Is hypoxic pulmonary vasoconstriction important during single lung ventilation in the lateral decubitus position?

Hypoxic pulmonary vasoconstriction (HPV) has not been demonstrated in human single lung anaesthesia in the lateral decubitus position (LDP). The purpose of this study was to determine whether (1) HPV occurs in the non-dependent, non-ventilated lung, and (2) if the infusion of sodium nitroprusside (SNP) inhibits HPV. During intravenous anaesthesia the tracheas of seven patients were intubated with double lumen endotracheal tubes. Standard monitors plus radial and pulmonary arterial catheters were placed. Patients were positioned in the LDP and haemodynamic and gas exchange data were recorded for each of three stages; I: two-lung ventilation, II: single, dependent lung ventilation (1 LV) and III: 1LV with infusion of SNP. In stage II the PaO2 decreased from 531 +/- 42 mmHg to 285 +/- 42 mmHg (P < 0.05) and Qs/Qt increased from 12.3 +/- 2.7 to 29.0 +/- 6.3% (P < 0.05). With SNP infusion there was a 30% increase in cardiac index (CI) (P < 0.05). The SNP infusion was not associated with changes in Qs/Qt or PaO2. This model demonstrates changes in Qs/Qt and PaO2 associated with single-lung ventilation in ASA I and II patients in the LDP but we were unable to demonstrate inhibition of HPV by SNP.