Pulmonary perfusion is more uniform in the prone than in the supine position: scintigraphy in healthy humans

MR Perfusion Imaging of the Lung

Lung perfusion assessment is critical for diagnosing and monitoring a variety of respiratory conditions. MRI perfusion provides a radiation-free technique, making it an ideal choice for longitudinal imaging in younger populations. This review focuses on the techniques and applications of MRI perfusion, including contrast-enhanced (CE) MRI and non-CE methods such as arterial spin labeling (ASL), fourier decomposition (FD), and hyperpolarized 129-Xenon (129-Xe) MRI. ASL leverages endogenous water protons as tracers for a non-invasive measure of lung perfusion, while FD offers simultaneous measurements of lung perfusion and ventilation, enabling the generation of ventilation/perfusion mapsHyperpolarized 129-Xe MRI emerges as a novel tool for assessing regional gas exchange in the lungs. Despite the promise of MRI perfusion techniques, challenges persist, including competition with other imaging techniques and the need for additional validation and standardization. In conditions such as cystic fibrosis and lung cancer, MRI has displayed encouraging results, whereas in diseases like chronic obstructive pulmonary disease, further validation remains necessary. In conclusion, while MRI perfusion techniques hold immense potential for a comprehensive, non-invasive assessment of lung function and perfusion, their broader clinical adoption hinges on technological advancements, collaborative research, and rigorous validation.

Prone Positioning and Molecular Biomarkers in COVID and Non-COVID ARDS: A Narrative Review

Prone positioning (PP) represents a therapeutic intervention with the proven capacity of ameliorating gas exchanges and ventilatory mechanics indicated in acute respiratory distress syndrome (ARDS). When PP is selectively applied to moderate-severe cases of ARDS, it sensitively affects clinical outcomes, including mortality. After the COVID-19 outbreak, clinical application of PP peaked worldwide and was applied in 60% of treated cases, according to large reports. Research on this topic has revealed many physiological underpinnings of PP, focusing on regional ventilation redistribution and the reduction of parenchymal stress and strain. However, there is a lack of evidence on biomarkers behavior in different phases and phenotypes of ARDS. Patients response to PP are, to date, decided on PaO2/FiO2 ratio improvement, whereas scarce data exist on biomarker tracking during PP. The purpose of this review is to explore current evidence on the clinical relevance of biomarkers in the setting of moderate-severe ARDS of different etiologies (i.e., COVID and non-COVID-related ARDS). Moreover, this review focuses on how PP may modulate biomarkers and which biomarkers may have a role in outcome prediction in ARDS patients.

The Pulmonary Vasculature

The pulmonary circulation is a low-pressure, low-resistance circuit whose primary function is to deliver deoxygenated blood to, and oxygenated blood from, the pulmonary capillary bed enabling gas exchange. The distribution of pulmonary blood flow is regulated by several factors including effects of vascular branching structure, large-scale forces related to gravity, and finer scale factors related to local control. Hypoxic pulmonary vasoconstriction is one such important regulatory mechanism. In the face of local hypoxia, vascular smooth muscle constriction of precapillary arterioles increases local resistance by up to 250%. This has the effect of diverting blood toward better oxygenated regions of the lung and optimizing ventilation-perfusion matching. However, in the face of global hypoxia, the net effect is an increase in pulmonary arterial pressure and vascular resistance. Pulmonary vascular resistance describes the flow-resistive properties of the pulmonary circulation and arises from both precapillary and postcapillary resistances. The pulmonary circulation is also distensible in response to an increase in transmural pressure and this distention, in addition to recruitment, moderates pulmonary arterial pressure and vascular resistance. This article reviews the physiology of the pulmonary vasculature and briefly discusses how this physiology is altered by common circumstances.

Prone Vs. Supine Position Ventilation in Intubated COVID-19 Patients: A Systematic Review and Meta-Analysis

Whether prone positioning of patients undergoing mechanical ventilation for COVID-19 pneumonia has benefits over supine positioning is not clear. We conducted a systematic review with meta-analysis to determine whether prone versus supine positioning during ventilation resulted in different outcomes for patients with COVID-19 pneumonia. We searched Ovid Medline, Embase, and Web of Science for prospective and retrospective studies up through April 2023. We included studies that compared outcomes of patients with COVID-19 after ventilation in prone and supine positions. The primary outcomes were three mortality measures: hospital, overall, and intensive care unit (ICU). Secondary outcomes were mechanical ventilation days, intensive care unit (ICU) length of stay, and hospital length of stay. We conducted risk of bias analysis and used meta-analysis software to analyze results. Mean difference (MD) was used for continuous data, and odds ratio (OR) was used for dichotomous data, both with 95% CIs. Significant heterogeneity (I²) was considered if I² was >50%. A statistically significant result was considered if the p-value was <0.05. Of 1787 articles identified, 93 were retrieved, and seven retrospective cohort studies encompassing 5216 patients with COVID-19 were analyzed. ICU mortality was significantly higher in the prone group (OR 2.22, 95% CI 1.43-3.43; p=0.0004). No statistically significant difference was observed between prone and supine groups for hospital mortality (OR, 0.95; 95% CI, 0.66-1.37; p=0.78) or overall mortality (OR, 1.08; 95% CI, 0.72-1.64; p=0.71). Studies that analyzed primary outcomes had significant heterogeneity. Hospital length of stay was significantly higher in the prone than in the supine group (MD, 6.06; 95 % CI, 3.15-8.97; p<0.0001). ICU length of stay and days of mechanical ventilation did not differ between the two groups. In conclusion, mechanical ventilation with prone positioning for all patients with COVID-19 pneumonia may not provide a mortality benefit over supine positioning.

Postoperative Respiratory Failure and Advanced Ventilator Settings

Postoperative respiratory failure has a multifactorial etiology, of which atelectasis is the most common mechanism. Its injurious effects are magnified by surgical inflammation, high driving pressures, and postoperative pain. Chest physiotherapy and noninvasive ventilation are good options to prevent progression of respiratory failure. Acute respiratory disease syndrome is a late and severe finding, which is associated with high morbidity and mortality. If present, proning is a safe, effective, and underutilized therapy. Extracorporeal membrane oxygenation is an option only when traditional supportive measures have failed.

Prone position: how understanding and clinical application of a technique progress with time

Historical background

The prone position was first proposed on theoretical background in 1974 (more advantageous distribution of mechanical ventilation). The first clinical report on 5 ARDS patients in 1976 showed remarkable improvement of oxygenation after pronation. 

Pathophysiology

The findings in CT scans enhanced the use of prone position in ARDS patients. The main mechanism of the improved gas exchange seen in the prone position is nowadays attributed to a dorsal ventilatory recruitment, with a substantially unchanged distribution of perfusion. Regardless of the gas exchange, the primary effect of the prone position is a more homogenous distribution of ventilation, stress and strain, with similar size of pulmonary units in dorsal and ventral regions. In contrast, in the supine position the ventral regions are more expanded compared with the dorsal regions, which leads to greater ventral stress and strain, induced by mechanical ventilation.

Outcome in ARDS

The number of clinical studies paralleled the evolution of the pathophysiological understanding. The first two clinical trials in 2001 and 2004 were based on the hypothesis that better oxygenation would lead to a better survival and the studies were more focused on gas exchange than on lung mechanics. The equations better oxygenation = better survival was disproved by these and other larger trials (ARMA trial). However, the first studies provided signals that some survival advantages were possible in a more severe ARDS, where both oxygenation and lung mechanics were impaired. The PROSEVA trial finally showed the benefits of prone position on mortality supporting the thesis that the clinical advantages of prone position, instead of improved gas exchange, were mainly due to a less harmful mechanical ventilation and better distribution of stress and strain. In less severe ARDS, in spite of a better gas exchange, reduced mechanical stress and strain, and improved oxygenation, prone position was ineffective on outcome.

Prone position and COVID-19

The mechanisms of oxygenation impairment in early COVID-19 are different than in typical ARDS and relate more on perfusion alteration than on alveolar consolidation/collapse, which are minimal in the early phase. Bronchial shunt may also contribute to the early COVID-19 hypoxemia. Therefore, in this phase, the oxygenation improvement in prone position is due to a better matching of local ventilation and perfusion, primarily caused by the perfusion component. Unfortunately, the conditions for improved outcomes, i.e. a better distribution of stress and strain, are almost absent in this phase of COVID-19 disease, as the lung parenchyma is nearly fully inflated. Due to some contradictory results, further studies are needed to better investigate the effect of prone position on outcome in COVID-19 patients.

Graphical Abstract

Lobar pulmonary perfusion quantification with dual-energy CT angiography: Interlobar variability and relationship with regional clot burden in pulmonary embolism

Purpose

Semi-automated lobar segmentation tools enable an anatomical assessment of regional pulmonary perfusion with Dual-Energy CTA (DE-CTA). We aimed to quantify lobar pulmonary perfusion with DE-CTA, analyze the perfusion distribution among the pulmonary lobes in subjects without cardiopulmonary diseases and assess the correlation between lobar perfusion and regional endoluminal clots in patients with acute pulmonary embolism (PE).

Methods

We evaluated 151 consecutive subjects with suspected PE and without cardiopulmonary comorbidities. DE-CTA derived perfused blood volume (PBV) of each pulmonary lobe was measured applying a semi-automated lobar segmentation technique. In patients with PE, blood clot location was assessed, and CT-based vascular obstruction index of each lobe (CTOI_lobe) was calculated and classified into three groups: CTOI_lobe= 0, low CTOI_lobe (1–50%) and high CTOI_lobe (>50%).

Results

Among patients without PE (103/151, 68.2%), median lobar PBV was 13.7% (IQR 10.2–18.0%); the right middle lobe presented lower PBV when compared to all the other lobes (p < .001). In patients with PE (48/151, 31.8%), lobar PBV was 12.6% (IQR 9.6–15.7%), 13.7% (IQR 10.1–16.7%) and 6.5% (IQR 5.1–10.2%) in the lobes with CTOI_lobe= 0, low CTOI_lobe and high CTOI_lobe scores, respectively, with a significantly decreased PBV in the lobes with high CTOI_lobe score (p < .001). ROC analysis of lobar PBV for prediction of high CTOI_lobe score revealed AUC of 0.847 (95%CI 0.785–0.908).

Conclusion

Pulmonary perfusion was heterogeneously distributed along the pulmonary lobes in patients without cardiopulmonary diseases. In patients with PE, the lobes with high vascular obstruction score (CTOI_lobe> 50%) presented a decreased lobar perfusion.

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Semi-automated tools enable assessment of lobar perfusion with Dual-Energy CTA.

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The pulmonary perfusion is heterogeneously distributed along the pulmonary lobes.

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Lobar perfusion was decreased only in the lobes with high vascular obstruction index.

Mechanisms of oxygenation responses to proning and recruitment in COVID-19 pneumonia

PURPOSE

This study aimed at investigating the mechanisms underlying the oxygenation response to proning and recruitment maneuvers in coronavirus disease 2019 (COVID-19) pneumonia.

METHODS

Twenty-five patients with COVID-19 pneumonia, at variable times since admission (from 1 to 3 weeks), underwent computed tomography (CT) lung scans, gas-exchange and lung-mechanics measurement in supine and prone positions at 5 cmH₂O and during recruiting maneuver (supine, 35 cmH₂O). Within the non-aerated tissue, we differentiated the atelectatic and consolidated tissue (recruitable and non-recruitable at 35 cmH₂O of airway pressure). Positive/negative response to proning/recruitment was defined as increase/decrease of PaO₂/FiO₂. Apparent perfusion ratio was computed as venous admixture/non aerated tissue fraction.

RESULTS

The average values of venous admixture and PaO₂/FiO₂ ratio were similar in supine-5 and prone-5. However, the PaO₂/FiO₂ changes (increasing in 65% of the patients and decreasing in 35%, from supine to prone) correlated with the balance between resolution of dorsal atelectasis and formation of ventral atelectasis (p = 0.002). Dorsal consolidated tissue determined this balance, being inversely related with dorsal recruitment (p = 0.012). From supine-5 to supine-35, the apparent perfusion ratio increased from 1.38 ± 0.71 to 2.15 ± 1.15 (p = 0.004) while PaO₂/FiO₂ ratio increased in 52% and decreased in 48% of patients. Non-responders had consolidated tissue fraction of 0.27 ± 0.1 vs. 0.18 ± 0.1 in the responding cohort (p = 0.04). Consolidated tissue, PaCO₂ and respiratory system elastance were higher in patients assessed late (all p < 0.05), suggesting, all together, "fibrotic-like" changes of the lung over time.

CONCLUSION

The amount of consolidated tissue was higher in patients assessed during the third week and determined the oxygenation responses following pronation and recruitment maneuvers.

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.

Awake prone positioning in patients with hypoxemic respiratory failure due to COVID-19: the PROFLO multicenter randomized clinical trial

Background

The effect of awake prone positioning on intubation rates is not established. The aim of this trial was to investigate if a protocol for awake prone positioning reduces the rate of endotracheal intubation compared with standard care among patients with moderate to severe hypoxemic respiratory failure due to COVID-19.

Methods

We conducted a multicenter randomized clinical trial. Adult patients with confirmed COVID-19, high-flow nasal oxygen or noninvasive ventilation for respiratory support and a PaO₂/FiO₂ ratio ≤ 20 kPa were randomly assigned to a protocol targeting 16 h prone positioning per day or standard care. The primary endpoint was intubation within 30 days. Secondary endpoints included duration of awake prone positioning, 30-day mortality, ventilator-free days, hospital and intensive care unit length of stay, use of noninvasive ventilation, organ support and adverse events. The trial was terminated early due to futility.

Results

Of 141 patients assessed for eligibility, 75 were randomized of whom 39 were allocated to the control group and 36 to the prone group. Within 30 days after enrollment, 13 patients (33%) were intubated in the control group versus 12 patients (33%) in the prone group (HR 1.01 (95% CI 0.46–2.21), P = 0.99). Median prone duration was 3.4 h [IQR 1.8–8.4] in the control group compared with 9.0 h per day [IQR 4.4–10.6] in the prone group (P = 0.014). Nine patients (23%) in the control group had pressure sores compared with two patients (6%) in the prone group (difference − 18% (95% CI − 2 to − 33%); P = 0.032). There were no other differences in secondary outcomes between groups.

Conclusions

The implemented protocol for awake prone positioning increased duration of prone positioning, but did not reduce the rate of intubation in patients with hypoxemic respiratory failure due to COVID-19 compared to standard care.

Trial registration

ISRCTN54917435. Registered 15 June 2020 (https://doi.org/10.1186/ISRCTN54917435).

Supplementary Information

The online version contains supplementary material available at 10.1186/s13054-021-03602-9.

Awake prone positioning for hypoxaemic respiratory failure: past, COVID-19 and perspectives

Prone positioning reduces mortality in the management of intubated patients with moderate-to-severe acute respiratory distress syndrome. It allows improvement in oxygenation by improving ventilation/perfusion ratio mismatching.

Because of its positive physiological effects, prone positioning has also been tested in non-intubated, spontaneously breathing patients, or “awake” prone positioning. This review provides an update on awake prone positioning for hypoxaemic respiratory failure, in both coronavirus disease 2019 (COVID-19) and non-COVID-19 patients. In non-COVID-19 acute respiratory failure, studies are limited to a few small nonrandomised studies and involved patients with different diseases. However, results have been appealing with regard to oxygenation improvement, especially when combined with noninvasive ventilation or high-flow nasal cannula.

The recent COVID-19 pandemic has led to a major increase in hospitalisations for acute respiratory failure. Awake prone positioning has been used with the aim to prevent intensive care unit admission and mechanical ventilation. Prone positioning in conscious, non-intubated COVID-19 patients is used in emergency departments, medical wards and intensive care units.

Several trials reported an improvement in oxygenation and respiratory rate during prone positioning, but impacts on clinical outcomes, particularly on intubation rates and survival, remain unclear. Tolerance of prolonged prone positioning is an issue. Larger controlled, randomised studies are underway to provide results concerning clinical benefit and define optimised prone positioning regimens.

In intubated ARDS patients, prone positioning reduces mortality. On spontaneously breathing patients, oxygenation improved during awake prone positioning. Relevant data on clinical outcomes are expected, especially on COVID-19 patients.https://bit.ly/2PU3B6v

Changes in respiratory mechanics of artificial pneumothorax two-lung ventilation in video-assisted thoracoscopic esophagectomy in prone position

We aimed to clarify the changes in respiratory mechanics and factors associated with them in artificial pneumothorax two-lung ventilation in video-assisted thoracoscopic esophagectomy in the prone position (PP-VATS-E) for esophageal cancer. Data of patients with esophageal cancer, who underwent PP-VATs-E were retrospectively analyzed. Our primary outcome was the change in the respiratory mechanics after intubation (T1), in the prone position (T2), after initiation of the artificial pneumothorax two-lung ventilation (T3), at 1 and 2 h (T4 and T5), in the supine position (T6), and after laparoscopy (T7). The secondary outcome was identifying factors affecting the change in dynamic lung compliance (Cdyn). Sixty-seven patients were included. Cdyn values were significantly lower at T3, T4, and T5 than at T1 (p < 0.001). End-expiratory flow was significantly higher at T4 and T5 than at T1 (p < 0.05). Body mass index and preoperative FEV_1.0% were found to significantly influence Cdyn reduction during artificial pneumothorax and two-lung ventilation (OR [95% CI]: 1.29 [1.03–2.24] and 0.20 (0.05–0.44); p = 0.010 and p = 0.034, respectively]. Changes in driving pressure were nonsignificant, and hypoxemia requiring treatment was not noted. This study suggests that in PP-VATs-E, artificial pneumothorax two-lung ventilation is safer for the management of anesthesia than conventional one-lung ventilation (UMIN Registry: 000042174).

Management of ARDS - What Works and What Does Not

Acute respiratory distress syndrome (ARDS) is a clinically and biologically heterogeneous disorder associated with a variety of disease processes that lead to acute lung injury with increased non-hydrostatic extravascular lung water, reduced compliance, and severe hypoxemia. Despite significant advances, mortality associated with this syndrome remains high. Mechanical ventilation remains the most important aspect of managing patients with ARDS. An in-depth knowledge of lung protective ventilation, optimal PEEP strategies, modes of ventilation and recruitment maneuvers are essential for ventilatory management of ARDS. Although, the management of ARDS is constantly evolving as new studies are published and guidelines being updated; we present a detailed review of the literature including the most up-to-date studies and guidelines in the management of ARDS. We believe this review is particularly helpful in the current times where more than half of the acute care hospitals lack in-house intensivists and the burden of ARDS is at large.

Effects of Prone Ventilation on Oxygenation, Inflammation, and Lung Infiltrates in COVID-19 Related Acute Respiratory Distress Syndrome: A Retrospective Cohort Study

Patients receiving mechanical ventilation for coronavirus disease 2019 (COVID-19) related, moderate-to-severe acute respiratory distress syndrome (CARDS) have mortality rates between 76-98%. The objective of this retrospective cohort study was to identify differences in prone ventilation effects on oxygenation, pulmonary infiltrates (as observed on chest X-ray (CXR)), and systemic inflammation in CARDS patients by survivorship and to identify baseline characteristics associated with survival after prone ventilation. The study cohort included 23 patients with moderate-to-severe CARDS who received prone ventilation for ≥16 h/day and was segmented by living status: living (n = 6) and deceased (n = 17). Immediately after prone ventilation, PaO2/FiO2 improved by 108% (p < 0.03) for the living and 150% (p < 3 × 10-4) for the deceased. However, the 48 h change in lung infiltrate severity in gravity-dependent lung zones was significantly better for the living than for the deceased (p < 0.02). In CXRs of the lower lungs before prone ventilation, we observed 5 patients with confluent infiltrates bilaterally, 12 patients with ground-glass opacities (GGOs) bilaterally, and 6 patients with mixed infiltrate patterns; 80% of patients with confluent infiltrates were alive vs. 8% of patients with GGOs. In conclusion, our small study indicates that CXRs may offer clinical utility in selecting patients with moderate-to-severe CARDS who will benefit from prone ventilation. Additionally, our study suggests that lung infiltrate severity may be a better indicator of patient disposition after prone ventilation than PaO2/FiO2.

Successful awake proning is associated with improved clinical outcomes in patients with COVID-19: single-centre high-dependency unit experience

The SARS-CoV-2 can lead to severe illness with COVID-19. Outcomes of patients requiring mechanical ventilation are poor. Awake proning in COVID-19 improves oxygenation, but on data clinical outcomes is limited. This single-centre retrospective study aimed to assess whether successful awake proning of patients with COVID-19, requiring respiratory support (continuous positive airways pressure (CPAP) or high-flow nasal oxygen (HFNO)) on a respiratory high-dependency unit (HDU), is associated with improved outcomes. HDU care included awake proning by respiratory physiotherapists. Of 565 patients admitted with COVID-19, 71 (12.6%) were managed on the respiratory HDU, with 48 of these (67.6%) requiring respiratory support. Patients managed with CPAP alone 22/48 (45.8%) were significantly less likely to die than patients who required transfer onto HFNO 26/48 (54.2%): CPAP mortality 36.4%; HFNO mortality 69.2%, (p=0.023); however, multivariate analysis demonstrated that increasing age and the inability to awake prone were the only independent predictors of COVID-19 mortality. The mortality of patients with COVID-19 requiring respiratory support is considerable. Data from our cohort managed on HDU show that CPAP and awake proning are possible in a selected population of COVID-19, and may be useful. Further prospective studies are required.

Using the prone position could help to combat the development of fast hypoxia in some patients with COVID-19

The world is facing an explosive COVID‐19 pandemic. Some cases rapidly develop deteriorating lung function, which causes deep hypoxaemia and requires urgent treatment. Many centres have started treating patients in the prone position, and oxygenation has improved considerably in some cases. Questions have been raised regarding the mechanisms behind this. The mini review provides some insights into the role of supine and prone body positions and summarises the latest understanding of the responsible mechanisms. The scope for discussion is outside the neonatal period and entirely based on experimental and clinical experiences related to adults. The human respiratory system is a complex interplay of many different variables. Therefore, this mini review has prioritised previous and ongoing research to find explanations based on three scientific areas: gravity, lung structure and fractal geometry and vascular regulation. It concludes that gravity is one of the variables responsible for ventilation/perfusion matching but in concert with lung structure and fractal geometry, ventilation and regulation of lung vascular tone. Since ventilation distribution does not change between supine and prone positions, the higher expression of nitric oxide in dorsal lung vessels than in ventral vessels is likely to be the most important mechanism behind enhanced oxygenation in the prone position.

A case of improved oxygenation in SARS-CoV-2 positive patient on nasal cannula undergoing prone positioning

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causing coronavirus disease 2019 (COVID-19) has resulted in significant morbidity and mortality worldwide. It has placed societal and financial burden on the globe. Its rapid progressions from mild URI symptoms to severe acute respiratory distress syndrome (ARDS) in a matter of days is the underlying reason as to why the world is struggling to keep up with ventilator production. In this case report, we went about proning a corona virus positive patient for 6-8hrs as a potential early intervention to prevent progression to ARDS. Our patient was initially in acute hypoxemic respiratory failure and placed on nasal cannula. He was started on hydroxychloroquine and azithromycin with no improvement of symptoms. However within the span of few hours of proning he experienced significant symptomatic relief with improvement of oxygenation. His oxygen saturation improved drastically and eventually was taken off of nasal cannula and discharged within span of one day of proning.

Inhibition of Constitutive Nitric Oxide Synthase Does Not Influence Ventilation-Perfusion Matching in Normal Prone Adult Sheep With Mechanical Ventilation

BACKGROUND

Local formation of nitric oxide in the lung induces vasodilation in proportion to ventilation and is a putative mechanism behind ventilation-perfusion matching. We hypothesized that regional ventilation-perfusion matching occurs in part due to local constitutive nitric oxide formation.

METHODS

Ventilation and perfusion were analyzed in lung regions (≈1.5 cm) before and after inhibition of constitutive nitric oxide synthase with N-nitro-L-arginine methyl ester (L-NAME) (25 mg/kg) in 7 prone sheep ventilated with 10 cm H2O positive end-expiratory pressure. Ventilation and perfusion were measured by the use of aerosolized fluorescent and infused radiolabeled microspheres, respectively. The animals were exsanguinated while deeply anesthetized; then, lungs were excised, dried at total lung capacity, and divided into cube units. The spatial location for each cube was tracked and fluorescence and radioactivity per unit weight determined.

RESULTS

After administration of L-NAME, pulmonary artery pressure increased from a mean of 16.6-23.6 mm Hg, P = .007 but PaO2, PaCO2, and SD log(V/Q) did not change. Distribution of ventilation was not influenced by L-NAME, but a small redistribution of perfusion from ventral to dorsal lung regions was observed. Perfusion to regions with the highest ventilation (fifth quintile of the ventilation distribution) remained unchanged after L-NAME.

CONCLUSIONS

We found minimal or no influence of constitutive nitric oxide synthase inhibition by L-NAME on the distributions of ventilation and perfusion, and ventilation-perfusion in prone, anesthetized, ventilated, and healthy adult sheep with normal gas exchange.

Evaluation of pulmonary perfusion by SPECT imaging using an endothelial cell tracer in supine humans and dogs

Background

Pulmonary perfusion is not spatially homogeneously distributed, and its variations could be of diagnostic value in lung vascular disease. PulmoBind is a ligand of the adrenomedullin receptor densely expressed in endothelial cells of lung capillaries. The aim of this study was to evaluate spatial distribution of human lung perfusion by using this novel molecular tracer of the pulmonary vascular endothelium.

Methods

Normal humans (n = 19) enrolled into the PulmoBind phase I trial were studied (Clinicaltrials.gov.NCT01539889). They were injected with ^99mTc-PulmoBind for SPECT imaging. Results were compared with ^99mTc-PulmoBind in quadruped mammals (dogs, n = 5). Imaging was performed in the supine position and distribution of activity was determined as a function of cumulative voxels along the different anatomical planes.

Results

PulmoBind uptake in humans was 58 ± 1 % (mean ± SEM) of the injected dose. Dorsal activity was 18.1 ± 2.1 % greater than ventral, and caudal activity was 25.7 ± 1.6 % greater than cranial. Lateral activity was only mildly higher than medial by 7.0 ± 1.0 %. In supine dogs, similar but higher PulmoBind gradients were present: dorsal 28.6 ± 2.5 %, caudal 34.1 ± 5.0 % and lateral 18.1 ± 2.0 %.

Conclusions

The perfused pulmonary circulation of supine humans, assessed by an adrenomedullin receptor ligand, is not homogeneously distributed with more prominent distribution in dorsal and caudal regions. It is qualitatively similar to a supine quadruped mammal confirming the presence of a microcirculatory gravitational perfusion gradient detectable with this tracer. Future studies are needed to determine if this novel endothelial cell tracer could be used to detect physiologic and pathologic variations of lung perfusion such as in pulmonary hypertension.

Clinical trial

ClinicalTrial.gov, NCT01539889

S2e guideline: positioning and early mobilisation in prophylaxis or therapy of pulmonary disorders : Revision 2015: S2e guideline of the German Society of Anaesthesiology and Intensive Care Medicine (DGAI)

The German Society of Anesthesiology and Intensive Care Medicine (DGAI) commissioneda revision of the S2 guidelines on "positioning therapy for prophylaxis or therapy of pulmonary function disorders" from 2008. Because of the increasing clinical and scientificrelevance the guidelines were extended to include the issue of "early mobilization"and the following main topics are therefore included: use of positioning therapy and earlymobilization for prophylaxis and therapy of pulmonary function disorders, undesired effects and complications of positioning therapy and early mobilization as well as practical aspects of the use of positioning therapy and early mobilization. These guidelines are the result of a systematic literature search and the subsequent critical evaluation of the evidence with scientific methods. The methodological approach for the process of development of the guidelines followed the requirements of evidence-based medicine, as defined as the standard by the Association of the Scientific Medical Societies in Germany. Recently published articles after 2005 were examined with respect to positioning therapy and the recently accepted aspect of early mobilization incorporates all literature published up to June 2014.

Perfluoropropane gas as a magnetic resonance lung imaging contrast agent in humans

BACKGROUND

Fluorine-enhanced MRI is a relatively inexpensive and straightforward technique that facilitates regional assessments of pulmonary ventilation. In this report, we assess its suitability through the use of perfluoropropane (PFP) in a cohort of human subjects with normal lungs and subjects with lung disease.

METHODS

Twenty-eight subjects between the ages of 18 and 71 years were recruited for imaging and were classified based on spirometry findings and medical history. Imaging was carried out on a Siemens TIM Trio 3T MRI scanner using two-dimensional, gradient echo, fast low-angle shot and three-dimensional gradient echo, volumetric, interpolated, breath-hold examination sequences for proton localizers and PFP functional scans, respectively. Respiratory waveforms and physiologic signals of interest were monitored throughout the imaging sessions. A region-growing algorithm was applied to the proton localizers to define the lung field of view for analysis of the PFP scans.

RESULTS

All subjects tolerated the gas mixture well with no adverse side effects. Images of healthy lungs demonstrated a homogeneous distribution of the gas with sufficient signal-to-noise ratios, while lung images from asthmatic and emphysematous lungs demonstrated increased heterogeneity and ventilation defects.

CONCLUSIONS

Fluorine-enhanced MRI using a normoxic PFP gas mixture is a well-tolerated, radiation-free technique for regionally assessing pulmonary ventilation. The inherent physical characteristics and applicability of the gaseous agent within a magnetic resonance setting facilitated a clear differentiation between normal and diseased lungs.

Thoracoscopic esophagectomy using prone positioning

Thoracotomic esophagectomy followed by cervical and abdominal procedures has been conventionally performed as the best curable operative procedure for treating invasive thoracic esophageal carcinoma. Despite improvements in the survival rate, the procedure is associated with significant operative morbidity and mortality rates due to the extreme invasiveness of an extensive dissection of the lymph nodes. Minimally invasive esophagectomy (MIE) was developed to reduce surgical invasiveness. Recently, the use of thoracoscopic esophagectomy performed in the prone position has stimulated new interest in minimally invasive approaches. However, the advantages and disadvantages of this technique are not well known. In this review, the literature to date, including series and comparative studies of minimally invasive esophagectomy performed in the prone position, is summarized, and the various lessons learned and controversies surrounding this technique are addressed.

Study of Stress Induced Failure of the Blood-gas Barrier and the Epithelial-epithelial Cells Connections of the Lung of the Domestic Fowl, Gallus gallus Variant Domesticus after Vascular Perfusion

Complete blood-gas barrier breaks (BGBBs) and epithelial-epithelial cells connections breaks (E-ECCBs) were enumerated in the lungs of free range chickens, Gallus gallus variant domesticus after vascular perfusion at different pressures. The E-ECCBs surpassed the BGBBs by a factor of ~2. This showed that the former parts of the gas exchange tissue were structurally weaker or more vulnerable to failure than the latter. The differences in the numbers of BGBBs and E-ECCBs in the different regions of the lung supplied with blood by the 4 main branches of the pulmonary artery (PA) corresponded with the diameters of the blood vessels, the angles at which they bifurcated from the PA, and the positions along the PA where they branched off. Most of the BGBBs and the E-ECCBs occurred in the regions supplied by the accessory- and the caudomedial branches: the former is the narrowest branch and the first blood vessel to separate from the PA while the latter is the most direct extension of the PA and is the widest. The E-ECCBs appeared to separate and fail from tensing of the blood capillary walls, as the perfusion- and intramural pressures increased. Compared to the mammalian lungs on which data are available, i.e., those of the rabbit, the dog, and the horse, the blood-gas barrier of the lung of free range chickens appears to be substantially stronger for its thinness.

Respiratory function during anesthesia: effects on gas exchange

Anaesthesia causes a respiratory impairment, whether the patient is breathing spontaneously or is ventilated mechanically. This impairment impedes the matching of alveolar ventilation and perfusion and thus the oxygenation of arterial blood. A triggering factor is loss of muscle tone that causes a fall in the resting lung volume, functional residual capacity. This fall promotes airway closure and gas adsorption, leading eventually to alveolar collapse, that is, atelectasis. The higher the oxygen concentration, the faster will the gas be adsorbed and the aleveoli collapse. Preoxygenation is a major cause of atelectasis and continuing use of high oxygen concentration maintains or increases the lung collapse, that typically is 10% or more of the lung tissue. It can exceed 25% to 40%. Perfusion of the atelectasis causes shunt and cyclic airway closure causes regions with low ventilation/perfusion ratios, that add to impaired oxygenation. Ventilation with positive end-expiratory pressure reduces the atelectasis but oxygenation need not improve, because of shift of blood flow down the lung to any remaining atelectatic tissue. Inflation of the lung to an airway pressure of 40 cmH2O recruits almost all collapsed lung and the lung remains open if ventilation is with moderate oxygen concentration (< 40%) but recollapses within a few minutes if ventilation is with 100% oxygen. Severe obesity increases the lung collapse and obstructive lung disease and one-lung anesthesia increase the mismatch of ventilation and perfusion. CO2 pneumoperitoneum increases atelectasis formation but not shunt, likely explained by enhanced hypoxic pulmonary vasoconstriction by CO2. Atelectasis may persist in the postoperative period and contribute to pneumonia.

Gas exchange under altered gravitational stress

Efficient gas exchange in the lung depends on the matching of ventilation and perfusion. However, the human lung is a readily deformable structure and as a result gravitational stresses generate gradients in both ventilation and perfusion. Nevertheless, the lung is capable of withstanding considerable change in the applied gravitational load before pulmonary gas exchange becomes impaired. The postural changes that are part of the everyday existence for most bipedal species are well tolerated, as is the removal of gravity (weightlessness). Increases in the applied gravitational load result only in a large impairment in pulmonary gas exchange above approximately three times that on the ground, at which point the matching of ventilation to perfusion is so impaired that efficient gas exchange is no longer possible. Much of the tolerance of the lung to alterations in gravitation stress comes from the fact that ventilation and perfusion are inextricably coupled. Deformations in the lung that alter ventilation necessarily alter perfusion, thus maintaining a degree of matching and minimizing the disruption in ventilation to perfusion ratio and thus gas exchange.

The gravitational distribution of ventilation-perfusion ratio is more uniform in prone than supine posture in the normal human lung

The gravitational gradient of intrapleural pressure is suggested to be less in prone posture than supine. Thus the gravitational distribution of ventilation is expected to be more uniform prone, potentially affecting regional ventilation-perfusion (Va/Q) ratio. Using a novel functional lung magnetic resonance imaging technique to measure regional Va/Q ratio, the gravitational gradients in proton density, ventilation, perfusion, and Va/Q ratio were measured in prone and supine posture. Data were acquired in seven healthy subjects in a single sagittal slice of the right lung at functional residual capacity. Regional specific ventilation images quantified using specific ventilation imaging and proton density images obtained using a fast gradient-echo sequence were registered and smoothed to calculate regional alveolar ventilation. Perfusion was measured using arterial spin labeling. Ventilation (ml·min(-1)·ml(-1)) images were combined on a voxel-by-voxel basis with smoothed perfusion (ml·min(-1)·ml(-1)) images to obtain regional Va/Q ratio. Data were averaged for voxels within 1-cm gravitational planes, starting from the most gravitationally dependent lung. The slope of the relationship between alveolar ventilation and vertical height was less prone than supine (-0.17 ± 0.10 ml·min(-1)·ml(-1)·cm(-1) supine, -0.040 ± 0.03 prone ml·min(-1)·ml(-1)·cm(-1), P = 0.02) as was the slope of the perfusion-height relationship (-0.14 ± 0.05 ml·min(-1)·ml(-1)·cm(-1) supine, -0.08 ± 0.09 prone ml·min(-1)·ml(-1)·cm(-1), P = 0.02). There was a significant gravitational gradient in Va/Q ratio in both postures (P < 0.05) that was less in prone (0.09 ± 0.08 cm(-1) supine, 0.04 ± 0.03 cm(-1) prone, P = 0.04). The gravitational gradients in ventilation, perfusion, and regional Va/Q ratio were greater supine than prone, suggesting an interplay between thoracic cavity configuration, airway and vascular tree anatomy, and the effects of gravity on Va/Q matching.

Imaging for lung physiology: what do we wish we could measure?

The role of imaging as a tool for investigating lung physiology is growing at an accelerating pace. Looking forward, we wished to identify unresolved issues in lung physiology that might realistically be addressed by imaging methods in development or imaging approaches that could be considered. The role of imaging is framed in terms of the importance of good spatial and temporal resolution and the types of questions that could be addressed as these technical capabilities improve. Recognizing that physiology is fundamentally a quantitative science, a recurring emphasis is on the need for imaging methods that provide reliable measurements of specific physiological parameters. The topics included necessarily reflect our perspective on what are interesting questions and are not meant to be a comprehensive review. Nevertheless, we hope that this essay will be a spur to physiologists to think about how imaging could usefully be applied in their research and to physical scientists developing new imaging methods to attack challenging questions imaging could potentially answer.

Influence of abdominal pressure on respiratory and abdominal organ function

PURPOSE OF REVIEW

Intra-abdominal hypertension (IAH) and abdominal compartment syndrome (ACS) have been realized as severe complications in the intensive care patient. Laparoscopic surgery in older and more obese patients increases the risk of IAH and ACS.

RECENT FINDINGS

The incidence of IAH may be larger than thought of being approximately one-third of mechanically ventilated intensive care patients. In shock/trauma, three-fourths of all patients may suffer from IAH. Kidney and liver may dysfunction and the gut barrier may be impeded, permitting spread of inflammation to other organs. IAH and ACS have an impact on respiratory mechanics and may impede ventilation and require higher ventilation pressures than under normal conditions. Prone position and alternating (asynchronous) ventilation may moderate the IAH. In addition, surgical decompression should be considered.

SUMMARY

In view of the frequent occurrence of IAH in intensive care, the need of better understanding of the mechanisms behind IAH is a prerequisite for better treatment. Respiratory mechanics are affected but may also indicate routes of ventilatory treatment to lower IAH.

Thoracoscopic esophagectomy in the prone position

BACKGROUND

Minimally invasive esophageal surgery has arisen in an attempt to reduce the significant complications associated with esophagectomy. Despite proposed technical and physiological advantages, the prone position technique has not been widely adopted. This article reviews the current status of prone thoracoscopic esophagectomy.

METHODS

A systematic literature search was performed to identify all published clinical studies related to prone esophagectomy. Medline, EMBASE and Google Scholar were searched using the keywords "prone," "thoracoscopic," and "esophagectomy" to identify articles published between January 1994 and September 2010. A critical review of these studies is given, and where appropriate the technique is compared to the more traditional minimally invasive technique utilising the left lateral decubitus position.

RESULTS

Twelve articles reporting the outcomes following prone thoracoscopic oesophagectomy were tabulated. These studies were all non-randomised single-centre prospective or retrospective studies of which four compared the technique to traditional minimally invasive surgery. Although prone esophagectomy is demonstrated as being both feasible and safe, there is no convincing evidence that it is superior to other forms of esophageal surgery. Most authors comment that the prone position is associated with superior surgical ergonomics and theoretically offers a number of physiological benefits.

CONCLUSION

The ideal approach within minimally invasive esophageal surgery continues to be a subject of debate since no single method has produced outstanding results. Further clinical studies are required to see whether ergonomic advantages of the prone position can be translated into improved patient outcomes.

Effects of posture on regional pulmonary blood flow in rats as measured by PET

Using small animal PET with (68)Ga-radiolabeled human albumin microspheres (Ga-68-microspheres), we investigated the effect of posture on regional pulmonary blood flow (PBF) in normal rats. This in vivo method is noninvasive and quantitative, and it allows for repeated longitudinal measurements. The purpose of the experiment was to quantify spatial differences in PBF in small animals in different postures. Two studies were performed in anesthetized, spontaneously breathing Wistar rats. Study 1 was designed to determine PBF in the prone and supine positions. Ga-68-microspheres were given to five prone and eight supine animals. We found that PBF increased in dorsal regions of supine animals (0.75) more than in prone animals (0.70; P = 0.037), according to a steeper vertical gradient of flow in supine than in prone animals. No differences in spatial heterogeneity were detected. Study 2 was designed to determine the effects of tissue distribution on PBF measurements. Because microspheres remained fixed in the lung, PET was performed on animals in the position in which they received Ga-68-microsphere injections and thereafter in the opposite posture. The distribution of PBF showed a preference for dorsal regions in both positions, but the distribution was dependent on the position during administration of the microspheres. We conclude that PET using Ga-68-microspheres can detect and quantify regional PBF in animals as small as the rat. PBF distributions differed between the prone and supine postures and were influenced by the distribution of lung tissue within the thorax.

Iloprost inhalation redistributes pulmonary perfusion and decreases arterial oxygenation in healthy volunteers

BACKGROUND

Previous studies have shown that ventilation-perfusion matching is improved in the prone as compared with that in the supine position. Regional differences in the regulation of vascular tone may explain this. We have recently demonstrated higher production of nitric oxide in dorsal compared with ventral human lung tissue. The purpose of the present study was to investigate regional differences in actions by another vasoactive mediator, namely prostacyclin. The effects on gas exchange and regional pulmonary perfusion in different body positions were investigated at increased prostacyclin levels by inhalation of a synthetic prostacyclin analogue and decreased prostacyclin levels by unselective cyclooxygenase (COX) inhibition.

METHODS

In 19 volunteers, regional pulmonary perfusion in the prone and supine position was assessed by single photon emission computed tomography using (99m)Tc macro-aggregated albumin before and after inhalation of iloprost, a stable prostacyclin analogue, or an intravenous infusion of a non-selective COX inhibitor, diclofenac. In addition, gas distribution was assessed in seven subjects using (99m)Tc-labelled ultra-fine carbon particles before and after iloprost inhalation in the supine position.

RESULTS

Iloprost inhalation decreased arterial PaO(2) in both prone (from 14.2+/-0.5 to 11.7+/-1.7 kPa, P<0.01) and supine (from 13.7+/-1.4 to 10.9+/-2.1 kPa, P<0.01) positions. Iloprost inhalation redistributed lung perfusion from non-dependent to dependent lung regions in both prone and supine positions, while ventilation in the supine position was distributed in the opposite direction. No significant effects of non-selective COX inhibition were found in this study.

CONCLUSIONS

Iloprost inhalation decreases arterial oxygenation and results in a more gravity-dependent pulmonary perfusion in both supine and prone positions in healthy humans.

Thoracoscopic esophagectomy for esophageal cancer: feasibility and safety of robotic assistance in the prone position

OBJECTIVE

To assess the feasibility and safety of robot-assisted thoracoscopic esophagectomy for esophageal cancer in the prone position.

METHODS

Twenty-one patients underwent robot-assisted thoracoscopic esophagectomy in the prone position by a surgical oncologist who had no prior experience with thoracoscopic esophagectomy. Hemodynamic and respiratory parameters were serially recorded to monitor changes in prone positioning.

RESULTS

All thoracoscopic procedures were completed with a robot-assisted technique followed by cervical esophagogastrostomy. R0 resection was achieved in 20 patients (95.2%), and the number of dissected nodes was 38.0 + or - 14.2. Robot console time was significantly reduced from 176.3 + or - 12.3 minutes in the initial 6 patients (group 1) to 81.7 + or - 16.5 minutes in the latter 15 patients (group 2) (P = .000). In group 2, there was less blood loss (P = .018), more patients could be extubated in the operating room (P = .004), and the number of dissected mediastinal nodes tended to be increased (P = .093). There was no incidence of pneumonia or 90-day mortality. Major complications included anastomotic leakage in 4 patients, vocal cord palsy in 6 patients, and intra-abdominal bleeding in 1 patient. The prone position led to an elevation of central venous pressure and mean pulmonary arterial pressure and a decrease in static lung compliance. However, cardiac index and mean arterial pressure were well maintained with the acceptable range of partial pressure of arterial oxygen and carbon dioxide.

CONCLUSION

Robotic assistance in the prone position is technically feasible and safe. Prone positioning was well tolerated, but preoperative risk assessment and meticulous anesthetic manipulation should be carried out.

Effect of recruitment and body positioning on lung volume and oxygenation in acute lung injury model

The mechanism of oxygenation improvement after recruitment manoeuvres or prone positioning in acute lung injury or acute respiratory distress syndrome is still unclear. We tried to determine the mechanism responsible for the effects of recruitment manoeuvres or prone positioning on lung aeration using a whole lung computed tomography scan in an oleic acid induced acute lung injury canine model. Twelve adult mongrel dogs were allocated into either the supine group (n=6) or the prone group (n=6). After the establishment of acute lung injury, three recruitment manoeuvres were performed at one-hour intervals. Haemodynamic and ventilatory variables, arterial blood gas analyses and CT scans of the whole lung were obtained 90 minutes after oleic acid injection and five minutes before and after each recruitment manoeuvre. Recruitment manoeuvres in the supine position improved oxygenation (P=0.025) that correlated with increase of the poorly- and well-aerated dorsal (dependent) lung volume (r=0.436, P=0.016). Prone positioning increased oxygenation (P=0.004) that also correlated with increase of the poorly- and well-aerated dorsal (nondependent) lung volume (r=0.787, P<0.001). However, the recruitment manoeuvre in the prone position had no effect on oxygenation despite an increase in ventral (dependent) lung volume. The increase in PaO2 after recruitment manoeuvres in the supine position or after prone positioning is related to the increase of the poorly- and well-aerated dorsal lung.

Hypoxic pulmonary vasoconstriction does not contribute to pulmonary blood flow heterogeneity in normoxia in normal supine humans

We hypothesized that some of the heterogeneity of pulmonary blood flow present in the normal human lung in normoxia is due to hypoxic pulmonary vasoconstriction (HPV). If so, mild hyperoxia would decrease the heterogeneity of pulmonary perfusion, whereas it would be increased by mild hypoxia. To test this, six healthy nonsmoking subjects underwent magnetic resonance imaging (MRI) during 20 min of breathing different oxygen concentrations through a face mask [normoxia, inspired O(2) fraction (Fi(O(2))) = 0.21; hypoxia, Fi(O(2)) = 0.125; hyperoxia, Fi(O(2)) = 0.30] in balanced order. Data were acquired on a 1.5-T MRI scanner during a breath hold at functional residual capacity from both coronal and sagittal slices in the right lung. Arterial spin labeling was used to quantify the spatial distribution of pulmonary blood flow in milliliters per minute per cubic centimeter and fast low-angle shot to quantify the regional proton density, allowing perfusion to be expressed as density-normalized perfusion in milliliters per minute per gram. Neither mean proton density [hypoxia, 0.46(0.18) g water/cm(3); normoxia, 0.47(0.18) g water/cm(3); hyperoxia, 0.48(0.17) g water/cm(3); P = 0.28] nor mean density-normalized perfusion [hypoxia, 4.89(2.13) ml x min(-1) x g(-1); normoxia, 4.94(1.88) ml x min(-1) x g(-1); hyperoxia, 5.32(1.83) ml x min(-1) x g(-1); P = 0.72] were significantly different between conditions in either imaging plane. Similarly, perfusion heterogeneity as measured by relative dispersion [hypoxia, 0.74(0.16); normoxia, 0.74(0.10); hyperoxia, 0.76(0.18); P = 0.97], fractal dimension [hypoxia, 1.21(0.04); normoxia, 1.19(0.03); hyperoxia, 1.20(0.04); P = 0.07], log normal shape parameter [hypoxia, 0.62(0.11); normoxia, 0.72(0.11); hyperoxia, 0.70(0.13); P = 0.07], and geometric standard deviation [hypoxia, 1.88(0.20); normoxia, 2.07(0.24); hyperoxia, 2.02(0.28); P = 0.11] was also not different. We conclude that HPV does not affect pulmonary perfusion heterogeneity in normoxia in the normal supine human lung.

Anaesthesia in the prone position

Prone positioning of patients during anaesthesia is required to provide operative access for a wide variety of surgical procedures. It is associated with predictable changes in physiology but also with a number of complications, and safe use of the prone position requires an understanding of both issues. We have reviewed the development of the prone position and its variants and the physiological changes which occur on prone positioning. The complications associated with this position and the published techniques for various practical procedures in this position will be discussed. The aim of this review is to identify the risks associated with prone positioning and how these risks may be anticipated and minimized.

Effect of body position on lung sounds in healthy young men

BACKGROUND

The effect of body position on the generation of abnormal respiratory sounds (eg, snoring and stridor) is well recognized. Postural effects on normal lung sounds have been studied in less detail but need to be clarified if respiratory acoustic measurements are to be used effectively in clinical practice.

METHODS

Lung sounds and airflow were recorded in six healthy male subjects. Two acoustic sensors were placed over corresponding sites of the right and left chest, first anteriorly and then on the back. Subjects were studied in sitting, supine, prone, and lateral decubitus positions. Lung sound intensity (LSI) was determined at flows of 0.4 to 0.6 L/s and 0.8 to 1.2 L/s within frequency bands of 150 to 300 Hz and 300 to 600 Hz.

RESULTS

LSI was greater over the dependent lungs in the lateral decubitus positions. In the sitting position, LSI was greater on the left compared with the right posterior lung at the same airflow within the same frequency bands. Compared with sitting, neither the supine nor prone positions caused a significant change in LSI.

CONCLUSIONS

Our study confirms previously reported asymmetries of normal lung sounds. The insignificant change of flow-specific LSI between the upright and horizontal positions in healthy subjects is encouraging for the clinical use of respiratory acoustic measurements. Further studies should address postural effects on lung sounds in patients with acute lung injury and other lung pathologies.