Residency · Residency · Emergency Medicine
Point-of-Care Ultrasound: Core ED Applications
Overview
Principles
Point-of-care ultrasound (POCUS) is a bedside imaging tool performed and interpreted by the treating clinician in real time. It is goal-directed, answering specific binary clinical questions such as "Is there free fluid?" or "Is the heart contracting?" It is integrated into the clinical workflow and performed at the bedside during the evaluation, not intended to replace comprehensive imaging but to guide immediate clinical decisions. ACEP recognizes 12 core ED ultrasound applications, and residency training requires competency in these.
Physics Basics
Higher frequency probes, typically linear probes operating at 7 to 15 MHz, provide better resolution with less penetration and are used for superficial structures, vessels, soft tissue, and procedural guidance. Lower frequency probes, typically curvilinear probes at 2 to 5 MHz, offer better penetration with lower resolution and are used for abdominal organs, the aorta, and the lungs. Phased array probes at 1 to 5 MHz have a small footprint that allows cardiac imaging through the intercostal rib spaces.
FAST Examination (Focused Assessment with Sonography for Trauma)
Standard Views
The FAST examination consists of four standard views. The right upper quadrant (RUQ) view of Morrison's pouch (the hepatorenal recess) is the most sensitive single view for detecting free fluid. The probe is placed between the liver and right kidney, with additional assessment above the diaphragm to evaluate the right hemithorax. The left upper quadrant (LUQ) view assesses the splenorenal recess between the spleen and left kidney, also checking above the diaphragm for left hemithorax fluid. Fluid often accumulates inferior to the spleen. The suprapubic view uses both transverse and sagittal orientations to identify fluid posterior to the bladder (the pouch of Douglas in females and the rectovesical space in males). The subxiphoid view evaluates the pericardium for effusion and looks for right ventricular diastolic collapse, which indicates tamponade.
Extended FAST (eFAST)
The extended FAST adds bilateral anterior thoracic views to evaluate for pneumothorax. The presence of lung sliding (the shimmering movement of the pleural line with respiration) and comet tails or B-lines rules out pneumothorax. The lung point, where lung sliding transitions from present to absent, is highly specific for pneumothorax. The eFAST is more sensitive than supine chest X-ray for detecting pneumothorax.
Interpretation
Free fluid appears as an anechoic (black) stripe between organs. A positive FAST in an unstable patient indicates the need for operative intervention. A negative FAST in an unstable patient does not exclude injury and should prompt consideration of other bleeding sources, a repeat examination, or diagnostic peritoneal lavage. A positive FAST in a stable patient warrants CT for injury characterization.
Cardiac Ultrasound
Core Views
The four core cardiac views are the parasternal long axis (PLAX), which evaluates left ventricular function, right ventricular size, pericardial effusion, and the mitral and aortic valves; the parasternal short axis (PSAX), which assesses LV function (the "squeeze"), RV size relative to the LV, and the D-sign indicating RV pressure overload; the apical four-chamber (A4C) view, which shows all four chambers and allows assessment of relative RV/LV size, gross wall motion, and ejection fraction estimation; and the subxiphoid view, which is best for identifying pericardial effusion, RV free wall collapse, and IVC assessment.
Key Applications
In cardiac arrest, cardiac ultrasound can identify potentially reversible causes including tamponade, PE, and hypovolemia, guide CPR quality, coordinate echo assessment with pulse checks, and determine whether organized cardiac activity is present. For pericardial effusion and tamponade, anechoic fluid around the heart is the primary finding, with RV diastolic collapse being the most specific sign, RA systolic collapse the most sensitive, and IVC plethora an associated feature. RV strain suggesting PE manifests as RV dilation (RV:LV ratio greater than 1:1 in the A4C view), septal bowing into the LV (the D-sign in the PSAX view), and McConnell's sign (RV free wall akinesis with preserved apical motion), which is specific for acute PE. LV function can be estimated visually, categorizing the heart as hyperdynamic, normal, or reduced, and this estimation correlates well with formal echocardiography when performed by trained operators.
IVC Assessment
The IVC is measured in the subxiphoid long-axis view, 1 to 2 cm from the RA-IVC junction. A small, collapsing IVC (less than 1 cm with more than 50 percent collapse with respiration) suggests low central venous pressure and likely fluid responsiveness. A plethoric IVC (greater than 2 cm with less than 50 percent collapse) suggests elevated CVP from conditions such as tamponade, RV failure, tension pneumothorax, or volume overload. Important limitations include unreliability in mechanically ventilated patients (positive pressure reverses the normal respiratory physiology), athletic individuals, and patients on positive-pressure ventilation.
Lung Ultrasound
Normal Findings
Lung sliding is the shimmering movement of the pleural line with respiration, representing the visceral pleura sliding against the parietal pleura. It is best seen in B-mode and produces the "seashore sign" on M-mode. A-lines are horizontal hyperechoic reverberation artifacts below the pleural line that are equidistant and parallel to the pleural line, representing normal air-filled lung.
Pathologic Findings
B-lines are vertical hyperechoic artifacts that extend from the pleural line to the bottom of the screen without fading, obliterating A-lines. They arise from interstitial fluid. Three or more B-lines per rib space in two or more zones bilaterally constitutes interstitial syndrome, which may represent pulmonary edema, ARDS, pneumonia, or fibrosis. Bilateral diffuse B-lines most commonly indicate cardiogenic pulmonary edema in the ED setting. Focal B-lines suggest pneumonia or contusion.
Absent lung sliding suggests pneumothorax, though it can also be seen with mainstem intubation, pleurodesis, severe ARDS, and apnea. On M-mode, the "barcode" or "stratosphere" sign (parallel horizontal lines) replaces the normal "seashore sign." The lung point is the transition point where lung sliding is present on one side and absent on the other. It is pathognomonic for pneumothorax with a specificity approaching 100 percent.
Consolidation appears as tissue-like "hepatization" of the lung parenchyma, with air bronchograms visible as hyperechoic dots or lines within the consolidated area. It is seen in pneumonia and atelectasis. Pleural effusion appears as anechoic fluid above the diaphragm and can be used to estimate volume and guide thoracentesis.
Multiple studies show that lung ultrasound is more sensitive and specific than chest X-ray for pneumothorax (90 to 95 percent versus 40 to 50 percent for supine CXR), pleural effusion, pulmonary edema (B-line pattern), and pneumonia (consolidation). The arguments for replacing CXR include faster results, no radiation, bedside availability, and greater accuracy. The arguments against include operator dependence, training requirements, inability to visualize the mediastinum or bony structures, and a lack of universal adoption as standard of care. In current practice, lung ultrasound complements chest X-ray, and in many scenarios it is obtained first and may make the CXR unnecessary.
RUSH Protocol (Rapid Ultrasound in Shock)
Systematic Approach to Undifferentiated Shock
The RUSH protocol provides a systematic bedside approach to undifferentiated shock, organized into three domains. The "Pump" assessment evaluates the heart: a hyperdynamic LV with small chambers suggests hypovolemia or distributive shock; a poorly squeezing LV indicates cardiogenic shock; RV dilation with the D-sign suggests obstructive shock from PE; and pericardial effusion with RV collapse indicates tamponade.
The "Tank" assessment evaluates volume status. The IVC is assessed as small and collapsing (empty tank) versus plethoric (full tank or obstructive cause). The lungs are evaluated for B-lines (wet lungs indicating volume overload or ARDS) versus A-lines (dry lungs). FAST views look for free fluid (hemorrhage, representing a "leaking tank"), and pleural effusion is assessed.
The "Pipes" assessment evaluates the vessels. The aorta is assessed for aneurysm (an AAA greater than 3 cm, usually ruptured if greater than 5 cm with symptoms). The femoral and popliteal veins are assessed with compression to evaluate for DVT (a non-compressible vein indicates DVT and suggests possible PE).
Aortic Ultrasound
Technique
A curvilinear probe is used to obtain both transverse and longitudinal views. The aorta is evaluated in three segments: proximal (below the xiphoid), mid (at the umbilicus), and distal (at the bifurcation and iliac arteries). The diameter is measured from outer wall to outer wall in the anteroposterior dimension on the transverse view.
Key Findings
A normal aortic diameter is less than 3 cm. An aneurysm is 3 cm or greater. An AAA larger than 5 cm or any symptomatic AAA (with back pain, flank pain, or hypotension) carries a high rupture risk. Free fluid combined with an AAA and hemodynamic instability represents a ruptured AAA until proven otherwise, and emergent surgical consultation should be obtained without delaying for CT.
Procedural Guidance
Ultrasound-Guided Procedures
Ultrasound guidance is used for central venous access, with the IJ being the most commonly ultrasound-guided site. Real-time visualization reduces complications for IJ, subclavian, and femoral approaches. For difficult peripheral IV access, the long-axis approach is preferred for catheter threading. Lumbar puncture guidance identifies the midline, the intervertebral space, and the depth to the ligamentum flavum. Thoracentesis and paracentesis benefit from real-time guidance that reduces the risk of pneumothorax and organ injury. Nerve blocks use fascial plane identification, and regional anesthesia applications are expanding. Peritonsillar abscess drainage uses ultrasound to differentiate abscess from cellulitis and guide needle placement. Pericardiocentesis is performed via the subxiphoid approach with real-time guidance.
Soft Tissue Ultrasound
Applications
Abscess appears as a hypoechoic or anechoic fluid collection with posterior acoustic enhancement, while cellulitis shows a "cobblestone" pattern of edematous subcutaneous tissue without a discrete collection. Foreign bodies appear as hyperechoic structures with posterior acoustic shadowing, and wood, glass, and metal can all be detected. Tendon evaluation can identify rupture and tenosynovitis. Fracture detection relies on identifying cortical disruption, and it is particularly useful in pediatric fractures where X-ray findings may be subtle.
<image>A comprehensive RUSH protocol diagram divided into three sections. "Pump" section: four cardiac views (PLAX, PSAX, A4C, subxiphoid) with normal findings and pathologic findings for each shock type — hyperdynamic small heart (hypovolemic), dilated poorly contracting LV (cardiogenic), dilated RV with D-sign (obstructive/PE), pericardial effusion with RV collapse (tamponade). "Tank" section: IVC assessment (small collapsing = empty tank, plethoric = full/obstructed), lung ultrasound (A-lines = dry, B-lines = wet), FAST views (free fluid = leaking tank). "Pipes" section: aorta transverse view with measurement showing normal less than 3 cm vs. aneurysm greater than 3 cm, and femoral vein compression test for DVT (compressible = normal, non-compressible = DVT).</image>
<image>A lung ultrasound findings panel with six images. Image 1: Normal A-lines — horizontal reverberation artifacts below the pleural line with lung sliding present. Image 2: B-lines — multiple vertical hyperechoic artifacts extending from the pleural line to the bottom of the screen, indicating interstitial fluid (pulmonary edema). Image 3: M-mode "seashore sign" — normal lung sliding showing granular pattern below the pleural line. Image 4: M-mode "stratosphere/barcode sign" — absent lung sliding showing only horizontal parallel lines, indicating pneumothorax. Image 5: Lung point — the transition zone between present and absent lung sliding, pathognomonic for pneumothorax. Image 6: Consolidation — tissue-like hepatization pattern with air bronchograms visible as hyperechoic dots within the consolidated lung.</image>
<image>An aortic ultrasound technique diagram showing the probe positions for three segments: proximal (epigastric, below the xiphoid), mid (periumbilical), and distal (bifurcation level). For each position, a corresponding ultrasound image shows: normal aorta (transverse, less than 3 cm with measurement calipers), and an abnormal image showing an abdominal aortic aneurysm (greater than 5 cm with intraluminal thrombus). An annotation shows the correct measurement technique: outer wall to outer wall in the anteroposterior dimension. A clinical decision box states: symptomatic AAA + hemodynamic instability = ruptured until proven otherwise → emergent surgery, do not delay for CT.</image>
Clinical Pearls
POCUS answers specific binary clinical questions at the bedside and does not replace comprehensive imaging but guides immediate management decisions. Morrison's pouch (RUQ) is the most sensitive single FAST view for detecting free fluid and should always be included. Lung ultrasound is more sensitive than supine chest X-ray for pneumothorax, and absent lung sliding combined with a lung point is diagnostic. Bilateral diffuse B-lines indicate interstitial fluid (pulmonary edema) and can rapidly differentiate CHF from COPD in the dyspneic patient. The RUSH protocol provides a systematic bedside approach to undifferentiated shock through the Pump, Tank, and Pipes framework. IVC assessment is a surrogate for volume status but has significant limitations and should not be used as the sole determinant for fluid resuscitation. An AAA larger than 5 cm with symptoms (back or flank pain) and hemodynamic instability represents a ruptured AAA until proven otherwise, and surgery should be called before CT. McConnell's sign (RV free wall akinesis with preserved apical contractility) is specific for acute PE and can support thrombolytic decision-making in the appropriate clinical context.
References
- American College of Emergency Physicians. Ultrasound guidelines: emergency, point-of-care, and clinical ultrasound guidelines in medicine. Ann Emerg Med. 2017;69:e27-e54.
- Perera P, et al. The RUSH exam: Rapid Ultrasound in Shock. Emerg Med Clin North Am. 2010;28:29-56.
- Lichtenstein DA, Meziere GA. Relevance of lung ultrasound in the diagnosis of acute respiratory failure (BLUE protocol). Chest. 2008;134:117-125.
- Melniker LA, et al. Randomized controlled clinical trial of point-of-care, limited ultrasonography for trauma in the emergency department (PLUS). Ann Emerg Med. 2006;48:227-235.
- Volpicelli G, et al. International evidence-based recommendations for point-of-care lung ultrasound. Intensive Care Med. 2012;38:577-591.
