Pulmonary & Respiratory Medicine

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

Abstract

Kavitha Bagavathy, Michael Fong, Luanda Grazette, Zhanghua Chen, Ahmet Baydur

Rationale: Inspiratory muscle endurance as expressed by the tension-time index of the diaphragm [TTI=(Pdi/ Pdimax) × (Ti/Ttot)] in normal subjects and cardiorespiratory disorders requires the use of esophageal and gastric balloons. A noninvasive technique can be used in which the ratio of tidal volume to vital capacity (Vt/VC) is substituted for Pdi/Pdimax, with the resulting relationship [(Ti/Ttot) × (Vt/FVC)] called the breathing intolerance index (BIT). The response to medical management of BIT in patients with cardiomyopathy with chronic heartfailure has not been assessed before and after medical management.
Objectives: To compare control of ventilation and BIT in patients with stable dilated cardiomyopathy at rest and exercise, and to analyze BIT, oxygen uptake and carbon dioxide elimination at baseline and approximately one year after initiating medical management.
Methods: Control of ventilation and BIT were assessed in 24 patients (mean age 55.5 years; 17 males) at rest and at peak exercise during bicycle ergometry, at baseline and approximately 14 months later.
Results: Median peak VO2 was 12.9 mL/kg/min and 14.3 mL/kg/min at baseline and followup, respectively (p<0.036, adjusted for age, gender and BMI). It increased 4.3 times from rest to peak exercise at baseline and 4.7 times at followup (NS). Peak V’O2 increased by 10.5% between baseline and followup (p=0.036 after adjusting for age, sex and BMI). BIT did not change significantly. Peak V’O2/BIT increased significantly from baseline to follow-up (p=0.008, adjusted for age, sex and BMI). No patients died or experienced acute heart failure during the study.
Conclusions: Peak V’O2 in relation to non-invasively measured peak tension-time index of the respiratory muscles (BIT) increases significantly after one year of medical management, indicating increased efficient oxygen utilization as cardiac function improves. BIT is useful for noninvasively assessing inspiratory muscle endurance and relating oxygen uptake to ventilation in patients with dilated cardiomyopathy and chronic congestive heart failure