This system provides the advantages of the Mapleson A, D, and E systems. It can be used efficiently for spontaneous and controlled ventilation in both children and adults.
It consists of two parallel lengths of 15 mm bore tubing; one delivers fresh gas and the other carries exhaled gas. One end of the tubing connects to the patient via a Y-connection and the other end contains the Humphrey block (Fig. 32.24). The Humphrey block (Fig 32.25) consists of an APL valve, a lever to select spontaneous or controlled ventilation, a reservoir bag, a port to connect a ventilator and a safety pressure relief valve which opens at a pressure above 6 kPa.
When the lever is in the A mode (up), the reservoir bag is connected to the breathing system as it would be in the Mapleson A system. The breathing hose connecting the bag to the patient is the inspiratory limb. The expired gases travel along the other tubing back to the APL valve, which is connected to the scavenging system.
With the lever in the D/E mode (down), the reservoir bag and the APL valve are isolated from the breathing system. What was the inspiratory limb in the A mode now delivers gas to the patient. The hose returning gas to the Humphrey block now functions as a reservoir to the T-piece. This hose would open to atmosphere via a port adjacent to the bag mount, but in practice this port is connected to a ventilator such as the Nuffield Penlon.
In adults, an appropriate FGF is 50-60 ml kg-1 min-l in spontaneously breathing patients and 70 ml kg-1 min-1 in ventilated patients.
Was this article helpful?