RELATIONSHIP BETWEEN TIDAL VOLUME AND DEADSPACE DURING HIGH FREQUENCY VENTILATION

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Abstract
Physiological (VDphys) and anatomical (VDanat) deadspaces were measured in seven anaesthetized, paralysed and intubated adult patients ventilated at normal and high frequencies. To maintain a constant PaCO2 while increasing the ventilation frequency from 15 to 120 b.p.m., the mean VT was decreased from 454±62 ml (mean±SD) to 117±9 ml. The mean VDanat and VDphys decreased from 130± 11 ml to 74 ± 12 ml and 165±24ml to 92±3 ml, respectively, by 80 b.p.m., above which frequency there was no further significant reduction in either. The mean VD/VT ratio increased from 0.36±0.04 to 0.76±0.05. This study showed that the deadspace volume measured conventionally was not a constant factor, was mainly a function of VT and was a determinant of tidal and minute volume requirement even during high frequency ventilation. The variable VDphys showed a wide variation between subjects, and appeared to have a mean minimal value of approximately 1.1 ml kg −1 at 80 b.p.m. in adult human subjects with a tracheal tube in situ — a value about half the VDphys measured at conventional normal tidal volumes and ventilation frequencies.

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