Breathing Intolerance Index (BIT) and its Relation to Exercise Data: Noninvasive Assessment of Inspiratory Muscle Endurance during Rest and Exercise in Patients with Chronic Obstructive Pulmonary Disease and Cardiovascular Disorders

Abstract

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.

Breathing Intolerance Index and Control of Ventilation, a Non-invasive Method for Evaluating Inspiratory Muscle Endurance at Rest and Exercise, in Patients with Cardiomyopathy: One Year Follow-up

“…They determined that OUEP was the strongest predictor of early mortality. We previously reported that median peak V'O 2 for patients with chronic heart failure was 54% lower than in control subjects [9], similar to findings by Sietsema et al [22] who showed that peak V'O 2 (corrected for weight) was 62% lower than in normal control subjects. Since BIT was not significantly different between normal control subjects and those with heart failure, median V'O 2 /BIT was 46% that of healthy controls, similar to changes in V'O 2 / V' E described by Sun et al [28].…”

“…Resting BIT indices in our patients were not in the range that would predict acute respiratory failure [6]. With exercise, BIT did not increase as much as in healthy subjects, mainly because their Vt increased by only 60% as much as in the controls [9]. Nevertheless, the ventilatory reserves of our patients were greater than those of the neuromuscular and asthma patients reported by Koga et al [6].…”

“…Indeed, none of our patients experienced acute decompensation or died during the period of study, despite borderline values of V'O 2 max. In an earlier study, we found that in patients with chronic heart failure resting V' E was higher than in healthy control subjects [9], as have others [28]. V' E , however, increased by only as half as much as it did in control subjects at peak exercise [9], reflecting decreased strength and endurance of respiratory muscles.…”

“…Koga et al [6] found that during resting breathing, BIT index was, on average, 0.186 in patients with asthma and restrictive thoracic disorders requiring nocturnal noninvasive positive-pressure ventilation, significantly greater than in non-ventilator users (0.087) and nonsmoking healthy volunteers (0.05), the last values being similar to those of our cardiomyopathy patients at rest (0.045 and 0.047 at baseline and followup, respectively). We previously reported resting BIT values in patients with chronic heart failure to be higher than in control subjects, primarily because of lower VCs [9]. Resting BIT indices in our patients were not in the range that would predict acute respiratory failure [6].…”

“…In 2016, our group [9] found that in patients with cardiomyopathy, resting BIT was 50% higher than in healthy control subjects. Even though there was a threefold increase in BIT in these patients at peak exercise, the difference in BIT at peak exercise was not statistically significant between cohorts, suggesting that respiratory muscle endurance was not compromised in the cardiomyopathy patients.…”