A reservoir bag for the Triservice anaesthetic apparatus

Abstract

The Triservice anaesthetic apparatus (TSA) has always included an oxygen supplementation system comprising a reservoir component distal to a supplementary oxygen supply. Houghton's original account of the apparatus [1] included a Laerdal Resusci Folding Bag Mark 2 but predated the introduction of the Laerdal Reusable 2600 ml Reservoir attachment (Laerdal Medical Ltd, Orpington, UK), with overpressure and entrainment valves, which now comes as a standard component of the Laerdal Resuscitator. Instead, Houghton used a 0.75 m length of open tubing as a reservoir, placed, with a Sander's T-piece, distal to the Triservice vapourisers during anaesthesia (Fig. 1), or distal to the self-inflating bag during resuscitation. Since the introduction of the Laerdal Reusable 2600 ml Reservoir attachment, it has routinely replaced the 0.75 m open tube in the resuscitation configuration but, to my knowledge, its use during anaesthesia has never been described. Original Triservice apparatus. I suggest changing the TSA reservoir component from the traditional open tubing to the 2600 ml reservoir bag (Fig. 2). The twin valve component of the reservoir system must be included to provide protection against overpressure and to allow air entrainment with minimal resistance. I have bench-tested this valve system at the Multi-National Division (South-east) (MND (SE)) Field Hospital in Iraq. Using the Laerdal self-inflating bag as an artificial lung in drawover configuration and the Datex S/5 spirometry system as a pressure monitor, no difference in the resistance of the system was detected at tidal volumes of 250, 500, 750 and 1000 ml when compared with the open tubing reservoir. Triservice anaesthetic apparatus with Laerdal Reusable 2600 ml Reservoir. This simple modification would appear to offer some advantages over the open tubing system, although its use would be limited to spontaneously breathing patients under anaesthetic and pre-oxygenation. Pre-oxygenation with the TSA has been shown to be unsatisfactory at minute volumes greater than the flow of oxygen into the apparatus [2]. Nonetheless, it remains common practice to use either the TSA or the Laerdal Resuscitator with high flow supplementary oxygen for this purpose. This technique consumes more oxygen than using an alternative system, such as a Bain, as suggested by Lowe and McFadzean [2], but does not require the carriage of additional equipment or a complete change of the breathing system after induction of anaesthesia. If the 2600 ml reservoir is used distal to the Triservice vapourisers instead of open tubing, it will give a clear indication of when the oxygen reservoir is being emptied during inspiration and air entrainment is taking place. Oxygen flow can then be more accurately adjusted to match minute volume and just prevent air entrainment during pre-oxygenation. This minimises the consumption of valuable compressed oxygen supplies – a significant advantage in field conditions. Furthermore, the 2600 ml reservoir bag may offer another advantage when maintaining anaesthesia in a spontaneously breathing patient with the TSA. Oxygen supplementation is mandatory during spontaneous ventilation [3]. Fortuitously, this will cause some movement of the reservoir bag during inspiration and provide a more obvious visual indication of respiratory rate from the breathing system itself. The use of this system has so far been limited to pre-oxygenation of patients undergoing non-emergency surgery at the MND (SE) Field Hospital while the ethical implications of its more widespread use are clarified. Fractional inspired oxygen concentrations of greater than 0.9 have consistently been achieved at supplementary oxygen flow rates of less then 8 l.min−1. The substitution of the open tube reservoir with a Laerdal reservoir bag in the TSA may offer a further improvement in this reliable and well-established piece of military anaesthetic equipment.