Figure 2. The sequential gas delivery circuit (SGD) operation.

The valve manifold has an inspiratory valve (1), an expiratory valve (2) and a cross-over valve (3). The latter has a slightly greater opening pressure than the other valves and, when open allows gas to cross from the expiratory to the inspiratory limb. During expiration, gas delivered by the gas blender enters the inspiratory gas reservoir (green dotted line), and exhaled gas enters the expired gas reservoir (blue dashed line).During inspiration, gas is drawn from the stored blender gas in the inspiratory gas reservoir (green dotted lines). If the inspired volume exceeds the volume in the inspiratory reservoir, the balance comes from the expiratory gas reservoir via the cross-over valve (blue dashed line).