Positional Therapy and Regional Pulmonary Ventilation

Abstract

Background Prone ventilation redistributes lung inflation along the gravitational axis; however, localized, nongravitational effects of body position are less well characterized. The authors hypothesize that positional inflation improvements follow both gravitational and nongravitational distributions. This study is a nonoverlapping reanalysis of previously published large animal data. Methods Five intubated, mechanically ventilated pigs were imaged before and after lung injury by tracheal injection of hydrochloric acid (2 ml/kg). Computed tomography scans were performed at 5 and 10 cm H2O positive end-expiratory pressure (PEEP) in both prone and supine positions. All paired prone–supine images were digitally aligned to each other. Each unit of lung tissue was assigned to three clusters (K-means) according to positional changes of its density and dimensions. The regional cluster distribution was analyzed. Units of tissue displaying lung recruitment were mapped. Results We characterized three tissue clusters on computed tomography: deflation (increased tissue density and contraction), limited response (stable density and volume), and reinflation (decreased density and expansion). The respective clusters occupied (mean ± SD including all studied conditions) 29.3 ± 12.9%, 47.6 ± 11.4%, and 23.1 ± 8.3% of total lung mass, with similar distributions before and after lung injury. Reinflation was slightly greater at higher PEEP after injury. Larger proportions of the reinflation cluster were contained in the dorsal versus ventral (86.4 ± 8.5% vs. 13.6 ± 8.5%, P < 0.001) and in the caudal versus cranial (63.4 ± 11.2% vs. 36.6 ± 11.2%, P < 0.001) regions of the lung. After injury, prone positioning recruited 64.5 ± 36.7 g of tissue (11.4 ± 6.7% of total lung mass) at lower PEEP, and 49.9 ± 12.9 g (8.9 ± 2.8% of total mass) at higher PEEP; more than 59.0% of this recruitment was caudal. Conclusions During mechanical ventilation, lung reinflation and recruitment by the prone positioning were primarily localized in the dorso-caudal lung. The local effects of positioning in this lung region may determine its clinical efficacy. Editor’s Perspective What We Already Know about This Topic What This Article Tells Us That Is New

Diminishing Efficacy of Prone Positioning With Late Application in Evolving Lung Injury

“…In early injury, switching position from supine to prone caused opposing changes in lung aeration in ventral versus dorsal lung territories: a posterior-caudal region of the lungs was reinflated, whereas the anterior territory was deflated (Fig. 3, left panels), as in previous observations by our group (14) and others (24). These changes were likely due to shifts of the gravitational and nongravitational forces that deform the lungs (25).…”

“…To quantify these evolving responses, we performed a detailed analysis of the effects of both prone positioning and PEEP on regional lung inflation. We used our registration method to map lung density shifts with voxel-level resolution (14). In early injury, switching position from supine to prone caused opposing changes in lung aeration in ventral versus dorsal lung territories: a posterior-caudal region of the lungs was reinflated, whereas the anterior territory was deflated (Fig.…”

“…Using paired prone-supine CT images in a large animal model of early lung injury, we found that reaeration and recruitment by the prone position were chiefly localized in posterior lung regions near the diaphragm (14). In the current study, we used a similar imaging approach in ventilated pigs to test the hypothesis that lung injury progression diminishes prone positioning’s ability to reaerate the lungs.…”

Clinical evaluation of prone position ventilation in the treatment of acute respiratory distress syndrome induced by sepsis

“…In the supine position, under the influence of gravity, the blood flow may remain distributed on the dorsal side, and the proportion of ventilated blood flow becomes imbalanced. Thus, the supine position may be ineffective for ventilation in patients with severe lung consolidation in gravity-dependent parts[ 14 , 15 ]. In a study by Walter et al [ 16 ], prone position ventilation for patients with ARDS resulted in significant improvement in lung compliance, shortening of mechanical ventilation time, and improvement in oxygenation.…”

Perioperative Pulmonary Atelectasis: Part I. Biology and Mechanisms

“…30,32 Perioperative chest wall reshaping is influenced by body position, with proning allowing for recruitment of dorso-caudal lung. 33…”