Acute Respiratory Failure

Depression of pulmonary metabolic function by cardiopulmonary bypass procedures increases levels of circulating norepinephrine

We measured plasma levels of endogenous norepinephrine radioenzymatically in mixed venous and arterial blood simultaneously sampled from anesthetized dogs before, during, and after 1 to 4 hours of total cardiopulmonary bypass (CPB) or 2 hours of left heart bypass. Prior to bypass, arterial levels of norepinephrine were 0.41 +/- 0.04 ng/ml and pulmonary extraction of norepinephrine was 25 +/- 3% (N = 20). During bypass, norepinephrine levels significantly increased from control to 1.41 +/- 0.15 (1 or 2 hours of CPB; N = 8) and 1.97 +/- 0.30 (3 or 4 hours of CPB; N = 8) or 0.97 +/- 0.29 (2 hours of left heart bypass) ng/ml. Restoration of normal pulmonary blood flow was associated with a rapid and significant decrease in arterial levels of norepinephrine, which, after 1 or 2 hours of CPB or 2 hours of left heart bypass, returned to levels obtained before bypass. However, arterial levels of norepinephrine remained higher than 1 ng/ml in the 3-hour recovery period after prolonged bypass. In these animals, pulmonary extraction of norepinephrine was significantly less than control. These data suggest that the lung's ability to remove norepinephrine is altered by CPB and that the severity of the alterations is directly related to pump time. The impairment in extraction allows higher than normal concentrations of norepinephrine to enter the arterial circulation and may contribute to systemic hypertension after bypass.

Mechanisms of lung vascular injury after intravascular coagulation

Pulmonary intravascular coagulation and the resultant microembolization increase lung vascular permeability to proteins. The increase in permeability is mediated by the activation of cellular and humoral factors after intravascular coagulation. In particular, intravascular coagulation results in sequestration and activation of leukocytes, which appears to be of primary importance in mediating the lung vascular injury. In addition, fibrin entrapment and the generation of fibrin-degradation products after fibrinolysis also contribute to the vascular injury. The classical inflammatory agents, such as prostaglandin, histamine, bradykinin, and serotonin, may modulate the degree of injury after pulmonary vascular thrombosis, but they do not appear to be the primary mediators of the injury. Platelet aggregation did not mediate the lung vascular injury after intravascular coagulation, but was responsible for the pulmonary gas-exchange impairment that occurs with pulmonary vascular thrombosis.