analog-to-digital converter and stored in a laptop computer for subsequent analysis

Fig. 2

Accuracy of automatically measured P0.1 displayed on the ventilator screen (P0.1vent). a Correlation between P0.1vent and reference P0.1 (P0.1ref). The horizontal dotted lines represent P0.1 reference values of 2.5, 5 and 10 cm H2O, respectively. The diagonal dotted line represents the identity line (y = x). b Bland and Altman plots of differences between P0.1vent and P0.1ref versus their mean. The dotted lines represent the mean bias and the limits of agreement

Fig. 3

Relationship between automatically measured P0.1 displayed by the ventilator (P0.1vent) and P0.1 measured on the airway pressure during automated P0.1 measurements (P0.1aw). a Correlation between P0.1vent and P0.1aw. The dotted line represents the identity line (y = x). b Bland and Altman plots of differences between P0.1vent and P0.1aw versus their mean. The dotted lines represent the mean bias and the limits of agreement

Fig. 4

P0.1 measurements (P0.1circuit) according to the position of the airway occlusion. P0.1circuit measurements were performed with occlusions at different positions (i.e., at 0, 195, 360 and 690 cm from the airway opening). The second compartment of the test lung was occluding using a hermetic plug placed at the airway opening or on a circuit tube of 195, 360 or 690 cm connected to the airway opening. Dotted lines represent P0.1 values of 2.5, 5 and 10 cm H2O, respectively

Fig. 5

Automatically measured P0.1 displayed by the ventilator (P0.1vent), P0.1 measured on the airway pressure during automated P0.1 measurement (P0.1aw) and P0.1 measurement corrected for the occlusion location (P0.1vent corrected). P0.1vent corrected = P0.1vent + ∆P0.1circuit with ∆P0.1circuit corresponding to the difference between P0.1circuit measured with an occlusion performed at 360 cm from the airway opening and P0.1ref (reference P0.1) measured with an occlusion performed at the airway opening