Magnitude of the Second Gas Effect on Arterial Sevoflurane Partial Pressure

Author:

Peyton Philip J.1,Horriat Maryam2,Robinson Gavin J. B.3,Pierce Robert4,Thompson Bruce R.5

Affiliation:

1. Associate Professor.

2. Registrar, Department of Anaesthesia, Austin Hospital, and Department of Surgery, Austin Hospital and University of Melbourne.

3. Honorary Anaesthetist, Department of Anaesthesia and Peri-operative Medicine, The Alfred Hospital, Melbourne, Australia.

4. Professor, Institute for Breathing and Sleep, Austin Hospital and University of Melbourne.

5. Head, Department of Allergy, Immunology and Respiratory Medicine, The Alfred Hospital and Monash University.

Abstract

Background A number of studies have demonstrated a faster rate of increase in end-expired partial pressure as a fraction of inspired (Pa/Pi) for volatile agents in the presence of high concentrations of nitrous oxide, consistent with the second gas effect. However, no study has demonstrated a similar effect on arterial blood concentrations. Methods The authors compared arterial and end-tidal partial pressures of sevoflurane (Pa/Pisevo and Pa/Pisevo) in 14 patients for 30 min after introduction of either 70% nitrous oxide or nitrous oxide-free gas mixtures to determine the magnitude of the second gas effect. Blood partial pressures were measured using a double headspace equilibration technique. Results Both Pa/Pisevo and Pa/Pisevo were significantly higher in the nitrous oxide group than in the control group (P < 0.001 on two-way analysis of variance). This difference was significantly greater (P < 0.05) for Pa/Pisevo (23.6% higher in the nitrous oxide group at 2 min, declining to 12.5% at 30 min) than for Pa/Pisevo (9.8% higher in the nitrous oxide group at 2 min) and was accompanied by a significantly lower Bispectral Index score at 5 min (40.7 vs. 25.4; P = 0.004). Conclusion Nitrous oxide uptake exerts a significant second gas effect on arterial sevoflurane partial pressures. This effect is two to three times more powerful than the effect on end-expired partial pressures. The authors explain how this is due to the influence of ventilation-perfusion scatter on the distribution of blood flow and gas uptake in the lung.

Publisher

Ovid Technologies (Wolters Kluwer Health)

Subject

Anesthesiology and Pain Medicine

Reference30 articles.

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