Mechanical ventilation with air-oxygen mixtures during total intravenous anaesthesia. An evaluation of the Penlon Nuffield Series 400 ventilator.

Mechanical ventilation with air-oxygen mixtures during total intravenous anaesthesia is discussed with the associated problem of obtaining a medically suitable source of air in British rooms. Independently powered ventilators capable of entraining filtered room air were thought to be the best solution. The Penlon Nuffield Series 400 ventilator was modified and assessed for use in this mode. An entrainment adapter and circuit was devised which allowed the ventilator to entrain more than sufficient ambient air, premixed with added oxygen for patient ventilation even under adverse conditions. The entrainment system ensured good mixing of the entrained air and oxygen and allowed accurate determination of oxygen concentration in the patient system using a suitable nomogram and without an oxygen analyser. An assessment of one-way patient valves was also undertaken. The Laerdal valve was found to be the most suitable for use with the system described.