Unilateral lung edema: effects on pulmonary gas exchange, hemodynamics, and pulmonary perfusion distribution

Positive End-expiratory Pressure Ventilation Increases Extravascular Lung Water Due to a Decrease in Lung Lymph Flow

Positive end-expiratory pressure (PEEP) is used to improve gas exchange, increase functional residual capacity, recruit air spaces, and decrease pulmonary shunt in patients suffering from respiratory failure. The effect of PEEP on extravascular lung water (EVLW), however, is still not fully understood. This study was designed as a prospective laboratory experiment to evaluate the effects of PEEP on EVLW and pulmonary lymph flow (QL) under physiologic conditions.

Twelve adult sheep were operatively prepared to measure haemodynamics of the systemic and pulmonary circulation, and to assess EVLW. In addition, the lung lymphatic duct was cannulated and a tracheostomy performed. The animals were then mechanically ventilated in the awake-state without end-expiratory pressure (PEEP 0). After a two-hour baseline period, PEEP was increased to 10 cmH2O for the duration of two hours, and then reduced back to 0 cmH2O. Cardiopulmonary variables, QL, and arterial blood gases were recorded intermittently; EVLW was determined two hours after each change in PEEP.

The increase in PEEP resulted in a decrease in QL (7±1 vs 5±1 ml/h) and an increase in EVLW (498±40 vs 630±58 ml; P<0.05 each) without affecting cardiac output. As PEEP was decreased back to baseline, QL increased significantly (5±1 vs 10±2 ml/h), whereas EVLW returned back to baseline.

This study suggests that institution of PEEP produces a reversible increase in EVLW that is linked to a decrease in QL.

Anaphylaxis followed by unilateral lung opacity and hypocomplementemia in a young female

A 36-year-old woman was stung in the right wrist by a bee, suffered typical anaphylaxis, and was transferred to a local hospital. After a few hours, which corresponded to late-onset reaction, she developed shortness of breath and weakness and was transferred to the emergency department, where the diagnosis of anaphylaxis was confirmed. Serum complement levels, components C3 and C4, were undetectable. Flexible bronchoscopy excluded lung hemorrhage. She was on the ventilator for 4 days and was fully awake during that time. After the treatment, her improvement was rapid, and she fully recovered. Three weeks after she had been stung by a bee, skin prick tests to bee, wasp, and yellow jacket venom were done, and all tests came back negative as well as IgE and IgG4 antibodies to the same venoms. Coagulation factors and the complement, including C1q inhibitor, were normal. Occurrence of complement activation and consumption could point to the immune complexes as basis of pathophysiological mechanism. It remains unclear why such a reaction would involve only the right lung and why no detectable immune complexes were discovered. The clinical picture in the presented case resembled acute respiratory distress syndrome, but the exact nature of lung consolidation remains puzzling. The most likely explanation is a very rare case of airway obstruction. To our knowledge, this is the only reported case of anaphylaxis associated with undetectable serum complement levels. The potential role and diagnostic significance of hypocomplementemia in cases of anaphylaxis should be further investigated.

A closed-loop model of the ovine cardiovascular system

We develop a closed-loop model of the ovine cardiovascular system, with the long term goal of developing a platform for simulating the hemodynamic efficacy of cardiopulmonary assist devices implanted and tested in this animal. The modeling of the systemic circulation is distributed and divided into subsystem circulations. Nonlinear aspects of the systemic venous system are included, such as the pressure-volume relation (PVR) of the systemic veins. In addition, a lumped model of the neural system controlling blood pressure is incorporated. The complete model provides good approximations to measured data from healthy reclining sheep. Moreover, it can predict the hemodynamic changes that accompany pre-load variation upon standing, and the short-term neurally mediated cardiovascular responses that attend this imposed orthostatic stress. We conclude that the model can serve as a simulation platform for evaluating cardiovascular and pulmonary assist devices.

Hemodynamic analysis and design of a paracorporeal artificial lung device

We have extended our model of the ovine pulmonary circulation to include a model of a paracorporeal artificial lung (AL) and its attachments to the natural pulmonary circulation in two configurations: in series and in parallel. Our model of the natural lung (NL) circulation is first shown to be in agreement with hemodynamic and input impedance data from the open literature. We then study design efficacy of the AL in terms of its housing and attachments. A sensitivity analysis of the modified pulmonary circulation model reveals that there are three key parameters: inlet graft length (IGL) and the compliances of the inlet compliance chamber (CC) and housing of the artificial lung. Based on literature reports, we assume the right ventricle is well-matched to the impedance of the natural pulmonary circulation and adjust the parameters of the modeled AL circuit to achieve the best least-squares fit to natural pulmonary input impedance data. Best-fit parameters produce impedance curves that fit natural impedance well, particularly below 3 Hz, where both compliance and graft length have their largest effects. Of these parameters, the impedance profile is most sensitive to IGL. However, the compliances are important, as well, particularly at low frequencies.