Measurement Error of Cardiac Output Determined by Nitrous Oxide Rebreathing and Impedance Cardiography in Healthy Adults

Abstract

Cardiac output (Q̇) is a fundamental physiological variable but remains challenging to measure. Exercise Q̇ is commonly measured by inert gas rebreathing using acetylene or nitrous oxide (Q̇N2O-IGR) and impedance cardiography (Q̇IC), but device measurement error has not been assessed at different workloads, or in females. This study determined the precision of Q̇N2O-IGR (Innocor, COSMED Inc., USA) and Q̇IC (Physioflow Enduro, Manatec Biomedical, France) in duplicate in sixty adults (30 females; 22±5 years; V̇O2max: 41.2±8.6 mL.kg-1.min-1) during upright rest and cycling at 50W and 90% peak power output (PPO) (277±71W). Measurement variance was higher for Q̇N2O-IGR vs. Q̇IC (p<0.0001), at 90% PPO compared to 50 W at rest (p<0.001), and in males vs. females at 50W with Q̇N2O-IGR (p=0.005). At rest the typical error (TE), coefficient of variation (CV), and intraclass correlation (ICC) were 0.6. L/min (12.4%), 11.6±8.5% and 0.75[0.62-0.82] for Q̇N2O-IGR and 0.4 L/min (6.9%), 7.5±8.4, and 0.87 [0.79-0.92] for Q̇IC. At 50W, the TE, CV, and ICC were 0.8 L/min (7.7%), 5.4±5.8%, and 0.79 [0.68-0.87] for Q̇N2O-IGR and 0.5 L/min (5.6%), 3.7±3.8%, and 0.85 [0.76-0.91] for Q̇IC. At 90% PPO, TE, CV, and ICC were 1.2 L/min (7.5%), 5.4±4.3%, and 0.89 [0.82-0.93] for Q̇N2O-IGR and 0.7 L/min (4.0%), 5.0±3.2%, 0.96 [0.93-0.98] for Q̇IC. Duplicate Q̇N2O-IGR differed up to ~4 L/min (38%) and Q̇IC ~3 L/min (24%). In conclusion, group-level measurement precision was generally better for Q̇IC than Q̇N2O-IGR but depended on workload and sex. Duplicate Q̇N2O-IGR and Q̇IC differed substantially therefore repeat measures are important.