Ultrasound guidance of vascular catheterization

Technical concepts The basic principles of ultrasound are covered in Chapter 11, but for the sake of completion they will be re-reviewed here. There are two basic kinds of ultrasound applicable to vascular access, Doppler and B-mode (also known as two-dimensional [2D]). The Doppler principle may be used to determine the velocity of moving objects, such as red blood cells within a vessel. This velocity information may be displayed as a velocity-versus-time spectrum or may be converted into an audio signal, in which different velocities are rendered as different pitches. Anesthesiologists are most familiar with this technique when it is used to “hear” a pulse to measure blood pressure or the presence or absence of circulation in an extremity (e.g. following vascular bypass). Signals obtained from arteries and veins sound distinctively different and thus can be used to help assist in the identification of vascular structures.Doppler ultrasound can be used alone or combined with 2D ultrasound. With the exception of color Doppler, Doppler ultrasound alone is currently rarely used to assist in vascular catheterization. US-guided regional anesthesia requires knowledge of the anatomy as well as basic ultrasound principles, which will allow practitioners to choose their equipment wisely. A number of linear and curved array US probes are available, and newer echogenic needles with distance markers continue to improve visualization. Portable US machines are available that will provide diagnostic-quality images for block performance.