FAST

(Focussed Assessment with Sonography for Trauma)

 

Mr. R.Gangahar

Consultant in Emergency Medicine

Rochdale Infirmary.

 

Assessment of the abdomen in the poly trauma patient can be difficult and is often unreliable as a result of distracting injury, altered level of consciousness and non-specific signs and symptoms. However, the consequence of missing a major abdominal injury can be catastrophic. The assessment tools available to clinicians are CT Scanning, Diagnostic Peritoneal Lavage (DPL) and USS. The use of CT scan is reserved for the stable patient and USS has replaced DPL as the initial screening tool for severe abdominal trauma in more than 80% of centres in North America.

The use of ultrasound in the assessment of abdominal trauma was first reported in the 1970’s. Initial results with regards sensitivity and specificity were disappointing primarily because organ specific injury was being investigated. In the 1990’s Grace Rozycki et al published a series of articles documenting the use of ultrasound to identify FREE FLUID in the abdomen in patients with blunt abdominal trauma. The reported sensitivity and specificity was comparable to that of CT Scanning and Diagnostic Peritoneal lavage.

The technique involves the use of a low frequency (3-5 MHz) probe to investigate 4 regions for the presence of Free Fluid. The 4 regions have been called the 4 P’s.

                               Perihepatic

                               Perisplenic

                               Pericardium

                               Pelvis

The area where blood is most likely to accumulate is the hepato-renal pouch and is therefore investigated first. The Perihepatic space is investigated in long section by placing the probe in the 10 intercostal space in the anterior axillary line. The image should reveal the liver and right kidney and fluid should be looked for at the interface between the two organs.

The Peri-splenic window can be difficult to obtain as the spleen lies more superior and posterior than may be expected. The probe is placed along the Posterior axillary line in the most inferior two or three intercostals spaces. Fluid is looked for in the spleno-renal fossa and in the posterior sub-phrenic space.

The sub-xiphoid Pericardial view looks for blood in the pericardial sac. The probe is placed transverse below the xiphoid process and directed towards the left shoulder. A pericardial effusion is recognised as an anechoic stripe between the ventricular wall and pericardium.

The Pelvic view must be performed with a full bladder. The probe is placed in the transverse axis above the symphysis pubis and angled down to the pelvis.  Fluid is commonly detected posterior to the bladder and is differentiated from intra-luminal fluid which has a corrugated appearance and may move with peristalsis.

Any FAST scan that does not visualise all four regions is sub-optimal and inadequate and cannot be used to inform decision-making.

 

Advantages of FAST

 

Non-Invasive

Repeatable     

Non-ionising                NO CONTRA-INDICATIONS

Near patient

Versatile

Reliable (see table below)

Cheap (in the grand scheme of things!)

 

FAST SCAN SHOULD BE USED ONLY AS A “RULE IN” TOOL.

COMPARISON OF DIAGNOSTIC TOOLS

 

 

SENSITIVITY %

SPECIFICITY %

DPL

95

99

CT

88

99

FAST

85

97

 

Potential Limitations

 

Operator dependant.

No organ specificity.

Rib shadowing and artefacts reduce accuracy.

Obesity, gaseous bowel distension and sub-cutaneous emphysema make investigation difficult.

Unsuitable for hollow viscus injury.

Low sensitivity for penetrating abdominal injury.

 

Ultrasound is operator dependant and there was initial scepticism among Radiologists about the ability of non-radiologists to perform trauma ultrasound. Rozycki compared a series of papers describing FAST by non-radiologists and radiologists. The results are summarised below.

 

Rozycki et al

Non-Radiologist

Radiologist

Sensitivity

93.4%

90.8%

Specificity

98.7%

99.2%

Accuracy

97.5%

97.8%

 

 

Extended FAST Examination e-FAST

 

The Emergency Physician must efficiently address life threatening thoracic injury which accounts for 25% of trauma deaths. In this respect the FAST examination can be extended to include the evaluation of both Haemothorax and Pnemothorax.

Haemothorax

Sisley et al demonstrated that thoracic sonography using the same probe used for the FAST exam could accurately detect traumatic effusions. US was 97.5% sensitive and 99.7% specific compared to chest radiograph’s 92.5% and 99.7%. Ma and Mateer in a separate study demonstrated a 96%sensitivity and 100% specificity.  This experience has led investigators to augment the standard FAST with routine views of the pleural space.

Pneumothorax

 

Normal respiration is associated with a gliding or sliding of the pleural layers upon one another, known as Lung Sliding and best seen at the bases. Comet tail artefacts are reverberation artefacts produced by water filled interlobular septa under the visceral pleura. The absence of Lung Slide and Comet tail artefact establishes the diagnosis of Pneumothorax.

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