Comparison of electrical velocimetry and transoesophageal Doppler echocardiography for measuring stroke volume and cardiac output†   C. Schmidt, G. Theilmeier, H. Van Aken, P. Korsmeier, S.P. Wirtz, E. Berendes, A. Hoffmeier, A. Meissner  British Journal of Anaesthesia  Volume 95, Issue 5, Pages 603-610 (November 2005) DOI: 10.1093/bja/aei224 Copyright © 2005 British Journal of Anaesthesia Terms and Conditions

Fig 1 Doppler echocardiographic method of calculating cardiac output at the level of the aortic valve. (a) Short-axis view during end-systole with aortic valve open and multiplane angle 38°. Aortic valve cusps appear as almost straight lines describing an equilateral triangle. The cusp adjacent to the atrial septum is the non-coronary cusp (NCC), the most anterior cusp is the right coronary cusp (RCC) and the remaining cusp is the left coronary cusp (LCC). The length of each cusp is measured using the echocardiograph software. IAS, interatrial septum; LA, left atrium; RA, right atrium; RVOT, right ventricular outflow tract. (b) Deep transgastric long-axis view of the left ventricle (LV) for Doppler quantification of instantaneous flow velocities across the aortic valve (AV). The left ventricular outflow tract (LVOT), the AV and the ascending aorta (AA) are aligned parallel to the Doppler beam. IVS, interventricular septum; MV, mitral valve; PW, posterior wall of the LV; RV, right ventricle. (c) Continuous-wave Doppler recording at the level of the aortic valve. With Doppler data, velocity is shown in the form of a spectral display per unit time, conventionally in metres per second. The contours of the spectral Doppler signals are measured by planimetry using the echocardiography system software. As a result the velocity–time integral is displayed in units of centimetres. British Journal of Anaesthesia 2005 95, 603-610DOI: (10.1093/bja/aei224) Copyright © 2005 British Journal of Anaesthesia Terms and Conditions

Fig 2 Electrical velocimetry to calculate bioimpedance cardiac output. (a) A small sinusoidal current is applied to two standard ECG electrodes at the base of the neck and inferior aspect of the thorax. Two additional electrodes 5 cm inside the stimulating electrodes record the changing impedance over that area of the thorax. A left-sided electrode configuration was chosen to allow insertion of a central venous catheter via the right internal jugular vein. (b) ECG impedance waveform ([–dZ(t)]), first derivative of the impedance waveform (dZ(t)/dt) and pulse oximetry ( S p O 2 ) in a representative patient. Although the impedance waveform is shown to have a positive upslope at early systole, [–dZ(t)] actually has a negative sign, showing decreased impedance to alternating current flow and increased conductivity. The increased thoracic conductivity is caused by the systolic pumping of blood into the great vessels from the ventricles. Because blood is the most highly conductive substance in the thorax, periodic increases in conductivity caused by ventricular systole are registered as decreases in impedance to current flow. However, traditionally [–dZ(t)] is shown inverted to demonstrate the affinity of the waveform shape with the shape of the arterial blood pressure waveform. The figure illustrates how the first derivative of the impedance waveform (dZ(t)/dt) is used with an ECG to determine the beginning of electrical systole (point Q), aortic valve opening (point B), maximal deflection of the dZ(t)/dt waveform (point C) and the closing of the aortic valve (point X). Stroke volume and cardiac output are calculated from these fiducial points and displayed on the screen of the monitoring device. LVET, left ventricular ejection time. British Journal of Anaesthesia 2005 95, 603-610DOI: (10.1093/bja/aei224) Copyright © 2005 British Journal of Anaesthesia Terms and Conditions

Fig 3 (a) Scatterplot comparing cardiac outputs measured with transoesophageal Doppler echocardiography (TOE–CO) and electrical velocimetry (EV–CO). (b) Bias plot of the difference in measurements of TOE–CO and EV–CO against the mean of the two results. The plot shows the limits of agreement (n=37 matched measurements). The mean difference between the results (bias) was 0.18 litre min−1. The lower and upper limits of agreement were −0.99 litre min−1 and +1.36 litre min−1, respectively. British Journal of Anaesthesia 2005 95, 603-610DOI: (10.1093/bja/aei224) Copyright © 2005 British Journal of Anaesthesia Terms and Conditions