What is acute lung injury? What is ARDS?

Consequences of vascular flow on lung injury induced by mechanical ventilation

To investigate whether the magnitude of blood flow contributes to ventilator-induced lung injury, 14 sets of isolated rabbit lungs were randomized for perfusion at either 300 (Group A: n = 7) or 900 ml/ min (Group B: n = 7) while ventilated with 30 cm H2O peak static pressure. Control lungs (Group C: n = 7) were ventilated with lower peak static pressure (15 cm H2O) and perfused at 500 ml/min. Weight gain, changes in the ultrafiltration coefficient (DeltaKf) and lung static compliance (CL), and extent of hemorrhage (scored by histology) were compared. Group B had a larger decrease in CL (-13 +/- 11%) than Groups A (2 +/- 6%) and C (5 +/- 5%) (p < 0.05). Group B had more hemorrhage and gained more weight (16.2 +/- 9.5 g) than Groups A (8.7 +/- 3.4 g) and C (1.6 +/- 1.0 g) (p < 0.05 for each pairwise comparison between groups). Finally, Kf (g . min-1 . cm H2O-1 . 100 g-1) increased the most in Group B (DeltaKf = 0.26 +/- 0. 20 versus 0.17 +/- 0.10 in Group A and 0.05 +/- 0.04 in Group C; p < 0.05 for B versus C). We conclude that the intensity of lung perfusion contributes to ventilator- induced lung injury in this model.

The effects of nitric oxide inhalation on respiratory mechanics and gas exchange during endotoxaemia in the pig

BACKGROUND

In the adult respiratory distress syndrome, nitric oxide (NO) inhalation improves oxygenation through reducing ventilation-perfusion mismatching, but detailed information on the pulmonary effects of NO inhalation in septic shock is scarce. The present study investigated the effects of inhaled NO on alveolar dead space (Vdalv) and venous admixture as well as on respiratory system compliance (Crs) and respiratory system resistance (Rrs) in a porcine model of septic shock. Protective effects of NO are discussed.

METHODS

Thirteen anaesthetised and ventilated pigs were given an infusion of endotoxin for an observation time of 220 min to induce acute lung injury (ALI). In the NO-early group (n=6), an inhalation of 60 ppm NO was started simultaneously with the endotoxin infusion and continued for 190 min. In 7 control/NO-late animals, 60 ppm NO was administered for 30 min following 190 min of endotoxin infusion. Haemodynamics, single-breath CO2-, pressure-, and flow signals were recorded.

RESULTS

Endotoxin induced haemoconcentration, pulmonary vasoconstriction, and a decrease in Crs, while venous admixture, Vdalv, and Rrs increased. In the NO-early group, the pulmonary vasoconstriction was attenuated, no increase in pulmonary venous admixture or in Vdalv was seen before cessation of NO, and the improvements in oxygenation outlasted the NO inhalation. In the control/NO-late group, the NO inhalation reversed the changes in dead space and venous admixture. NO had no effect on the changes in respiratory mechanics.

CONCLUSION

In porcine ALI, 60 ppm NO diminishes pulmonary vasoconstriction and improves gas exchange by reducing pulmonary venous admixture and alveolar dead space, but does not prevent a fall in Crs. NO inhalation may help prevent long-lasting pulmonary failure.

Acute lung injury in the medical ICU: comorbid conditions, age, etiology, and hospital outcome

The independent effects of chronic disease, age, severity of illness, lung injury score (LIS) and etiology, and preceding nonpulmonary organ-system dysfunction (OSD) on the outcome of acute lung injury (ALI) have not been examined in an exclusively medical-intensive-care-unit (MICU) population. Therefore, 107 consecutive MICU patients with ALI (76% with acute respiratory distress syndrome [ARDS]) were prospectively investigated. The impact of comorbidities, age > 65 yr, acute physiology score (APS), LIS, etiology of ALI, and OSD on hospital survival were studied. The overall mortality was 62 of 107 patients (58%), including 47 (58%) with ARDS. With univariate analysis, age > 65 yr, organ transplantation, human immunodeficiency virus (HIV) infection, active malignancy, chronic steroid use, and a septic or aspiration-related etiology of ALI were associated with a > or = 1.2-fold greater relative risk (RR) of hospital mortality. With multiple logistic regression, independent predictors of hospital death were age > 65 yr, organ transplantation, HIV infection, cirrhosis, active malignancy, and sepsis. APS, LIS, aspiration-related etiology of ALI, preceding OSD, and other comorbidities were not independently predictive of hospital death. Multivariate analysis of the ARDS cohort showed similar results, although cirrhosis and malignancy did not reach statistical significance. We conclude that comorbid conditions, older age, and sepsis etiology are independent predictors of hospital death in exclusively MICU patients with ALI (76% of whom satisfied criteria for ARDS). These factors should be considered in analyzing studies of new therapies and interpreting trends in mortality for ALI and ARDS.

Positron emission tomography measurements of pulmonary vascular permeability with Ga-68 transferrin or C-11 methylalbumin

OBJECTIVE

To compare estimates of pulmonary endothelial barrier integrity obtained using two different tracer proteins.

DESIGN

Measure the pulmonary transcapillary escape rate with gallium (Ga)-68 labeled transferrin (PTCER(tf)) and positron emission tomography (PET) imaging and compare the results to similar measurements obtained with C-11 labeled methylalbumin (PTCER(alb)).

SETTING

Laboratory investigation.

SUBJECTS

Mongrel dogs.

INTERVENTIONS

No intervention in one group of dogs (n = 3); oleic-acid induced lung injury in another set (n = 4).

MEASUREMENTS AND MAIN RESULTS

Although PTCER(tf) was consistently higher than PTCER(alb) (mean difference: 50 x 10(-4)/min), the overall correlation between the two methods, after normalizing for differences in regression slope and intercept among the individual dogs, was excellent (r2 = .67).

CONCLUSION

The data support the continued use of PET and Ga-68 transferrin as an appropriate means of evaluating and quantifying lung injury.

Pulmonary function and radiographic abnormalities related to neurological outcome after aneurysmal subarachnoid hemorrhage

OBJECT

This observational study is based on a consecutive series of 207 patients with aneurysmal subarachnoid hemorrhage who were treated within 7 days of their most recent bleed. The purpose of the study was to evaluate the effect of respiratory failure on neurological outcome.

METHODS

Pulmonary function was assessed by determination of parameters describing pulmonary oxygen transport and exchange, by using composite scores for quantification of lung injury (lung injury score [LIS]) and mechanical ventilator settings (PIF score). Pulmonary function was related to the Hunt and Hess (H & H) grade assigned to the patient at hospital admission (p < 0.001). The pattern and time course of lung injury differed significantly between patients with H & H Grade I or II, Grade III, and Grade IV or V. Hunt and Hess grade, Fisher computerized tomography grade, intracranial pressure, cerebral perfusion pressure, LIS, ratio of PaO2 to the fraction of inspired oxygen (FiO2), and the ratio of the alveolar-minus-arterial oxygen tension difference (AaDO2) to FiO2 were related to neurological outcome (p < 0.001). The LIS on the day of maximum lung injury remained an independent predictor of outcome (p = 0.01) in a stepwise logistic regression analysis. The probability of poor neurological outcome significantly increased with both decreasing cerebral perfusion pressure and increasing severity of lung injury.

CONCLUSIONS

The overall mortality rate was 22.2% (46 of 207 patients). Subarachnoid hemorrhage and its neurological sequelae accounted for the principal mortality in this series. Medical (nonneurological and nontreatment-related) complications accounted for 37% of all deaths. Systemic inflammatory response syndrome with associated multiple organ dysfunction syndrome was the leading cause of death from medical complications. The authors conclude that respiratory failure is related to neurological outcome, although it is not commonly the primary cause of death from medical complications.

Radionuclide assessment of pulmonary microvascular permeability

The literature has been reviewed to evaluate the technique and clinical value of radionuclide measurements of microvascular permeability and oedema formation in the lungs. Methodology, modelling and interpretation vary widely among studies. Nevertheless, most studies agree on the fact that the measurement of permeability via pulmonary radioactivity measurements of intravenously injected radiolabelled proteins versus that in the blood pool, the so-called pulmonary protein transport rate (PTR), can assist the clinician in discriminating between permeability oedema of the lungs associated with the adult respiratory distress syndrome (ARDS) and oedema caused by an increased filtration pressure, for instance in the course of cardiac disease, i.e. pressure-induced pulmonary oedema. Some of the techniques used to measure PTR are also able to detect subclinical forms of lung microvascular injury not yet complicated by permeability oedema. This may occur after cardiopulmonary bypass and major vascular surgery, for instance. By paralleling the clinical severity and course of the ARDS, the PTR method may also serve as a tool to evaluate new therapies for the syndrome. Taken together, the currently available radionuclide methods, which are applicable at the bedside in the intensive care unit, may provide a gold standard for detecting minor and major forms of acute microvascular lung injury, and for evaluating the severity, course and response to treatment.

Influence of prone position on the extent and distribution of lung injury in a high tidal volume oleic acid model of acute respiratory distress syndrome

OBJECTIVE

To evaluate the influence of body position on the extent and distribution of experimental lung damage in an oleic acid canine model of acute respiratory distress syndrome, using mechanical ventilation with high tidal volumes and positive end-expiratory pressure (PEEP).

DESIGN

Prospective, randomized study.

SETTING

Experimental animal laboratory.

SUBJECTS

Twelve anesthetized and paralyzed dogs.

INTERVENTIONS

Ninety minutes after lung injury was induced by injection of oleic acid, 12 animals were randomized to be ventilated for 4 hrs, in either the supine (supine group, n = 6) or prone (prone group, n = 6) positions, using the same ventilatory pattern (F10(2) 0.6, PEEP > or = 10 cm H2O, and a tidal volume that generated a peak transpulmonary pressure of 35 cm H2O when implemented in the supine position). Regardless of randomization to position, the tidal volumes, F10(2), and PEEP were kept constant and the pulmonary artery occlusion pressure was maintained between 4 and 6 mm Hg for the duration of the study.

MEASUREMENTS AND MAIN RESULTS

At the end of the protocol, the lungs were excised for gravimetric determination (wet/dry weight ratio) and histologic examination (histologic score). Changes over time in the static pressure-volume curve of the lungs (obtained in the supine position) were also used as end-point variables. At baseline, hemodynamic and respiratory variables did not differ between groups. Just before randomization to position (90 mins after oleic acid injection), both groups presented similar lung static pressure-volume curves. Pulmonary artery occlusion pressure (4.3 +/- 1.9 vs. 4.8 +/- 1.3 mm Hg [supine vs. prone group]), cardiac output (4.1 +/- 0.4 vs. 5.2 +/- 1.3 L/min [supine vs. prone group]), and venous admixture (36.7 +/- 20.7% vs. 28.3 +/- 19.4% [supine vs. prone group]) were also not significantly (p > .05) different when measured in the supine position. At the end of the experiment, lung gravimetric data in the two experimental groups were not statistically different, suggesting a similar extent of edema. Histologic abnormalities, however, were less in the prone group than in the supine group (p < .01), due primarily to marked differences in extent and severity in the dependent regions of the lungs. Static lung compliance improved over time in the prone group (34 +/- 9 to 46 +/- 19 mL/cm H2O)(p = .02), but not in the supine group (34 +/- 6 to 36 +/- 6 mL/cm H2O).

CONCLUSIONS

After oleic acid-induced lung injury, animals ventilated with high tidal volume and PEEP undergo less extensive histologic change in the prone position than in the supine position. The prone position alters the distribution of histologic abnormalities.

Adjuncts to mechanical ventilation

Adjunctive ventilatory strategies have been developed to improve oxygenation and carbon dioxide (CO2) removal during mechanical ventilation of critically ill patients. These techniques allow clinicians to attain their clinical goals at lower levels of ventilatory support. In this article, the authors discuss extracorporeal CO2 removal, venovenous intravena caval oxygenator, and tracheal gas insufflation as adjuncts to CO2 removal and nitric oxide, surfactant replacement therapy, perfluorocarbon-associated gas exchange, and prone positioning as adjuncts to oxygenation.

Current concepts of sepsis and acute lung injury

Acute respiratory distress syndrome continues to be a vexing clinical problem with no specific therapy. Epidemiologic and basic sciences have advanced our understanding of the clinical syndrome and have brought us to the brink of effective intervention strategies. This article carefully examines the current state of knowledge, with reference to acute lung injury and current efforts, to arrive at effective pharmacologic approaches.

Acute respiratory distress syndrome

The acute respiratory distress syndrome (ARDS) is a serious and complex clinical problem that often threatens the lives of patients. Emerging clinical data suggest that the survival of patients with this disorder may have improved during the last two decades, presumably because of advances in supportive medical care. Among the supportive therapies used to treat patients with ARDS, none is more complex than mechanical ventilation. New strategies for administering mechanical ventilation to patients with ARDS may reduce the occurrence of iatrogenic volotrauma and oxygen toxicity, accounting in part for the recently observed improvements in patient survival. Prevention and cure of ARDS have remained elusive goals because of the lack of specific therapies directed against the known pathogenic factors. Ongoing investigations are aimed at identifying specific therapies to interrupt the mechanisms of inflammation and lung injury responsible for this syndrome. Until such therapies become available, clinicians caring for patients with ARDS should attempt to minimize additional morbidity and mortality resulting from nosocomial infections and iatrogenic injuries.