Measuring Stroke Volume: Impedance Cardiography vs Phase-Contrast Magnetic Resonance Imaging.

Background: Determination of cardiac output requires measurement of both heart rate and stroke volume. Techniques for measuring heart rate are widespread, and 1 technique for bedside monitoring of stroke volume is electrical impedance cardiography.

Objectives: To determine the accuracy and precision of stroke volume measured via impedance cardiography and whether the technique can be used to detect trends.

Methods: Eleven healthy research participants (22-52 years old) were examined with simultaneous impedance cardiography and phase-contrast magnetic resonance imaging at rest and during exercise. Bland-Altman analysis with repeated-measures correction was used to compare stroke volumes determined with the 2 methods. The suitability of impedance cardiography for detecting trends in stroke volume was analyzed by using the Critchley radial limits of agreement method.

Results: Phase-contrast magnetic resonance imaging indicated a mean stroke volume of 87 (SD, 16) mL at rest; in 9 volunteers, it changed during exercise ( = .04 to < .001); in 2 volunteers, it did not ( = .32, = .06). For the range of stroke-volume measurements (60-122 mL), impedance cardiography yielded underestimates of stroke volumes at the low end (bias, -17 mL) and overestimates at the high end (bias, +17 mL; < .001). Corresponding 95% limits of agreement were 64 mL, a 73% overestimate or underestimate of stroke volume at rest. Critchley radial limits of agreement indicated poor concordance of stroke-volume trends.

Conclusions: Impedance cardiography had low accuracy and precision in measuring absolute stroke volume and was a poor detector of stroke-volume trends.