These observations show a beneficial impact of APRV on oxygenation in patients with Covid-19 related acute respiratory failure. The increase in P/F ratio was observed after 6 hours of APRV after switching from Volume Control or Pressure Control modes.
Implementation of APRV in the ICU and APRV settings
In the two ICUs that implemented the inverse-ratio APRV, few physicians were familiar with the procedure. The strategy to initially set APRV in each patient for only 6 hours was dictated by the fact that during the 24-hour period in the ICU, the attending physician was not always familiar with APRV, especially at the beginning of the Covid-19 pandemic. Over the course of the pandemic, certain patients remained on APRV for longer than 6 hours when more physicians and nurses received basic training. This training was particularly focused on the management of T-low duration in order to achieve a tidal volume by observation of the Flow Curve. P-low was seldom set to 0 cmH2O due to concerns of alveolar pressure reaching P-low (in the case of inappropriately long T-low) and was set to 5 cmH2O as described by Zhou et al1. Nurses were instructed to alert the physician if a drop in VT and/or minute ventilation was observed, with ventilator alarms being set accordingly. Individual APRV settings optimized after 6 hours of APRV are reported in Additional file 2.
Inverse-ratio APRV and oxygenation
The main finding of the present study is the increase in P/F ratio noted in most of the patients after a 6-hour APRV trial. The impact of such an improvement in oxygenation is significant as patients would otherwise require other adjuvant therapies, such as prone positioning, which can be cumbersome with a high patient volume during a pandemic. It is worth noting that Covid-19 pneumonia patients have been reported to have a high recruitment potential, thus possibly explaining this PaO2:FiO2 increase 3,4.
APRV settings
APRV can refer to two types of settings. One is more conventional, with a shorter inspiratory time (short T-high) and a longer expiratory time (long T-low), which can be compared to BiPAP5. The other refers to inverse-ratio APRV in which the inspiratory time is longer (prolonged T-high) than the expiratory time (short T-low) and the expiratory phase lasts around 0.3 – 0.8 seconds with the aim of preventing alveolar pressure from reaching the set P-low and thus not allowing the lung to depressurize.
Inverse-ratio APRV
When setting APRV, we set T-low by observing the expiratory flow curve and ensuring that the expiratory flow is interrupted before reaching zero at 50-75% of the peak expiratory flow. Inverse-ratio APRV is a non-conventional ventilator mode in which tidal volume is delivered during a short decrease in pressure contrary to volume control or pressure support where VT is delivered during an increase in pressure from a set level of PEEP. Thus, in inverse-ratio APRV, the Functional Residual Capacity (FRC) is higher than in conventional ventilation. Spontaneous breathing remains possible if the patient is not paralyzed.
Time Controlled Adaptive Ventilation
Although we did not set APRV precisely with regards to the method of Time Controlled Adaptive Ventilation (TCAV), we tried to approach this technique as much as possible. TCAV is aimed at limiting Ventilator-Induced Lung Injury and has been proposed since 2005 6–8. In TCAV, expiration is interrupted before the expiratory flow reaches zero, at 75% of the slope of the expiratory flow curve, voluntarily creating an intrinsic PEEP and preventing cyclic alveolar opening and re-collapse 9–11. The beneficial impact of TCAV was suggested by experimental and animal studies, by limiting atelectrauma and stabilizing the alveoli, thus reducing Ventilator-Induced Lung Injury 12,13.
In the ICU, the use of APRV/TCAV remains limited as reported by a recent meta-analysis of 5 studies including 330 patient 14. Moreover, much of the data stem from only a few expert centers with a strong experience in TCAV protocols. At present, the largest study randomizing patients to inverse-ratio APRV or conventional ventilation is a trial conducted by Zhou et al. with 71 patients in the APRV group 1.