Effects of ventilatory pattern and body position on lung volume in dogs

¹ U.S. Army Medical Research and Nutrition Laboratory, Fitzsimons General Hospital, Denver, Colorado

The functional residual capacity (FRC) of paralyzed pump-ventilated dogs was measured by open-circuit helium dilution at various rates and volumes of ventilation and the FRC's of spontaneously ventilated dogs were compared after changes in body position. When tidal volume was constant, increases in rate and minute volume increased FRC. The FRC tended to increase with larger tidal volumes when minute ventilation was constant. An inclination of 30 degrees increased FRC 23.8% (mean for 41 dogs) above the FRC measured in the horizontal position, whereas a declination of 30 degrees decreased FRC by 65.8% (mean for 8 dogs). It is suggested that larger tidal or minute volumes open pulmonary units which are closed by surface tension forces when the volume or duration of inflation is less. Tilting head-up or head-down changes the position of the diaphragm and the abdominal contents in the thoracic ‘cylinder’ to accommodate more or less gas volume in the thorax.