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Correspondence |
Toronto, Ontario
I thank Drs. Vassiliadis and Barbetakis for their interest in the effects of lateral position on one-lung anesthesia. In their study of 23 patients they were not able to demonstrate any difference in oxygenation between left and right thoracotomies. Our previous study1 of 80 patients and another of 200 patients2 found that the mean PaO2 levels were higher during left thoracotomies.
Usually, the left lung is less perfused than the right and there is less shunt when the left lung is collapsed. Hurford et al.3 showed that the PaO2 during one-lung anesthesia varies with the proportion of perfusion to the ventilated lung. Given the data of Drs. Vassiliadis and Barbetakis, it would require 130 subjects to have adequate power to determine that the side of surgery did not have an effect on oxygenation during one-lung anesthesia.4
These authors found a trend towards poorer oxygenation during both two- and one-lung ventilation in the right-lateral position. This may be due to shunt in the dependent lung. The authors used 66% N2O in oxygen during two-lung ventilation. This mixture contributes to atelectasis, particularly in the dependent lung in the lateral position.5
Their observations on positioning and arterial end-tidal CO2 (Pa-ETCO2) gradients are interesting. At the onset of one-lung ventilation there is a sudden increase in deadspace that is ameliorated over the first several minutes as hypoxic pulmonary vasoconstriction redistributes blood flow back to the ventilated lung. The Pa-ETCO2 gradient during one-lung anesthesia is dynamic and measurements will depend on the time they are taken. Unfortunately the authors do not specify at what time during lung collapse they made their measurements.
References
1 Slinger P, Suissa S, Triolet W. Predicting arterial oxygenation during one-lung anaesthesia. Can J Anaesth 1992; 39: 1030–5.
2 Lewis JW Jr, Serwin JP, Gabriel FS, Bastanfar M, Jacobsen G. The utility of a double-lumen tube for one-lung ventilation in a variety of noncardiac thoracic surgical procedures. J Cardiothorac Vasc Anesth 1992; 6: 705–10.[Medline]
3 Hurford WE, Kolker AC, Strauss HW. The use of ventialtion/perfusion lung scans to predict oxygenation during one-lung anesthesia. Anesthesiology 1987; 67: 841–4.[Medline]
4 Lerman J. Study design in clinical research: sample size estimation and power analysis. Can J Anaesth 1996; 43: 184–91.
5 Rothen HU, Sporre B, Engberg G, Wegenius G, Hedenstierna G. Airway closure, atelectasis and gas exchange during general anaesthesia. Br J Anaesth 1998; 81: 681–6.
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