Background

Patient–ventilator dyssynchrony is frequently observed during assisted mechanical ventilation. However, the effects of expiratory muscle contraction on patient–ventilator interaction are underexplored. The authors hypothesized that active expiration would affect patient–ventilator interaction and they tested their hypothesis in a mixed cohort of invasively ventilated patients with spontaneous breathing activity.

Methods

This is a retrospective observational study involving patients on assisted mechanical ventilation who had their esophageal pressure (Peso) and gastric pressure monitored for clinical purposes. Active expiration was defined as gastric pressure rise (ΔPgas) greater than or equal to 1.0 cm H2O during expiratory flow without a corresponding change in diaphragmatic pressure. Waveforms of Peso, gastric pressure, diaphragmatic pressure, flow, and airway pressure (Paw) were analyzed to identify and characterize abnormal patient–ventilator interaction.

Results

76 patients were identified with Peso and gastric pressure recordings, of whom 58 demonstrated active expiration with a median ΔPgas of 3.4 cm H2O (interquartile range = 2.4 to 5.3) observed in this subgroup. Among these 58 patients, 23 presented the following events associated with expiratory muscle activity: (1) distortions in Paw and flow that resembled ineffective efforts, (2) distortions similar to autotriggering, (3) multiple triggering, (4) prolonged ventilatory cycles with biphasic inspiratory flow, with a median percentage (interquartile range) increase in mechanical inflation time and tidal volume of 54% (44 to 70%) and 25% (8 to 35%), respectively and (5) breathing exclusively by expiratory muscle relaxation. Gastric pressure monitoring was required to identify the association of active expiration with these events. Respiratory drive, assessed by the rate of inspiratory Peso decrease, was significantly higher in patients with active expiration (median [interquartile range] dPeso/dt: 12.7 [9.0 to 18.5] vs 9.2 [6.8 to 14.2] cmH2O/sec; P < 0.05).

Conclusions

Active expiration can impair patient–ventilator interaction in critically ill patients. Without documenting gastric pressure, abnormal patient–ventilator interaction associated with expiratory muscle contraction may be mistakenly attributed to a mismatch between the patient’s inspiratory effort and mechanical inflation. This misinterpretation could potentially influence decisions regarding clinical management.

Editor’s Perspective
What We Already Know about This Topic
  • Patient ventilator dyssynchrony is common during prolonged mechanical ventilation in critically ill patients and can be associated with adverse outcome and prolonged weaning
  • This can involve either inspiration or expiration, although less is known regarding the effects of expiratory musculature on it
  • Recognition requires careful inspection of ventilator waveforms along with esophageal and gastric pressures
What This Article Tells Us That Is New
  • In a retrospective analysis of a cohort of ventilated critically ill patients, waveforms and pressure tracings were evaluated by the investigators
  • Based on elevations of gastric pressure during expiratory flow, they categorized five unique groups based on expiratory muscle activity
  • Respiratory drive was significantly higher in subjects with active expiration
  • This approach holds promise to better clarify dyssynchrony with regard to which respiratory phase is predominant