One-lung ventilation physiology and the hypoxaemia management algorithm

Model answer

Physiology (5). OLV creates true shunt through the non-ventilated lung; Licker describes HPV redirecting flow from the upper non-ventilated hypoxic lung to the dependent ventilated lung to limit shunt and optimise V/Q, via a biphasic calcium response within seconds, modified by acid-base, expansion, volaemia, lung disease, age and anaesthetic drugs.^[1] Abe found propofol gave greater PaO2 and lower shunt than isoflurane/sevoflurane, with shunt falling after volatile-to-propofol conversion and PaO2 rising after 4 cm PEEP during propofol OLV.^[2] Pruszkowski nuance: BIS-titrated sevoflurane vs propofol had similar PaO2 (lowest 16.3 vs 17.7 kPa, ns; 4 per group SpO2 below 90%); 15 excluded for incorrect tube placement/BIS.^[3]

Algorithm (10). FiO2 1.0 and tell surgeon; fibreoptic position check first (Pruszkowski exclusions for incorrect placement^[3]); suction; PEEP to ventilated lung (Abe 4 cm PEEP point^[2]); CPAP to non-ventilated lung — Maroof CPAP 5–10 to non-ventilated lung rescued all 10 failing 100% oxygen^[6], Sentürk CPAP 5 + FiO2 0.5 beat FiO2 1.0 alone (PaO2 198 vs 181; shunt 30 vs 34%; surgery unimpaired)^[7]; recruitment; intermittent two-lung rescue per local protocol; consider propofol per Abe with Pruszkowski BIS nuance^[2][3]; DLT sizing by Brodsky tracheal-diameter method^[5]; low-volume protection per El Tahan (lower pressures but worse intra-OLV gas exchange; preserved post-OLV oxygenation; fewer infiltrates/ARDS; PPCs/HLOS similar)^[4].