Arterial oxygen partial pressures during nitrous oxide and xenon elimination

Introduction

A fall in arterial oxygen partial pressure (PaO₂) during nitrous oxide (N₂O) elimination is a well known phenomenon denominated as diffusive hypoxia that is caused by the relatively fast N₂O-elimination compared with the slower simultaneous N₂-uptake. On the one hand, the occurence of diffusive hypoxia during Xenon (Xe) elimination could be supposed because of the rapid recovery from an inhalational (Xe)-anesthesia that can be explained by the low blood/gas partition coefficient (λ_Xe = 0.l4, λ_N20 = 0.47). On the other hand, since diffusion of gas molecules is highly dependent on molecular weight (MW_N2 = 28, MW_N2O = 44, MW_Xe = 133), the velocity of Xe-elimination should not exceed N₂-uptake very much. Therefore we compared PaO₂ during the course of Xe- and N₂O elimination.

Methods

After approval by the institutional animal ethics committee 11 pigs (Xe group=6, N₂O=5) were anestethized and paralyzed. The animals were ventilated using a standard semiclosed anesthesia circuit (Cicero EM, Drägerwerk AG, Germany) with a fresh gas flow (70% Xe + 30% O₂ or 70% N₂O + 30% O₂) corresponding to the minute ventilation. Ventilatory parameters were: tidal volume (V_t) = 12 ml/kg BW, respiratory time (f) = 12/min, inspiration rate (Ti) = 1.5 s, inspiratory breath hold = 1.0 s. After equilibration (45 min) fresh gas composition was switched from Xe or N₂O respectively to nitrogen (N₂) keeping FiO₂ as well as the ventilatory parameters constant. PaO₂ was measured continuously throughout the equilibration period and up to 15 min after switching to N₂ by the continuous blood gas monitoring device Paratrend 7 (Biomedical Sensors, New York, NY).

Results

Data are mean ± SD, ^*P < 0.05 RS Mann Whitney.

Results

Data are mean ± SD, ^*P < 0.05 RS Mann Whitney.

Conclusions

Since PaO₂ decreased to a much lesser degree during Xe-elimination compared with N₂O. Diffusive hypoxia during recovery from Xe-anesthesia seems to be unlikely to occur.