Discuss pulmonary blood flow distribution, shunt flow, and partial pressure of oxygen in the arterial blood (PaO2) (fractional inspired O2 concentration [FIO
2] = 1.0) during single-lung ventilation in the lateral position.Answer:
When the nondependent lung is collapsed, HPV will ultimately increase pulmonary vascular resistance and decrease lung blood flow in this area. If no complicating factors exist, HPV should decrease blood flow to that lung by approximately 50%, causing a total reduction from 40% to 20%. The resulting nondependent/dependent lung blood flow ratio during OLV should be 20%:80%.
Once atelectatic, all the blood flowing into the nonventilated lung is shunt flow. Therefore, OLV creates an obligatory right-to-left transpulmonary shunt that was not present during double-lung ventilation. During single-lung ventilation, an ideal total shunt flow of 20% would be expected, if no shunt existed during double-lung ventilation (ignoring the normal 1% to 3% physiologic amount from the bronchial, pleural, and thebesian circulation). PaO2 with an FIO2 equal to 1 should be approximately 280 mmHg if hemodynamic and metabolic states are normal. Clinically, PaO2 (FIO2 = 1) ranges from 150 to 250 mmHg.
Reference(s):
- Lumb AB, Slinger P. Hypoxic pulmonary vasoconstriction: physiology and anesthetic implications. Anesthesiology. 2015;122:932-946.
- Marshall BE, Marshall C. Continuity of response to hypoxic pulmonary vasoconstriction. J Appl Physiol Respir Environ Exerc Physiol. 1980;59:189-196.