The 30-year evolution of airway pressure release ventilation (APRV)

Abstract

Airway pressure release ventilation (APRV) was first described in 1987 and defined as continuous positive airway pressure (CPAP) with a brief release while allowing the patient to spontaneously breathe throughout the respiratory cycle. The current understanding of the optimal strategy to minimize ventilator-induced lung injury is to "open the lung and keep it open". APRV should be ideal for this strategy with the prolonged CPAP duration recruiting the lung and the minimal release duration preventing lung collapse. However, APRV is inconsistently defined with significant variation in the settings used in experimental studies and in clinical practice. The goal of this review was to analyze the published literature and determine APRV efficacy as a lung-protective strategy. We reviewed all original articles in which the authors stated that APRV was used. The primary analysis was to correlate APRV settings with physiologic and clinical outcomes. Results showed that there was tremendous variation in settings that were all defined as APRV, particularly CPAP and release phase duration and the parameters used to guide these settings. Thus, it was impossible to assess efficacy of a single strategy since almost none of the APRV settings were identical. Therefore, we divided all APRV studies divided into two basic categories: (1) fixed-setting APRV (F-APRV) in which the release phase is set and left constant; and (2) personalized-APRV (P-APRV) in which the release phase is set based on changes in lung mechanics using the slope of the expiratory flow curve. Results showed that in no study was there a statistically significant worse outcome with APRV, regardless of the settings (F-ARPV or P-APRV). Multiple studies demonstrated that P-APRV stabilizes alveoli and reduces the incidence of acute respiratory distress syndrome (ARDS) in clinically relevant animal models and in trauma patients. In conclusion, over the 30 years since the mode's inception there have been no strict criteria in defining a mechanical breath as being APRV. P-APRV has shown great promise as a highly lung-protective ventilation strategy.

Keywords: APRV; ARDS; Lung protection; Ventilator-induced lung injury.

Fig. 1

Comparison of APRV pressure waveforms. Artistic depiction of airway pressure waveforms, all of which were defined as APRV, illustrating the significant variability in what has been defined as an APRV breath. Stock in 1987 used 60 % CPAP with T_Low of 1.27 s and a respiratory rate (RR) of 20 [2]. Davis in 1993 used a similar %CPAP, but decreased the RR by prolonging T_High and T_Low [3]. Gama de Abreau in 2010 simulated conventional ventilation with a prolonged T_Low and short T_High [4]. Finally, Roy in 2013 used a very brief adaptive T_Low and large T_High with 90 % CPAP [5]. Of note, though the ventilator pressure is set at zero, this does not reflect true pressure as the brief T_Low prevents full deflation of the lung, and thus prevents end-expiratory pressure from reaching zero. Figures a–c are examples of fixed-APRV (F-APRV) and figure d of personalized APRV (P-APRV)

Fig. 2

Method of setting expiratory duration (T_Low). a Typical personalized airway pressure release ventilation (P-APRV) airway pressure and flow curves. Correctly set P-APRV has a very brief release phase (time at low pressure—T_Low) and CPAP phase (time at high pressure—T_High) [6]. The T_High is ~90 % of each breath. The two other P-ARPV settings are the pressure at inspiration (P_High) and at expiration (P_Low). T_Low is sufficiently brief such that end-expiratory pressure (P_Low) never reaches 0 cmH₂O measured by the tracheal pressure (green line). b Maintain alveolar stability by adaptively adjusting the expiratory duration as directed by the expiratory flow curve. The rate of lung collapse is seen in the normal (slope 45°) and acutely injured lung (ARDS, slope 30°). ARDS causes a more rapid lung collapse due to decreased lung compliance. Our preliminary studies have shown that if the end-expiratory flow (EEF; −45 L/min) to the peak expiratory flow (PEF; −60 L/min) ratio is equal to 0.75, the resultant T_Low (0.5 s) is sufficient to stabilize alveoli [54, 55]. The lung with ARDS collapses more rapidly such that the EEF/PEF ratio of 75 % identifies an expiratory duration of 0.45 s as necessary to stabilize alveoli. Thus, this method of setting expiratory duration is adaptive to changes in lung pathophysiology and personalizes the mechanical breath to each individual patient