The Relationship of Resting Cardiopulmonary Function to Peak and Submaximal Cardiopulmonary Exercise Testing in Older Adults with Heart Failure

Abstract

Background: Heart failure (HF) is a challenging disease that affects more than five million people, half of whom are at least 75 years old. Peak oxygen consumption (VO2peak), the minute ventilation/carbon dioxide production (VE/VCO2) slope, and the 6MWT are powerful prognostic indicators of all-cause mortality and cardiovascular-related mortality. VO2peak and the VE/VCO2 slope, obtained during CPX, have been shown to be useful for monitoring the efficacy of symptom and therapeutic management. Peak cardiac output (Qpeak) would also be an excellent prognostic indicator but traditionally it has been difficult to measure since the measurement is usually highly invasive. At the University of Minnesota, we are able to measure Qpeak noninvasively using the acetylene washin method. Despite the body of evidence supporting these measures, CPX and the 6MWT are not routinely performed on an outpatient basis as a part of HF symptom assessment and management. Additionally, older patients with HF often do not recognize worsening symptoms which frequently lead to hospitalization. In order for nurses to maximize quality of life for patients with HF and affect morbidity and mortality, usable methods for the evaluation of therapeutic efficacy of symptom management and prescribed treatments must be available. N-terminal prohormone brain natriuretic peptide (NT-pro BNP), New York Heart Association (NYHA) classification, and inspiratory capacity, are all obtainable in an office visit and may explain enough variance in peak Q, VO2, the VE/VCO2 slope, 6MWT distance, or all three measurements to be useful in the outpatient setting. Objective: The purpose of this study was to explore the potential of a model that incorporates resting measures, NT-Pro BNP, NYHA classification, and inspiratory capacity, for the evaluation of therapeutic efficacy of symptom management and prescribed treatments for older patients with HF. We hypothesized that there is a relationship between Qpeak, VO2peak, the VE/VCO2 slope, and/or 6MWT distance with NT-pro BNP, NYHA classification, and inspiratory capacity. Method: Twenty-three older patients (mean age 73.6 + 4.5 years old) with HF underwent venipuncture, inspiratory capacity measurement, and performed the 6MWT and CPX per standardized protocol. Qpeak, VO2 peak and the VE/VCO2 slope measurements were recorded during the CPX. NYHA classification was obtained from chart review and assessment. Results: The strongest relationships were between inspiratory capacity and Qpeak (R = 0.77, p <0.0001), and between NT-pro BNP and the VE/VCO2 slope (R = 0.71, p <0.001). Additionally, there was a moderate relationship between NT-pro BNP and VO2peak (R = -0.47, p <0.03) and between inspiratory capacity and VO2peak (R = 0.51, p <0.02). Due to the lack of variance NYHA classification was not included in the regression analysis. The 6MWT distance did not correlate with NT-pro BNP or inspiratory capacity. NT-pro BNP accounted for 22% of variance in VO2peak and 50% of variance in the VE/VCO2 slope. Mean inspiratory capacity accounted for 59% of variance in Qpeak and 26% of variance in VO2peak. The combined measurements of inspiratory capacity and NT-pro BNP explained 42% of the variance in VO2peak (adjusted R2 = 0.42, F (2, 20) = 8.82, p < 0.002). A model of prediction for either Qpeak or the VE/VCO2 slope could not be constructed since only one predictor variable for each outcome variable was statistically significant.