Focussed Assessment with Sonography in Trauma (FAST)
In major trauma, it is crucial to identify the patient who may be bleeding into the peritoneum following blunt abdominal injuries. Although CT scan is very reliable, transferring a bleeding patient out of the emergency room may be hazardous. Diagnostic peritoneal lavage (DPL) has been used in this situation; however, it may be oversensitive leading to negative laparotomies. Besides, serial examinations to check for ongoing bleed is not possible.
FAST surveys the peritoneal and pericardial cavity for free fluid through standard views with the subject lying in the supine position. A curvilinear phased array probe of ...view middle of the document...
It is quick, non-invasive and does not require transfer out of the Emergency Room; besides, repeat examinations can be performed as appropriate. The specificity and accuracy of FAST has been reported to be 98-100%.
Aorta, liver, gall bladder and kidneys
The abdominal aorta can be assessed in the patient suspected to have an aneurysm by visualizing the entire length of the aorta. A diameter of less than 3 cm all along its path would help to exclude an aneurysm. If the examination is suspicious, further investigations like CT scan may be carried out. Bowel gas may be an impediment to the adequacy of viewing of the aorta. In acute kidney injury, US may be employed to exclude an obstructive cause. The renal resistive index may help to predict acute kidney injury and to prognosticate outcome. The liver and the gall bladder are well visualized on the US scan. A gall bladder wall thickness of more than 3 mm may suggest acute cholecystitis, although it represents a non specific sign that occurs in many different disease processes. The normal diameter of the common bile duct is less than 4 mm; a diameter of more than 7 mm may suggest biliary obstruction.
It may be important to monitor intracranial pressure (ICP), especially in traumatic brain injury, to detect any rise and direct appropriate therapeutic intervention. Conventionally, ICP is measured invasively by intra-parenchymal or intra-ventricular catheter insertion. Measurement of the optic nerve sheath diameter (ONSD) by ocular ultrasonography may be a useful tool to measure intracranial pressure at the bedside. A high frequency (7.5 MHz) linear probe is applied over the closed upper eye lid with the optic nerve sheath perpendicular to the scanning axis. The optic nerve sheath is measured 3 mm posterior to the globe. In adults with traumatic brain injury, an ONSD diameter cut off value of 5.7 mm predicted raised intracranial pressure of 20 mm Hg or more. In another prospective study, a single blinded operator measured ONSD in 27 adult patients with traumatic brain injury and compared it with invasive ICP measurements. A cut off of 5.2 mm had a sensitivity of 83% (95% CI = 35.9% to 99.6%) and specificity of 100% (95% CI = 83.9% to 100%) to predict ICP rise of more than 20 mm Hg.
Transcranial doppler Ultrasonography (TCD) may be used to diagnose brain death. TCD is performed using a low frequency probe – a 2-4 mHz echocardiography probe is suitable. The middle cerebral artery is most often studied; insonation is through the temporal window. The probe is placed about 2 cm above a line joining the tragus to the lateral canthus of the eye. The probe is placed at right angles to the skull with the probe marker pointing anteriorly. The middle cerebral artery signal is found at a depth of around 50 mm. Once a signal is obtained, pulse wave doppler is applied to measure velocity of flow. In cerebral circulatory arrest, typical flow patterns are seen. Diastolic flow...