Ultrasound Blocks for the Anterior Abdominal Wall

2. Ultrasound and Regional Anesthesia | 31
visualization. Tissue layers, nerves and vessels are more clearly
differentiated (Figure 2.1).
Ultrasound and the Needle
When inserted to perform a block, the needle may be
visualized dynamically with the use of either an “in-plane” or
“out-of-plane” approach. An in-plane approach is performed
when the needle is parallel to the long axis of the transducer
(LOX) (Figure 2.4). An out-of-plane approach is performed when
the needle is perpendicular to the long axis of the transducer or
parallel to the short axis (SOX). An out-of-plane approach may
over- or underestimate the depth of the needle (Marhofer 2010).
The needle axis must be parallel and also aligned with the axis of
the probe.
When injecting, local anesthetic spread must be monitored. If
anesthetic spread is not seen, intravascular injection or poor
visualization must be excluded. Needle electrostimulators may
confirm the presence of the nerve because of the twitching of
the muscles caused by the current. However, in abdominal
blocks this effect may not occur.
One of the problems with needle visualization is that
depending on the angle of insertion, some echoes are reflected
out of the plane of the transducer and thus lost (Figure 2.1). The
more the needle is parallel to the transducer, the more the
echoes will be captured from the transducer and the needle
visualized.
Equipment
Ultrasonography is a safe and effective form of imaging. Over
the past two decades, ultrasound equipment has become more
compact, of higher quality and less expensive (Figure 2.2). This
improvement has facilitated the growth of point-of-care
ultrasonography, that is, ultrasonography performed and