Study of circle system without absorber under controlled ventilation with limited fresh gas flow

A biphasic controlled randomised study was conducted to evaluate the differences in fresh gas flow with alterations in different anaesthetic parameters, on 15 adult female patients of American Society of Anesthetists grade I or II, who underwent elective gynaecological operations with identical premedication and anaesthetic technique in which absorber was by-passed. Intermittent positive pressure ventilation was maintained with a tidal volume of 10 ml/kg and fixed respiratory frequency of 16/minute with inspiratory:expiratory ratio of 1:2. N2O (67%) in O2, a non-depolarising muscle relaxant and a narcotic analgesic were used to maintain the anaesthesia, muscle relaxation and analgesia respectively. Phase I and II of the study were characterised by fresh gas flow of 81/minute and 5.61/minute respectively during steady state and data of both phases were collected every 30 seconds for 5 minutes. Arterial blood samples for arterial CO2 and O2 tension studies were taken towards the end of 5-minute period. In phase I a mean fractional concentration of end tidal CO2 of 3.43% +/- 0.42 (p less than 0.01) and a mean arterial tension of CO2 of 24.06 mm of Hg +/- 4.07 were obtained. In phase II mean fractional concentration of end tidal CO2 3.81% +/- 0.39 (p less than 0.01) and mean arterial O2 of 28 mm Hg +/- 2.32 were obtained. Mean arterial blood O2 saturation was above 99% and mean arterial O2 tension was above 160 mmHg in both the phases. The results were statistically significant.