Abstract
A volume cycled (Engstrom) and a pressure cycled respirator (Bird Mark VII) were compared in 20 patients in chronic respiratory failure with a carbon dioxide tension (Pco<sub>2</sub>) of over 50 mm Hg. The volume cycled respirator produced twice as great a reduction in Pco<sub>2</sub> (-7.5 mm Hg vs −3.5 mm Hg) and increase in oxygen tension (Pco<sub>2</sub> +8.3 mm Hg vs +3.6 mm Hg) as the pressure cycled respirator. With the volume cycled respirator the transpulmonary pressure and inspiratory flow changed rapidly, the peak inspiratory transpulmonary pressure occurring immediately after peak flow was reached. With the pressure cycled respirator transpulmonary pressure changed more slowly, peak inspiratory flow was less and peak transpulmonary pressure occurred at the end of inspiration. Peak mouth pressure was similar with both respirators (15 cm H<sub>2</sub>O). The volume cycled respirator produced a significant increase in peak inspiratory flow and transpulmonary pressure and a decrease in pulmonary resistance while the only significant change with the pressure cycled respirator was an increase in peak expiratory flow.
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