Effects of prone position and positive end-expiratory pressure on lung perfusion and ventilation

Influence of Prone Position on Regional Ventilation/Perfusion Matching in Patients With ARDS Over Time: A Prospective Physiological Study

Background: We sought to investigate the short- and long-term effects of prone positioning (PP) on ventilation/perfusion matching in patients with ARDS using contrast-enhanced electrical impedance tomography (EIT). Methods: EIT measurements were performed in 18 mechanically ventilated subjects with ARDS before PP (supine position [SP]), 1 h after turning subjects to PP (PP1), 3 h after PP (PP3), 9 h after (PP9), 16 h after PP (PP16; the end of PP), and 3 h after returning to the supine position (Re-SP3). Results: The P a O 2 /F I O 2 increased gradually during the PP period (110.68 vs 158.44 vs 210.15 vs 215.22 vs 236.04 vs 163.77 mm Hg, mean values at SP, PP1, PP3, PP9, PP16, and Re-SP3, respectively, P < .001). Global ventilation/perfusion matched percent significantly increased within PP duration (54.13% vs 63.15% vs 63.02% vs 63.75% vs 66.63% vs 57.42, P < .005). Compared with SP, dorsal ventilation significantly increased at PP1 (P < .001) and increased gradually during PP. However, the dorsal flow commenced to improve at PP9 and persisted in enhancement until PP16 (40.61% vs 48.78% vs 50.56%, mean values at PP3, PP9, and PP16, respectively, P < .05). There was a significant reduction in global Shunt-EIT percentage within PP duration, primarily localized in the dorsal area. Dead Space-EIT percentage remained unchanged during PP. Conclusions: Oxygenation remained improved or maintained throughout the 16-h duration of PP. Ventilation is susceptible to immediate gravitational effects; however, changes in blood flow may occur later after 9 h, which supports prolonged PP treatment. The shunt continuously decreases, but no significant changes were observed for dead space. Trial registration: ClinicalTrials.gov, NCT04725227. Registered on January 25, 2021.

Effect of continuous positive airway pressure on the respiratory system: a comprehensive review

Background

CPAP is characterized by the application of a constant and continuous positive pressure into the patient’s airway. By delivering a constant pressure during both inspiration and expiration, CPAP increases functional residual capacity and opens collapsed or under ventilated alveoli, thus decreasing right to left intrapulmonary shunt and improving oxygenation in obese individuals.

Main body of abstract

Obesity is characterized by several alterations in the mechanics of the respiratory system that tend to further exaggerate impairment of gas exchange rendering these patients prone to perioperative complications, such as hypoxemia, hypercapnia, and atelectasis. Interestingly, CPAP has been advocated as an efficacious modality for prevention and treatment of postoperative atelectasis considered to be the most common postoperative respiratory complication. In OSA, the CPAP device works to splint the airway open and prevent the collapse of the upper airway that is the cardinal event of OSA leading improvement of sleep, quality of life and the reduction of the risks of the cardiovascular and neurocognitive side effects associated with the disease. Besides such a beneficial effect, there are other physiological benefits to CPAP: greater end-expiratory lung volume and consequent increase in oxygen stores, increased tracheal traction to improve upper airway patency and decrease in cardiac after load.

Conclusion

Due to various physiological benefits on the respiratory system CPAP therapy is crucial for the prevention postoperative complications particularly related to obesity and the cornerstone for the treatment of moderate to severe obstructive sleep apnea.

Imaging the acute respiratory distress syndrome: past, present and future

In patients with the acute respiratory distress syndrome (ARDS), lung imaging is a fundamental tool in the study of the morphological and mechanistic features of the lungs. Chest computed tomography studies led to major advances in the understanding of ARDS physiology. They allowed the in vivo study of the syndrome's lung features in relation with its impact on respiratory physiology and physiology, but also explored the lungs' response to mechanical ventilation, be it alveolar recruitment or ventilator-induced lung injuries. Coupled with positron emission tomography, morphological findings were put in relation with ventilation, perfusion or acute lung inflammation. Lung imaging has always been central in the care of patients with ARDS, with modern point-of-care tools such as electrical impedance tomography or lung ultrasounds guiding clinical reasoning beyond macro-respiratory mechanics. Finally, artificial intelligence and machine learning now assist imaging post-processing software, which allows real-time analysis of quantitative parameters that describe the syndrome's complexity. This narrative review aims to draw a didactic and comprehensive picture of how modern imaging techniques improved our understanding of the syndrome, and have the potential to help the clinician guide ventilatory treatment and refine patient prognostication.

Prone positioning improves ventilation-perfusion matching assessed by electrical impedance tomography in patients with ARDS: a prospective physiological study

BACKGROUND

The physiological effects of prone ventilation in ARDS patients have been discussed for a long time but have not been fully elucidated. Electrical impedance tomography (EIT) has emerged as a tool for bedside monitoring of pulmonary ventilation and perfusion, allowing the opportunity to obtain data. This study aimed to investigate the effect of prone positioning (PP) on ventilation-perfusion matching by contrast-enhanced EIT in patients with ARDS.

DESIGN

Monocenter prospective physiologic study.

SETTING

University medical ICU.

PATIENTS

Ten mechanically ventilated ARDS patients who underwent PP.

INTERVENTIONS

We performed EIT evaluation at the initiation of PP, 3 h after PP initiation and the end of PP during the first PP session.

MEASUREMENTS AND MAIN RESULTS

The regional distribution of ventilation and perfusion was analyzed based on EIT images and compared to the clinical variables regarding respiratory and hemodynamic status. Prolonged prone ventilation improved oxygenation in the ARDS patients. Based on EIT measurements, the distribution of ventilation was homogenized and dorsal lung ventilation was significantly improved by PP administration, while the effect of PP on lung perfusion was relatively mild, with increased dorsal lung perfusion observed. The ventilation-perfusion matched region was found to increase and correlate with the increased PaO₂/FiO₂ by PP, which was attributed mainly to reduced shunt in the lung.

CONCLUSIONS

Prolonged prone ventilation increased dorsal ventilation and perfusion, which resulted in improved ventilation-perfusion matching and oxygenation.

TRIAL REGISTRATION

ClinicalTrials.gov, NCT04725227. Registered on 25 January 2021.

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

OBJECTIVES

It is not known how lung injury progression during mechanical ventilation modifies pulmonary responses to prone positioning. We compared the effects of prone positioning on regional lung aeration in late versus early stages of lung injury.

DESIGN

Prospective, longitudinal imaging study.

SETTING

Research imaging facility at The University of Pennsylvania (Philadelphia, PA) and Medical and Surgical ICUs at Massachusetts General Hospital (Boston, MA).

SUBJECTS

Anesthetized swine and patients with acute respiratory distress syndrome (acute respiratory distress syndrome).

INTERVENTIONS

Lung injury was induced by bronchial hydrochloric acid (3.5 mL/kg) in 10 ventilated Yorkshire pigs and worsened by supine nonprotective ventilation for 24 hours. Whole-lung CT was performed 2 hours after hydrochloric acid (Day 1) in both prone and supine positions and repeated at 24 hours (Day 2). Prone and supine images were registered (superimposed) in pairs to measure the effects of positioning on the aeration of each tissue unit. Two patients with early acute respiratory distress syndrome were compared with two patients with late acute respiratory distress syndrome, using electrical impedance tomography to measure the effects of body position on regional lung mechanics.

MEASUREMENTS AND MAIN RESULTS

Gas exchange and respiratory mechanics worsened over 24 hours, indicating lung injury progression. On Day 1, prone positioning reinflated 18.9% ± 5.2% of lung mass in the posterior lung regions. On Day 2, position-associated dorsal reinflation was reduced to 7.3% ± 1.5% (p < 0.05 vs Day 1). Prone positioning decreased aeration in the anterior lungs on both days. Although prone positioning improved posterior lung compliance in the early acute respiratory distress syndrome patients, it had no effect in late acute respiratory distress syndrome subjects.

CONCLUSIONS

The effects of prone positioning on lung aeration may depend on the stage of lung injury and duration of prior ventilation; this may limit the clinical efficacy of this treatment if applied late.

Positive End-Expiratory Pressure, Pleural Pressure, and Regional Compliance during Pronation: An Experimental Study

Rationale: The physiological basis of lung protection and the impact of positive end-expiratory pressure (PEEP) during pronation in acute respiratory distress syndrome are not fully elucidated. Objectives: To compare pleural pressure (Ppl) gradient, ventilation distribution, and regional compliance between dependent and nondependent lungs, and investigate the effect of PEEP during supination and pronation. Methods: We used a two-hit model of lung injury (saline lavage and high-volume ventilation) in 14 mechanically ventilated pigs and studied supine and prone positions. Global and regional lung mechanics including Ppl and distribution of ventilation (electrical impedance tomography) were analyzed across PEEP steps from 20 to 3 cm H₂O. Two pigs underwent computed tomography scans: tidal recruitment and hyperinflation were calculated. Measurements and Main Results: Pronation improved oxygenation, increased Ppl, thus decreasing transpulmonary pressure for any PEEP, and reduced the dorsal-ventral pleural pressure gradient at PEEP < 10 cm H₂O. The distribution of ventilation was homogenized between dependent and nondependent while prone and was less dependent on the PEEP level than while supine. The highest regional compliance was achieved at different PEEP levels in dependent and nondependent regions in supine position (15 and 8 cm H₂O), but for similar values in prone position (13 and 12 cm H₂O). Tidal recruitment was more evenly distributed (dependent and nondependent), hyperinflation lower, and lungs cephalocaudally longer in the prone position. Conclusions: In this lung injury model, pronation reduces the vertical pleural pressure gradient and homogenizes regional ventilation and compliance between the dependent and nondependent regions. Homogenization is much less dependent on the PEEP level in prone than in supine positon.

A rational approach on the use of extracorporeal membrane oxygenation in severe hypoxemia: advanced technology is not a panacea

Veno-venous extracorporeal membrane oxygenation (ECMO) is a helpful intervention in patients with severe refractory hypoxemia either because mechanical ventilation cannot ensure adequate oxygenation or because lung protective ventilation is not feasible. Since ECMO is a highly invasive procedure with several, potentially devastating complications and its implementation is complex and expensive, simpler and less invasive therapeutic options should be first exploited. Low tidal volume and driving pressure ventilation, prone position, neuromuscular blocking agents and individualized ventilation based on transpulmonary pressure measurements have been demonstrated to successfully treat the vast majority of mechanically ventilated patients with severe hypoxemia. Veno-venous ECMO has a place in the small portion of severely hypoxemic patients in whom these strategies fail. A combined analysis of recent ARDS trials revealed that ECMO was used in only 2.15% of patients (n = 145/6736). Nevertheless, ECMO use has sharply increased in the last decade, raising questions regarding its thoughtful use. Such a policy could be harmful both for patients as well as for the ECMO technique itself. This narrative review attempts to describe together the practical approaches that can be offered to the sickest patients before going to ECMO, as well as the rationale and the limitations of ECMO. The benefit and the drawbacks associated with ECMO use along with a direct comparison with less invasive therapeutic strategies will be analyzed.

Therapeutic benefits of proning to improve pulmonary gas exchange in severe respiratory failure: focus on fundamentals of physiology

New Findings

What is the topic of this review?

The use of proning for improving pulmonary gas exchange in critically ill patients.

What advances does it highlight?

Proning places the lung in its ‘natural’ posture, and thus optimises the ventilation‐perfusion distribution, which enables lung protective ventilation and the alleviation of potentially life‐threatening hypoxaemia in COVID‐19 and other types of critical illness with respiratory failure.

Abstract

The survival benefit of proning patients with acute respiratory distress syndrome (ARDS) is well established and has recently been found to improve pulmonary gas exchange in patients with COVID‐19‐associated ARDS (CARDS). This review outlines the physiological implications of transitioning from supine to prone on alveolar ventilation‐perfusion (V˙A--Q˙) relationships during spontaneous breathing and during general anaesthesia in the healthy state, as well as during invasive mechanical ventilation in patients with ARDS and CARDS. Spontaneously breathing, awake healthy individuals maintain a small vertical (ventral‐to‐dorsal) V˙A/Q˙ ratio gradient in the supine position, which is largely neutralised in the prone position, mainly through redistribution of perfusion. In anaesthetised and mechanically ventilated healthy individuals, a vertical V˙A/Q˙ ratio gradient is present in both postures, but with better V˙A--Q˙ matching in the prone position. In ARDS and CARDS, the vertical V˙A/Q˙ ratio gradient in the supine position becomes larger, with intrapulmonary shunting in gravitationally dependent lung regions due to compression atelectasis of the dorsal lung. This is counteracted by proning, mainly through a more homogeneous distribution of ventilation combined with a largely unaffected high perfusion dorsally, and a consequent substantial improvement in arterial oxygenation. The data regarding proning as a therapy in patients with CARDS is still limited and whether the associated improvement in arterial oxygenation translates to a survival benefit remains unknown. Proning is nonetheless an attractive and lung protective manoeuvre with the potential benefit of improving life‐threatening hypoxaemia in patients with ARDS and CARDS.

Prone Position Impairs Oxygen Supply-Demand Balance During Systemic Hypoxia in Rabbits

Ventilation in the prone position improves the prognosis of patients with severe acute respiratory distress syndrome (ARDS). Contraindications to ventilation in this position include unstable systemic circulation. Only a few reports exist on the effects of prone ventilation in respiratory failure on systemic circulation. This animal study compared systemic hemodynamic changes between supine and prone positions in anesthetized rabbits under acute systemic hypoxia (breathing 15% O₂). Cardiac output and the systemic O₂ extraction ratio increased under the hypoxia, but only in the supine group. Besides, the rate pressure product was higher in the prone group than in the supine group. This study showed that prone ventilation increases myocardial O₂ consumption and suppresses compensatory mechanisms to maintain aerobic metabolism during systemic hypoxia. First of all, it will be necessary to examine the effect of prone ventilation on the O₂ supply-demand balance in the ARDS model.

Positional Therapy and Regional Pulmonary Ventilation

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

Hemodynamic effects of extended prone position sessions in ARDS

Background

Hemodynamic response to prone position (PP) has never been studied in a large series of patients with acute respiratory distress syndrome (ARDS). The primary aim of this study was to estimate the rate of PP sessions associated with cardiac index improvement. Secondary objective was to describe hemodynamic response to PP and during the shift from PP to supine position.

Methods

The study was a single-center retrospective observational study, performed on ARDS patients, undergoing at least one PP session under monitoring by transpulmonary thermodilution. PP sessions performed more than 10 days after ARDS onset, or with any missing cardiac index measurements before (T₁), at the end (T₃), and after the PP session (T₄) were excluded. Changes in hemodynamic parameters during PP were tested after statistical adjustment for volume of fluid challenges, vasopressor and dobutamine dose at each time point to take into account therapeutic changes during PP sessions.

Results

In total, 107 patients fulfilled the inclusion criteria, totalizing 197 PP sessions. Changes in cardiac index between T₁ and T₂ (early response to PP) and between T₁ and T₃ (late response to PP) were significantly correlated (R² = 0.42, p < 0.001) with a concordance rate amounting to 85%. Cardiac index increased significantly between T₁ and T₃ in 49 sessions (25% [95% confidence interval (CI_95%) 18–32%]), decreased significantly in 46 (23% [CI_95% 16–31%]), and remained stable in 102 (52% [CI_95% 45–59%]). Global end-diastolic volume index (GEDVI) increased slightly but significantly from 719 ± 193 mL m⁻² at T₁ to 757 ± 209 mL m⁻² at T₃ and returned to baseline values at T₄. Cardiac index and oxygen delivery decreased slightly but significantly from T₃ to T₄, without detectable increase in lactate level. Patients who increased their cardiac index during PP had significantly lower CI, GEDVI, global ejection fraction at T₁, and received significantly more fluids than patients who did not.

Conclusion

PP is associated with an increase in cardiac index in 18% to 32% of all PP sessions and a sustained increase in GEDVI reversible after return to supine position. Return from prone to supine position is associated with a slight hemodynamic impairment.

Electronic supplementary material

The online version of this article (10.1186/s13613-018-0464-9) contains supplementary material, which is available to authorized users.

Lung volumes and lung volume recruitment in ARDS: a comparison between supine and prone position

Background

The use of positive end-expiratory pressure (PEEP) and prone position (PP) is common in the management of severe acute respiratory distress syndrome patients (ARDS). We conducted this study to analyze the variation in lung volumes and PEEP-induced lung volume recruitment with the change from supine position (SP) to PP in ARDS patients.

Methods

The investigation was conducted in a multidisciplinary intensive care unit. Patients who met the clinical criteria of the Berlin definition for ARDS were included. The responsible physician set basal PEEP. To avoid hypoxemia, FiO₂ was increased to 0.8 1 h before starting the protocol. End-expiratory lung volume (EELV) and functional residual capacity (FRC) were measured using the nitrogen washout/washin technique. After the procedures in SP, the patients were turned to PP and 1 h later the same procedures were made in PP.

Results

Twenty-three patients were included in the study, and twenty were analyzed. The change from SP to PP significantly increased FRC (from 965 ± 397 to 1140 ± 490 ml, p = 0.008) and EELV (from 1566 ± 476 to 1832 ± 719 ml, p = 0.008), but PEEP-induced lung volume recruitment did not significantly change (269 ± 186 ml in SP to 324 ± 188 ml in PP, p = 0.263). Dynamic strain at PEEP decreased with the change from SP to PP (0.38 ± 0.14 to 0.33 ± 0.13, p = 0.040).

Conclusions

As compared to supine, prone position increases resting lung volumes and decreases dynamic lung strain.

Electronic supplementary material

The online version of this article (10.1186/s13613-018-0371-0) contains supplementary material, which is available to authorized users.

Reduced pulmonary blood flow in regions of injury 2 hours after acid aspiration in rats

BACKGROUND

Aspiration-induced lung injury can decrease gas exchange and increase mortality. Acute lung injury following acid aspiration is characterized by elevated pulmonary blood flow (PBF) in damaged lung areas in the early inflammation stage. Knowledge of PBF patterns after acid aspiration is important for targeting intravenous treatments. We examined PBF in an experimental model at a later stage (2 hours after injury).

METHODS

Anesthetized Wistar-Unilever rats (n = 5) underwent unilateral endobronchial instillation of hydrochloric acid. The PBF distribution was compared between injured and uninjured sides and with that of untreated control animals (n = 6). Changes in lung density after injury were measured using computed tomography (CT). Regional PBF distribution was determined quantitatively in vivo 2 hours after acid instillation by measuring the concentration of [(68)Ga]-radiolabeled microspheres using positron emission tomography.

RESULTS

CT scans revealed increased lung density in areas of acid aspiration. Lung injury was accompanied by impaired gas exchange. Acid aspiration decreased the arterial pressure of oxygen from 157 mmHg [139;165] to 74 mmHg [67;86] at 20 minutes and tended toward restoration to 109 mmHg [69;114] at 110 minutes (P < 0.001). The PBF ratio of the middle region of the injured versus uninjured lungs of the aspiration group (0.86 [0.7;0.9], median [25%;75%]) was significantly lower than the PBF ratio in the left versus right lung of the control group (1.02 [1.0;1.05]; P = 0.016).

CONCLUSIONS

The PBF pattern 2 hours after aspiration-induced lung injury showed a redistribution of PBF away from injured regions that was likely responsible for the partial recovery from hypoxemia over time. Treatments given intravenously 2 hours after acid-induced lung injury may not preferentially reach the injured lung regions, contrary to what occurs during the first hour of inflammation. Please see related article: http://dx.doi.org/10.1186/s12871-015-0014-z.

Effect of body position on ventilation distribution during PEEP titration in a porcine model of acute lung injury using advanced respiratory monitoring and electrical impedance tomography

Background

Lung failure after acute lung injury remains a challenge in different clinical settings. Various interventions for restoration of gas exchange have been investigated. Recruitment of collapsed alveoli by positive end expiratory pressure (PEEP) titration and optimization of ventilation-perfusion ratio by prone positioning have been extensively described in animal and clinical trials. This animal study was conducted to investigate the effects of PEEP and positioning by means of advanced respiratory monitoring including gas exchange, respiratory mechanics, volumetric capnography and electrical impedance tomography.

Methods

After induction of acute lung injury by oleic acid and lung lavage, 12 domestic pigs were studied in randomly assigned supine or prone position during a PEEP titration trial with maximal PEEP of 30 mbar.

Results

Induction of lung injury resulted in significant deterioration of oxygenation [partial pressure of arterial oxygen/inspiratory fraction of oxygen (PaO₂/FiO₂): p = 0.002] and ventilation [partial pressure of arterial carbon dioxide (PaCO₂): p = 0.002] and elevated alveolar dead-space ratios (Valv/Vte: p = 0.003) in both groups. Differences in the prone and the supine group were significant for PaCO₂ at incremental PEEP 10 and 20 and at decremental PEEP 20 (20d) and 10 (10d), for PaO₂/FiO₂ at PEEP 10 and 10d and for alveolar dead space at PEEP 10d. Electrical impedance tomography revealed homogenous ventilation distribution in prone position during PEEP 20, 30 and 20d.

Conclusions

Prone position leads to improved oxygenation and ventilation parameters in a lung injury model. Respiratory monitoring with measurement of alveolar dead space and electrical impedance tomography may visualize optimized ventilation in a PEEP titration trial.

Prone position

PURPOSE OF REVIEW

Prone position can prevent ventilator-induced lung injury in acute respiratory distress syndrome (ARDS) patients receiving conventional mechanical ventilation and, hence, may have the potential to improve survival from this basis. Even though no single randomized controlled trial has proven benefit on patient outcome until recently, two meta-analyses, one on grouped data and the other on individual data, have shown that patients with PaO2/FIO2 ratio less than 100 mmHg at the time of inclusion did benefit from prone position. As a fifth trial completed recently has shown a significant reduction in mortality in patients with severe and confirmed ARDS from using prone position, the purpose of this review is to revisit prone positioning in ARDS in the light of these new findings.

RECENT FINDINGS

In this trial done in patients with severe ARDS severity criteria (PaO2/FIO2 ratio less than 150 mmHg with positive end expiratory pressure of 5 cmH2O or more, FIO2 of 60% or more and tidal volume around 6 ml/kg predicted body weight) confirmed 12-24 h after the onset of ARDS, the day 28 mortality in the supine group (229 patients) was 32.8 versus 16% in the prone group (237 patients) (P < 0.001). Significant reduction in mortality was confirmed at day 90.

SUMMARY

From the combined results of the two meta-analyses and the last randomized controlled trial, there is a very strong signal to use prone position in patients with severe ARDS, as early as possible and for long sessions.

Mechanisms of the effects of prone positioning in acute respiratory distress syndrome

INTRODUCTION

Prone positioning has been used for many years in patients with acute respiratory distress syndrome (ARDS). The initial reason for prone positioning in ARDS patients was improvement in oxygenation. It was later shown that mechanical ventilation in the prone position can be less injurious to the lung and hence the primary reason to use prone positioning is prevention of ventilator-induced lung injury (VILI).

MATERIAL AND METHODS

A large body of physiologic benefits of prone positioning in ARDS patients accumulated but these failed to translate into clinical benefits. More recently, meta-analyses and randomized controlled trial in a specific subgroup of ARDS patients demonstrated that prone positioning can improve survival. This review covers the effects of prone positioning on oxygenation, respiratory mechanics, and VILI.

CONCLUSIONS

We conclude with the effects of prone positioning on patient outcome, in particular on survival.

The impact of tidal volume on pulmonary complications following minimally invasive esophagectomy: a randomized and controlled study

BACKGROUND

Minimally invasive esophagectomy (MIE) has been advantageous for lowering pulmonary complications compared with open approaches.(1) However, pulmonary complications remain the most common morbidity after surgical resection of esophageal cancer.(2,3) The aim of this prospective, randomized, controlled, clinical trial was designed to see whether low tidal volume (VT) could further minimize pulmonary complications after MIE.

METHODS

Between June 2011 and July 2012, a total of 101 patients who underwent MIE received left-lung ventilation during thoracoscopic esophagectomy. All patients received left-lung ventilation during thoracoscopic esophagectomy. Patients were randomly assigned to a low VT (5 mL/kg + 5 cm H2O positive end-expiratory pressure) preserved ventilation (PV) group (n = 53) and a conventional VT (8 mL/kg) controlled ventilation (CV) group (n = 48) in the thoracic stage. Alveolar lavage fluid was harvested from the ventilated lung at intubation and at 18 hours after surgery for analysis of interleukin (IL)-1ß, IL-6, and IL-8 levels. Clinical characteristics, including patient demographics, operation features, and changes in oxygenation index, were recorded and analyzed. Pulmonary complications were identified and statistically compared between the 2 groups.

RESULTS

The clinical characteristics and operation features were comparable between the 2 groups. IL-1ß, IL-6, and IL-8 expressions in preoperative alveolar lavage fluid were similar between the 2 groups. Significantly lower IL expressions were observed in the PV group than those in the CV group at 18 hours after MIE (IL-1ß, 25.42 ± 31.01 vs 94.96 ± 118.24 pg/mL; IL-6, 30.86 ± 75.78 vs 92.99 ± 72.90 pg/mL; IL-8, 258.75 ± 188.24 vs 403.95 ± 151.44 pg/mL; all P < .05). The 18-hour postoperative oxygenation index was lower in the CV group than that in the PV group (292.85 ± 28.74 vs 326.35 ± 34.43; P = .046). Pulmonary complications were observed in 18 cases of our series, occurring more frequently on the ventilation side (right, 6 cases; and left, 12 cases). All patients were cured by conservative therapy without severe sequelae. The occurrence of pulmonary complications in the PV group was lower than that in the CV group (9.43% vs 27.08%; P = .021).

CONCLUSIONS

Lung injury due to intraoperative single-lung ventilation may contribute to pulmonary complications after MIE. Low VT ventilation could decrease ventilation-associated lung inflammation, thus minimizing pulmonary complications after MIE. Further studies, based on a larger volume of populations, are required to confirm these findings.

Effects of ventilation strategy on distribution of lung inflammatory cell activity

Introduction

Leukocyte infiltration is central to the development of acute lung injury, but it is not known how mechanical ventilation strategy alters the distribution or activation of inflammatory cells. We explored how protective (vs. injurious) ventilation alters the magnitude and distribution of lung leukocyte activation following systemic endotoxin administration.

Methods

Anesthetized sheep received intravenous endotoxin (10 ng/kg/min) followed by 2 h of either injurious or protective mechanical ventilation (n = 6 per group). We used positron emission tomography to obtain images of regional perfusion and shunting with infused ¹³N[nitrogen]-saline and images of neutrophilic inflammation with ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG). The Sokoloff model was used to quantify ¹⁸F-FDG uptake (K_i), as well as its components: the phosphorylation rate (k₃, a surrogate of hexokinase activity) and the distribution volume of ¹⁸F-FDG (F_e) as a fraction of lung volume (K_i = F_e × k₃). Regional gas fractions (f_gas) were assessed by examining transmission scans.

Results

Before endotoxin administration, protective (vs. injurious) ventilation was associated with a higher ratio of partial pressure of oxygen in arterial blood to fraction of inspired oxygen (PaO₂/FiO₂) (351 ± 117 vs. 255 ± 74 mmHg; P < 0.01) and higher whole-lung f_gas (0.71 ± 0.12 vs. 0.48 ± 0.08; P = 0.004), as well as, in dependent regions, lower shunt fractions. Following 2 h of endotoxemia, PaO₂/FiO₂ ratios decreased in both groups, but more so with injurious ventilation, which also increased the shunt fraction in dependent lung. Protective ventilation resulted in less nonaerated lung (20-fold; P < 0.01) and more normally aerated lung (14-fold; P < 0.01). K_i was lower during protective (vs. injurious) ventilation, especially in dependent lung regions (0.0075 ± 0.0043/min vs. 0.0157 ± 0.0072/min; P < 0.01). ¹⁸F-FDG phosphorylation rate (k₃) was twofold higher with injurious ventilation and accounted for most of the between-group difference in K_i. Dependent regions of the protective ventilation group exhibited lower k₃ values per neutrophil than those in the injurious ventilation group (P = 0.01). In contrast, F_e was not affected by ventilation strategy (P = 0.52). Lung neutrophil counts were not different between groups, even when regional inflation was accounted for.

Conclusions

During systemic endotoxemia, protective ventilation may reduce the magnitude and heterogeneity of pulmonary inflammatory cell metabolic activity in early lung injury and may improve gas exchange through its effects predominantly in dependent lung regions. Such effects are likely related to a reduction in the metabolic activity, but not in the number, of lung-infiltrating neutrophils.

Impact of the Prone Position in an Animal Model of Unilateral Bacterial Pneumonia Undergoing Mechanical Ventilation

Background:

The prone position (PP) has proven beneficial in patients with severe lung injury subjected to mechanical ventilation (MV), especially in those with lobar involvement. We assessed the impact of PP on unilateral pneumonia in rabbits subjected to MV.

Methods:

After endobronchial challenge with Enterobacter aerogenes, adult rabbits were subjected to either “adverse” (peak inspiratory pressure = 30 cm H2O, zero end-expiratory pressure; n = 10) or “protective” (tidal volume = 8 ml/kg, 5 cm H2O positive end-expiratory pressure; n = 10) MV and then randomly kept supine or turned to the PP. Pneumonia was assessed 8 h later. Data are presented as median (interquartile range).

Results:

Compared with the supine position, PP was associated with significantly lower bacterial concentrations within the infected lung, even if a “protective” MV was applied (5.93 [0.34] vs. 6.66 [0.86] log10 cfu/g, respectively; P = 0.008). Bacterial concentrations in the spleen were also decreased by the PP if the “adverse” MV was used (3.62 [1.74] vs. 6.55 [3.67] log10 cfu/g, respectively; P = 0.038). In addition, the noninfected lung was less severely injured in the PP group. Finally, lung and systemic inflammation as assessed through interleukin-8 and tumor necrosis factor-α measurement was attenuated by the PP.

Conclusions:

The PP could be protective if the host is subjected to MV and unilateral bacterial pneumonia. It improves lung injury even if it is utilized after lung injury has occurred and nonprotective ventilation has been administered.

Thoracoscopic enucleation of esophageal leiomyoma in prone position and single lumen endotracheal intubation

INTRODUCTION

Esophageal leiomyomas are the most common benign tumors of the esophagus. Surgical enucleation is warranted for symptomatic patients. Thoracoscopic enucleation is the preferable approach for being less invasive by avoiding the discomfort and complications associated to larger thoracic incisions. The purpose of this study was to review our experience with enucleation of esophageal leiomyoma using a prone-position thoracoscopy technique.

METHODS

Between January 2009 and July 2012, ten patients underwent resection of esophageal leiomyoma by thoracoscopy approach in prone position. Indications for surgical treatment were symptomatic tumors (dysphagia). All patients were followed postoperatively for at least 3 months with contrast x-ray of the esophagus. After single-lumen endotracheal intubation (nonselective intubation) in supine, patients were placed in prone position. Pneumothorax was kept at 6 to 8 mmHg using CO2 insufflation. A myotomy was performed over the tumor using hook cautery carefully protecting the mucosa from injuries. The myotomy was closed with continuous sutures.

RESULTS

The procedures were completed in the prone position in all cases, without any conversion. Mean operative time was 89.2 ± 28.7 minutes. Bleeding was negligible, and there were no intraoperative or postoperative complications. No intensive care unit support was needed for any patient. Chest x-ray in the first postoperative day showed no significant changes in any patient. The mean hospital stay was 3.2 days. Contrast x-ray of the esophagus was normal in all patients at 3 months postoperatively.

CONCLUSIONS

Thoracoscopic enucleation of esophageal leiomyoma is a feasible, simple, and safe procedure. Thoracoscopy in the prone position with CO2 insufflation allows the use of usual technique of intubation and also provides optimal operative field. The advantages of the thoracoscopic approach are less postoperative discomfort and lower risk of complications from open thoracotomy (especially pulmonary).

Relationship between regional lung compliance and ventilation homogeneity in the supine and prone position

BACKGROUND

The prone position (PP) improves ventilation homogeneity in acute respiratory distress syndrome. The aim of this study was to investigate whether the alleviation of ventilation inhomogeneity in PP was due to changes in regional lung compliance.

METHODS

Ten lung-lavaged piglets were mechanically ventilated in supine position (SP) and in PP. In each position, positive end-expiratory pressure (PEEP) was reduced from 20 to 6 cmH(2)O in steps of 2 cmH(2)O every 10 min after full lung recruitment. Respiratory mechanics, blood gas, haemodynamic data and whole-lung computed tomography scans were recorded at each PEEP. The compliances of normally aerated (C(normal)) and newly recruited (C(recruited)) lung regions were calculated. Open lung PEEP (OL-PEEP) was defined as the lowest PEEP to maintain full lung recruitment.

RESULTS

At OL-PEEP, PP significantly increased normally aerated lung regions, decreased poorly aerated and hyperinflated lung regions and decreased tidal recruitment and hyperinflation. C(normal) was significantly reduced in PP compared with SP (12.8 ± 4.2 ml/cmH(2)O vs. 20.1 ± 6.2 ml/cmH(2)O, P < 0.001), whereas C(recruited) was increased in PP (13.9 ± 3.9 ml/cmH(2)O vs. 9.4 ± 2.4 ml/cmH(2)O, P < 0.001). C(normal) was correlated with hyperinflated lung regions at end-expiration (rho = 0.67) and end-inspiration (rho = 0.56) at OL-PEEP. C(recruited) was correlated with normally (r(2) = 0.36) and poorly aerated lung regions (rho = -0.58) at OL-PEEP.

CONCLUSION

This surfactant-depleted model shows that the improvement of ventilation homogeneity in PP is related to an increase in C(recruited) and a decrease in C(normal).

Prone positioning improves oxygenation in adult burn patients with severe acute respiratory distress syndrome

BACKGROUND

Prone positioning (PP) improves oxygenation and may provide a benefit in patients with acute respiratory distress syndrome (ARDS). This approach adds significant challenges to patients in intensive care by limiting access to the endotracheal or tracheostomy tube and vascular access. PP also significantly complicates burn care by making skin protection and wound care more difficult. We hypothesize that PP improves oxygenation and can be performed safely in burn patients with ARDS.

METHODS

PP was implemented in a burn intensive care unit for 18 patients with severe refractory ARDS. The characteristics of these patients were retrospectively reviewed to evaluate the impact of PP on Pao2:FiO2 ratio (PFR) during the first 48 hours of therapy. Each patient was considered his or her own control before initiation of PP, and trends in PFR were evaluated with one-way analysis of variance. Secondary measures of complications and mortality were also evaluated.

RESULTS

Mean PFR before PP was 87 (± 38) with a mean sequential organ failure assessment score of 11 (± 4). PFR improved during 48 hours in 12 of 14 survivors (p < 0.05). Mean PFR was 133 (± 77) immediately after PP, 165 (± 118) at 6 hours, 170 (± 115) at 12 hours, 214 (± 126) at 24 hours, 236 (± 137) at 36 hours, and 210 (± 97) at 48 hours. At each measured time interval except the last, PFR significantly improved. There were no unintended extubations. Facial pressure ulcers developed in four patients (22%). Overall, 14 survived 48 hours (78%), 12 survived 28 days (67%), and six survived to hospital discharge (33%).

CONCLUSIONS

PP improves oxygenation in burn patients with severe ARDS and was safely implemented in a burn intensive care unit. Mortality in this population remains high, warranting investigation into additional complementary rescue therapies.

LEVEL OF EVIDENCE

Therapeutic study, level IV.

Imaging regional PAO2 and gas exchange

Several methods allow regional gas exchange to be inferred from imaging of regional ventilation and perfusion (V/Q) ratios. Each method measures slightly different aspects of gas exchange and has inherent advantages and drawbacks that are reviewed. Single photon emission computed tomography can provide regional measure of ventilation and perfusion from which regional V/Q ratios can be derived. PET methods using inhaled or intravenously administered nitrogen-13 provide imaging of both regional blood flow, shunt, and ventilation. Electric impedance tomography has recently been refined to allow simultaneous measurements of both regional ventilation and blood flow. MRI methods utilizing hyperpolarized helium-3 or xenon-129 are currently being refined and have been used to estimate local PaO(2) in both humans and animals. Microsphere methods are included in this review as they provide measurements of regional ventilation and perfusion in animals. One of their advantages is their greater spatial resolution than most imaging methods and the ability to use them as gold standards against which new imaging methods can be tested. In general, the reviewed methods differ in characteristics such as spatial resolution, possibility of repeated measurements, radiation exposure, availability, expensiveness, and their current stage of development.

Positron emission tomography: a tool for better understanding of ventilator-induced and acute lung injury

PURPOSE OF REVIEW

PET has recently gained traction among several groups of investigators as an imaging tool to study lung pathophysiology in vivo noninvasively on a regional basis. This review aims to present the major findings of PET studies on acute lung injury (ALI) and ventilator-induced lung injury (VILI) with a perspective relevant to the physiologist-intensivist.

RECENT FINDINGS

Using various tracers, PET has been used to investigate the relationship between the distributions of pulmonary perfusion, ventilation and aeration, and the effect of positive end-expiratory pressure, recruitment maneuvers, prone positioning, and endotoxin on these distributions in ALI. More recently, PET with 2-[18F]fluoro-2-deoxy-D-glucose has been used to measure regional neutrophil metabolic activation in ALI and VILI. Because gas exchange impairment and inflammation are two hallmarks of ALI and VILI, these studies have provided significant insights into the pathophysiology of these conditions.

SUMMARY

PET is a versatile imaging tool for physiologic investigation. By imaging the regional effects of interventions commonly performed in critically ill patients with ALI, PET has improved our understanding of the mechanism by which such interventions can exert their positive or negative effects as well as of the pathophysiology of ALI and VILI.

Lateral position with the remaining lung uppermost improves matching of pulmonary ventilation and perfusion in pneumonectomized pigs

BACKGROUND

Pneumonectomy is a major surgery. Severe hypoxemia sometimes occurs after pneumonectomy. Effective gas exchange depends on perfect pulmonary ventilation (V˙(A)) and perfusion (Q˙) matching. The effect of position on V˙(A)/Q˙ matching after pneumonectomy is not clear. We therefore conducted this study to examine the effects of supine, left lateral decubitus (LLD), and right lateral decubitus (RLD) positions on V˙(A)/Q˙ matching and gas exchange after pneumonectomy in a porcine model.

METHODS

Twelve pigs were anesthetized and mechanically ventilated; six pigs received right pneumonectomy and six pigs received left pneumonectomy. The positions of the pigs were changed to supine, LLD, and RLD in random order after pneumonectomy. We applied intravenous and aerosolized high-resolution fluorescent microsphere technique (FMT) to mark V˙(A) and Q˙ in conjunction with arterial blood gas analysis to study these variables at different positions. Mechanical ventilation was kept constant throughout.

RESULTS

Different positions after pneumonectomy lead to significant changes in heterogeneity and matching of V˙(A)/Q˙. In right pneumonectomized pigs, the highest PaO(2), lowest V˙(A)/Q˙heterogeneity, and highest matching of V˙(A)/Q˙ was in RLD. In left pneumonectomized pigs, the highest PaO(2), lowest V˙(A)/Q˙ heterogeneity, and highest matching of V˙(A)/Q˙ was in LLD.

CONCLUSIONS

The lateral position with the remaining lung uppermost leads to the highest V˙(A)/Q˙ matching and best gas exchange after pneumonectomy.

Electrical impedance tomography compared to positron emission tomography for the measurement of regional lung ventilation: an experimental study

Introduction

Electrical impedance tomography (EIT), which can assess regional lung ventilation at the bedside, has never been compared with positron-emission tomography (PET), a gold-standard to quantify regional ventilation. This experiment systematically compared both techniques in injured and non-injured lungs.

Methods

The study was performed in six mechanically ventilated female piglets. In normal lungs, tidal volume (V_T) was randomly changed to 6, 8, 10 and 15 ml/kg on zero end-expiratory pressure (ZEEP), then, at V_T 10 ml/kg, positive end-expiratory pressure (PEEP) was randomly changed to 5, 10 and 15 cmH₂O. Afterwards, acute lung injury (ALI) was subsequently created in three animals by injecting 3 ml/kg hydrochloric acid into the trachea. Then at PEEP 5 cmH₂O, V_T was randomly changed to 8 and 12 ml/kg and PEEP of 10 and 15 cmH₂O applied at V_T 10 ml/kg. EIT and PET examinations were performed simultaneously. EIT ventilation (V_TEIT) and lung volume (V_L) were measured in the anterior and posterior area of each lung. On the same regions of interest, ventilation (V_PET) and aerated lung volume (VA_atten) were determined with PET.

Results

On ZEEP, V_TEIT and V_PET significantly correlated for global (V_TEIT = VPET - 2E-13, R² = 0.95, P < 0.001) and regional (V_TEIT = 0.81V_PET+7.65, R² = 0.63, P < 0.001) ventilation over both conditions. For ALI condition, corresponding R² were 0.91 and 0.73 (P < 0.01). Bias was = 0 and limits of agreement were -37.42 and +37.42 ml/min for global ventilation over both conditions. These values were 0.04 and -29.01 and +29.08 ml/min, respectively, for regional ventilation. Significant correlations were also found between V_L and VA_atten for global (V_L = VA_atten+1E-12, R² = 0.93, P < 0.0001) and regional (V_L = 0.99VA_atten+0.92, R² = 0.65, P < 0.001) volume. For ALI condition, corresponding R² were 0.94 (P < 0.001) and 0.54 (P < 0.05). Bias was = 0 and limits of agreement ranged -38.16 and +38.16 ml for global ventilation over both conditions. These values were -0.24 and -31.96 to +31.48 ml, respectively, for regional ventilation.

Conclusions

Regional lung ventilation and volume were accurately measured with EIT in healthy and injured lungs and validated by simultaneous PET imaging.

Prone position prevents regional alveolar hyperinflation and mechanical stress and strain in mild experimental acute lung injury

Prone position may delay the development of ventilator-induced lung injury (VILI), but the mechanisms require better elucidation. In experimental mild acute lung injury (ALI), arterial oxygen partial pressure (Pa O2), lung mechanics and histology, inflammatory markers [interleukin (IL)-6 and IL-1 beta], and type III procollagen (PCIII) mRNA expressions were analysed in supine and prone position. Wistar rats were randomly divided into two groups. In controls, saline was intraperitoneally injected while ALI was induced by paraquat. After 24-h, the animals were mechanically ventilated for 1-h in supine or prone positions. In ALI, prone position led to a better blood flow/tissue ratio both in ventral and dorsal regions and was associated with a more homogeneous distribution of alveolar aeration/tissue ratio reducing lung static elastance and viscoelastic pressure, and increasing end-expiratory lung volume and Pa O2. PCIII expression was higher in the ventral than dorsal region in supine position, with no regional changes in inflammatory markers. In conclusion, prone position may protect the lungs against VILI, thus reducing pulmonary stress and strain.