The effect of positive end expiratory pressure after three types of open heart surgery

Cardiopulmonary function of cats with respiratory distress induced by N-nitroso N-methylurethane

The purposes of this study were to determine the effects of positive end-expiratory pressure (PEEP) and end-expiratory lung volume on systemic blood flow, whether PEEP levels yielding maximum systemic oxygen transport are associated with maximum lung compliance, and the effects of end-expiratory lung volume on pulmonary resistance to gas flow, in an animal model of respiratory distress. Twelve cats were inoculated with 12 mg/kg N-Nitroso N-Methylurethane (NNNMU) to induce respiratory distress. The NNNMU caused a 76% decrease in disaturated phosphatidyl-choline of lung lavage, a 34% decrease in functional residual capacity (FRC), an 80% decrease in lung compliance, an 88% increase in pulmonary resistance to gas flow, a 43% decrease in PaO2, and a 37% decrease in oxygen consumption. Systemic blood flow and systemic oxygen transport were not significantly altered by the chemically induced respiratory distress. PEEP levels of 5.1 +/- 0.8 cm H2O returned end-expiratory lung volume to normal FRC levels. Increases in PEEP caused systemic blood flow to decrease even when end-expiratory lung volume was below or equal to normal FRC levels but did not significantly affect systemic oxygen transport, lung compliance, or pulmonary resistance. We conclude that in cats with NNNMU-induced respiratory distress: PEEP causes decreases in systemic blood flow, lung compliance and systemic oxygen transport are not clear indicators of optimal PEEP level, and returning end-expiratory lung volume to normal FRC does not significantly reduce pulmonary resistance to gas flow.

The influence of PEEP ventilation on organ blood flow and peripheral oxygen delivery

In an experimental study including 10 dogs with intact lungs (group I) and 10 dogs with oleic acid-induced pulmonary edema (group II), the effects of PEEP 10 and 20 (cm H20) on central hemodynamics, lung function, organ blood flow and tissue PO2 (liver and skeletal muscle) were examined. In both groups, PEEP resulted in a significant fall of cardiac output (CO) despite volume substitution. In group I, flow was redistributed favouring brain, heart, kidneys, and adrenals, at the expense of stomach, pancreas and and thyroid gland. Animals with pulmonary edema and consequently lower absolute values of CO showed a more uniform reduction of organ perfusion sparing only brain and heart PEEP 10 led to a marginal improvement of tissue oxygenation, whereas PEEP 20 resulted in a deterioration of local PO2 associated with a decrease in O2 transport. It is concluded that whenever PEEP is applied, a reduction and redistribution of CO may occur possibly jeopardizing tissue oxygenation and thus interfering with organ function.

[The effect of PEEP ventilation on hemodynamics and regional blood flow (author's transl)]

The beneficial effects of PEEP on lung function may be counteracted by its hemodynamic sequelae induced by a reduction of venous return due to the elevated intrathoracic pressure, and by an increased right ventricular afterload secondary to the rise of pulmonary vascular resistance. PEEP redistributes cardiac output in favor of brain, heart, adrenals and intestines, whereas the perfusion of stomach, pancreas and thyroid is diminished out of proportion to the fall of cardiac output. Total renal blood flow is relatively little affected; however, redistribution of intrarenal blood flow will result in a marked salt-water-retention. Reduction of hepatic artery flow, at higher levels of PEEP, may jeopardize liver tissue oxygenation. - Under clinical conditions, individual differences regarding preexisting cardiopulmonary and peripheral-vascular diseases may modify the PEEP-induced hemodynamic alterations in a wide range.