“[Inspiratory occlusion pressure and airway occlusion pressure are] simple and noninvasive methods to monitor the risk of lung and diaphragm injury in patients with acute respiratory failure.”

Image: A. Johnson, Vivo Visuals Studio.

Image: A. Johnson, Vivo Visuals Studio.

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Monitoring and managing spontaneous breathing during mechanical ventilation is a routine clinical challenge for clinicians caring for patients with acute respiratory failure. When the respiratory muscles contract in synchrony with the ventilator, the pressure applied to the lung by the respiratory muscles adds to the pressure applied by the ventilator, increasing the total pressure applied across the lung. Respiratory effort is often excessive in patients with acute respiratory failure,1  and the resulting high lung-distending pressures may further worsen lung injury (a phenomenon referred to as patient self-inflicted lung injury).2  Vigorous respiratory efforts can also cause diaphragm myotrauma. On the other hand, when respiratory effort is insufficient, patients are at high risk for diaphragm disuse atrophy. Given increasing evidence of the physiologic and clinical relevance of these problems, respiratory effort merits close attention in mechanically ventilated patients.3  Traditionally, monitoring respiratory effort and lung-distending pressure during spontaneous breathing requires esophageal manometry to assess pleural pressure swings. However, this technique is not routinely employed in clinical practice as it requires dedicated equipment and expertise to correctly acquire and interpret the signals. Simple, noninvasive techniques using ventilator-based maneuvers to monitor respiratory effort would therefore be of great value.

In this edition of Anesthesiology, de Vries et al.4  evaluated the performance of two noninvasive techniques for monitoring respiratory effort and lung-distending pressure: the inspiratory occlusion pressure (Pocc), and the airway occlusion pressure in the first 100 ms (P0.1). Using data from 38 patients enrolled in a randomized trial testing a strategy to facilitate safe spontaneous breathing during mechanical ventilation, they demonstrate the utility of these measurements to monitor diaphragmatic effort and lung-distending pressure during mechanical ventilation. Specifically, they show that Pocc and P0.1 can accurately detect (1) very low diaphragmatic effort, (2) very high diaphragmatic effort, and (3) potentially injurious levels of transpulmonary driving pressure or transpulmonary mechanical power. There are several key novel features of this work. First, the authors directly quantify diaphragmatic effort, rather than respiratory effort; this is relevant since transdiaphragmatic pressure is the relevant index of muscular activity. Second, the authors validate Pocc and P0.1 with reference to a prolonged recording of the reference standard measurement (1 h vs. a few minutes, as in previous studies); this suggests that these measurements reflect lung- and diaphragm-protective targets over hours, rather than merely minutes. Third, the authors derived and validated a method for predicting lung-distending pressure from P0.1. Fourth, they also derived and validated a method for computing transpulmonary mechanical power from P0.1 and Pocc. Overall, the work by de Vries et al. provides definitive confirmation of the relevance of these simple, noninvasive maneuvers to monitor respiratory effort and lung-distending pressure during assisted mechanical ventilation and highlight their potential utility to guide a lung- and diaphragm-protective ventilation strategy.

Airway occlusion pressure in the first 100 ms is a well-known maneuver described several decades ago by Whitelaw et al.,5  but Pocc was described only recently6  and is not as widely employed in clinical practice. Pocc measures the magnitude of the pressure generated by the respiratory muscles against the occluded airway. An end-expiratory occlusion maneuver is applied and maintained for the duration of a single breath (fig. 1). The maneuver is somewhat analogous to measurement of maximal inspiratory pressure on the ventilator (also sometimes called negative inspiratory force), except that the occlusion is being used to estimate the pressure generated during tidal breathing rather than during maximal volitional efforts. The resulting pressure deflection generated by the respiratory muscles can be used to estimate the pleural pressure swing using an empirically derived correction factor. The lung-distending pressure (dynamic transpulmonary driving pressure) during spontaneous breathing can then be estimated by adding the measured airway pressure swing (peak pressure—positive end-expiratory pressure) to the estimated pleural pressure swing. This study confirms and significantly extends our knowledge of the value and interpretation of this technique. A simple Pocc calculator for use at the bedside is available at https://pocc.coemv.ca.

Fig. 1.

Theoretical factors affecting the relationship between respiratory drive (P0.1) and respiratory effort (Pocc). In contrast to respiratory effort, respiratory drive is not affected by respiratory system mechanics or respiratory muscle dysfunction. The Pocc will therefore vary according inspiratory time (A vs. B) and according to respiratory muscle strength (A vs. C), while P0.1 is relatively unaffected across each clinical scenario. Paw, airway pressure; Pocc, inspiratory occlusion pressure; P0.1, airway occlusion pressure in the first 100 ms.

Fig. 1.

Theoretical factors affecting the relationship between respiratory drive (P0.1) and respiratory effort (Pocc). In contrast to respiratory effort, respiratory drive is not affected by respiratory system mechanics or respiratory muscle dysfunction. The Pocc will therefore vary according inspiratory time (A vs. B) and according to respiratory muscle strength (A vs. C), while P0.1 is relatively unaffected across each clinical scenario. Paw, airway pressure; Pocc, inspiratory occlusion pressure; P0.1, airway occlusion pressure in the first 100 ms.

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One of the important features of the work by de Vries et al. is that they provide a head-to-head comparison of the utility of Pocc and P0.1 for monitoring respiratory effort and lung-distending pressure. Airway occlusion pressure in the first 100 ms is technically a measure of the respiratory drive, but since respiratory effort (the amplitude of respiratory muscle force generation during a given breath) is dependent on the presence and magnitude of respiratory drive, it makes sense that drive and effort would be correlated. de Vries et al. confirm a previous finding that P0.1 can accurately detect insufficient respiratory effort but is less accurate to detect elevated respiratory effort.7  Various factors can explain why elevated respiratory effort is less reliably detected by P0.1. The relationship between respiratory drive and respiratory effort depends on (1) the inspiratory time (for a given level of respiratory drive—a longer inspiration will lead to greater peak respiratory effort) and (2) diaphragm strength or force-generating capacity: a patient with diaphragmatic weakness may have elevated respiratory drive but be capable of generating only relatively small respiratory efforts (fig. 1). Despite this limitation, the authors found that P0.1 can detect elevated lung-distending pressure with reasonable accuracy using an empirically derived correction factor. Overall, P0.1 and Pocc provide complementary information about spontaneous breathing. Of note, Esnault et al. reported that both P0.1 and Pocc predicted a higher risk of failed transition from controlled to assisted ventilation in COVID-19 acute respiratory distress syndrome.8 

The study by de Vries et al. substantially strengthens the growing body of data on the validity of both P0.1 and Pocc as simple and noninvasive methods to monitor the risk of lung and diaphragm injury in patients with acute respiratory failure. Analogous to routine measurement of plateau pressure and driving pressure during passive ventilation, P0.1 and Pocc provide invaluable information on the safety and appropriateness of mechanical ventilation. The impact of a lung- and diaphragm-protective ventilation strategy guided by these noninvasive measurements on patient-centered outcomes remains to be determined in clinical trials (www.practicalplatform.org), but clinicians may find them immediately useful for monitoring mechanical ventilation in routine clinical practice.

Dr. Goligher is a member of the Clinical Advisory Board of LungPacer (Exton, Pennsylvania), which markets pulmonary therapy products. He reports receiving personal fees from Vyaire (Mettawa, Illinois), which markets ventilators and other critical care products; Getinge (Solna, Sweden), a medical technology company that markets ventilators; and BioAge (Richmond, California), which markets health and wellness products and services. Dr. Dianti is not supported by, nor maintains any financial interest in, any commercial activity that may be associated with the topic of this article.

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