Spatial distribution of ventilation and perfusion in anesthetized dogs in lateral postures

Effects of Body Position and Hypovolemia on the Regional Distribution of Pulmonary Perfusion During One-Lung Ventilation in Endotoxemic Pigs

Background: The incidence of hypoxemia during one-lung ventilation (OLV) is as high as 10%. It is also partially determined by the distribution of perfusion. During thoracic surgery, different body positions are used, such as the supine, semilateral, lateral, and prone positions, with such positions potentially influencing the distribution of perfusion. Furthermore, hypovolemia can impair hypoxic vasoconstriction. However, the effects of body position and hypovolemia on the distribution of perfusion remain poorly defined. We hypothesized that, during OLV, the relative perfusion of the ventilated lung is higher in the lateral decubitus position and that hypovolemia impairs the redistribution of pulmonary blood flow. Methods: Sixteen juvenile pigs were anesthetized, mechanically ventilated, submitted to a right-sided thoracotomy, and randomly assigned to one of two groups: (1) intravascular normovolemia or (2) intravascular hypovolemia, as achieved by drawing ~25% of the estimated blood volume (n = 8/group). Furthermore, to mimic thoracic surgery inflammatory conditions, Escherichia coli lipopolysaccharide was continuously infused at 0.5 μg kg^-1 h^-1. Under left-sided OLV conditions, the animals were further randomized to one of the four sequences of supine, left semilateral, left lateral, and prone positioning. Measurements of pulmonary perfusion distribution with fluorescence-marked microspheres, ventilation distribution by electrical impedance tomography, and gas exchange were then performed during two-lung ventilation in a supine position and after 30 min in each position and intravascular volume status during OLV. Results: During one-lung ventilation, the relative perfusion of the ventilated lung was higher in the lateral than the supine position. The relative perfusion of the non-ventilated lung was lower in the lateral than the supine and prone positions and in semilateral compared with the prone position. During OLV, the highest arterial partial pressure of oxygen/inspiratory fraction of oxygen (PaO₂/F _I O ₂) was achieved in the lateral position as compared with all the other positions. The distribution of perfusion, ventilation, and oxygenation did not differ significantly between normovolemia and hypovolemia. Conclusions: During one-lung ventilation in endotoxemic pigs, the relative perfusion of the ventilated lung and oxygenation were higher in the lateral than in the supine position and not impaired by hypovolemia.

Lung lobe torsion in 15 dogs: Peripheral band sign on ultrasound

The diagnosis of lung lobe torsion in dogs is usually based on radiological, endoscopic, and CT features. Few ultrasonographic descriptions have been published. The purpose of this multicenter, retrospective, and prospective observational study was to investigate the presence of a hypoechoic area forming a pulmonary band or line at the periphery of the twisted lobe on ultrasonography and assess its significance by comparing it to CT and histological findings. Fifteen dogs with lung lobe torsion confirmed surgically or postmortem were included. All had received ultrasonography and CT examinations; 13 had additional histopathological examination performed. In 14 cases, thoracic ultrasonography revealed a peripheral hypoechoic band, overlying areas of scattered, hyperreflecting interfaces in the affected lobe. On CT, central emphysema was surrounded by a peripheral, soft tissue attenuation band, affecting the periphery in 14 cases. No band was observed in one case, in which the lobe was entirely consolidated. Histological examination yielded a comparable peripheral band, consisting of a thickened visceral pleura with or without hemorrhagic necrosis of the underlying pulmonary parenchyma. This peripheral band may be related to the specific fractal organization of airways and vessels, which plays an important role in lung perfusion and ventilation and makes the lung periphery more prone to ischemia. Our findings suggest that the presence of a peripheral hypoechoic band, associated with central emphysema in a noncollapsed lung lobe on ultrasonography, is suggestive of compromised blood supply and air flow, and lung lobe torsion should therefore be suspected.

Ventilation/Perfusion Relationships and Gas Exchange: Measurement Approaches

Ventilation-perfusion ( V ˙ A / Q ˙ ) matching, the regional matching of the flow of fresh gas to flow of deoxygenated capillary blood, is the most important mechanism affecting the efficiency of pulmonary gas exchange. This article discusses the measurement of V ˙ A / Q ˙ matching with three broad classes of techniques: (i) those based in gas exchange, such as the multiple inert gas elimination technique (MIGET); (ii) those derived from imaging techniques such as single-photon emission computed tomography (SPECT), positron emission tomography (PET), magnetic resonance imaging (MRI), computed tomography (CT), and electrical impedance tomography (EIT); and (iii) fluorescent and radiolabeled microspheres. The focus is on the physiological basis of these techniques that provide quantitative information for research purposes rather than qualitative measurements that are used clinically. The fundamental equations of pulmonary gas exchange are first reviewed to lay the foundation for the gas exchange techniques and some of the imaging applications. The physiological considerations for each of the techniques along with advantages and disadvantages are briefly discussed. © 2020 American Physiological Society. Compr Physiol 10:1155-1205, 2020.

Effect of recumbency and body condition score on open-lung positive end-expiratory pressure and respiratory system compliance following a stepwise lung recruitment manoeuvre in healthy dogs during general anaesthesia

The aim was to assess the effects of recumbency and body condition score (BCS) on open-lung positive end-expiratory pressure (OL-PEEP) and quasistatic respiratory system compliance (Crs) following stepwise lung recruitment manoeuvre (RM) in healthy dogs under general anaesthesia. Thirty-four dogs were anaesthetised and mechanically ventilated (tidal volume of 10 mL/kg) without PEEP for 1 min (baseline). A stepwise RM was then performed and the individual OL-PEEP was subsequently applied. The Crs was registered at baseline and every 10-min for 50 min after RM. Dogs were classified into either dorsal or lateral recumbency groups, and as normal (score 4-5/9) or high (≥6/9) BCS groups. The OL-PEEP was higher in lateral than in dorsal recumbency (P = .002), but differences were not observed between normal and high BCS (P = .865). The Crs was increased from baseline at all time points after RM in all groups. The Crs did not differ between dorsally and laterally recumbent dogs at any time point. However, the baseline Crs was significantly lower in dogs with a high BCS than in those with a normal BCS (P < .001); therefore, the absolute change from baseline was considered when comparing Crs after the RM and it was similar in both BCS groups. In conclusion, in anaesthetised healthy dogs the OL-PEEP following RM was lower when dogs were positioned in dorsal than in lateral recumbency. The Crs after RM remained unchanged regardless of the dogs' recumbency. A stepwise RM followed by OL-PEEP could compensate for the potential negative impact of moderately increased BCS on Crs.

Supine, prone, right and left gravitational effects on human pulmonary circulation

BACKGROUND

Body position can be optimized for pulmonary ventilation/perfusion matching during surgery and intensive care. However, positional effects upon distribution of pulmonary blood flow and vascular distensibility measured as the pulmonary blood volume variation have not been quantitatively characterized. In order to explore the potential clinical utility of body position as a modulator of pulmonary hemodynamics, we aimed to characterize gravitational effects upon distribution of pulmonary blood flow, pulmonary vascular distension, and pulmonary vascular distensibility.

METHODS

Healthy subjects (n = 10) underwent phase contrast cardiovascular magnetic resonance (CMR) pulmonary artery and vein flow measurements in the supine, prone, and right/left lateral decubitus positions. For each lung, blood volume variation was calculated by subtracting venous from arterial flow per time frame.

RESULTS

Body position did not change cardiac output (p = 0.84). There was no difference in blood flow between the superior and inferior pulmonary veins in the supine (p = 0.92) or prone body positions (p = 0.43). Compared to supine, pulmonary blood flow increased to the dependent lung in the lateral positions (16-33%, p = 0.002 for both). Venous but not arterial cross-sectional vessel area increased in both lungs when dependent compared to when non-dependent in the lateral positions (22-27%, p ≤ 0.01 for both). In contrast, compared to supine, distensibility increased in the non-dependent lung in the lateral positions (68-113%, p = 0.002 for both).

CONCLUSIONS

CMR demonstrates that in the lateral position, there is a shift in blood flow distribution, and venous but not arterial blood volume, from the non-dependent to the dependent lung. The non-dependent lung has a sizable pulmonary vascular distensibility reserve, possibly related to left atrial pressure. These results support the physiological basis for positioning patients with unilateral pulmonary pathology with the "good lung down" in the context of intensive care. Future studies are warranted to evaluate the diagnostic potential of these physiological insights into pulmonary hemodynamics, particularly in the context of non-invasively characterizing pulmonary hypertension.

Ventilation/Perfusion Matching: Of Myths, Mice, and Men

Despite a huge range in lung size between species, there is little measured difference in the ability of the lung to provide a well-matched air flow (ventilation) to blood flow (perfusion) at the gas exchange tissue. Here, we consider the remarkable similarities in ventilation/perfusion matching between species through a biophysical lens and consider evidence that matching in large animals is dominated by gravity but in small animals by structure.

Assessment Of Changes In Regional Xenon-Ventilation, Perfusion, And Ventilation-Perfusion Mismatch Using Dual-Energy Computed Tomography After Pharmacological Treatment In Patients With Chronic Obstructive Pulmonary Disease: Visual And Quantitative Analysis

Purpose

To assess changes in regional ventilation (V), perfusion (Q), and V-Q mismatch in patients with chronic obstructive pulmonary disease (COPD) after pharmacologic treatment using combined xenon-enhanced V and iodine-enhanced Q dual-energy CT (DECT).

Patients and methods

Combined V and Q DECT were performed at baseline and after three-month pharmacologic treatment in 52 COPD patients. Anatomically co-registered virtual non-contrast images, V, Q, and V/Q_ratio maps were obtained. V/Q pattern was visually determined to be matched, mismatched, or reversed-mismatched and compared with the regional parenchymal disease patterns of each segment. DECT parameters for V, Q, and V-Q imbalance were quantified.

Results

The parenchymal patterns on CT were not changed at follow-up. The segments with matched V/Q pattern were increased (80.2% to 83.6%) as the segments with reversed-mismatched V/Q pattern were decreased with improving ventilation (17.6% to 13.8%) after treatment. Changes of V/Q patterns were mostly observed in segments with bronchial wall thickening. Compared with patients without bronchial wall thickening, the quantified DECT parameters of V-Q imbalance were significantly improved in patients with bronchial wall thickening (p < 0.05). Changes in forced expiratory volume in one second after treatment were correlated with changes in the quantified DECT parameters (r = 0.327–0.342 or r = −0.406 and −0.303; p < 0.05).

Conclusion

DECT analysis showed that the V-Q imbalance was improved after the pharmacological treatment in COPD patients, although the parenchymal disease patterns remained unchanged. This improvement of V-Q imbalance may occur mostly in the areas with bronchial wall thickening.

Systemic air embolism depicted on systematic whole thoracic CT acquisition after percutaneous lung biopsy: Incidence and risk factors

OBJECTIVES

To evaluate the incidence and risk factors of systemic air embolism (SAE) depicted on systematic whole thoracic CT performed after percutaneous lung biopsy.

METHODS

A total of 559 CT-guided lung biopsies performed between April 2014 and May 2016 were retrospectively evaluated. SAE was defined by the presence of air in the aorta or left cardiac cavities seen on whole thorax CT images acquired after needle withdrawal. Analyzed data focused on patient (age, sex, spirometry data, emphysema on CT, therapeutics received), target lesion (location, depth, size and feature) and procedure (patient position, length of intrapulmonary needle path, number of pleural passes and of biopsy samples, operator's experience). A regression logistic model was used to identify risk factors of SAE.

RESULTS

SAE was observed after 27 of the 559 lung biopsies, corresponding to a radiological incidence of 4.8% (95%CI: 3.3-7.0). Clinical incidence was 0.17% (n = 1). For 21/27 patients (78%), a targeted acquisition in the nodule area would not have included the cardiac cavities meaning SAE would have been missed. On multivariate analysis, the independent risk factors were needle path length through ventilated lung (OR: 1.13, 95%CI: 1.02-1.25, p = 0.024), number of samples (OR: 1.48, 95%CI: 1.01-2.17, p = 0.046) and prone position (OR: 3.12, 95%CI: 1.11-8.31, p = 0.031) or right-sided lateral decubitus (OR: 6.15, 95%CI: 1.66-22.85, p = 0.005).

CONCLUSIONS

Asymptomatic systemic air embolism can be depicted in almost 5% of post biopsy CT examinations, when they are not limited to the targeted nodule area but include the entire thorax.

Impact of four different recumbencies on the distribution of ventilation in conscious or anaesthetized spontaneously breathing beagle dogs: An electrical impedance tomography study

The aim was to examine the effects of recumbency and anaesthesia on distribution of ventilation in beagle dogs using Electrical Impedance Tomography (EIT). Nine healthy beagle dogs, aging 3.7±1.7 (mean±SD) years and weighing 16.3±1.6 kg, received a series of treatments in a fixed order on a single occasion. Conscious dogs were positioned in right lateral recumbency (RLR) and equipped with 32 EIT electrodes around the thorax. Following five minutes of equilibration, two minutes of EIT recordings were made in each recumbency in the following order: RLR, dorsal (DR), left (LLR) and sternal (SR). The dogs were then positioned in RLR, premedicated (medetomidine 0.01, midazolam 0.1, butorphanol 0.1 mg kg^-1 iv) and pre-oxygenated. Fifteen minutes later anaesthesia was induced with 1 mg kg^-1 propofol iv and maintained with propofol infusion (0.1–0.2 mg kg^-1 minute^-1 iv). After induction, the animals were intubated and allowed to breathe spontaneously (FIO₂ = 1). Recordings of EIT were performed again in four recumbencies similarly to conscious state. Centre of ventilation (COV) and global inhomogeneity (GI) index were calculated from the functional EIT images. Repeated-measures ANOVA and Bonferroni tests were used for statistical analysis (p < 0.05). None of the variables changed in the conscious state. During anaesthesia left-to-right COV increased from 46.8±2.8% in DR to 49.8±2.9% in SR indicating a right shift, and ventral-to-dorsal COV increased from 49.8±1.7% in DR to 51.8±1.1% in LLR indicating a dorsal shift in distribution of ventilation. Recumbency affected distribution of ventilation in anaesthetized but not in conscious dogs. This can be related to loss of respiratory muscle tone (e.g. diaphragm) and changes in thoracic shape. Changing position of thoraco-abdominal organs under the EIT belt should be considered as alternative explanation of these findings.

Successful 1:1 proportion ventilation with a unique device for independent lung ventilation using a double-lumen tube without complications in the supine and lateral decubitus positions. A pilot study

INTRODUCTION

Adequate blood oxygenation and ventilation/perfusion matching should be main goal of anaesthetic and intensive care management. At present, one of the methods of improving gas exchange restricted by ventilation/perfusion mismatching is independent ventilation with two ventilators. Recently, however, a unique device has been developed, enabling ventilation of independent lungs in 1:1, 2:1, 3:1, and 5:1 proportions. The main goal of the study was to evaluate the device's utility, precision and impact on pulmonary mechanics. Secondly- to measure the gas distribution in supine and lateral decubitus position.

MATERIALS AND METHODS

69 patients who underwent elective thoracic surgery were eligible for the study. During general anaesthesia, after double lumen tube intubation, the aforementioned control system was placed between the anaesthetic machine and the patient. In the supine and lateral decubitus (left/right) positions, measurements of conventional and independent (1:1 proportion) ventilation were performed separately for each lung, including the following: tidal volume, peak pressure and dynamic compliance.

RESULTS

Our results show that conventional ventilation using Robertshaw tube in the supine position directs 47% of the tidal volume to the left lung and 53% to the right lung. Furthermore, in the left lateral position, 44% is directed to the dependent lung and 56% to the non-dependent lung. In the right lateral position, 49% is directed to the dependent lung and 51% to the non-dependent lung. The control system positively affected non-dependent and dependent lung ventilation by delivering equal tidal volumes into both lungs with no adverse effects, regardless of patient's position.

CONCLUSIONS

We report that gas distribution is uneven during conventional ventilation using Robertshaw tube in the supine and lateral decubitus positions. However, this recently released control system enables precise and safe independent ventilation in the supine and the left and right lateral decubitus positions.

Regional lung volume differences between the side-lying and semi-prone positions

[Purpose] This study aimed to clarify the differences in regional lung volume between the semi-prone (Sim's position) and side-lying position, and the optimal position for increasing lung volume. [Methods] Measurements were performed in both positions on both sides. Sim's position was inclined 45° forward from the side-lying position. A 1.5-T system with a fast advanced spin-echo sequence in the coronal plane was used for magnetic resonance imaging. [Results] The two positions did not significantly differ in total lung capacity and its subdivisions on both sides, except the left lung in the right side-lying position and right Sim's position. In the nondependent lung, the percentage lung volume of the dorsal segment was significantly higher in the right Sim's position than in the right side-lying position. However, no significant difference was observed between the left side-lying and left Sim's position. [Conclusion] The heart was displaced ventrally by gravity in Sim's position and leaned on the ventral parapet. The spaces for the expansion of the ventral and dorsal segments of the lung were decreased and increased in Sim's position, respectively. With a nondependent left lung, the increase in the percentage lung volume of the dorsal segment was greater in Sim's position than in the side-lying position.

Effects of a 1:1 inspiratory to expiratory ratio on respiratory mechanics and oxygenation during one-lung ventilation in patients with low diffusion capacity of lung for carbon monoxide: a crossover study

STUDY OBJECTIVE

To investigate the effects of a 1:1 inspiratory-to-expiratory (I:E) ventilation ratio on oxygenation and respiratory mechanics during one-lung ventilation (OLV) in patients with low diffusion capacity of lung for carbon monoxide (DLCO).

DESIGN

Prospective, randomized, crossover study.

SETTING

Operating room, university hospital.

PATIENTS

Twenty-six patients with a preoperative DLCO less than 80% who were scheduled for lung lobectomy requiring OLV under general anesthesia.

INTERVENTIONS

In the first group (n = 13), OLV was begun with a 1:1 I:E ratio, which was switched to a 1:2 I:E ratio after 30 minutes. In the second group (n = 13), the modes of ventilation were performed in the opposite order. Pressure-controlled ventilation with 5 cm H2O of positive end-expiratory pressure and a tidal volume of 5 to 8 mL/kg was applied during OLV.

MEASUREMENTS

Arterial and central venous blood gas analyses were recorded and used to calculate intrapulmonary shunt fraction and physiologic dead space. These measurements were taken at 4 time points: 10 minutes after two-lung ventilation in the lateral decubitus position, 30 minutes after initiation of OLV, 30 minutes after switching the I:E ratio, and 10 minutes after two-lung ventilation was resumed.

MAIN RESULTS

There was no difference in arterial oxygen tension during OLV between the 2 groups (P = .429). Arterial carbon dioxide tension and peak airway pressure were lower in the 1:1 group than in the 1:2 group (P = .003; P = .008). Physiologic dead space was also decreased in the 1:1 I:E ratio group (P = .003). Mean airway pressure and dynamic compliance were higher in the 1:1 group (P = .003; P = .007).

CONCLUSIONS

Pressure-controlled ventilation with a 1:1 I:E ventilation ratio did not improve oxygenation in patients with low DLCO during OLV compared with a 1:2 I:E ventilation ratio. However, it did provide benefits in terms of respiratory mechanics and increased the efficiency of alveolar ventilation during OLV.

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.

A comparison of conventional surfactant treatment and partial liquid ventilation on the lung volume of injured ventilated small lungs

As an alternative to surfactant therapy (ST), partial liquid ventilation (PLV) with perfluorocarbons (PFC) has been considered as a treatment for acute lung injury (ALI) in newborns. The instilled PFC is much heavier than the instilled surfactant and the aim of this study was to investigate whether PLV, compared to ST, increases the end-expiratory volume of the lung (VL). Fifteen newborn piglets (age <12 h, mean weight 678 g) underwent saline lung lavage to achieve a surfactant depletion. Thereafter animals were randomized to PLV (n = 8), receiving PFC PF5080 (3M, Germany) at 30 mL kg(-1), and ST (n = 7) receiving 120 mg Curosurf®. Blood gases, hemodynamics and static compliance were measured initially (baseline), immediately after ALI, and after 240 min mechanical ventilation with either technique. Subsequently all piglets were killed; the lungs were removed in toto and frozen in liquid N2. After freeze-drying the lungs were cut into lung cubes (LCs) with edge lengths of 0.7 cm, to calculate VL. All LCs were weighed and the density of the dried lung tissue was calculated. No statistically significant differences between treatment groups PLV and ST (means ± SD) were noted in body weight (676 ± 16 g versus 679 ± 17 g; P = 0.974) or lung dry weight (1.64 ± 0.29 g versus 1.79 ± 0.48 g; P = 0.48). Oxygenation index and ventilatory efficacy index did not differ significantly between both groups at any time. VL (34.28 ± 6.13 mL versus 26.22 ± 8.1 mL; P < 0.05) and the density of the dried lung tissue (48.07 ± 5.02 mg mL(-1) versus 69.07 ± 5.30 mg mL(-1); P < 0.001), however, differed significantly between the PLV and ST groups. A 4 h PLV treatment of injured ventilated small lungs increased VL by 30% and decreased lung density by 31% compared to ST treatment, indicating greater lung distension after PLV compared to ST.

One-lung ventilation and arterial oxygenation

PURPOSE OF REVIEW

Hypoxemia during one-lung ventilation (OLV) has become less common; however, it may still occur in about 10% of cases. We review recent developments which may affect the incidence and treatment of hypoxemia during OLV.

RECENT FINDINGS

Changes in surgical techniques are affecting oxygenation during OLV. The increased use of the supine position may adversely affect the prevalence of hypoxemia, whereas the increased application of thoracoscopic techniques is limiting the treatment options. Treatment options such as global or selective recruitment maneuvers and drug effects of dexmedetomidine and epoprostenol on arterial oxygenation during OLV are discussed. Capnometry prior to, or early during OLV, may in fact be able to predict the degree of hypoxemia during OLV. Persistent controversies surrounding the effect of epidural anesthesia, ventilatory modalities and gravity are reviewed.

SUMMARY

Interesting concepts have emerged from case reports and small studies on the treatment and prediction of hypoxemia during OLV. Definitive studies on the most effective ventilatory mode remain elusive. End-organ effects of OLV are an exciting new concept that may shape clinical practice and research going forward.

The physical basis of ventilator-induced lung injury

Although mechanical ventilation (MV) is a life-saving intervention for patients with acute respiratory distress syndrome (ARDS), it can aggravate or cause lung injury, known as ventilator-induced lung injury (VILI). The biophysical characteristics of heterogeneously injured ARDS lungs increase the parenchymal stress associated with breathing, which is further aggravated by MV. Cells, in particular those lining the capillaries, airways and alveoli, transform this strain into chemical signals (mechanotransduction). The interaction of reparative and injurious mechanotransductive pathways leads to VILI. Several attempts have been made to identify clinical surrogate measures of lung stress/strain (e.g., density changes in chest computed tomography, lower and upper inflection points of the pressure-volume curve, plateau pressure and inflammatory cytokine levels) that could be used to titrate MV. However, uncertainty about the topographical distribution of stress relative to that of the susceptibility of the cells and tissues to injury makes the existence of a single 'global' stress/strain injury threshold doubtful.

Gravity is an important determinant of oxygenation during one-lung ventilation

BACKGROUND

The role of gravity in the redistribution of pulmonary blood flow during one-lung ventilation (OLV) has been questioned recently. To address this controversial but clinically important issue, we used an experimental approach that allowed us to differentiate the effects of gravity from the effects of hypoxic pulmonary vasoconstriction (HPV) on arterial oxygenation during OLV in patients scheduled for thoracic surgery.

METHODS

Forty patients with chronic obstructive pulmonary disease scheduled for right lung tumour resection were randomized to undergo dependent (left) one-lung ventilation (D-OLV; n=20) or non-dependent (right) one-lung ventilation (ND-OLV; n=20) in the supine and left lateral positions. Partial pressure of arterial oxygen (PaO2) was measured as a surrogate for ventilation/perfusion matching. Patients were studied before surgery under closed chest conditions.

RESULTS

When compared with bilateral lung ventilation, both D-OLV and ND-OLV caused a significant and equal decrease in PaO(2) in the supine position. However, D-OLV in the lateral position was associated with a higher PaO2 as compared with the supine position [274.2 (77.6) vs. 181.9 (68.3) mmHg, P<0.01, analysis of variance (ANOVA)]. In contrast, in patients undergoing ND-OLV, PaO2 was always lower in the lateral as compared with the supine position [105.3 (63.2) vs. 187 (63.1) mmHg, P<0.01, ANOVA].

CONCLUSION

The relative position of the ventilated vs. the non-ventilated lung markedly affects arterial oxygenation during OLV. These data suggest that gravity affects ventilation-perfusion matching independent of HPV.

The physical basis of ventilator-induced lung injury

Although mechanical ventilation (MV) is a life-saving intervention for patients with acute respiratory distress syndrome (ARDS), it can aggravate or cause lung injury, known as ventilator-induced lung injury (VILI). The biophysical characteristics of heterogeneously injured ARDS lungs increase the parenchymal stress associated with breathing, which is further aggravated by MV. Cells, in particular those lining the capillaries, airways and alveoli, transform this strain into chemical signals (mechanotransduction). The interaction of reparative and injurious mechanotransductive pathways leads to VILI. Several attempts have been made to identify clinical surrogate measures of lung stress/strain (e.g., density changes in chest computed tomography, lower and upper inflection points of the pressure-volume curve, plateau pressure and inflammatory cytokine levels) that could be used to titrate MV. However, uncertainty about the topographical distribution of stress relative to that of the susceptibility of the cells and tissues to injury makes the existence of a single 'global' stress/strain injury threshold doubtful.

Pulmonary perfusion in the prone and supine postures in the normal human lung

Prone posture increases cardiac output and improves pulmonary gas exchange. We hypothesized that, in the supine posture, greater compression of dependent lung limits regional blood flow. To test this, MRI-based measures of regional lung density, MRI arterial spin labeling quantification of pulmonary perfusion, and density-normalized perfusion were made in six healthy subjects. Measurements were made in both the prone and supine posture at functional residual capacity. Data were acquired in three nonoverlapping 15-mm sagittal slices covering most of the right lung: central, middle, and lateral, which were further divided into vertical zones: anterior, intermediate, and posterior. The density of the entire lung was not different between prone and supine, but the increase in lung density in the anterior lung with prone posture was less than the decrease in the posterior lung (change: +0.07 g/cm(3) anterior, -0.11 posterior; P < 0.0001), indicating greater compression of dependent lung in supine posture, principally in the central lung slice (P < 0.0001). Overall, density-normalized perfusion was significantly greater in prone posture (7.9 +/- 3.6 ml.min(-1).g(-1) prone, 5.1 +/- 1.8 supine, a 55% increase; P < 0.05) and showed the largest increase in the posterior lung as it became nondependent (change: +71% posterior, +58% intermediate, +31% anterior; P = 0.08), most marked in the central lung slice (P < 0.05). These data indicate that central posterior portions of the lung are more compressed in the supine posture, likely by the heart and adjacent structures, than are central anterior portions in the prone and that this limits regional perfusion in the supine posture.

Distribution of blood flow and ventilation in the lung: gravity is not the only factor

Current textbooks in anaesthesia describe how gravity affects the regional distribution of ventilation and blood flow in the lung, in terms of vertical gradients of pleural pressure and pulmonary vascular pressures. This concept fails to explain some of the clinical features of disturbed lung function. Evidence now suggests that gravity has a less important role in the variation of regional distribution than structural features of the airways and blood vessels. We review more recent studies that used a variety of methods: external radioactive counters, measurements using inhaled and injected particles, and computer tomography scans. These give a higher spatial resolution of regional blood flow and ventilation. The matching between ventilation and blood flow in these small units of lung is considered; the effects of microgravity, increased gravity, and different postures are reviewed, and the application of these findings to conditions such as acute lung injury is discussed. Down to the scale of the acinus, there is considerable heterogeneity in the distribution of both ventilation and blood flow. However, the matching of blood flow with ventilation is well maintained and may result from a common pattern of asymmetric branching of the airways and blood vessels. Disruption of this pattern may explain impaired gas exchange after acute lung injury and explain how the prone position improves gas exchange.

Developmental signals do not further accentuate nonuniform postpneumonectomy compensatory lung growth

Mechanical forces imposed on lung tissue constitute major stimuli for normal lung development and postpneumonectomy (PNX) compensatory growth and remodeling. Superimposing developmental signals on PNX signals augments compensatory alveolar growth but exaggerates airway-parenchymal dissociation (i.e., dysanaptic lung growth); the latter tends to offset benefits derived from the former. In adult dogs after PNX, lobar expansion and growth of the remaining lobes were markedly non-uniform (Ravikumar et al. J Appl Physiol 97:1567-1574, 2004). We hypothesized that superimposing developmental and post-PNX signals further accentuates nonuniformity of lobar growth. We used high-resolution computed tomography (HRCT) to follow regional lung expansion and growth in foxhounds undergoing right PNX at 2.5 mo of age compared with litter-matched control (Sham) animals; scans were performed 4 and 10 mo following surgery, i.e., before and after somatic maturity. Air and tissue volumes were measured in each lobe; tissue volume estimated by HRCT includes air-free tissue and blood in small vessels <1 mm. Interlobar nonuniformity of tissue volume was absent at 4 mo but evident 10 mo after PNX; growth of the remaining left lower lobe gradually lagged behind other lobes. At maturity, nonuniformity of lobar growth in pneumonectomized puppies was similar to that previously reported in pneumonectomized adults. We conclude that superimposing developmental and post-PNX signals enhances some aspects of compensatory lung growth and remodeling without altering its nonuniform spatial distribution.

A System for Open-Access He Human Lung Imaging at Very Low Field

We describe a prototype system built to allow open-access very-low-field MRI of human lungs using laser-polarized (3)He gas. The system employs an open four-coil electromagnet with an operational B(0) field of 4 mT, and planar gradient coils that generate gradient fields up to 0.18 G/cm in the x and y direction and 0.41 G/cm in the z direction. This system was used to obtain (1)H and (3)He phantom images and supine and upright (3)He images of human lungs. We include discussion on challenges unique to imaging at 50 -200 kHz, including noise filtering and compensation for narrow-bandwidth coils.

Effect of pulmonary perfusion on the slopes of single-breath test of CO2

The objective of this study was to evaluate the effects of lung perfusion on the slopes of phases II ( SII) and III ( SIII) of a single-breath test of CO2 (SBT-CO2). Fourteen patients submitted to cardiac surgery were studied during weaning from cardiopulmonary bypass (CPB). Pump flow was decreased in 20% steps, from 100% (total CPB = 2.5 l·min−1·m−2) to 0%. This maneuver resulted in a progressive and opposite increase in pulmonary blood flow (PBF) while maintaining ventilator settings constant. SBT-CO2, respiratory, and hemodynamic variables remained unchanged before and after CPB, reflecting a constant condition at those stages. SIII was similar before and after CPB (19.6 ± 2.8 and 18.7 ± 2.1 mmHg/l, respectively). SIII was lowest during 20% PBF (8.6 ± 1.9 mmHg/l) and increased in proportion to PBF until exit from CPB (15.6 ± 2.2 mmHg/l; P < 0.05). Similarly, SII and the CO2 area under the curve increased from 163 ± 41 mmHg/l and 4.7 ± 0.6 ml, respectively, at 20% PBF to 313 ± 32 mmHg/l and 7.9 ± 0.6 ml ( P < 0.05) at CPB end. When SII and SIII were normalized by the mean percent expired CO2, they remained unchanged during the protocol. In summary, the changes in PBF affect the slopes of the SBT-CO2. Normalizing SII and SIII eliminated the effect of changes in the magnitude of PBF on the shape of the SBT-CO2 curve.

Cellular stress failure in ventilator-injured lungs

The clinical and experimental literature has unequivocally established that mechanical ventilation with large tidal volumes is injurious to the lung. However, uncertainty about the micromechanics of injured lungs and the numerous degrees of freedom in ventilator settings leave many unanswered questions about the biophysical determinants of lung injury. In this review we focus on experimental evidence for lung cells as injury targets and the relevance of these studies for human ventilator-associated lung injury. In vitro, the stress-induced mechanical interactions between matrix and adherent cells are important for cellular remodeling as a means for preventing compromise of cell structure and ultimately cell injury or death. In vivo, these same principles apply. Large tidal volume mechanical ventilation results in physical breaks in alveolar epithelial and endothelial plasma membrane integrity and subsequent triggering of proinflammatory signaling cascades resulting in the cytokine milieu and pathologic and physiologic findings of ventilator-associated lung injury. Importantly, though, alveolar cells possess cellular repair and remodeling mechanisms that in addition to protecting the stressed cell provide potential molecular targets for the prevention and treatment of ventilator-associated lung injury in the future.

Residual heterogeneity of intra- and interregional pulmonary perfusion in short-term microgravity

We hypothesized that the perfusion heterogeneity in the human, upright lung is determined by nongravitational more than gravitational factors. Twelve and six subjects were studied during two series of parabolic flights. We used cardiogenic oscillations of O2/SF6 as an indirect estimate of intraregional perfusion heterogeneity ( series 1) and phase IV amplitude (P4) as a indirect estimate of interregional perfusion heterogeneity ( series 2). A rebreathing-breath holding-expiration maneuver was performed. In flight, breath holding and expiration were performed either in microgravity (0 G) or in hypergravity. Controls were performed at normal gravity (1 G). In series 1, expiration was performed at 0 G. Cardiogenic oscillations of O2/SF6 were 19% lower when breath holding was performed at 0 G than when breath holding was performed at 1 G [means (SD): 1.7 (0.3) and 2.3 (0.6)% units] ( P = 0.044). When breath holding was performed at 1.8 G, values did not differ from 1-G control [2.6 (0.8)% units, P = 0.15], but they were 17% larger at 1.8 G than at 1 G. In series 2, expiration was performed at 1.7 G. P4 changed with gravity ( P < 0.001). When breath holding was performed at 0 G, P4 values were 45 (46)% of control. When breath holding was performed at 1.7 G, P4 values were 183 (101)% of control. We conclude that more than one-half of indexes of perfusion heterogeneity at 1 G are caused by nongravitational mechanisms.

Spontaneous breathing affects the spatial ventilation and perfusion distribution during mechanical ventilatory support

OBJECTIVE

In acute respiratory failure, gas exchange improves with spontaneous breathing during airway pressure release ventilation (APRV). The mechanisms for this improvement are not fully clear. We have shown that APRV with spontaneous breathing reopens nonaerated lung tissue in dorsal juxtadiaphragmatic regions. We hypothesized that spontaneous breathing during APRV may redistribute ventilation and perfusion toward these reopened regions.

DESIGN

Prospective, randomized, controlled study.

SETTING

Animal research laboratory

SUBJECTS

Twenty controlled mechanically ventilated pigs.

INTERVENTIONS

Lung injury was induced by injection of oleic acid into the central circulation; thereafter, pigs were randomized to APRV with or without spontaneous breathing. To induce spontaneous breathing during APRV with spontaneous breathing, the mechanical respiratory rate was decreased by 50% in this group.

MEASUREMENTS AND MAIN RESULTS

We measured respiratory mechanics, hemodynamics, gas exchange including the multiple inert gas elimination technique, and the spatial ventilation and perfusion distribution using single photon emission tomography. At similar minute ventilation and airway pressures, shunt remained stable during APRV with spontaneous breathing, whereas it increased during APRV without spontaneous breathing during the 2-hr study period (p = .006). Single photon emission tomography showed more ventilation (p < .001) and pulmonary blood (p < .025) flow in dorsal, juxtadiaphragmatic lung regions when spontaneous breathing was present.

CONCLUSIONS

The beneficial effects of spontaneous breathing on intrapulmonary shunt and oxygenation are explained both by increased ventilation of aerated dependent lung tissue and by opening up nonaerated tissue so that ventilation is distributed to a larger share of the lung. Redistribution of perfusion is possibly secondary to the altered ventilation. The overall effect is a more efficient use of available lung tissue for gas exchange.

Effect of posture on regional gas exchange in pigs

Although recent high-resolution studies demonstrate the importance of nongravitational determinants for both pulmonary blood flow and ventilation distributions, posture has a clear impact on whole lung gas exchange. Deterioration in arterial oxygenation with repositioning from prone to supine posture is caused by increased heterogeneity in the distribution of ventilation-to-perfusion ratios. This can result from increased heterogeneity in regional blood flow distribution, increased heterogeneity in regional ventilation distribution, decreased correlation between regional blood flow and ventilation, or some combination of the above (Wilson TA and Beck KC, J Appl Physiol 72: 2298-2304, 1992). We hypothesize that, although repositioning from prone to supine has relatively small effects on overall blood flow and ventilation distributions, regional changes are poorly correlated, resulting in regional ventilation-perfusion mismatch and reduction in alveolar oxygen tension. We report ventilation and perfusion distributions in seven anesthetized, mechanically ventilated pigs measured with aerosolized and injected microspheres. Total contributions of pulmonary structure and posture on ventilation and perfusion heterogeneities were quantified by using analysis of variance. Regional gradients of posture-mediated change in ventilation, perfusion, and calculated alveolar oxygen tension were examined in the caudocranial and ventrodorsal directions. We found that pulmonary structure was responsible for 74.0 +/- 4.7% of total ventilation heterogeneity and 63.3 +/- 4.2% of total blood flow heterogeneity. Posture-mediated redistribution was primarily oriented along the caudocranial axis for ventilation and along the ventrodorsal axis for blood flow. These mismatched changes reduced alveolar oxygen tension primarily in the dorsocaudal lung region.