Hypoxemia and lung water in acute myocardial infarction

Effects of positive end-expiratory pressure in an experimental model of acute myocardial infarct in wistar rats

Our purpose in this study was to access the pulmonary effects of mechanical ventilation with positive end-expiratory pressure (PEEP; 10 cmH2O) or without PEEP (zero PEEP-ZEEP) in a rat model of acute myocardial infarction that resulted in hypotension but not in pulmonary congestion.

METHODS

Wistar rats were anesthetized (1.5% isoflurane) and myocardial infarct was induced by ligature of the anterior interventricular coronary artery. Rats with myocardial infarct were compared with sham-operated (Sham) and closed thorax groups.

RESULTS AND CONCLUSION

There was a significant decrease in MAP in the acute myocardial infarct group (92.5 +/- 4.2 mmHg) when compared with closed chest group (113.0 +/- 4.4 mmHg). There was no significant difference between acute myocardial infarct and Sham groups in PEEP or ZEEP. Mechanical ventilation for 120 min resulted in a significant increase in respiratory system elastance in the groups ventilated with ZEEP (2.59 +/- 0.17 and 2.32 +/- 0.17 cmH2O.mL, Sham and acute myocardial infarct groups, respectively). This effect of mechanical ventilation was not observed in the presence of PEEP in both groups. There was no significant increase in the amount of perivascular pulmonary edema measured in all groups studied. Mean airspace linear intercept and lung tissue distortion index also did not show statistically significant difference between Sham and acute myocardial infarct groups. We conclude that in this experimental model of acute myocardial infarct (12.4 +/- 4.1% area of necrotic tissue and 26.4 +/- 4.0% area of ischemic tissue), there was a protective pulmonary effect of PEEP.

Effect of furosemide on hemodynamics and lung water in acute pulmonary edema secondary to myocardial infarction

Hemodynamic studies were carried out in 19 patients with left ventricular failure complicating acute myocardial infarction. Fourteen patients were studied before and after the intravenous administration of 0.5 mg/kg of furosemide, and five patients served as a control group. Serial measurements included intracardiac pressures, cardiac output and lung water by a double isotope technique. A significant reduction was noted in right atrial (P less than 0.005), pulmonary arterial (P less than 0.0005) and pulmonary wedge pressures (P less than 0.0005) after administration of furosemide. Only the change in right atrial pressure was significantly different from that in the control group (P less than 0.05). Lung water was not changed in 4 patients studied 2 hours after administration of furosemide but was significantly changed in the remaining 10 patients studied 4 to 24 hours after furosemide (P = 0.0001). This change was also significantly different from values in the control group (P less than 0.05). The patients with no reduction in excess lung water also had a smaller reduction in pulmonary wedge pressure and a lower pretreatment stroke work index than the other patients. The mobilization of excess lung water in patients with acute myocardial infarction complicated by left ventricular failure has several features. Despite a prompt diuresis, the reduction in lung water is delayed for at least several hours after the administration of furosemide and may be related to the degree of left ventricular dysfunction. Venodilation may be a major result of treatment with furosemide.

Clinical significance of pulmonary function tests. Pulmonary function after uncomplicated myocardial infarction

Derangement of pulmonary function following myocardial infarction is related to the severity of hemodynamic dysfunction. Abnormalities of pulmonary function appear even in patients without clinical or radiologic evidence of congestive failure. There is a reduction in vital capacity and rates of air flow. There is evidence for dysfunction of "small airways" and diminished ventilation to dependent parts of the lung. Total lung capacity may be normal or reduced, and residual volume may be increased slightly in uncomplicated myocardial infarction. Residual volume falls with more pronounced pulmonary congestion and edema. Distribution of pulmonary perfusion is altered after myocardial infarction, with a shift of perfusion away from the dependent parts of the lung (bases) towards the apices. Pulmonary gas exchange is impaired, with hypoxemia (due to both ventilation-perfusion inequality and increased shunting); and the diffusing capacity for carbon monoxide is diminished. Dead space is increased. The basic pathophysiologic mechanism responsible for abnormalities of pulmonary function is increased pulmonary water, which may be very minimal with uncomplicated myocardial infarction and stay primarily in the pulmonary interstitial space, but becomes progressively more severe with eventual alveolar flooding and marked impairment of pulmonary function.