Pulmonary gas exchange and ventilation-perfusion relationships during hypocapnia and thoracotomy in anaesthetized dogs.

The effects of hypocapnia and thoracotomy, both individually and combined, on pulmonary gas exchange and distribution of ventilation-perfusion ratio (Va/Q) were studied in anesthetized and paralyzed mongrel dogs by the six inert gas elimination technique. Normocapnia (PaCO2 35 mmHg) and hypocapnia (PaCO2 20 mmHg) were produced sequentially by varying the inspired CO2 concentration. Thoracotomy was performed at the fourth intercostal space. When ventilation was changed from normocapnia to hypocapnia without thoracotomy, PaO2 decreased from 160 +/- 10 to 147 +/- 11 mmHg and Qs/Qt increased from 0.0 +/- 0.0 to 0.6 +/- 0.7%. However, no change was observed in perfusion distribution following thoracotomy during normocapnia, PaO2 decreased from 160 +/- 10 to 113 +/- 15 mmHg together with a shift of perfusion toward the low Va/Q region. However, no change was observed in Qs/Qt. When ventilation was changed from normocapnia to hypocapnia with thoracotomy, PaO2 decreased from 113 +/- 15 to 98 +/- 12 mmHg and Qs/Qt increased from 0.3 +/- 0.8 to 3.4 +/- 2.0%. After thoracotomy, a shift of perfusion toward the low Va/Q region was observed, which was probably responsible for the decrease in PaO2. The decrease in PaO2 during hypocapnia was due to an increase in the true shunt rather than the development of low Va/Q region. Hypocapnia combined with thoracotomy produced a further reduction of PaO2 and a greater increase in Qs/Qt.