The fresh gas flow decoupling valve and the potential for leaks in the anaesthetic circle breathing system

A fresh gas flow decoupling system is a valve located between the fresh gas flow inlet and ventilator in a circle system breathing circuit. It prevents the fresh gas or oxygen flush from being added to the tidal volume during inspiration by diverting it to the reservoir bag. Thus, a fresh gas flow decoupling system ensures the delivery of an accurate tidal volume and prevents barotrauma and volutrauma [1]. Depending on the relative positions of the circle system’s different components (carbon dioxide absorber, bag and adjustable pressure‐limiting valve, ventilator, fresh gas flow inlet) and the type of ventilation used (mechanical or manual), the fresh gas flow decoupling valve may, under certain circumstances, mask massive leaks in the circle system.

We experienced a leak in the circle circuit of a Primus® anaesthesia workstation (Dräger Medical, Lübeck, Germany), caused by a defect in a carbon dioxide absorber inserted intra‐operatively. The leak was not initially apparent during mechanical ventilation but became obvious when manual ventilation proved impossible. The design of the circle circuit (Fig. 1) makes it easy to understand why manual ventilation is impossible when there is a leak in the carbon dioxide absorber. Firstly, the carbon dioxide absorber is located between the fresh gas flow inlet and the reservoir bag. Fresh gas flow or an oxygen flush has to move past the leak in the carbon dioxide absorber before reaching the bag, which explains why the bag cannot fill. When the bag is squeezed, all the gas immediately escapes the circuit through the leak in the carbon dioxide absorber. Secondly, the fresh gas flow decoupling valve, located between the piston ventilator and the carbon dioxide absorber, isolates the piston ventilator and the patient from a potentially leaky carbon dioxide absorber, making mechanical ventilation possible despite the leak. During exhalation, ambient air can be drawn from the leak into the circuit. This inflow might dilute the inspiratory gas and thus reduce the fraction of inspired oxygen and anaesthetic gas concentration [2, 3].

Figure 1.

Gas flow diagrams during inspiration under (a) mechanical and (b) manual ventilation on the Primus® anaesthesia workstation with the leak in its absorber. (a) Under mechanical ventilation, the fresh gas flow decoupling valve covers the leak and enables delivery of the tidal volume. (b) Under manual ventilation, the green arrows represent the gas flow direction taken when squeezing the bag or when attempting a rapid oxygen flush. V, volume measurement; P, pressure measurement; PEEP, positive end‐expiratory pressure; APL, adjustable pressure‐limiting. Modified with the permission of Dräger Switzerland. Animation available at https://static.draeger.com/trainer/primus_trainer_en/start.html#id=D1100 (accessed 03/28/2021).

Leaks solely apparent on manual ventilation have previously been described in workstations sharing similar circuit configurations (Perseus A500® and Fabius®, Dräger Medical, Lübeck, Germany) [2, 3]. In other companies’ circuits, the bag, adjustable pressure‐limiting valve and the ventilator share a common connection with the circuit [4]. In this configuration, both mechanical and manual ventilation are immediately impaired if the carbon dioxide absorber leaks. Identifying problems fast enough to avoid hypoxaemia or awareness can be very difficult in challenging anaesthetic environments. To mitigate this, anaesthetists must always have available a self‐inflating bag, an alternative oxygen source and the equipment needed to switch to intravenous anaesthesia if required [5].