Figure 25-11.

Pressure (closed-type) plethysmograph.

The subject breathes through a shutter/pneumotachygraph. The shutter is open during tidal breathing and for measurements of airway resistance, and closed for measurements of thoracic gas volume. When the shutter is closed, mouth pressure (equal to alveolar pressure at no flow) is measured by a pressure transducer (1). The pneumotachygraph measures airflow with another transducer (2), and the flow signal is integrated to volume electronically. The plethysmograph pressure is measured by a third transducer (3). The signals from the three transducers are processed by a computer. Excess box pressure caused by temperature changes when the subject sits in the closed box is vented through a valve.