Experimental myocardial infarction. 8. Chronotropic augmentation of cardiac function in left ventricular failure of acute and healing stages in intact conscious dogs

Abrupt Deflation after Sustained Inflation Causes Lung Injury

RATIONALE

Ventilator management in acute respiratory distress syndrome usually focuses on setting parameters, but events occurring at ventilator disconnection are not well understood.

OBJECTIVES

To determine if abrupt deflation after sustained inflation causes lung injury.

METHODS

Male Sprague-Dawley rats were ventilated (low Vt, 6 ml/kg) and randomized to control (n = 6; positive end-expiratory pressure [PEEP], 3 cm H₂O; 100 min) or intervention (n = 6; PEEP, 3-11 cm H₂O over 70 min; abrupt deflation to zero PEEP; ventilation for 30 min). Lung function and injury was assessed, scanning electron microscopy performed, and microvascular leak timed by Evans blue dye (n = 4/group at 0, 2, 5, 10, and 20 min after deflation). Hemodynamic assessment included systemic arterial pressure (n = 6), echocardiography (n = 4), and right (n = 6) and left ventricular pressures (n = 6).

MEASUREMENTS AND MAIN RESULTS

Abrupt deflation after sustained inflation (vs. control) caused acute lung dysfunction (compliance 0.48 ± 1.0 vs. 0.82 ± 0.2 m/cm H₂O, oxygen saturation as measured by pulse oximetry 67 ± 23.5 vs. 91 ± 4.4%; P < 0.05) and injury (wet/dry ratio 6.1 ± 0.6 vs. 4.6 ± 0.4; P < 0.01). Vascular leak was absent before deflation and maximal 5-10 minutes thereafter; injury was predominantly endothelial. At deflation, left ventricular preload, systemic blood pressure, and left ventricular end-diastolic pressure increased precipitously in proportion to the degree of injury. Injury caused later right ventricular failure. Sodium nitroprusside prevented the increase in systemic blood pressure and left ventricular end-diastolic pressure associated with deflation, and prevented injury. Injury did not occur with gradual deflation.

CONCLUSIONS

Abrupt deflation after sustained inflation can cause acute lung injury. It seems to be mediated by acute left ventricular decompensation (caused by increased left ventricular preload and afterload) that elevates pulmonary microvascular pressure; this directly injures the endothelium and causes edema, which is potentiated by the surge in pulmonary perfusion.

Experimental myocardial infarction. X. Efficacy of glucagon in acute and healing phase in intact conscious dogs: effects on hemodynamics and myocardial oxygen consumption

This study was designed to test the efficacy of glucagon in the treatment of hemodynamic abnormalities of acute and healing experimental canine myocardial infarction. Myocardial infarction was produced in intact, conscious dogs by gradual inflation of a balloon cuff device implanted around the left anterior descending coronary artery 1 to 2 weeks prior to the study. Hemodynamic and metabolic effects of 50 µg/kg of glucagon were assessed serially in the control state, 1 hour after myocardial infarction and again 1 week later. In the control state glucagon improved cardiac performance and increased myocardial oxygen consumption. One hour after acute myocardial infarction glucagon improved cardiac performance and reduced the degree of left ventricular failure, without any increase in myocardial oxygen consumption. Similar effects of glucagon were noted in the healing phase of myocardial infarction. It is postulated that in this animal model in the presence of heart failure due to myocardial infarction there are reciprocal changes in the factors that increase myocardial oxygen consumption (glucagon-induced inotropy) and decrease oxygen consumption (fall in ventricular end-diastolic volume and wall stress), resulting in no net change in oxygen requirement.