The anatomical and alveolar deadspaces during respirator treatment. Influence of respiratory frequency, minute volume and tracheal pressure.

In 10 patients receiving prolonged respirator treatment, respiratory deadspace was measured at 12, 18 and 24 b.p.m. and with two different minute volumes. The physiological deadspace (the sum of apparatus, anatomical and alveolar deadspaces) was derived using the Bohr equation, and the anatomical and alveolar deadspaces were measured by carbon dioxide analysis. Tracheal pressure was measured concurrently. The physiological deadspace to tidal volume ratio was increased on increasing the frequency, and was slightly reduced by an increase in minute volume. The mean values for anatomical deadspace ranged from 144 to 277 ml under the different conditions studied. Neither frequency nor minute volume had any influence on the VD/VT ratio. The volume of the alveolar deadspace was the less than half of the anatomical deadspace. The VD/VT ratio did not vary with frequency or minute volume. A relationship between tracheal end inspiratory pressure and anatomical deadspace was found, with a correlation coefficient of 0.80. The slope of the regression line indicated a high airway compliance of approximately 10 ml/cm H2O.