This is the first meta-analysis focusing on the effect of prone position in surgical and trauma patients. Our study demonstrated the significant improvement in P/F ratio after proning. This benefit was shown in both overall and subgroup analysis. Prone position in surgical and trauma patients also significantly decreased mortality and mechanical ventilator days. There was no effect on cardiac index regarding two studies. Despite a low rate of serious complications from prone positioning in our systematic review, minor complications, particularly facial edema, were frequently reported among all studies
Ventilator-induced lung injury (VILI) associated with barotrauma, volutrauma, atelectrauma, and biotrauma demonstrated largely influenced to mortality on ARDS.24, 25 Lung-protective mechanical ventilation (MV) strategies have demonstrated improved survival in patients with ARDS over the past decades.26 However, adjusting mechanical ventilation alone may not successfully improve outcomes, including oxygenation, MV day, ICU length of stay and mortality. Consequently, there are alternative methods proposed to help improve outcomes, including prone positioning.27
Prone positioning (PP) is one of current ARDS managements, has been studies in many large randomized controlled trials (RCT) to access aforementioned benefits.4, 5, 28–30 The only absolute contraindication of prone positioning is an unstable spinal fracture, while relative contraindications that should be individually selected include hemodynamic instability, trauma related injuries (open abdominal wounds, increased intracranial pressure, and unstable long bone or pelvic fracture), and late-term pregnancy.31
In our study, there was significant oxygenation improvement (increased P/F ratio) in patients who underwent PP compared to supine positioning (SP), (mean difference 79.26; 95% CI, 53.38-105.13; 10 studies; n = 624 patients). Mortality benefit remained controversial among the studies. One systematic review and meta-analysis included eight RCTs and evaluated the effect of prone positioning on 28-day mortality. It demonstrated a non-significant reduction in mortality in favour of PP; however, a subgroup analysis of patients with ≥ 12 hours of PP found a significantly lower mortality in this group.31 This mortality reduction was most marked among patients with moderate to severe ARDS. In our study, mortality was significantly improved in PP, (RR 0.48, 95% CI, 0.35–0.67; 6 studies, n = 362), although our included studies were heterogeneous in terms of the duration of PP, varying between < 12 hours (6 studies) and ≥ 12 hours (9 studies).
The PROSEVA study showed a significant reduction in 28 and 90 days on MV (14 ± 9 days and 33 ± 34 days, respectively).5As in PROSEVA trial, our results indicated a significant reduction in MV days days in PP (mean difference − 2.59, 95% CI, -4.21-0.97; 3 studies; n = 165). While ICU LOS in PROSEVA study showed a trend in favour of PP (P = 0.05) ICU LOS, similar to our study demonstrated no significant difference between two groups (mean difference − 2.23, 95% CI, -5.33-0.87; 4 studies, n = 212).
In terms of hemodynamic aspect, Jozwiak et al demonstrated that the microcirculatory effect of prone position result from three basic mechanisms: an increase in intraabdominal pressure, improvement in arterial oxygenation, and lung recruitment. These three effects can lead to significant increase in cardiac preload, decrease right ventricular afterload, and increase in left ventricular preload. However, cardiac output will increase only in preload reserve patients. While our study did not show increased cardiac output, this result might be explained by the lack of preload assessment data. However, other hemodynamic parameters, especially the decrease of pulmonary vascular resistance and the improvement of intrapulmonary shunt were similarly demonstrated.
Complications from prone positioning must also be noted, particularly since trauma and surgical patients may be at increased risk for morbidity from position changes due to the presence of fractures, surgical incisions, or increased support lines and devices. Adverse events such as facial swelling, loss of venous access, device displacement and pressure sore can occur during transition to and from prone position and during prone positioning itself; however, they can be attenuated with program training. It has been suggested that endotracheal tube obstruction and vasopressor requirement increased with prone position, while the incidence of barotrauma and ventilator-associated pneumonia and unexplained central catheter or endotracheal tube removal were not significant different between groups.32
In this study, we reported surgical complications that ranged from 3–42%, particularly involving abdominal and sternal wound dehiscences related to prone position; however, it was not significantly different compared to supine position. In addition, there were some studies reported the incidence of intraabdominal hypertension or abdominal compartment syndrome in our study. Though prone position can increase intraabdominal pressure, the effect is small. There was no previous reported significant increased intraabdominal pressure resulting in abdominal compartment after prone position.33–35 The possible explanation are the improper location of the cushion with abdominal compression, and the undetected preexisting intraabdominal hypertension.
Although this is the first meta-analysis of a large number of surgical and trauma patients with ARDS and prone positioning, we must acknowledge the limitations of this study. These include clinical and methodologic heterogeneity among included studies. We attempted to correct for some of these differences via subgroup analyses, but the possibility of bias from the data heterogeneity must be considered. A large multicenter prospective study is warranted.
In summary, prone position can significantly improve the P/F ratio and has a mortality benefit among surgical and trauma patients who developed acute respiratory distress syndrome. It can cause minor complications, such as facial edema. There was no significant difference in local wound complications compared to those with supine position. Prone position may be an effective rescue therapy for surgical and trauma patients.