A novel non-invasive method to detect excessively high respiratory effort and dynamic transpulmonary driving pressure during mechanical ventilation

Abstract

Background: Excessive respiratory muscle effort during mechanical ventilation may cause patient self-inflicted lung injury and load-induced diaphragm myotrauma, but there are no non-invasive methods to reliably detect elevated transpulmonary driving pressure and elevated respiratory muscle effort during assisted ventilation. We hypothesized that the swing in airway pressure generated by respiratory muscle effort under assisted ventilation when the airway is briefly occluded (ΔPocc) could be used as a highly feasible non-invasive technique to screen for these conditions.

Methods: Respiratory muscle pressure (Pmus), dynamic transpulmonary driving pressure (ΔPL,dyn, the difference between peak and end-expiratory transpulmonary pressure), and ΔPocc were measured daily in mechanically ventilated patients in two ICUs in Toronto, Canada. A conversion factor to predict ΔPL,dyn and Pmus from ΔPocc was derived and validated using cross-validation. External validity was assessed in an independent cohort (Nanjing, China).

Results: Fifty-two daily recordings were collected in 16 patients. In this sample, Pmus and ΔPL were frequently excessively high: Pmus exceeded 10 cm H2O on 84% of study days and ΔPL,dyn exceeded 15 cm H2O on 53% of study days. ΔPocc measurements accurately detected Pmus > 10 cm H2O (AUROC 0.92, 95% CI 0.83-0.97) and ΔPL,dyn > 15 cm H2O (AUROC 0.93, 95% CI 0.86-0.99). In the external validation cohort (n = 12), estimating Pmus and ΔPL,dyn from ΔPocc measurements detected excessively high Pmus and ΔPL,dyn with similar accuracy (AUROC ≥ 0.94).

Conclusions: Measuring ΔPocc enables accurate non-invasive detection of elevated respiratory muscle pressure and transpulmonary driving pressure. Excessive respiratory effort and transpulmonary driving pressure may be frequent in spontaneously breathing ventilated patients.

Keywords: Acute lung injury; Artificial respiration; Mechanical ventilation; Myotrauma; Respiratory monitoring.

Conflict of interest statement

Dr. Goligher’s laboratory receives support in the form of equipment from Getinge, and Dr. Goligher has received speaking honoraria from Getinge. The other authors declare that they have no competing interests.

Figures

Fig. 1

Representative tracings obtained during the airway occlusion maneuver. Flow, airway pressure (Paw), esophageal pressure (Pes), and diaphragm electrical activity (Edi) were recorded while a one-way end-expiratory occlusion permitting expiratory flow but not inspiratory flow (black arrow) was applied at a random interval. Transpulmonary pressure (PL), obtained by digital subtraction of Pes from Paw, signifies the dynamic stress applied to the lung. Chest wall elastic recoil pressure (ΔPcw) was estimated by multiplying tidal volume by predicted chest wall elastance. Inspiratory effort was quantified by the peak inspiratory muscle pressure, Pmus, estimated as the difference between ΔPcw and ΔPes (baseline Pmus is 0 cm H2O by definition). Note that peak Edi did not differ between occluded and non-occluded breaths

Fig. 2

Distribution of ΔPL (dynamic transpulmonary driving pressure) and Pmus (respiratory muscle pressure) during mechanical ventilation. Pressures frequently exceeded “probably excessive” and “definitely excessive” thresholds (dotted and dashed lines, respectively) irrespective of the duration of the study or the mode of ventilation. While peak and driving airway pressures were lower under partially assisted modes of ventilation (p < 0.001 for both comparisons), transpulmonary pressure swings were not significantly different (p = 0.16)

Fig. 3

Discriminative accuracy assessed by receiver operating characteristic curves. Threshold values are shown as points on the ROC curves. Pmus, respiratory muscle pressure; ΔPL, dynamic transpulmonary driving pressure

Fig. 4

Proposed clinical algorithm for monitoring respiratory muscle pressure (Pmus) and dynamic transpulmonary pressure swings (ΔPL) based on the negative deflection in airway pressure during an end-expiratory airway occlusion maneuver (ΔPocc). Pes, esophageal pressure