4.2 Airway Pressure Release Ventilation in Moderate-Severe ARDS

APRV, as a protective ventilation strategy based on the open lung concept, features “continuous positive airway pressure combined with brief pressure release.” It aims to maintain alveolar recruitment and stability, promote uniform gas distribution, prevent repeated alveolar opening/collapse, and maximize VILI reduction [1,2]. This article explores APRV’s application value in moderate-to-severe ARDS and whether its physiological advantages translate into clinical prognostic benefits.

I. Physiological Advantages of APRV and Clinical Research Evidence

APRV’s physiological effect on ARDS alveoli is achieved by maintaining sufficiently long plateau time and higher mean airway pressure to recruit collapsed alveoli, achieve alveolar homogenization, and improve oxygenation. This is validated in acute lung injury animal models: compared to low tidal volume ventilation, mice ventilated with APRV showed lighter lung injury and better alveolar uniformity [3,4].

Clinical research provides strong support. The author’s team used EIT to study moderate-to-severe ARDS patients, finding APRV improves dorsal lung ventilation. As ventilation time prolonged, perfusion and V/Q matching in dorsal lung tissue significantly increased, and ventilation heterogeneity was markedly improved [5]. This suggests APRV improves oxygenation by optimizing dorsal V/Q relationships; through dual pressure and time effects, APRV improves ventilation heterogeneity, possessing potential to reduce VILI. Meta-analyses of multiple RCTs show APRV-treated patients have significant improvements in oxygenation index, lung compliance, and ventilator-free days, with a trend toward reduced mortality [6,7].

To explore whether clinical advantages stem from superior lung function improvement, the author’s team conducted a randomized controlled study combining EIT [8]. Over 24 h, APRV patients showed marked dorsal region ventilation improvement, significantly increased V/Q matching, while abdominal dead space ventilation was unchanged. Ventilation heterogeneity index decreased significantly; centralization index increased. Oxygenation index and static respiratory system compliance were significantly superior to controls; PaCO₂ significantly reduced. However, mean airway pressure was significantly higher in the APRV group, and no significant mortality difference existed between groups.

II. Key Factors Affecting APRV Benefits and Precautions

Although EIT research reveals significant physiological advantages of APRV in moderate-to-severe ARDS (improved oxygenation, optimized V/Q matching), and systematic reviews confirm association with shortened ICU stay without increased barotrauma or hemodynamic instability [7,9], existing studies have not reached consensus on mortality reduction. This contradiction highlights the complexity of translating physiological advantages into prognostic benefits. A key factor is the current lack of unified parameter setting standards. Given ARDS heterogeneity [10], individualized precise titration is the core challenge.

Core APRV parameters (high pressure, high pressure time, low pressure, low pressure time) all influence therapeutic effect. Setting methods vary greatly across researchers, making direct comparison difficult and introducing potential risks. Insufficient high pressure or short high pressure time may fail to achieve effective alveolar recruitment; excessively long high pressure time may increase intrathoracic pressure, causing ventral overdistension and increased right heart afterload. Low pressure time is particularly critical: insufficient release time triggers excessive auto-PEEP; excessive release time causes re-collapse of recruited alveoli. Low pressure time is based on time-controlled adaptive ventilation (TCAV) [11,12], with individualized Tlow setting by analyzing expiratory flow waveform. The EEF/PEF ratio is commonly used as the termination criterion, with 75% recommended as the routine threshold. However, in COPD or increased airway resistance conditions, 75% EEF may cause overdistension from excessive auto-PEEP, decreased expiratory tidal volume, right heart injury, and hemodynamic instability. Pulmonary CT and airway resistance status should be fully evaluated for individualized settings.

Based on ARDS heterogeneity and APRV’s recruitment nature, phenotype-guided individualized settings should be determined for different ARDS subtypes. Before APRV, lung recruitability must be assessed. Non-recruitable patients may experience increased Paw, overdistension, and worsened hemodynamics in hypovolemia. For non-focal ARDS, low pressure time adjustment optimizes auto-PEEP for gradual stable recruitment; for focal ARDS, EIT monitoring of expansion/collapse rates seeks balance between high pressure and auto-PEEP. Intra-abdominal hypertension or reduced chest wall compliance patients require high pressure titration, potentially combined with esophageal pressure monitoring and prone positioning. For patients with small shunt ratio and large dead space ratio (massive resuscitation phase, extensive microthrombosis), preserving spontaneous breathing during APRV lowers Paw, promotes venous return, and reduces dead space.

III. Summary

APRV’s physiological advantages in moderate-to-severe ARDS translate into prognostic benefits to a certain extent, excelling in oxygenation improvement, shortened mechanical ventilation time, and ICU stay. However, due to lack of unified parameter standards and phenotype-based individualized strategies, the core of future application lies in exploring “individualized” and “titration-based” ventilation through high-quality research, combining bedside real-time monitoring (esophageal pressure, EIT) for dynamic precise adjustments.

(Zou Xiaojing, Shang You, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology)

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