Rationale: Diaphragmatic tension-time index [TTdi = (Pdi/Pdimax)(Ti/ Ttot)] assesses inspiratory muscle endurance during rest and exercise. However, the procedure is invasive and not practical for general use. Recently, Tidal Volume/Vital Capacity (Vt/VC) has been substituted for Pdi/Pdimax, to assess need for assisted ventilation in chronic respiratory disorders. This technique has not been used to assess respiratory muscle endurance in other conditions during rest and exercise.Objective: To compare control of ventilation and BIT in cardiorespiratory disorders and controls and to assess their relation to oxygen uptake (V'O 2 ) and annual acute decompensations.Methods: BIT and V'O 2 were assessed in patients with stable CVD (n = 20), RD (n = 20) and C (n = 20) during bicycle ergometry. ANOVA assessed differences in variables amongst cohorts at rest and peak exercise. Results:Resting BIT was [median (25 th , 75 th percentiles)] 0.06 (0.04, 0.07), 0.06 (0.04, 0.09) and 0.04 (0.03, 0.05) for CVD, COPD and controls, respectively. Compared to resting, BIT increased by 3.0-to 4.6-fold at peak exercise in all cohorts (p < 0.0001). V'O 2 max exceeded resting values by 7.5-, 4.6-, and 9.4-fold in COPD, CVD and C, respectively (all p < 0.0001). When corrected for BIT (V'O 2 /BIT), controls exhibited significantly greater (V'O 2 /BIT) max than CVD and COPD (p < 0.0001). Amongst patients who acutely decompensated, BIT was higher only in resting CVD patients (p < 0.019). Conclusions:BIT is useful for evaluating respiratory muscle effort during rest and exercise in CVD and COPD. Oxygen uptake fails to increase in proportion to BIT in these conditions as much as in controls, reflecting impaired oxygen utilization. Its potential usefulness in predicting acute decompensations should be assessed in larger prospective studies.