Pediatric emergency medicine trisk 1121
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Most ultrasound imaging is performed in B-mode (brightness), which is the
standard, two-dimensional representation of the reflected ultrasound waves. As
described above, each pixel represents the intensity and distance (determined by
time) of a returning sound wave. M-mode (motion) essentially creates a single
line of a sound wave through an object of interest, and then displays that image
with respect to time (on the horizontal access). M-mode is useful to document
movement of a structure, such as cardiac valves or the fetal heart.
Finally, D-mode (Doppler) and Color Flow Doppler sense the movement of the
ultrasound waves as they encounter a moving medium, represented by either
color changes or sound from the ultrasound machine. Color Flow Doppler
ultrasound is especially useful when identifying vessels and discerning between
arterial and venous flow. The colors represent flow away from or toward the
probe and do not correlate with arterial or venous flow ( Video 131.1 ).
FIGURE 131.3 Image of femoral anatomy. Note the marker on the screen (arrow ) is a green
dot . By convention, the marker should always be on the left, upper side of the monitor.
DIAGNOSTIC APPLICATIONS
Focused Assessment With Sonography in Trauma
The use of sonography in trauma was one of the very first applications of
emergency POCUS, and many now consider it a standard part of the evaluation of
the injured patient. The basic sonographic question when performing the FAST
examination is “Is there free fluid in the peritoneum or pericardium?” The
enhanced FAST, or eFAST, includes the additional evaluation of the pleural space
for pneumothorax or hemothorax. The overarching principle of the FAST
examination is that hemoperitoneum or hemopericardium is an indication of
organ injury in the setting of blunt or penetrating torso trauma. Blood in the
abdomen or thorax will appear hypoechoic or anechoic (dark) against the
hyperechoic (bright) background of the internal organs (Fig. 131.4 ). Thus, the
detection of peritoneal or pericardial fluid by sonography may be evidence of
injury to the abdominal organs or heart, respectively.
Although computed tomography (CT) remains the study of choice for the
stable pediatric patient with suspected intra-abdominal injury, the FAST scan has
several distinct advantages. First, it can be performed immediately at the bedside
and is interpreted by the person performing the test, who is directly aware of the
clinical context. Second, there is no exposure to the ionizing radiation of CT, and
sedation is not needed for FAST. Additionally, the FAST scan can be repeated
with serial examinations if the patient’s condition changes. Finally, for unstable
patients, CT may not be a viable option, and the FAST scan can frequently
provide valuable information that may guide therapeutic or operative
interventions.
Research pertaining to the FAST scan has been plentiful, mainly focused in the
adult population, with several pediatric studies. Published data indicate that the
sensitivity of FAST scan in children is not as robust as in adults but the specificity
remains very high. Thus, a positive FAST scan should always prompt either
further investigation or therapeutic intervention. A negative FAST scan does not
necessarily obviate the need for CT scan but can still be valuable in patients with
a low pretest probability of intra-abdominal injury.
FIGURE 131.4 A positive FAST examination. Note the anechoic stripe of fluid (arrows ) in
Morison pouch, between the liver and kidney.
Anatomy
When supine, there are several dependent areas of the peritoneal cavity where
blood or fluid has a tendency to accumulate (Fig. 131.5 ). The hepatorenal recess,
also known as Morison pouch, is the potential space located between the liver and
right kidney. The splenorenal recess is the space located between the spleen and
left kidney and no fluid should be seen here in the healthy person. The
rectovesical pouch (male patients) and the pouch of Douglas (female patients) are
formed by the space between the rectum and bladder or uterus, respectively.
These potential spaces form the basis of the FAST abdominal views.
FIGURE 131.5 A: CT cross section of abdominal anatomy. Note the dependent areas of
Morison pouch (MP) and the splenorenal recess (SR). B: Pelvic anatomy. Note the dependent
rectovesical pouch (RP) between the posterior wall of the bladder and stool-filled rectum.
