Spontaneous breathing with the bain circuit at low flow rates: A case report

Abstract

THE BAL'q CIRCUIT was introduced because its simplicity of design and lightness made it especially adaptable for operations on the head and neck3 Under conditions of controlled ventilation, physiological Pacoe values obtain with low flow rates. It has been claimed that the same flow rates can be used with spontaneous respiration, and that the Paco2 still stays within the physiological range.l.2However, it has been determined that rebreathing occurs if the Mapleson-D circuit is used with flow rates equal to or less than minute volume, and that indeed flow rates exceeding twice the minute volume are necessary to prevent rebreathing. 3 A patient can compensate for some degree of rebreathing by increasing minute volume and, in fact, with contxolled ventilation higher than normal tidal and minute volumes are actually doing just that. But the qnestion arises whether under anaesthesia all patients can compensate for the degree of rebreathing which occurs when the Bain circuit is used with low fresh gas flow. Even if they can compensate, is the price which must be paid in increased work of breathing justified? It was my impression that some patients were not compensating well enough and at times exhibited some degree of distress when spontaneous breathing was permitted with the Bain circuit. Paco~ values in the range of 55 to 65 torr were occasionally found. But it was impossible to determine the exact reason for this because of the unknown effect of the narcotic and inhalation agents used.It has been shown that with the Mapleson-A or Magill circuit rebreathing does not occur with spontaneous respiration if the fresh gas flow is at ]east 70 per cent of the minute volume. 4,5 If it were possible to compare the response of a patient to a gas mixture delivered by the Bain circuit at 70 per cent minute volume to the response with a Magill circuit at the same flow rates under the same conditions, then some questions might be answered.In a clinical setting, one needs a fairly long operation where the operative and anaesthetic conditions remain reasonably constant. Such an opportunity arose at our hospital. CASE Rm'OlaTA 29-year-old fit male weighing 80 kilograms was scheduled for finger re-implantation following traumatic amputation. The patient was given a standard premedication of meperidine 50 mg and promethazine 25 mg one hour before the operation. Only mild sedation resulted from this.Anaesthesia was induced with thiopentone 4 mg/kg body weight followed by succinycholine 80 mg and tracheal intubation and was then maintained with nitrous oxide (66~o) and oxygen supplemented by 1 per cent halothane.

Comparison of the Bain system and Uniflow universal anaesthetic breathing systems in spontaneously breathing young pigs

“…The specific advantages of their anaesthetic breathing system over those used previously was that it allowed valves and scavenging equipment to be sited at the anaesthetic machine, rather than at the subject's head. Despite controversy over the minimum flows required for safe use, particularly with spontaneous breathing (Bain & Spoerel 1972, 1977a,b; Mansell 1976; Conway et al. 1977; Soliman & Laberge 1978; Spoerel et al.…”