Airway Pressure Release Ventilation in COVID-19: There's More to This Than Meets the Eye.

To the Editor:

We read with great interest the article by Ibarra-Estrada et al (1). We congratulate the authors for their fascinating randomized controlled trial. However, we would like to highlight specific points regarding the study.

The authors have taken the high pressure (P-high) based on the plateau pressure (Pplat) in the previous volume-controlled ventilation mode. According to the authors, this derivation of P-high on airway pressure release ventilation (APRV) mode is based on a previously unpublished protocol. The APRV group had tidal volume (TV) of 7.4 ± 1.1, 8.1 ± 1.3, and 8.6 ± 1.0 on days 3, 5, and 7, respectively. Although the TV generated on APRV cannot be equated with the TV generated on low tidal volume (LTV) ventilation, it still places patients at risk of ventilator-induced lung injury due to the high TV generated (2). APRV encourages spontaneous breathing efforts. This may increase the final end-inspiratory transpulmonary pressure much higher than the set P-high of 30. We suggest using 2–5 cm H2O above mean airway pressure (Pmean) to limit adverse events such as barotraumas. Our suggestion is based on the physiologic concept that under normal conditions, the Pmean correlates with the mean alveolar pressure, which is in turn a surrogate marker of the stresses on the lung parenchyma with ventilation (3).

A high frequency of severe hypercapnia in the APRV group (42%) was seen, which was also statistically significant (p = 0.009). Patients with severe obstructive lung disease are not ideal candidates for APRV. The high inspiratory time in APRV leads to hypercapnia. A few case reports have used APRV in resistant cases of chronic obstructive pulmonary disorder to reduce hypercapnia. However, in such cases, multiple adjustments in the ventilatory settings of APRV have been made to achieve control of the hypercapnia (4). No such adjustments were accommodated in the trial protocol for COPD patients. Another reason for the lower incidence of hypercapnia in LTV group could be allowing for the Pplat target limit to be exceeded in case of severe respiratory acidosis (pH < 7.15).

A significant advantage of APRV over LTV is that it allows for spontaneous ventilation with patient efforts. Ibarra-Estrada et al (1), in this study, were able to achieve deficient spontaneous minute ventilation (MVspont) in the APRV group (zero up to day 5, first time by day 7). This is in stark contrast to the study by Zhou et al (5) who have achieved it by day 3. Zhou et al (5) also targeted MVspont, approximately 30% total minute ventilation in their study protocol. The low MVspont could be a consequence of using neuromuscular blockade in a large proportion (93%) of patients of APRV group in this study, compared with study by Zhou et al (5) who used it in only 2.8% of patients in APRV group. A second reason could be the absence of any targets for achieving MVspont in the study protocol. Deficient MVspont in APRV group may be one of the reasons why no improvement in ventilator-free days was noted.