2 Cardiology Research and Practice Heart rate Cardiac output Tissue perfusion Stroke volume Contractility Preload Afterload Inotropic state Venous return Atrial pressure Intrathoracic pressure Intravascular volume Vascular resistance Vasomotor tone Figure 1: General scheme of hemodynamics. or the tension on the myocardial wall during end-diastole, is determined largely by the venous pressure hence intravas- cular volume. Cardiac contractility is inﬂuenced by (1) the inotropic state, (2) preload by way of the Frank-Starling mechanism, and (3) to a lesser extent, heart rate and rhythm. LV geometry may also aﬀect stroke volume and is best de- picted by TTE when compared to blind invasive monitoring system. It is apparent that all these factors are intimately connected. Left and right heart function aside, the expression “hemodynamics” is often used to denote the assessments of cardiac output, ﬂuid status, and intravascular pressures, the latter often being used as surrogates for afterload (e.g., ar- terial blood pressure) and preload (e.g., central venous pres- sure and pulmonary artery occlusion pressure). While vas- cular resistance can be assessed by various methods, it is based on information derived from other measurements, such as cardiac output and mean arterial blood pressure. It is therefore not a direct measurement and its clinical value is unclear. 3. Hemodynamic Assessments by TTE The use of TTE in hemodynamic assessment is an attractive approach because the procedure is noninvasive and a fo- cused assessment takes less than 20 minutes. However, the biggest drawbacks are (1) it is not a continuous monitoring technique and (2) study quality can be limited by a num- ber of factors including patient’s position and habitus, co- morbidities, mechanical ventilation, operator expertise, and machine quality. Fortunately, reasonable study quality can usually be obtained in majority of the cases provided that the operator is reasonable skillful such as attained level 1 or basic ultrasound training [3]. Most hemodynamic parameters and other useful information can be extracted even with suboptimal image quality. A standard TTE provides vital information about the heart function (Table 1), the esti- mation of cardiac output and assessment of preload (ﬂuid status) and ﬂuid responsiveness. 3.1. Measurement of Cardiac Output (CO). Cardiac output is the most often used surrogate for monitoring hemodynamic in intensive care unit (ICU). It is used for guiding treatment especially in patients with shock. TTE can provide a point estimate (“snapshot”) of the CO. CO can be determined by either 2D volumetric methods such as the Simpson’s method or Doppler echocardiography. Unfortunately, the 2D image qualities of the critically ill are usually suboptimal hence precluding the use of Simpson’s method. Instead, CO can be more reliably determined using Doppler TTE. CO is measured at the left ventricular outﬂow tract (LVOT), and is based on the mathematical relation of CO = SV × HR, where SV and HR are stroke volume and heart rate, respectively. Echocardiographically, three parameters are needed to work out the CO: (a) LVOT velocity time integral (VTILVOT), (b) LVOT cross-sectional area (CSA), and (c) HR [4]. The VTI is the summation of all velocities per heartbeat and is re- presented by the area under the curve for each heartbeat. The LVOT velocity is obtained by placing the pulsed-wave Doppler sample gate in the LVOT in apical-5-chamber