Visualization of alveolar recruitment in a porcine model of unilateral lung lavage using 3He-MRI

Electrical impedance tomography for non-invasive assessment of stroke volume variation in health and experimental lung injury

BACKGROUND

Functional imaging by thoracic electrical impedance tomography (EIT) is a non-invasive approach to continuously assess central stroke volume variation (SVV) for guiding fluid therapy. The early available data were from healthy lungs without injury-related changes in thoracic impedance as a potentially influencing factor. The aim of this study was to evaluate SVV measured by EIT (SVV_EIT) against SVV from pulse contour analysis (SVV_PC) in an experimental animal model of acute lung injury at different lung volumes.

METHODS

We conducted a randomized controlled trial in 30 anaesthetized domestic pigs. SVV_EIT was calculated automatically analysing heart-lung interactions in a set of pixels representing the aorta. Each initial analysis was performed automatically and unsupervised using predefined frequency domain algorithms that had not previously been used in the study population. After baseline measurements in normal lung conditions, lung injury was induced either by repeated broncho-alveolar lavage (n=15) or by intravenous administration of oleic acid (n=15) and SVV_EIT was remeasured.

RESULTS

The protocol was completed in 28 animals. A total of 123 pairs of SVV measurements were acquired. Correlation coefficients (r) between SVV_EIT and SVV_PC were 0.77 in healthy lungs, 0.84 after broncho-alveolar lavage, and 0.48 after lung injury from oleic acid.

CONCLUSIONS

EIT provides automated calculation of a dynamic preload index of fluid responsiveness (SVV_EIT) that is non-invasively derived from a central haemodynamic signal. However, alterations in thoracic impedance induced by lung injury influence this method.

Effects of pulmonary acid aspiration on the regional pulmonary blood flow within the first hour after injury: An observational study in rats

INTRODUCTION

Gastric aspiration events are recognized as a major cause of pneumonitis and the development of acute respiratory distress syndrome. The first peak in the inflammatory response has been observed one hour after acid-induced lung injury in rats. The spatial pulmonary blood flow (PBF) distribution after an acid aspiration event within this time frame has not been adequately studied. We determined therefore PBF pattern within the first hour after acid aspiration.

METHODS

Anesthetized, spontaneous breathing rats (n = 8) underwent unilateral endobronchial hydrochlorid acid instillation so that the PBF distributions between the injured and non-injured lungs could be compared. The signal intensity of the lung parenchyma after injury was measured by magnetic resonance tomography. PBF distribution was determined by measuring the concentration of [68Ga]-radiolabeled microspheres using positron emission tomography.

RESULTS

Following acid aspiration, magnetic resonance images revealed increased signal intensity in the injured regions accompanied by reduced oxygenation. PBF was increased in all injured lungs (171 [150; 196], median [25%; 75%]) compared to the blood flow in all uninjured lungs (141 [122; 159], P = 0.0078).

CONCLUSIONS

From the first minute until fifty minutes after acid-induced acute lung injury, the PBF was consistently increased in the injured lung. These blood flow elevation was accompanied by significant hypoxemia.

Monitoring Lung Volumes During Mechanical Ventilation

Respiratory inductive plethysmography (RIP) is a non-invasive method of measuring change in lung volume which is well-established as a monitor of tidal ventilation and thus respiratory patterns in sleep medicine. As RIP is leak independent, can measure end-expiratory lung volume as well as tidal volume and is applicable to both the ventilated and spontaneously breathing patient, there has been a recent interest in its use as a bedside tool in the intensive care unit.

Translational applications of hyperpolarized 3He and 129Xe

Clinical magnetic resonance imaging of the lung is technologically challenging, yet over the past two decades hyperpolarized noble gas ((3)He and (129)Xe) imaging has demonstrated the ability to measure multiple pulmonary functional biomarkers. There is a growing need for non-ionizing, non-invasive imaging techniques due to increased concern about cancer risk from ionizing radiation, but the translation of hyperpolarized gas imaging to the pulmonary clinic has been stunted by limited access to the technology. New developments may open doors to greater access and more translation to clinical studies. Here we briefly review a few translational applications of hyperpolarized gas MRI in the contexts of ventilation, diffusion, and dissolved-phase imaging, as well as comparing and contrasting (3)He and (129)Xe gases for these applications. Simple static ventilation MRI reveals regions of the lung not participating in normal ventilation, and these defects have been observed in many pulmonary diseases. Biomarkers related to airspace size and connectivity can be quantified by apparent diffusion coefficient measurements of hyperpolarized gas, and have been shown to be more sensitive to small changes in lung morphology than standard clinical pulmonary functional tests and have been validated by quantitative histology. Parameters related to gas uptake and exchange and lung tissue density can be determined using (129)Xe dissolved-phase MRI. In most cases functional biomarkers can be determined via MRI of either gas, but for some applications one gas may be preferred, such as (3)He for long-range diffusion measurements and (129)Xe for dissolved-phase imaging. Greater access to hyperpolarized gas imaging coupled with newly developing therapeutics makes pulmonary medicine poised for a potential revolution, further adding to the prospects of personalized medicine already evidenced by advancements in molecular biology. Hyperpolarized gas researchers have the opportunity to contribute to this revolution, particularly if greater clinical application of hyperpolarized gas imaging is realized.

Overview of current lung imaging in acute respiratory distress syndrome

Imaging plays a key role in the diagnosis and follow-up of acute respiratory distress syndrome (ARDS). Chest radiography, bedside lung ultrasonography and computed tomography scans can provide useful information for the management of patients and detection of prognostic factors. However, imaging findings are not specific and several possible differential diagnoses should be taken into account. Herein we will review the role of radiological techniques in ARDS, highlight the plain radiological and computed tomography findings according to the pathological stage of the disease (exudative, inflammatory and fibroproliferative), and summarise the main points for the differential diagnosis with cardiogenic oedema, which is still challenging in the acute stage.

Hyperpolarized gas diffusion MRI for the study of atelectasis and acute respiratory distress syndrome

Considerable uncertainty remains about the best ventilator strategies for the mitigation of atelectasis and associated airspace stretch in patients with acute respiratory distress syndrome (ARDS). In addition to several immediate physiological effects, atelectasis increases the risk of ventilator-associated lung injury, which has been shown to significantly worsen ARDS outcomes. A number of lung imaging techniques have made substantial headway in clarifying the mechanisms of atelectasis. This paper reviews the contributions of computed tomography, positron emission tomography, and conventional MRI to understanding this phenomenon. In doing so, it also reveals several important shortcomings inherent to each of these approaches. Once these shortcomings have been made apparent, we describe how hyperpolarized (HP) gas MRI--a technique that is uniquely able to assess responses to mechanical ventilation and lung injury in peripheral airspaces--is poised to fill several of these knowledge gaps. The HP-MRI-derived apparent diffusion coefficient (ADC) quantifies the restriction of (3) He diffusion by peripheral airspaces, thereby obtaining pulmonary structural information at an extremely small scale. Lastly, this paper reports the results of a series of experiments that measured ADC in mechanically ventilated rats in order to investigate (i) the effect of atelectasis on ventilated airspaces, (ii) the relationship between positive end-expiratory pressure (PEEP), hysteresis, and the dimensions of peripheral airspaces, and (iii) the ability of PEEP and surfactant to reduce airspace dimensions after lung injury. An increase in ADC was found to be a marker of atelectasis-induced overdistension. With recruitment, higher airway pressures were shown to reduce stretch rather than worsen it. Moving forward, HP MRI has significant potential to shed further light on the atelectatic processes that occur during mechanical ventilation.

Acute respiratory distress syndrome induction by pulmonary ischemia-reperfusion injury in large animal models

Acute respiratory distress syndrome (ARDS) is a common critical pulmonary complication after esophagectomy and other thoracic surgeries (e.g., lung transplantation, pulmonary thromboendarterectomy). Direct pulmonary ischemia-reperfusion injury (PIRI) is known to play the main role in induction of ARDS in these cases. Large animal models are an appropriate choice for ARDS as well as PIRI study because of their physiological and anatomic similarities to the human body. With regard to large animal models, we reviewed different methods of inducing in situ direct PIRI and the commonly applied methods for diagnosing and monitoring ARDS or PIRI in an experimental research setting.

Effect of positive end-expiratory pressure on regional ventilation distribution during mechanical ventilation after surfactant depletion

BACKGROUND

Ventilator-induced lung injury occurs due to exaggerated local stresses, repeated collapse, and opening of terminal air spaces in poorly aerated dependent lung, and increased stretch in nondependent lung. The aim of this study was to quantify the functional behavior of peripheral lung units in whole-lung lavage-induced surfactant depletion, and to assess the effect of positive end-expiratory pressure.

METHODS

The authors used synchrotron imaging to measure lung aeration and regional specific ventilation at positive end-expiratory pressure of 3 and 9 cm H2O, before and after whole-lung lavage in rabbits. Respiratory mechanical parameters were measured, and helium-washout was used to assess end-expiratory lung volume.

RESULTS

Atelectatic, poorly, normally aerated, hyperinflated, and trapped regions could be identified using the imaging technique used in this study. Surfactant depletion significantly increased atelectasis (6.3±3.3 [mean±SEM]% total lung area; P=0.04 vs. control) and poor aeration in dependent lung. Regional ventilation was distributed to poorly aerated regions with high (16.4±4.4%; P<0.001), normal (20.7±5.9%; P<0.001 vs. control), and low (5.7±1.2%; P<0.05 vs. control) specific ventilation. Significant redistribution of ventilation to normally aerated nondependent lung regions occurred (41.0±9.6%; P=0.03 vs. control). Increasing positive end-expiratory pressure level to 9 cm H2O significantly reduced poor aeration and recruited atelectasis, but ventilation redistribution persisted (39.2±9.5%; P<0.001 vs. control).

CONCLUSIONS

Ventilation of poorly aerated dependent lung regions, which can promote the local concentration of mechanical stresses, was the predominant functional behavior in surfactant-depleted lung. Potential tidal recruitment of atelectatic lung regions involved a smaller fraction of the imaged lung. Significant ventilation redistribution to aerated lung regions places these at risk of increased stretch injury.

Effects of surfactant depletion on regional pulmonary metabolic activity during mechanical ventilation

Inflammation during mechanical ventilation is thought to depend on regional mechanical stress. This can be produced by concentration of stresses and cyclic recruitment in low-aeration dependent lung. Positron emission tomography (PET) with (18)F-fluorodeoxyglucose ((18)F-FDG) allows for noninvasive assessment of regional metabolic activity, an index of neutrophilic inflammation. We tested the hypothesis that, during mechanical ventilation, surfactant-depleted low-aeration lung regions present increased regional (18)F-FDG uptake suggestive of in vivo increased regional metabolic activity and inflammation. Sheep underwent unilateral saline lung lavage and were ventilated supine for 4 h (positive end-expiratory pressure = 10 cmH(2)O, tidal volume adjusted to plateau pressure = 30 cmH(2)O). We used PET scans of injected (13)N-nitrogen to compute regional perfusion and ventilation and injected (18)F-FDG to calculate (18)F-FDG uptake rate. Regional aeration was quantified with transmission scans. Whole lung (18)F-FDG uptake was approximately two times higher in lavaged than in nonlavaged lungs (2.9 ± 0.6 vs. 1.5 ± 0.3 10(-3)/min; P < 0.05). The increased (18)F-FDG uptake was topographically heterogeneous and highest in dependent low-aeration regions (gas fraction 10-50%, P < 0.001), even after correction for lung density and wet-to-dry lung ratios. (18)F-FDG uptake in low-aeration regions of lavaged lungs was higher than that in low-aeration regions of nonlavaged lungs (P < 0.05). This occurred despite lower perfusion and ventilation to dependent regions in lavaged than nonlavaged lungs (P < 0.001). In contrast, (18)F-FDG uptake in normally aerated regions was low and similar between lungs. Surfactant depletion produces increased and heterogeneously distributed pulmonary (18)F-FDG uptake after 4 h of supine mechanical ventilation. Metabolic activity is highest in poorly aerated dependent regions, suggesting local increased inflammation.

Influence of propofol and volatile anaesthetics on the inflammatory response in the ventilated lung

BACKGROUND

The immunomodulatory effects of volatile anaesthetics in vitro and the protective effect of propofol in lung injury spurred us to study the effects of volatile anaesthetics and propofol on lung tissue in vivo.

METHODS

Twenty-seven pigs were randomized to 4-h general anaesthesia with propofol (8 mg/kg/h, group P, n=9), sevoflurane [minimum alveolar concentration (MAC)=1.0, group S, n=9) or desflurane (MAC=1.0, group D, n=9). Four healthy animals served as the no-ventilation group. Bronchoalveolar lavage fluid (BALF) was obtained to measure the cell counts, platelet-activating factor acetylhydrolase (PAF-AcH), phospholipase A(2) (PLA(2)) and superoxide dismutase (SOD) activity. Lung tissues were evaluated histologically and for caspase-3 expression.

RESULTS

Volatile anaesthetics reduced PAF-AcH levels without affecting PLA(2) activity and resulted in decreased alveolar macrophage and increased lymphocyte counts in BALF (sevoflurane: 29 ± 23%; desflurane: 26 ± 6%, both P<0.05 compared with 4 ± 2% in the no-ventilation group). These findings were accompanied by atelectasis and inflammatory cells' infiltration in the inhalational anaesthetics groups. Also, sevoflurane reduced SOD activity and both sevoflurane and desflurane induced significant caspase-3 expression. In contrast, propofol resulted in a minor degree of inflammation and preserved BALF cells' composition without triggering apoptosis.

CONCLUSION

Halogenated anaesthetics seem to trigger an immune lymphocytic response in the lung, inducing significant apoptosis and impairment of PAF-AcH. In contrast, propofol preserves anti-inflammatory and anti-oxidant defences during mechanical ventilation, thus preventing the emergence of apoptosis.