Ventilator Waveforms May Give Clues to Expiratory Muscle Activity

To the Editor:

We read with great interest the article in the Journal by Plens and colleagues (1). Their study explored the incidence of end-expiratory lung volume (reflected by impedance measurement) increase after neuromuscular blockade (NMBA) and its association with expiratory muscle activity and fentanyl use. The seemingly counterintuitive phenomenon was not uncommon, according to their results.

With the progress in understanding patient self-inflicted lung injury and diaphragm-protective ventilation, the monitoring of respiratory effort has gained much attention (2). However, clinicians focus mainly on inspiratory effort but seldom talk about expiratory effort. Through investigating expiratory effort, the authors may shed light on “the dark side of the moon.” Given the potential benefits (unloading respiratory muscle, increasing lung volume, and avoiding tidal recruitment) of muscle relaxation in patients with acute respiratory distress syndrome (ARDS) with expiratory muscle activity, expiratory effort deserves more attention.

Although gastric pressure monitoring is the reference method for qualitative and quantitative evaluation of expiratory effort, it still requires specific tubes and ventilators to operate, which may limit its application in some ICUs. Ventilator waveforms, however, may give clues to respiratory effort (3). Usually, expiration is passive, so the flow–time curve shows an exponential decrease after the peak expiratory flow during expiration. Active expiration increases expiratory flow, forming an upward concavity on the curve. Just as inspiratory effort can be quantified by the “flow index” (4), the magnitude of expiratory effort may also be related to the deformation of the expiratory flow curve. The authors did not directly show the flow–time waveform in the main text. We wonder if there was a difference between responders and nonresponders in the expiratory flow waveform.

As Shi and colleagues (5) mentioned in their elegant review, expiratory muscle recruitment may give clues to inspiratory muscle overload. We present a case ventilated under pressure support mode with low support levels (Figure 1). Vt and respiratory rate were acceptable, but the ventilator waveforms suggested the existence of expiratory effort and the underlying abnormality. The patient was an 83-year-old man diagnosed with ARDS caused by viral pneumonia. His inspiratory effort was thought to be strong by measuring P_0.1 (the negative pressure measured 100 ms after the initiation of an inspiratory effort performed against a closed respiratory circuit), occlusion pressure, and pressure muscular index; therefore, he was prescribed NMBA.

Figure 1.

Case presentation: the patient was an 83-year-old man with acute respiratory distress syndrome caused by viral pneumonia (typical computed tomography slide shown in A), ventilated with pressure support level of 8 cm H₂O and positive end-expiratory pressure of 5 cm H₂O. Vt was approximately 500–550 ml, and the respiratory rate was approximately 14–16 breaths/min. Occlusion pressure equaled −25 cm H₂O (B). Mean P_0.1 was −5.8 cm H₂O (C), and pressure muscular index was 21.6 – (5 + 8) = 8.6 cm H₂O (not shown).

Despite the authors’ interesting discovery regarding expiratory effort, we wonder if the detection of expiratory muscle activity would provide extra value for NMBA decision making, because strong inspiratory effort was already recognized as an indication for (partial) muscle relaxation. In the study population, were there any patients with acceptable inspiratory effort but very strong expiratory effort? If the answer were yes, it would definitely enhance the importance of monitoring for expiratory effort.

In addition, we would also like to know whether other opioids, such as remifentanil, have effects on lung volume similar to those of fentanyl, because remifentanil is preferred for some patients with ARDS in our center. Finally, we thank the authors for their contributions in this field. We look forward to further unraveling the mysteries of expiratory effort in the future.