StandardsADE Exhaust Valve Function
Humphrey ADE system with new efficient self-regulating 4 phase exhaust valve
Fig 5
Standard exhaust valves on the Lack or Magill systems have “two phases” such that they are either “open” or “closed” with the valve seat lifting off a knife-edge rim. The new valve on the ADE-circle system is designed with a sleeve or chimney around the rim and opens and closes in four distinct phases. Such a simple design modification confers many advantages in efficiency and function. In addition to those described below, the orange spindle connected to the valve seat is visible at the top of the valve (Fig 5); this lifts with expiration enabling respiration to be monitored. Putting a finger on it closes the valve and allows manual ventilation without needing to screw the valve cap down.
Fig 6a
Phase 1 – early expiration (Fig 6a)
In the first phase the valve seat lifts as the patient breathes out, it lifts into the chimney but the valve still remains effectively closed until the seat reaches the top of the chimney. Consequently during the initial first phase of expiration, “dead space gas” can only flow into the inspiratory limb where it is conserved for the next breath. Clinical trails have shown a drop of up to 30% in FGFs required when compared to the Magill & Lack systems and over 75% to the Bain and T-piece.
Fig 6b
Phase 2 – mid expiration (Fig 6b)
Once the reservoir bag has filled with fresh gas and dead space gas, any further gas from the lungs can only pass into the expiratory limb. The pressure on the underside of the valve seat rises, lifting it above the top of the chimney. The valve now opens and allows what is now alveolar gas to escape into the shrouded casing from where it is scavenged. Resistance is low as the greater the flow the more the valve lifts and opens (see Fig 7). Small cats and kittens can easily breathe through it.
Fig 6c
Phase 3 – late expiration – PEEP effect (Fig 6c)
Another benefit of using the ADE valve is that it prevents lung collapse. This is a common complication in paediatric anaesthesia where it is known that lung volume falls during anaesthesia and partial collapse of alveoli can occur. By applying a small positive back pressure towards and at the end of expiration (PEEP), this collapse can be prevented or reduced. The new valve in phase 3 has this effect since towards the end of expiration the valve seat falls back into the chimney at a pressure of about 1 cm H2O thus preventing the last few milliliters of gas from escaping. The use of the ADE system for children with this valve is now a recommended technique over the T-piece and is likely to be of benefit for small animals.
Fig 6d
Phase 4 – inspiration (Fig 6d)
The fourth and final phase is the inspiratory cycle where the valve seat is held closed by the spring. As no gas can flow back to the patient along the expiratory limb during inspiration, alveolar gas containing carbon dioxide cannot return to the patient. Consequently the only gas inhaled is from the inspiratory limb which contains “dead space gas” and fresh gas.
Resistance through the ADE 4 phase valve
Fig 7
Once the valve seat reaches the top of the chimney it allows gas to escape, but the greater the flow the more the valve seat lifts up over the rim of the chimney and opens up. It is thus virtually resistance free and has been safely used in animals weighing as little as 1 kg, The maximum "PEEP" effect is about 1 cm H2O (arrow)