Non-invasive determination of cardiac output: comparison of a novel CW Doppler ultrasonic technique and inert gas rebreathing.

Background: Cardiac Output (CO) is an important parameter in the diagnosis and therapy of heart diseases. Inert gas rebreathing (IGR) and continuous wave Doppler ultrasound (CWD) are among the most promising newer techniques aiming at a non-invasive, point of care measurement of CO. A direct comparison of the two methods has not yet been carried out.

Methods: 63 consecutive patients were included in the study. CO was measured twice with both CWD and IGR to assess inter-method agreement and reproducibility. The statistical comparisons were performed as proposed by Bland and Altman.

Results: There was a significant correlation between the CO measurements by both methods (r=0.53, p<0.001). Bland-Altman analysis showed a good agreement of measurements with a bias of 0.4+/-1.0 l/min (mean+/-standard deviation). Both methods showed a good reproducibility. CWD measurements were not possible in 14% of patients while IGR measurements were not possible in 5% of patients (p=0.13).

Conclusion: The determination of CO by IGR and CWD revealed a good agreement and reproducibility with a low rate of impossible measurements, suggesting that IGR and CWD can be used interchangeably in the clinical setting.