The ventilation-perfusion relation and gas exchange in mitral valve disease and coronary artery disease. Implications for anesthesia, extracorporeal circulation, and cardiac surgery.

BACKGROUND

Patients with mitral valve disease (MVD) are at greater risk for respiratory complications after cardiac surgery compared with patients with coronary artery disease (CAD). The authors hypothesized that ventilation-perfusion (VA/Q) inequality is more pronounced in patients with MVD before and after induction of anesthesia and during and after surgery when extracorporeal circulation (ECC) is used.

METHODS

In patients with MVD (n = 12) or with CAD (n = 12), VA/Q distribution was determined using the multiple inert gas elimination technique. Intrapulmonary shunt (Qs/Qr) defined as regions with VA/Q < 0.005 [% of total perfusion (Qr)], perfusion of "low" VA/Q areas (0.005 < or = VA/Q < 0.1, [% of Qr]), ventilation of "high" VA/Q regions (10 < or = VA/Q < or = 100 [% of total ventilation VE]), and dead space (VA/Q > 100 [% of VE]) were calculated from the retention/excretion data of the inert gases. Recordings were obtained while patients spontaneously breathed air in the awake state, during mechanical ventilation after induction of anesthesia, after separation of patients from ECC, and 4 h after operation.

RESULTS

Qs/Qr was low in the awake state (MVD group, 3% +/- 3%; CAD group, 3% +/- 4%) and increased after induction of anesthesia to 10% +/- 8% (MVD group, P < 0.05) and 11% +/- 7% (CAD group, P < 0.01). Qs/Qr increased further after separation from ECC (MVD group, 24% +/- 9%, P < 0.01; CAD group, 23% +/- 7%, P < 0.01). Similarly, alveolar-arterial oxygen tension difference (PA-aO2) increased from 168 +/- 54 mmHg (anesthetized state) to 427 +/- 138 mmHg after ECC (MVD group, P < 0.01) and from 153 +/- 65 mmHg to 377 +/- 101 mmHg (CAD group, P < 0.01). In both groups, PA-aO2 was correlated with Qs/Qr. Four hours after operation, Qs/Qr had decreased significantly to 8% +/- 6% (CAD group) and 10% +/- 6% (MVD group). PA-aO2 and Qs/Qr showed no significant differences between the CAD and MVD groups.

CONCLUSIONS

Qs/Qr is the main pathophysiologic mechanism of gas exchange impairment during cardiac surgery for MVD or CAD. Impairment of pulmonary gas exchange secondary to general anesthesia, cardiac surgery, and ECC are comparable for patients undergoing myocardial revascularization or mitral valve surgery.