Pediatric emergency medicine trisk 1122
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In the supine patient, free fluid from the right upper quadrant (RUQ) will tend
to collect in Morison pouch first, whereas free fluid from the left upper quadrant
(LUQ) will often accumulate in the left subphrenic space initially (i.e., not the
splenorenal recess). The amount of intraperitoneal fluid needed for detection by
ultrasound has been reported to be as little as 100 mL in adults and will depend
on the source of the bleeding and patient positioning.
Technique
Probe selection is the first step. A low-frequency (2 to 5 MHz) probe should be
chosen for adequate penetration, most commonly a large footprint curvilinear
probe. In pediatric trauma, however, the smaller head of a phased array probe or
microconvex probe may be more useful to obtain images between the small
intercostal spaces.
There are four views of the FAST examination: (a) hepatorenal recess or
Morison pouch, (b) splenorenal recess, (c) pelvic/bladder view, and (d)
subxyphoid pericardial view (Fig. 131.6 ). Many practitioners also incorporate
views of the thorax to assess for hemothorax or pneumothorax, referred to as the
enhanced FAST or eFAST. The sonographer should perform the FAST
examination in a systematic manner in a standard sequence. This will allow
greater focus on image acquisition and optimization as the examination order
becomes routine.
A view of Morison pouch can be obtained by placing the probe in the coronal
plane (marker toward the patient’s head) in the anterior axillary line between the
seventh and ninth ribs on the patient’s right-hand side. If rib shadows prevent
optimal images, the probe can be rotated slightly in a counterclockwise fashion
such that it is oriented in between and parallel to the ribs. Once the hepatorenal
recess comes into view, the probe can be moved or fanned superiorly toward the
patient’s head and inferiorly toward the feet to visualize Morison pouch
completely, as well as the inferior portions of the liver and kidney ( Video
131.2 ). As mentioned earlier, blood will tend to accumulate in these dependent
portions of the peritoneal cavity initially ( Video 131.3 ).
The splenorenal recess is often more difficult to view. Because the left kidney
sits more superior and posterior than the right kidney, starting position for the
probe is the coronal plane (marker to the patient’s head) between the fifth and
seventh ribs in the posterior axillary line on the left. Rotation of the probe slightly
should help avoid rib shadows. In this view, blood will frequently accumulate
between the spleen and diaphragm, so it is important to visualize the superior
portion of the spleen in addition to the splenorenal junction ( Videos 131.4
[normal] and 131.5 [abnormal]).
The pelvic view is obtained by placing the probe transversely (marker to the
patient’s right), just above the symphysis pubis and angling the probe inferiorly
toward the feet. A full bladder will appear as a large anechoic structure and free
fluid will be seen either posterior to or superior to the bladder wall. For this
reason, a sagittal view visualizing the superior bladder wall is always necessary
for a complete examination ( Videos 131.6 [normal] and 131.7 [abnormal]).
Finally, the subxiphoid view of the heart is obtained. It is important to
remember that the heart sits slightly rotated in the thorax, with the right ventricle
most anterior and the left ventricle posterior and toward the patient’s left hip. The
transducer should lie almost parallel to the abdomen, just below the xiphoid
process, with the marker toward the patient’s right-hand side and probe angled
toward the left shoulder (Fig. 131.6 ). The probe can be slid rightward along the
inferior portion of the rib, using the liver as an acoustic window to avoid the
artifacts caused by air in the stomach. The normal pericardium is seen as a
hyperechoic (bright) line surrounding the heart (Fig. 131.7 ). Pericardial fluid will
appear as an anechoic collection between the myocardium and bright pericardium
( Video 131.8 ). A more detailed description of bedside echocardiography is
discussed in the next section.
FIGURE 131.6 Probe positions for the abdominal FAST scan. (A) Morison pouch, (B)
splenorenal recess, (C) pelvic, and (D) subcostal cardiac.
Pitfalls
For the Morison pouch and splenorenal recess views, the most common difficulty
arises from rib shadows. The probe can be rotated about 20 degrees such that its
orientation is parallel to the course of the ribs above and below. The probe can
also be moved either anterior or posterior to optimize images. It is also important
to recognize the inferior vena cava and the gallbladder in the RUQ scan. Both of
these structures typically appear anechoic and can mimic free fluid in Morison
pouch for the inexperienced sonographer.
The splenorenal recess is more difficult to visualize than Morison pouch
because of the relative superior position of the left kidney and smaller spleen size.
Often, the probe is not positioned posterior or cephalad enough.
A frequent pitfall with the pelvic view is the inability to visualize the bladder.
Sometimes the bladder is empty (i.e., when a Foley catheter has been placed).
More often, the probe is positioned too superior and should be slid and/or angled
toward the feet. Less commonly, the bladder is off of midline to the right or left.
There are several reasons why a sonographer may not view the heart during the
cardiac examination. First, the depth has not been adjusted from the abdominal
scans. The abdominal organs lie relatively closer to the skin than the heart does to
the subxiphoid process, and thus, the heart will often be deeper than the maximal
depth set on the monitor. By increasing the depth prior to the cardiac scan, the
heart will come into view easily. Second, the angle of the probe may be too steep.
Remember that from the subcostal position, the heart lies superiorly and the head
of the transducer must be pointed in that direction. Third, air from the stomach
can scatter the ultrasound beams, rendering the image unreadable. The probe
should be slid to the patient’s right, away from the stomach, thereby using the
liver as an acoustic window instead.
FIGURE 131.7 Normal subxiphoid cardiac view. RV, right ventricle; LV, left ventricle; RA,
right atrium; LA, left atrium.
Cardiac
Cardiac ultrasound as part of the FAST examination was one of the first
applications of POCUS. Subsequently, there has been increasing use of cardiac
examinations for nontraumatic conditions. The basic questions asked when
performing focused bedside echocardiography are (i) “Is the heart beating?” and
(ii) “Is there a pericardial effusion?” Consequently, the two most common
indications for bedside cardiac ultrasound are (i) evaluating for cardiac activity in
patients with cardiac arrest or pulseless electrical activity (PEA) and (ii) assessing
for pericardial effusions. Although these scenarios are much less common than in
