X-Y-T recorder with body plethysmograph for determining functional residual capacity from thoracic gas volume.

The advantages of determining thoracic gas volume and functional residual capacity (FRC) via the body plethysmograph have long been recognized.1 In our laboratory, we have found that employing a standard X-Y-T recorder in conjunction with the plethysmograph enables us to determine a more precise measurement of FRC from our measurement of thoracic gas volume. During plethysmographic study, the technician has the responsibility of determining when the subject is at an end resting tidal volume (TV) or FRC so that the mouth shutter can be closed and the panting maneuver initiated if the technician is to assume that the measured thoracic gas volume represents the subject’s FRC. In our laboratory, we have often had di culty determining when the patient was at true FRC. The operating manual for our plethysmograph (Cardio-Pulmonary Instruments 2000TB) suggests observing the oscilloscopic tracing that occurs during tidal breathing. When the tracing has stabilized and is consistently retracing itself, the shutter can be closed at end of expiration;2 however, this technique does not allow the technician to know at exactly what pulmonary volume the subject is when the shutter closes. Great variations in the measurement of FRC and thoracic gas volume will occur if the shutter closes at pulmonary volumes above or below FRC. By employing an X-Y-T recorder and by plotting volume on the Y axis and time on the X axis (in order to monitor tidal breathing), it is possible to more accurately assess when the patient has reached a stable TV. Since there is no volume