Stroke volume measurement during supine and upright cycle exercise by impedance cardiography

Abstract

This study evaluated impedance cardiography (ZCG) estimates of stroke volume (SV) during exercise. Seven subjects were studied at rest and during progressive cycle exercise in supine and upright positions. SV was determined by ZCG (SVzcg) during exercise and for the first 5 cardiac cycles following exercise. SVzcg was compared with separate measurements of SV by CO₂ rebreathing (SVco ₂). Static blood resistivity (ρ) was measured at each level of exercise. No significant differences were found between supine exercise and immediate post-exercise values for the peak of the first derivative of the impedance change (dZ/dt _max), left ventricular ejection time (LVET), or SVzcg. Small differences indZ/dt _max and SVzcg, but not LVET, were found in exercise to post-exercise cycling in the upright position. Intra-individual SVzcg and SVco ₂ were moderately correlated (upright meanr=0.64, supiner=0.42) from rest to 70% of peak$\dot Vo_2$. Similar correlations were found between Pulse-O₂ ($\dot Vo_2$/heart rate, used as an index to SV) and both SVzcg (uprightr=0.73, supiner=0.57) and SVco ₂ (uprightr=0.8, supiner=0.65). The ZCG parametersdZ/dt _max and LVET correlated better with Pulse-O₂ (dZ/dt _max: uprightr=0.92, supiner=0.73; LVET: uprightr=−0.9, supiner=−0.9). SVzcg calculated with the Kubicek equation performed as well as SVco ₂. ZCG might be a superior method if the inversely correlated parameters,dZ/dt _max and LVET, were not expressed as a product to calculate SV.