29.3 Abdominal, Vascular & Procedural Point-of-Care Ultrasound

Abdominal Free Fluid: The FAST Examination

The Focused Assessment with Sonography for Trauma answers one question: is there free fluid?

Limitations: FAST detects about 200 mL or more of free fluid, is insensitive for retroperitoneal injury, bowel injury, and solid organ laceration without free fluid, cannot distinguish blood from ascites or urine, and does not exclude ongoing bleeding. A negative FAST in an unstable patient does not exclude abdominal hemorrhage, and a positive FAST in a stable patient prompts CT rather than automatic laparotomy. Serial examinations add sensitivity.

Renal, Bladder and Ascites Applications

• Hydronephrosis: look for anechoic dilation of the renal pelvis and calyces. Grading from mild to severe guides urgency, and bilateral hydronephrosis or hydronephrosis in a single functioning kidney with acute kidney injury calls for urgent decompression. Caveat: hydronephrosis can be absent early, with volume depletion, or in retroperitoneal fibrosis, and a prominent extrarenal pelvis or parapelvic cysts can mimic it.
• Bladder volume: a simple estimate is $\text{height} \times \text{width} \times \text{depth} \times 0.7$ in centimeters, giving milliliters. This distinguishes anuria from retention and avoids unnecessary catheterization.
• Ascites and paracentesis: identify a pocket free of bowel, mark the site in the left or right lower quadrant lateral to the rectus sheath (avoiding the inferior epigastric vessels), and use color Doppler to check for vessels in the path. Ultrasound reduces bleeding and bowel injury.
• Pleural effusion and thoracentesis: confirm the fluid, identify the diaphragm and liver or spleen, choose a site above the diaphragm with the patient positioned consistently, and avoid draining more than about 1 to 1.5 liters at a session or stopping with chest discomfort, to limit re-expansion edema.

Aorta and Vascular Applications

• Abdominal aortic aneurysm: scan from the epigastrium to the bifurcation in transverse and longitudinal planes, measuring outer wall to outer wall. Above 3 cm defines an aneurysm; rupture risk rises sharply above 5.5 cm. In a hypotensive older patient with abdominal or back pain, finding an aneurysm is actionable even though ultrasound cannot confirm rupture, which usually requires CT in the stable patient or immediate operation in the unstable one.
• Aortic dissection clues: an intimal flap in the aortic root or abdominal aorta, a pericardial effusion, or a new aortic regurgitation jet; the definitive test remains CT angiography or transesophageal echocardiography.
• Deep vein thrombosis: compression ultrasound of the common femoral and popliteal veins (see the core POCUS section).
• Volume and flow adjuncts: inferior vena cava measurement, carotid or femoral artery Doppler velocity time integral change with a passive leg raise, and left ventricular outflow tract velocity time integral for stroke volume. Each has limits and should be combined with other data rather than used alone.

Hepatobiliary Applications

• Acalculous cholecystitis in the critically ill: a thickened gallbladder wall (above about 3.5 mm), pericholecystic fluid, sludge, and distension without stones, with a sonographic Murphy sign if the patient can report pain. This is an important cause of unexplained sepsis in ventilated patients on parenteral nutrition.
• Cholelithiasis and biliary dilation: hyperechoic stones with shadowing and a common bile duct above about 6 mm (rising with age and after cholecystectomy).
• Portal hypertension signs: a nodular liver, splenomegaly, ascites, recanalized umbilical vein, and portal flow reversal.
• Pneumoperitoneum and bowel: enhanced peritoneal stripe sign for free air, dilated loops with to-and-fro content in obstruction, and a thickened aperistaltic segment in ischemia. These are supportive rather than definitive findings.

Procedural Guidance

Adjunct ICU Applications

• Diaphragm assessment: measure thickness in the zone of apposition. A thickening fraction below about 20% to 30% during a spontaneous breathing trial predicts weaning failure, and excursion below about 10 to 15 mm suggests dysfunction. Unilateral paralysis shows paradoxical motion.
• Optic nerve sheath diameter: measured 3 mm behind the globe, a diameter above about 5 mm suggests raised intracranial pressure. It is a screening adjunct, not a substitute for invasive monitoring, and is operator-dependent.
• Gastric ultrasound: antral cross-sectional area estimates volume and aspiration risk before sedation or extubation, which is useful when fasting status is unknown.
• Soft tissue: differentiates cellulitis (cobblestoning) from abscess (a drainable fluid collection) and detects subcutaneous gas in necrotizing infection.
• Transcranial Doppler: assesses vasospasm after subarachnoid hemorrhage and cerebral blood flow patterns in brain death evaluation (where it is an ancillary test).

Governance and Pitfalls

• Image acquisition, interpretation, and documentation should be archived with a written report, because POCUS findings drive decisions and have medicolegal weight.
• Know the limits of the question asked: POCUS is a focused examination, not a comprehensive study, and a negative focused scan does not exclude disease.
• Common errors: mistaking a seminal vesicle or ovarian cyst for free fluid, calling perinephric fat free fluid, measuring the aorta inner wall to inner wall (which underestimates), missing posterior effusions by scanning too anteriorly, mistaking the gastric antrum for the aorta, and failing to repeat the scan when the clinical picture changes.
• Credentialing and quality assurance: training, image review, and documented competence support safe use, and findings that contradict the clinical picture should prompt a confirmatory study.