Pulmonary vasoconstriction due to impaired nitric oxide production after cardiopulmonary bypass

Effects of ischemia on pulmonary dysfunction after cardiopulmonary bypass

BACKGROUND

Pulmonary hypertension and lung injury secondary to cardiopulmonary bypass (CPB) are probably caused by a combination of ischemia and inflammation. This study was undertaken to investigate the potential ischemic effects of cessation of pulmonary arterial flow during CPB on pulmonary injury.

METHODS

Twenty neonatal piglets (2.5 to 3.1 kg) were randomly assigned to two groups. Group A (n = 10) underwent 90 minutes of CPB at full flow (100 mL x kg(-1) x min(-1)) and clamping of the main pulmonary artery (PA). Group B (n = 10) underwent 90 minutes of partial CPB (66 mL x kg(-1) x min(-1)) with continued mechanical ventilation and without clamping of the PA. All hearts were instrumented with micromanometers and a PA ultrasonic flow probe. Endothelial function was assessed by measuring endothelial-dependent relaxation (measured by change in pulmonary vascular resistance after PA infusion of acetylcholine) and endothelial-independent relaxation (measured by change in pulmonary vascular resistance after ventilator infusion of nitric oxide and PA infusion of sodium nitroprusside).

RESULTS

All groups exhibited signs of pulmonary injury after CPB as evidenced by significantly increased pulmonary vascular resistance, increased alveolar-arterial O2 gradients, and decreased pulmonary compliance (p<0.05); however, pulmonary injury was significantly worse in group A (p<0.05).

CONCLUSIONS

This study suggests that although exposure to CPB alone is enough to cause pulmonary injury, cessation of PA flow during CPB contributes significantly to this pulmonary dysfunction.

Inhibition of endogenous nitric oxide synthesis potentiates the effects of sodium nitroprusside but not of adenosine in experimental pulmonary hypertension

This study examined the systemic and pulmonary vasodilator effects of sodium nitroprusside (SNP) and adenosine during experimental pulmonary hypertension with and without inhibition of endogenous NO synthesis. Male New Zealand White rabbits were anesthetized and mechanically ventilated. The NO synthesis inhibitor NG-nitro-L-arginine methyl ester (L-NAME) was administered to 15 of the 28 rabbits. Pulmonary hypertension was then produced in all rabbits by U46619, a thromboxane A2 mimetic. SNP was infused in 14 rabbits (7 L-NAME, 7 control) at doses of 0.5-20 microg/kg/min; adenosine was infused in the other 14 rabbits (8 L- NAME, 6 control) at doses of 12.5-300 microg/kg/min. The U46619 dose required to produce pulmonary hypertension was significantly lower in the L-NAME group. SNP dose-dependently decreased pulmonary (Ppa) and systemic (Psa) artery pressures and systemic vascular resistance (SVR). Both Ppa and Psa were decreased more with SNP in the L-NAME than in the no L-NAME group. The SNP ED50 for the decrease in PVR was almost threefold lower in the L-NAME group. Adenosine dose-dependently decreased Ppa, Psa, PVR and SVR. The adenosine ED50 for the decreases in PVR and SVR were similar in the L-NAME group and the control group. We conclude that inhibition of endogenous NO synthesis shifts the dose-response curves for both the pulmonary and systemic vasodilator effects to the left for the nitrovasodilator SNP but not for the non-nitrovasodilator adenosine.

Alteration of the neonatal pulmonary physiology after total cardiopulmonary bypass

OBJECTIVES

The purpose of this study was to analyze the mechanisms associated with lung injury after cardiopulmonary bypass and to propose strategies of prevention.

METHODS

Thirty-two neonatal piglets underwent 90 minutes of hypothermic cardiopulmonary bypass without aortic cross-clamping. Five experimental groups were defined: group I had standard cardiopulmonary bypass (control), group II received continuous low-flow lung perfusion during cardiopulmonary bypass, group III treatment was similar to that of group I with maintenance of ventilation, group IV received pneumoplegia, and group V received nitric oxide ventilation (30 ppm) after cardiopulmonary bypass. Data drawn from hemodynamic and gas exchange values and muscular and pulmonary tissular levels of adenosine triphosphate (in micromoles per gram) and myeloperoxidase (in international units per 100 mg) were used for comparisons before and 30 and 60 minutes after cardiopulmonary bypass. Pulmonary and systemic vascular endothelial functions were assessed in vitro after cardiopulmonary bypass on isolated rings of pulmonary and iliac arteries.

RESULTS

Pulmonary vascular resistance index, cardiac index, and oxygen tension were better preserved in groups II, IV, and V. All groups disclosed a significant decrease in lung adenosine triphosphate levels and an increase in myeloperoxidase activity whereas these levels stayed within pre-cardiopulmonary bypass ranges in muscular beds. Endothelium-dependent relaxation was preserved in systemic arteries but was strongly affected in pulmonary arteries after cardiopulmonary bypass. None of the methods that aimed to protect the pulmonary vascular bed demonstrated any preservation of pulmonary endothelial function.

CONCLUSION

Cardiopulmonary bypass results in ischemia-reperfusion injury of the pulmonary vascular bed. Lung protection by continuous perfusion, pneumoplegia, or nitric oxide ventilation can prevent hemodynamic alterations after cardiopulmonary bypass but failed to prevent any of the biochemical disturbances.