Focused Assessment with Sonography for Trauma — sequential acoustic windows screening for free intraperitoneal fluid, pericardial effusion, and (eFAST) pneumothorax at the bedside
The RUQ (perihepatic) view interrogates Morison's pouch, the potential space between the posterior liver surface and the anterior pararenal fat/right kidney. In the supine patient this is anatomically continuous with the right paracolic gutter and subhepatic space, making it the most consistent single collection site for blood tracking down from injuries anywhere in the upper abdomen, pelvis, or diaphragm. Multiple prospective trauma-registry studies have found more free fluid detected here than in any other single FAST window, which is why sonographers are taught to scan it first and most thoroughly.
The transducer is placed in the right mid-to-posterior axillary line, typically between the 11th and 12th intercostal spaces, with the probe marker oriented cephalad so the resulting image is displayed in a coronal plane with the liver superficial (image top) and the kidney deeper. The examiner should not stop at a single static frame: the probe is fanned anteriorly and posteriorly through the entire hepatorenal interface, because small volumes of fluid can layer in only a portion of the recess and be missed on a single cut. A complete RUQ examination also sweeps cephalad to interrogate the right subphrenic space (between diaphragm and liver) and caudad along the inferior liver tip and right paracolic gutter, since isolated subdiaphragmatic or gutter collections can exist even when Morison's pouch itself appears dry.
A normal (negative) view shows the liver and kidney directly apposed, separated only by a thin hyperechoic interface (Gerota's fascia and perirenal fat) with no interposed anechoic stripe. A positive view shows a black (anechoic), non-compressible stripe of varying thickness splitting the liver-kidney interface; larger hemoperitoneum volumes extend this stripe around the inferior liver edge and into the paracolic gutter, and very large volumes can outline bowel loops floating free in fluid throughout the abdomen.
Perinephric fat can mimic a thin anechoic stripe in obese patients, and a preexisting simple renal or hepatic cyst can be mistaken for free fluid if the examiner does not trace its full contour and confirm posterior acoustic enhancement typical of a cyst rather than an irregular, gravity-dependent free-flowing collection. Rib shadowing from the costal margin can obscure the most cephalad portion of the recess; sliding the probe one interspace caudad or having the patient take a deep breath (dropping the diaphragm and liver into view) mitigates this. Because clotted blood can become more echogenic and isoechoic to adjacent soft tissue as it organizes over hours, a delayed re-scan of a patient with a strongly suggestive mechanism but an initially negative RUQ view is part of the rationale for serial FAST examinations discussed later in this module.
The LUQ (perisplenic) view is the anatomic mirror of the RUQ view, assessing the potential space between the spleen and left kidney. Because the spleen sits higher, more posterior, and more medial than the liver — tucked beneath the rib cage and often partly intrathoracic — this window is generally regarded as more technically demanding to obtain and, per several outcome series, slightly less sensitive for small-volume hemoperitoneum than Morison's pouch, even though the underlying pathophysiology (gravity-dependent tracking of blood into a perihepatic-type recess) is identical.
Because the spleen is smaller than the liver and sits higher under the ribs, the probe is generally placed one to two interspaces more cephalad and more posterior than the RUQ view, often requiring the sonographer to reach across or behind the patient. Positioning the probe marker cephalad again yields a coronal image, but the examiner should specifically fan to capture the splenodiaphragmatic (subphrenic) interface above the spleen in addition to the splenorenal interface below it, since in the left upper quadrant free fluid tends to collect preferentially in the subphrenic space before it fills the splenorenal recess — the reverse order of dependency compared with the right side. A left lateral decubitus tilt or a deep inspiratory breath-hold from the patient can substantially improve visualization by displacing the lung and rib shadow out of the acoustic window.
A positive left-sided view again appears as an anechoic stripe outlining the spleen, most often first seen superior to the spleen (subphrenic) or at the splenorenal junction, and can extend along the descending colon into the left paracolic gutter with larger volumes.
Although Morison's pouch alone captures the majority of hemoperitoneum in registry data, blunt splenic injury is at least as common as hepatic injury in blunt trauma, and a meaningful minority of patients with isolated splenic lacerations will have fluid confined predominantly to the left upper quadrant at the time of the initial scan. Omitting the LUQ view would therefore sacrifice sensitivity for exactly the injury pattern this window is best positioned to detect. As with the right side, a small simple perinephric or perisplenic fluid collection should be distinguished from artifact by confirming it is truly anechoic, non-septated in the acute setting, and tracks with the patient's position when feasible.
In a supine patient, the rectovesical pouch in males and the rectouterine pouch (Pouch of Douglas) in females represent the most gravitationally dependent portions of the entire peritoneal cavity below the diaphragm. In principle this makes the pelvic view the single most sensitive site for very small volumes of free fluid — some cadaveric instillation studies have identified collections here below 100 mL — but that theoretical sensitivity is entirely contingent on having an adequately distended urinary bladder to serve as an acoustic window, which is frequently not the case in trauma patients who have been catheterized and drained prior to scanning.
The probe is placed suprapubically and angled caudad into the pelvis, first in a transverse plane sweeping cephalad-to-caudad and then rotated ninety degrees into a sagittal plane sweeping side to side, in order to interrogate the entire posterior wall of the bladder and the cul-de-sac behind it. The fluid-filled bladder acts as an acoustic window, transmitting sound to the deeper pelvic structures with minimal attenuation; when the bladder has already been decompressed by a Foley catheter — extremely common in the trauma bay — the examiner should temporarily clamp the catheter tubing to allow the bladder to refill, or instill a small volume of sterile saline retrograde through the catheter, before concluding the pelvic view is truly negative.
A positive pelvic view demonstrates an anechoic stripe posterior to the bladder (rectovesical pouch) in males or posterior to the uterus (rectouterine pouch / Pouch of Douglas) in females; because this is the most dependent site, even a modest volume of free intraperitoneal fluid from an upper abdominal source may ultimately be most visible here after it has had time to redistribute, which is part of the rationale for re-examining the pelvis on any repeat or serial FAST.
A small amount of free fluid in the Pouch of Douglas is a normal finding in many healthy, non-pregnant women of reproductive age, related to physiologic peritoneal fluid and retrograde menstrual flow, and should not by itself be interpreted as pathologic hemoperitoneum in the absence of a plausible mechanism or other positive findings. This is one of the few caveats to FAST's general operating rule of "any free fluid after trauma is presumed pathologic until proven otherwise," and underscores why the pelvic view is interpreted in the context of the whole clinical picture rather than in isolation.
The subxiphoid (subcostal) cardiac view uses the left lobe of the liver as an acoustic window to image all four chambers of the heart and the pericardial sac that surrounds them, making it the fastest available bedside tool to identify traumatic pericardial effusion — a time-critical diagnosis in penetrating precordial injury and an important, if less common, consideration in severe blunt chest trauma.
The transducer is placed just below the xiphoid process, angled at a very shallow angle nearly parallel to the abdominal wall, aiming toward the patient's left shoulder, with the probe marker directed toward the patient's left so the heart appears with the liver in the near field and the right ventricle typically closest to the transducer. Adequate views require patience — this is often the technically hardest FAST window, particularly in patients with a large body habitus, subcutaneous emphysema, or overlying bowel gas, all of which scatter or block the ultrasound beam. When the subxiphoid window is unobtainable, a parasternal long-axis view (probe at the left sternal border, third or fourth intercostal space) is an accepted alternative for assessing the pericardium.
A positive view shows an anechoic (black) stripe circumferentially surrounding the heart within the pericardial sac, distinct from a simple pleural effusion, which will not wrap around the heart and instead layers posterior to the descending aorta on a parasternal view. Because the pericardium is a relatively fixed, non-compliant sac, even a modest volume of acutely accumulated blood — well under what would be tolerated in the peritoneal cavity — can raise intrapericardial pressure enough to impair diastolic filling. Sonographic clues to this tamponade physiology include diastolic collapse of the right ventricular free wall, a dilated and poorly collapsing inferior vena cava, and a swinging motion of the heart within the effusion.
The extended FAST (eFAST) protocol adds bilateral anterior thoracic views to the four classic abdominal/pericardial windows, using a high-frequency linear probe to assess the pleural interface for pneumothorax. In a supine trauma patient, free air rises to the least dependent portion of the thorax — the anterior chest wall — making this an anatomically ideal location to detect even a small anterior pneumothorax that may be entirely missed on a supine portable chest radiograph.
In B-mode, the normal pleural line appears as a bright horizontal hyperechoic line that shimmers back and forth with each respiratory cycle — "lung sliding" — produced by the visceral pleura sliding against the parietal pleura. Below this line, reverberation artifacts called A-lines (horizontal lines equally spaced at multiples of the pleural depth) are seen in both normal lung and pneumothorax, so A-lines alone are not diagnostic; sliding, or its absence, is the key discriminator. Because sliding can be subtle in real time, an M-mode tracing through the pleural line is used to document it objectively: normal sliding lung produces the "seashore sign," a smooth, wave-like pattern above the pleural line (representing the stationary chest wall) transitioning to a granular, sandy pattern below it (representing moving lung). When a pneumothorax separates the two pleural layers with air, sliding is abolished and the M-mode instead shows uniform horizontal lines both above and below the pleural line — the "barcode" or "stratosphere" sign.
When a pneumothorax is present but does not occupy the entire hemithorax, there is a specific transition zone on the chest wall where sliding lung (seashore sign) alternates with non-sliding lung (barcode sign) with each respiratory cycle, as the edge of the partially collapsed lung intermittently contacts the chest wall at that location. This transition, called the "lung point," is considered essentially pathognomonic for pneumothorax (specificity approaching 100% in most series) because no other condition reproduces it, and its location can even give a rough visual estimate of pneumothorax size — a lung point found far laterally implies a smaller pneumothorax than one found near the sternum. However, a large or complete pneumothorax may push the lung edge out of the entire accessible chest wall, so failure to find a lung point does not exclude pneumothorax; absent sliding without an identifiable lung point is still treated as a positive, clinically actionable finding when the pretest probability is high.
Important confounders include a mainstem intubation or complete atelectasis (both abolish sliding on the affected side without a pneumothorax being present), prior pleurodesis, and severe bullous emphysema — all of which can produce false-positive loss of sliding, so the eFAST pneumothorax finding, like every other FAST finding, is interpreted alongside the clinical exam rather than in isolation.