Critical Care and Resuscitation • Volume 19 Number 3 • September 2017 ORIGINAL ARTICLES 222 Comparison of continous-wave Doppler ultrasound monitor and echocardiography to assess cardiac output in intensive care patients Ahmad Elgendy, Ian M Seppelt and Andrew S Lane Echocardiography is commonly used in critically ill patients for haemodynamic assessment, and there are multiple emerging invasive and non-invasive bedside haemodynamic monitors available for the intensive care unit. In the 21st century, formal echocardiography is accepted as a gold standard for assessment of cardiac output and cardiac function, 1 but is time consuming and relies on the availability of appropriately trained personnel. Echocardiography provides reliable non-invasive information in real time for making critical decisions, such as about fluid management, vasoactive or inotropic drugs, and the need for pericardiocentesis. 2,3 Stroke volume (SV) is assessed echocardiographically by first measuring the diameter (and thus the radius) of the left ventricular outflow tract (LVOT) in the parasternal long axis window. Then flow is measured with pulse-wave (PW) Doppler ultrasound of LVOT in the apical five-chamber view, and the velocity–time integral (Vti) is calculated. SV is calculated from the equation: SV = πr²  Vti (LVOT) in which r = radius of the LVOT (in cm), calculating the SV in millilitres. 4 Cardiac output is calculated by multiplying SV by heart rate. Echocardiography can be used to assess volume status by measuring the inferior vena cava (IVC) diameters and looking for respiratory variation in patients who are predicted to be fluid responsive. An IVC collapsibility index [(maximum diameter – minimum diameter)  maximum diameter]  100 that exceeds 40% in spontaneously breathing patients, and an IVC distensibility index [(maximum diameter – minimum diameter)  minimum diameter]  100 that exceeds 15%, predicts fluid responsiveness in mechanically ventilated patients with a sensitivity and specificity of 90%. 5,6,7 PW Doppler scan of LVOT can be used in mechanically ventilated patients in sinus rhythm to assess respiratory changes in SV and predict fluid responsiveness. A variation of more than 12%–17% predicts a positive response to fluid. 8 The ultrasonic cardiac output monitor (USCOM) (Uscom Ltd) is a relatively new monitor based on the same suprasternal continuous-wave (CW) Doppler principles, and therefore measures the highest velocity in its directed path, which is across the valve (see uscom.com.au). The CW Doppler transducer is positioned suprasternally and the ABSTRACT Background: Continuous-wave Doppler (CWD) ultrasound through the left ventricular outflow tract is one modality used for non-invasive cardiac output measurement. The ultrasonic cardiac output monitor (USCOM) is a relatively new monitor which uses a small, transcutaneous ultrasound probe to measure cardiac output with CWD via the suprasternal window. It is faster and less complex to train new users than conventional echocardiography. In addition to stroke volume (SV), the USCOM can calculate stroke volume variation (SVV) and the Smith–Madigan inotropy index (SMII), which is an estimate of the pre-load independent contractility of the myocardium. Objective: To assess the level of agreement between cardiac output measured with conventional echocardiography and with USCOM. Methods: A prospective, observational, multicentre trial of patients admitted to the intensive care units of two hospitals. After excluding patients with aortic stenosis, any patient undergoing a clinically indicated echocardiogram also underwent a subsequent USCOM study for comparison. Results: We enrolled 121 patients in the study, with aortic stenosis the main reason for patient exclusion. Of the study patients, 63% were mechanically ventilated, 84% were in sinus rhythm, and the mean age of the study cohort was 66 years (SD, 17 years). There was a very strong correlation between SV as measured by the USCOM and by echocardiography. The mean difference in SV was 0.33 mL (SD, 5.62 mL), r 2 = 0.956, and Bland–Altman analysis confirmed no significant bias with acceptable limits of agreement between the methods. Patients who were fluid responsive had an SVV cut point on the receiver operating characteristic curve of 21%, and sensitivity and specificity of 95%. A low SMII (< 1.1 watts/m 2 ) calculated with the USCOM did not correlate well with low cardiac output status, with a sensitivity of only 69%. Conclusions: SV (and thus cardiac output) measured using the USCOM correlated well with echocardiographic cardiac output measurement, which suggests that the USCOM could be a valuable haemodynamic tool for assessment of cardiac output and fluid responsiveness in critically ill patients if patients with aortic stenosis are excluded. Inotropy, as a parameter of low cardiac output, was not useful in this cohort of patients. Crit Care Resusc 2017; 19: 222-229