In your June 2010 edition Nguyen et al. compared transthoracic echocardiography (TTE), using pulsed wave (PW) Doppler ultrasound, with continuous wave (CW) Doppler using the ultrasonic cardiac output monitor (USCOM), and found significant disagreement. This is surprising. Both Doppler methods measure the same flow-generated Doppler frequency shift, so any controlled comparison should yield identical measures, and both methods have high levels of utility in over 40 years of clinical practice [1]. So how did this lack of agreement occur? TTE is ‘‘conventional echocardiography’’ and is a technically difficult method based on image-guided PW Doppler, (‘‘Duplex Doppler’’). It is expensive, requires 2–3 years of training and experience before qualification and professional registration, has a 45 min examination time and generally requires cardiologic supervision [2]. Further, PW is a user-dependent method with high inter and intra-operator variability requiring precise methodology and adherence to rigid examination protocols and guidelines to achieve clinical effectiveness [3]. It is precisely these clinical limitations that encouraged the development of the USCOM. USCOM is a 2D independent, CW Doppler device with an anthropometric algorithm, automated signal tracing and wide transmit beam which increases reliability, reproducibility and applicability of the Doppler method and adds the benefits of generalisability theory to increase sensitivity [5]. Additionally, operational proficiency can be achieved in a few days by both medical and non-medical personnel, has a 5–10 min examination time [4], and has been extensively validated. TTE and USCOM use different Doppler modalities and any valid comparison demands rigorous methodology and a carefully designed experimental protocol if the results are to reflect the modalities being compared rather than flaws in the study design. Nguyen et al. found disagreement between the methods and concluded equivocally on the clinical utility of USCOM. However, examination of Doppler theory and practice may allow for a more positive conclusion. As cardiac output (CO) = cross sectional area (CSA)9 heart rate (HR)9 velocity time integral (vti), the source of error in this study is easily identified from table 2 of the paper. The study found that comparison Phillips RA, Smith BE. Challenges in comparison of Doppler CO measurement methods and the importance of understanding ultrasound theory and practice. J Clin Monit Comput 2011