Cardiology Research and Practice Table 1: Common cardiac information which can be provided by a standard TTE. Left heart Right heart Aorta Estimation of pressures Other Valvular pathologies (ii) Stenoses (iii) Prolapses (iv) Presence of vegetation (i) Dimensions: chamber sizes and thickness (ii) Left ventricular ejection fraction (iii) Regional wall motion abnormalities (i) Dimensions: chamber size and thickness (ii) Right ventricular systolic function: FAC or TAPSE (iii) Signs of pressure or volume overload (i) Regurgitations (i) Dilatation (ii) Dissection (i) Pulmonary artery systolic pressure (ii) Left ventricular ﬁlling pressure (iii) Transvalvular pressure gradients Pericardial eﬀusion and tamponade FAC: fractional area contraction; TAPSE: tricuspid annular plane systolic excursion. window. VTILVOT is obtained by manually tracing the Dop- pler velocity spectrum. The process of summation of the velocities is however automated. An average of 3 to 5 con- secutive VTILVOT is normally used to minimize variability. The CSA of the LVOT is calculated from the diameter of the LVOT obtained from the parasternal long axis window. Measurement of the diameter is done manually, but the calculation of area is automated. Heart rate is obtained by measuring the R-R interval. If accurately done, the TTE obtained CO is comparable to pulmonary artery catheter thermodilution method [5]. Limitations. The major limitations of TTE CO measurement are listed in Table 2. One of the major limitations is the lack of continuous tracking ability. Serial measurements can be done, but is laborious and “round-the-clock” availability of sonographers is a problem in many units. Sudden and rapid changes in hemodynamic status mean ﬁnding the right operator and setting up the ultrasound machine may be too late in some instances. Interobserver variability may also be an issue. Other limitations are related to measurement errors of which there are two: errors in LVOT diameter measurement and in Doppler velocity measurement (Doppler angle error). Measurement of LVOT diameter relies on obtaining a proper longitudinal plane of the LVOT. Slight angulation or lateral misplacement of the transducer will result in obtaining an oblique or tangential plane of the LVOT, hence underestimat- ing the LVOT diameter (Figure 2). Incorrectly identifying the tissue-blood interface may result in under- or overestimation of the diameter. Since the CSA is proportional to the square Cross-section of LVOT 3 Oblique plane Tangential plane Proper longitudinal plane Figure 2: Schematic diagram showing proper and improper ul- trasound longitudinal planes in measuring left ventricular outﬂow tract (LVOT) diameter. Oblique plane is resulted from a tilted angle from the proper plane, while a parallel shift in ultrasound plane results in a tangential planes. Both oblique and tangential planes give rise to underestimation of LVOT diameter. The same is true for measuring inferior vena cava diameter. Table 2: Limitations of cardiac output measurements in ICU by TTE. Cannot provide continuous monitoring Measurements and accuracies can be aﬀected by: (i) Patient’s position (ii) Patient’s condition: for example, lung hyperinﬂation, cutaneous emphysema, trauma, wound (iii) Eﬀects of mechanical ventilation (iv) Suboptimal ultrasound windows: poor image quality (v) Heart plane motion during measurements (vi) Doppler angle error: poor angle alignment (vii) Arrhythmias of the diameter [CSA = π × (diameter/2)2], any error made will also be squared. A 10% error in diameter will result in approximately 20% error in CSA, hence CO. Accurate Doppler measurements demand the ultra- sound beam being parallel to the blood ﬂow (i.e., Doppler angle = 0◦). Deviation from this will result in underestima- tion of blood ﬂow velocity. While a Doppler angle of 20◦ results in an acceptable 6% error, a 30◦ angle will end up with greater than 10% error. It is often forgotten that blood is ﬂowing in a three-dimensional perspective—the 2-dimen- sional (X -Y ) plane as seen on the screen plus a Z-plane which is perpendicular to the screen. Angle correction (for the X -Y plane) is seldom used in echocardiography as the error in Z-plane is unknown. The only remedy is to ensure a proper apical-5-chamber window is used for measurements, and the transducer should be tilted or moved around slightly