Focused Echocardiography in the ICU

Abstract

Technological advancements in portable ultrasound units have helped bring high-quality imaging to the bedside of the critically ill patient. The ability to obtain images of a quality that approaches traditional echocardiography imaging systems, including Doppler measurements, has enabled clinicians to obtain dynamic information about cardiac function and cardiopulmonary interaction that was not previously possible. Ultrasound examination, including focused echocardiography, has now become an integral part of care in many critical care units. The availability of transesophageal transducers for portable units has extended the utility of this diagnostic tool in critically ill patients.

Glossary

2D real-time imaging

see B-mode imaging.

Acoustic impedance

a measure of the resistance of tissue to the propagation of ultrasound. It is largely dependent on the density of the tissue and the speed of sound propagation through the tissue.

Apical window

Insonation window located at the apex of the heart, approachable from the transthoracic and transesophageal modalities.

B-mode imaging

two dimensional imaging based on scanning over an area and mapping intensity in two dimensions. By repeating the imaging at a sufficiently fast scan rate, a real-time image can be obtained.

Color flow Doppler imaging

A form of imaging in which Doppler velocity signals are measured over an area, converted to a color map, and overlaid onto the corresponding 2D image.

Compression waves

refers to the longitudinal waves associated with sound traveling through a medium. Longitudinal waves consist of alternating pressure deviations.

Continuous wave Doppler

A form of Doppler interrogation in which a continuous ultrasound is transmitted, with continuous receiving of the reflected waves and calculation of velocity. This mode is sensitive to velocities all along the beam, and thus is useful for finding the maximum velocity only but not where it is located.

Insonation window

A location on the surface of the body or in the esophagus/stomach that is free of interfering structures, allowing acquisition of an image.

M-mode imaging

an imaging mode consisting of a one-dimensional view recorded against time.

Parasternal window

Transthoracic insonation window located to the left of the sternum, about the fourth intercostal space.

Pulse repetition frequency

In pulsed Doppler or B-mode imaging, represents the rate at which the transducer is pulsed. Imaging of deeper structures requires more time for the ultrasound signal to be reflected from the deep structures, requiring a lower PRF, and less information that can be recorded.

Pulsed wave Doppler

A form of Doppler interrogation in which a pulse of ultrasound is transmitted, with the signal received during a specified time window corresponding to a known depth. This mode allows recording of velocities at a particular intracardiac location.

Reflection

A mechanism of ultrasound signal loss that occurs when a wave is reflected away from a tissue interface of differing acoustic impedances.

Scan area

The anatomic area that can be imaged at one time. For linear transducers, the scan area is a square below the transducer. For phased-array sector transducers such as that used in echocardiography, the scan area is pie-shaped, with the narrow angle directly under the transducer.

Subcostal window

Transthoracic insonation window located in the epigastrum below the xyphoid process.