The effect of hypocapnia on arterial oxygenation in thoracotomized dogs

The effect of hypocapnia on arterial oxygenation was investigated in unilaterally thoracotomized patients (N = 11) and dogs (N = 9) anesthetized with N2O-O2-enflurane. In patients, a change in PaCO2 from 39.7 +/- 1.4 to 28.2 +/- 1.7 mmHg produced a significant fall in PaO2 from 146 +/- 32 to 126 +/- 27 mmHg. In dogs, the change in PaCO2 from 38.7 +/- 0.8 mmHg (normocapnia) to 22.4 +/- 0.9 mmHg (hypocapnia) produced a significant decrease in PaO2 from 94 +/- 6 to 77 +/- 5 mmHg and a significant increase in pulmonary shunt (Qs/Qt) from 13.5 +/- 1.6 to 19.4 +/- 1.7%. Hypocapnia induced significant decreases in cardiac output and pulmonary arterial pressure; from 3.6 +/- 0.4 to 3.1 +/- 0.3 l.min-1, and from 20.8 +/- 1.3 to 17.5 +/- 1.0 mmHg, respectively. After the thoracotomy, the end-expiratory volume of the lung of the thoracotomized side became smaller. Therefore, a large fraction of low VA/Q regions might have existed in the lung of the studied patients and animals. Since hypocapnia induces an attenuation of hypoxic pulmonary vasoconstriction (HPV), an attenuation of HPV by hypocapnia might have occurred in the present study, which in turn produced a disproportionate increase in perfusion to low VA/Q regions, leading to the increase in Qs/Qt as observed in the present study.