Echocardiography can be performed in real time at the bedside, permits qualitative and quantitative image analysis, can easily be repeated for monitoring purposes, carries no risk to the patient, and is available at a low cost. It is an operator-dependent technique requiring well-trained personnel. The major prerequisite for an echocardiographic examination is an acoustic window that offers adequate access to the region of interest.

Echocardiography has a prominent role as a diagnostic procedure for cardiac and critical care patients. Transthoracic echocardiography with its different imaging modalities, including two-dimensional (B-mode) imaging, M-mode, and Doppler and color Doppler flow mapping, provides unique information for many clinical problems (Weyman 1994).

Two-dimensional echocardiography generates real-time tomographic images of cardiac and extracardiac structures by rapidly sweeping the ultrasound beam through the acoustic window and recording the intensity of the backscattered signal for every spatial location within the sector sweep. Images can be frozen at selected time points, such as end-systole and end-diastole, to permit measurements of distances and areas. Transthoracic imaging probes use frequencies between 2.5 and 3.5 MHz with better penetration but less resolution. In contrast, transesophageal probes are located very near the target structures and can employ higher frequencies (5 or 7 MHz) to generate images with a higher resolution.

M-mode images show the ultrasound signal along a particular scan line versus time and are used to obtain geometric measurements at selected time points during the cardiac cycle (e.g. wall thickness and ventricular dimensions at end-systolic and end-diastolic time periods). Estimates of myocardial function such as the shortening fraction or wall thickening can then be calculated from these M-mode measurements.

Doppler imaging provides information about the velocity of blood flow within cardiac chambers, across cardiac valves, and within vessels. Continous Doppler imaging records all velocities that are detected along an ultrasound beam irrespective of their location. Continuous Doppler imaging can measure high-velocity jets but is unable to localize the source of the velocity within the Doppler beam. In contrast, pulsed Doppler imaging constrains the signal acquisition to a small sample volume within the ultrasound beam and thus is able to determine precisely the location of the Doppler signal. However, pulsed Doppler imaging is unable to quantify high-velocity jets (> 2 m/s) which typically occur across stenotic cardiac valves.

Color Doppler flow mapping extends conventional pulsed Doppler imaging from one sampling point to an entire sector image and provides a two-dimensional sector map of Doppler velocities. For each spatial location in the two-dimensional sector, the Doppler signal is recorded and displayed as a colored pixel value which encodes the velocity and the direction of flow. With this color coding, the observer can assess the direction of flow (e.g. red towards the transducer, blue away from the transducer) and the characteristics of blood flow (yellow for turbulent flow). Color Doppler imaging can be used to assess the severity of valvular regurgitant flow, the location and direction of pathological flow channels, such as septal defects, aneurysms, or shunts, and the hemodynamic impact of intraluminal obstructions.

Transesophageal echocardiography

The transthoracic acoustic window is limited in patients on mechanical ventilation and also in patients with lung disease, skeletal deformities, or extreme obesity, and diagnostic information may be inadequate. Moreover, anatomical structures that are located deeper within the thoracic cavity can often be difficult to visualize with a transthoracic approach. In these cases, transesophageal echocardiography can be employed to image the heart through a posterior acoustic window and provides a more detailed view of many anatomical areas, including the left and right atria, the intra-atrial septum, and native or artificial valves. Transesophageal echocardiographic imaging is particularly helpful in hemodynamically unstable patients when a prompt diagnosis needs to be established for therapeutic decision-making.

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