LAUSR

BACKGROUND There is no established method to quantitatively measure the presence and the severity of respiratory instability (RI). The purpose of this pilot study was to propose a novel index of diurnal RI as a surrogate measure of clinical severity of heart failure (HF). METHODS AND RESULTS We prospectively enrolled 60 patients with symptomatic HF [70±14 years, 75% male, and New York Heart Association (NYHA) functional classes II-IV] who underwent right heart catheterization (RHC), and recorded diurnal respiration using a nasal pressure sensor during bed rest while awake within 2 days before or after RHC. Non-uniformity of the breath-by-breath respiratory slopes during 15min calculated as the ratio of peak expiratory amplitude to corresponding peak-to-peak interval was assessed by histogram-based frequency distribution measurement, and was defined as the "RI-index". The RI-index was significantly different among NYHA functional classes and was highest in NYHA class IV. The presence of atrial fibrillation (β coefficient: 0.300, p=0.01) and stroke volume index (β coefficient: -0.462, p<0.01) were independently associated with RI index among hemodynamic parameters. Furthermore, the high RI index above the median value was the independent predictor of the composite outcome of death from any cause, a life-threatening arrhythmia, and an unplanned hospitalization for worsening HF. CONCLUSIONS The RI index stratified functional severity of HF well, and was a significant independent predictor of poor outcomes.