The Second Gas Effect: Not a Second Rate Business
Philip J Peyton 1, Maryam Horriat 1, Gavin JB Robinson 2, Robert Pierce 1 Bruce R Thompson 11. Austin Hospital, Melbourne 2. The Alfred, Melbourne
Purpose
A number of studies have demonstrated a faster rate of rise in end-expired partial pressure as a fraction of inspired (PA/PI) for volatile agents in the presence of high concentrations of N2O, consistent with the second gas effect. However, the only study which measured arterial blood concentrations found no difference between patients breathing N2O or N2O-free gas mixtures, casting doubt on the validity of the second gas effect. We compared arterial and end-tidal partial pressures of sevoflurane (Pa/PI sevo and PA/PI sevo) in patients breathing N2O or N2O-free gas mixtures to determine the existence and magnitude of the second gas effect.
Method
With approval from the institutional HREC, 14 patients undergoing general surgery were randomised to receive 2% sevoflurane in either O2, or 33% O2/67% N2O. End-tidal and inspired sevoflurane concentrations were measured while simultaneously arterial blood samples were collected at 2, 5, 10 and 30 minutes. Blood sevoflurane partial pressures were measured by double headspace equilibration.
Result
Where N2O was present, the rate of rise was faster for PA/PI sevo and Pa/PI sevo (p < 0.05). At 2 minutes, Pa/PI sevo was 23.6% higher in the N2O group, declining to12.5% at 30 minutes. This difference was significantly greater than the effect on PA/PI sevo (p < 0.05), which was 9.8% higher at 2 minutes in the N2O group, and 3.9% at 30 minutes.
Conclusion
The second gas effect on arterial sevoflurane partial pressures is 2-3 times more powerful than the effect on end-expired partial pressures. This is explained by the influence of V/Q scatter on the distribution of blood flow and gas uptake in the lung, which serves to magnify the effect on arterial blood concentrations relative to those in expired alveolar gas. A second gas effect of this magnitude would be clinically significant in routine practice.
Time of Presentation
1530
