Introduction: The sustained maximal inspiratory pressure (SMIP) is a novel measure of inspiratory muscle performance that reflects the ability of the inspiratory muscles to maintain force over time from residual volume (RV) to total lung capacity (TLC) and has been described as single-breath work capacity. In chronic obstructive pulmonary disease SMIP has been found to be more highly correlated to a greater number of physiologic and functional measures than maximal inspiratory pressure (MIP). This study examined the relationship of SMIP and MIP to a variety of physiologic and functional measures in patients with heart failure (HF).
Methods: Fifty male Veterans (mean±SD age, BMI, LVEF = 69.9±8.0 years, 33.6±7.2 kg/m2, 40.1±14.4%, respectively) with HF underwent measurement of MIP and SMIP with the PrO2 device (Smithfield, RI) which were measured from RV during initial inhalation and to TLC, respectively. Pulmonary function and maximal expiratory pressure (MEP) were also assessed. Exercise capacity was assessed via cardiopulmonary exercise testing (CPET) and leg strength testing. Physical function was assessed with grip strength, 6-minute walk distance (6MWD), and sit-to-stand test (STS).
Results: The mean±SD MIP and SMIP were 66.3±24.8 cmH2O and 351.3±151.4 PTU, respectively. Both MIP and SMIP were significantly (p<0.05) correlated to many physiologic (r=.33-.48) and functional measures (r=.38-.62) with the greatest correlate to MIP being MEP (r=.58) and the greatest correlate to SMIP being FEV1 (r=.62). The greatest physiologic correlate to MIP and SMIP was CPET peak tidal volume with identical r-values (.48). Absolute and relative peak oxygen consumption (pVO2) were correlated to both MIP and SMIP with identical r-values for absolute pVO2 (.33) and slightly higher r-values between MIP and relative pVO2 (.37 vs .34). The greatest functional correlate to MIP and SMIP was leg strength/endurance (r=.53 and .46, respectively) followed by STS (r=.52 and .45, respectively) and 6MWD (r=.49 and .45, respectively).
Conclusions: MIP and SMIP are significantly correlated in a positive direction to many physiologic and functional measures supporting the role of inspiratory muscle training and further study of SMIP and MIP in HF.