[Drive-time constant (DTC): a new index of respiratory muscle endurance]

Mouth occlusion pressure (P0.1) and the function of respiratory muscle were measured in 6 unsuccessful weaning patients (group I), 15 successful weaning patients (group II) and 9 normal subjects (group III). The results showed that PImax in the three groups was 4.36 +/- 1.67kPa, 6.19 +/- 2.38kPa and 9.20 +/- 3.51kPa respectively. Pdimax was 5.39 +/- 1.62kPa, 6.90 +/- 2.41kPa and 11.46 +/- 3.16kPa respectively. TTdi was 0.24 +/- 0.07, 0.12 +/- 0.04 and 0.05 +/- 0.02 respectively. DTC, the product of P0.1/PImax and Ti/Ttot, was 0.06 +/- 0.01, 0.03 +/- 0.01 and 0.01 +/- 0.003 respectively. Both group I and group II had lower PImax and Pdimax but higher TTdi and DTC other than group III. Both PImax and Pdimax between group I and group II showed no significant difference, but TTdi and DTC of group I were higher than those of group II. If DTC 0.04 and TTdi 0.15 were used as the criteria for predicting whether or not the patients could perform a successful weaning trial, the values of sensitivity were 83% and 100%, and the values of specificity 93% and 87% respectively. We conclude that DTC as a noninvasive and convenient measurement is of clinical value in evaluating the endurance of respiratory muscles.