Whole-body impedance cardiography

Whole-body impedance cardiography (CircMon^R, JR Medical Ltd., Tallinn, Estonia), was used to determine beat-to-beat heart rate, cardiac output and aortic-to-popliteal pulse wave velocity (PWV)^19,20. SVR and left cardiac work (LCW) were calculated from the tonometric BP and cardiac output measured by the CircMon^R. Average supine central venous pressure is ~3–4 mmHg, while during upright position the value is close to zero mmHg^21–23. As central venous pressure was not measured in the present study, the formula for SVR estimation did not include this variable and was calculated as follows: SVR = 79.96 * mean arterial BP/cardiac output; 79.96 was the conversion factor from mmHg/L/min to dyn*s/cm⁵. LCW was calculated as 0.0143 × (mean arterial BP − pulmonary artery occlusion pressure) × cardiac output. Pulmonary artery occlusion pressure was assumed to be 6 mmHg (normal), and 0.0143 was the conversion factor of pressure from millimeter of mercury to centimeters of water, volume to density of blood, centimeters to meters, and conversion from gram to gram force¹⁶. Time-domain estimate of aortic characteristic impedance [(pressure at inflection point - diastolic aortic BP) * systolic time)/(stroke volume * 2)] was calculated according to Chemla et al. so that aortic BP and systolic time were derived from pulse wave analysis and stroke volume from impedance cardiography²⁴.

When using the CircMon^R whole body impedance cardiography, the cardiac output values of correlate well with the thermodilution method (bias 0.00 l/min, 95% CI −0.26 to 0.26) and the direct oxygen Fick method (bias −0.32 l/min, 95% CI −0.69 to 0.05)²⁵, corresponding upright reductions in cardiac output are observed when compared with thermodilution²¹, the upright stroke volume shows good correlation with 3-dimensional echocardiography (r = 0.781, bias 4.1 ml, 95% CI −2.2 to 10.4)¹⁶, and the PWV values show good correlation with the tonometric method (r = 0.82, bias 0.02 m/s, 95% CI −0.21 to 0.25)²⁰. Due to technical problems, PWV from one subject, aortic characteristic impedance and AIx@75 from 3 subjects supine and 3 subjects upright, time to the return of the reflected wave from 3 subjects supine and 4 subjects upright, were missing.