Principles of echocardiography for the anesthesiologist

Ultrasound-based diagnostic techniques are now part of the cardiological patients’ chart, while echocardiography is used more often for intraoperative diagnostic as well, as monitoring practices by the anesthesiologist. This article will address the basic physical principles of the ultrasound waves and describe the interactions of ultrasound waves with tissues that lead to the generation of two-dimensional and Doppler ultrasound images Sound waves and ultrasound: definitions Any physical medium can vibrate and produce sound. The sound waves are nothing but tissue vibrations, and are comprised of alternate compressions (areas with tightly packed molecules) and rarefactions (areas where molecules are spaced apart). Sound is depicted as a sine wave with the peak representing the maximum and the nadir the minimum pressure. Vacuum cannot produce sound. Sound waves are described by: period, frequency, speed, amplitude, power, intensity and wavelength. A single sound wave (one compression and one rarefaction) has a temporal (duration or period, sec) and a spatial (wavelength, λ, cm) dimension (figure 1). The number of sound waves in one second is termed frequency (f, Hz), and period × f = 1. Speed (c, m/sec) of sound is the propagation velocity of the sound through the carrying medium (for biologic tissue, such as the human body, c ≈ 1540 m/sec). Sounds of different frequencies travel at the same speed in the same medium, that is, it is the medium that determines the velocity of the sound. The sound velocity in a medium depends on the density and elasticity of that medium. The ability of tissue to transmit sound is called acoustic impedance (Z) and is related to the density of the tissue (ρ): Z = ρ × c. In general, as the density of a medium increases, the velocity of sound through the medium also increases (for example, sound travels faster through bone than liquids). Since c = f × λ, frequency and wavelength are inversely related, so that as the frequency increases, the wavelength decreases. This relationship is important in echocardiographic imaging, as detailed imaging is produced by λ