Noninvasive thoracic impedance cardiography (ICG) was recorded via standard tetrapolar spot electrode placement along the spine of each subject. One electrode was placed at the top of the neck, and a second electrode was placed roughly at the seventh vertebra of the cervical spine (approximately 5 cm from the first electrode). A distance of 25–35 cm (depending on subject height) was between the second and third electrodes from the uppermost electrode. The Biopac EBI100C system amplified the signals. Delta Z was derived using Acqknowledge 4.4. The software extracted the dZ/dt waveform, providing the following: C-point location using adaptive template matching, X-point location using the minimum dZ/dt 150–275 ms after the C-point, and B-point location using the minimum derivative in the C-QRS interval. Two independent scorers visually detected the B point. Each dZ/dt waveform was visually inspected twice by two researchers for accuracy and manually edited where necessary. Subjects’ height, weight, and distance between second and third electrodes were entered into the analysis routine. The Kubicek/Rho method for stroke volume calculation was selected.
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