26.3 Critical Care Point-of-Care Ultrasound (POCUS)

Focused Cardiac Ultrasound (FoCUS)

Point-of-care focused cardiac ultrasound is a goal-directed examination performed to evaluate acute, life-threatening hemodynamic instability. Rather than providing a comprehensive structural evaluation, FoCUS addresses five binary clinical questions: severe left ventricular (LV) systolic dysfunction, acute right ventricular (RV) enlargement and strain, pericardial tamponade physiology, severe intravascular hypovolemia, and gross valvular catastrophes.

Standard Acoustic Windows and Views

1. Parasternal Long Axis (PLAX): Probe placed in the 3rd or 4th left intercostal space adjacent to the sternum with the orientation marker pointing toward the patient's right shoulder. Visualizes the right ventricular outflow tract (anteriorly), the interventricular septum, the LV cavity, the posterior LV wall, the mitral valve, the left atrium, the aortic valve, the aortic root, and the descending thoracic aorta in cross-section.
   • Effusion Differentiation: Pericardial effusions track anterior to the descending thoracic aorta; pleural effusions track posterior to the descending thoracic aorta.
2. Parasternal Short Axis (PSAX): Probe rotated 90 degrees clockwise from PLAX with the marker directed toward the patient's left shoulder. Sweeping from base to apex evaluates the aortic valve level ("Mercedes-Benz" sign), mitral valve level ("fish mouth"), papillary muscle level (mid-ventricular "doughnut"), and apical level. Assesses regional LV contractility and interventricular septal geometry.
3. Apical Four-Chamber (A4C): Probe positioned at the cardiac apex (typically 5th intercostal space, anterior axillary line) with marker pointing toward the left axilla. Demonstrates the LV, RV, left atrium, right atrium, mitral valve, and tricuspid valve. Ideal for evaluating RV-to-LV size ratio and tricuspid annular displacement.
4. Subcostal (Subxiphoid) Four-Chamber: Probe positioned below the xiphoid process with the marker directed to the patient's left, angled flat toward the left shoulder using the liver as an acoustic window. The primary rescue window in mechanically ventilated patients with hyperinflated lungs.
5. Subcostal Inferior Vena Cava (IVC): Probe rotated 90 degrees sagittal from the subcostal four-chamber view with the marker directed cephalad. Visualizes the longitudinal entry of the IVC into the right atrium, measured $1$ to $2\text{ cm}$ caudal to the junction of the hepatic veins.

Left Ventricular Systolic Assessment

• Qualitative Visual Estimation: LV contractility is categorized as hyperdynamic (ejection fraction $> 70\%$, with end-systolic cavity obliteration), normal ($50\% - 70\%$), mildly reduced ($40\% - 49\%$), moderately reduced ($30\% - 39\%$), or severely reduced ($< 30\%$).
• E-Point Septal Separation (EPSS): Measured in the PLAX view using M-mode cursor alignment across the tip of the anterior mitral valve leaflet. EPSS represents the minimum distance between the peak early diastolic opening of the anterior leaflet (the E-point) and the interventricular septum. In a normal ventricle, the leaflet approaches or touches the septum (normal $\text{EPSS} < 7\text{ mm}$). When LV dilation or systolic impairment occurs, ventricular filling is reduced and the septum dilates away from the leaflet, widening the gap. An $\text{EPSS} > 7\text{ mm}$ indicates reduced ejection fraction, and an $\text{EPSS} > 15\text{ mm}$ strongly correlates with severe systolic dysfunction ($\text{LVEF} < 30\%$). Note: Mitral stenosis or aortic regurgitation (aortic jet deflecting the leaflet) cause false-positive EPSS elevations independent of LV function.

Right Ventricular Strain and Acute Cor Pulmonale

Under normal physiological conditions, the RV is triangular and its volume is significantly smaller than the LV. In the A4C view, the normal end-diastolic RV-to-LV basal diameter ratio is $< 0.6$. Signs of RV strain include:

• RV Dilation: An RV/LV basal diameter ratio $> 0.6$ indicates RV enlargement; an $\text{RV/LV ratio} > 1.0$ indicates severe RV dilation and acute cor pulmonale.
• McConnell's Sign: Severe hypokinesis or akinesia of the RV mid-free wall with preserved or hyperdynamic contraction of the RV apex. Highly specific ($94\% - 96\%$) for acute pulmonary embolism.
• Interventricular Septal Flattening ("D-Shaped" LV): Evaluated in the PSAX view at the papillary muscle level:
  • Isolated Diastolic Flattening: Indicates RV Volume Overload (e.g., tricuspid regurgitation, atrial septal defect).
  • Systolic and Diastolic Flattening: Indicates RV Pressure Overload (e.g., massive pulmonary embolism, acute respiratory distress syndrome with severe pulmonary vascular dysfunction).
• Tricuspid Annular Plane Systolic Excursion (TAPSE): M-mode measurement of the longitudinal excursion of the lateral tricuspid valve annulus toward the apex in A4C. A $\text{TAPSE} < 17\text{ mm}$ confirms RV systolic dysfunction.

Cardiac Tamponade Evaluation

Pericardial fluid appears as an anechoic space between the epicardium and parietal pericardium. Hemodynamic compromise progresses through specific ultrasound stages:

1. Early Diastolic RV Collapse: Indentation of the RV anterior free wall into the RV cavity during ventricular diastole (when RV pressure is lowest). This finding is highly specific for tamponade.
2. Late Diastolic / Systolic Right Atrial (RA) Collapse: Inversion of the RA free wall lasting for more than one-third ($> 33\%$) of the cardiac cycle. This is the earliest and most sensitive ultrasound sign of tamponade.
3. Plethoric, Non-Collapsible IVC: IVC diameter $> 2.1\text{ cm}$ with $< 50\%$ respiratory collapse (collapsibility index). An uncompromised, collapsing IVC virtually excludes cardiac tamponade (sensitivity approaching $100\%$), except in rare "low-pressure tamponade" in hypovolemic patients.
4. Doppler Respiratory Variation (Pulsus Paradoxus): Mitral inflow Doppler $E$-wave velocity variation $> 25\%$ or tricuspid inflow variation $> 40\%$ across the respiratory cycle.

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Lung Ultrasound: The BLUE Protocol

Lung ultrasound is governed by acoustic artifacts created by the interface of acoustic sound waves with air, water, and pleural membranes. The BLUE Protocol (Bedside Lung Ultrasound in Emergency) standardizes this evaluation in acute respiratory failure.

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Vascular Compression Ultrasound for DVT

Vascular POCUS for deep vein thrombosis (DVT) relies exclusively on complete mechanical coaptation (compressibility) of the vein lumen under gentle downward probe pressure. The evaluation employs either a 2-point protocol (examining the common femoral vein at the saphenofemoral junction and the popliteal vein down to its trifurcation) or an extended 3-point protocol (incorporating the deep femoral vein and the proximal femoral vein). A vein is normal if the anterior and posterior walls completely collapse and touch, completely obliterating the lumen. Non-compressibility is the diagnostic criterion. Acute thrombus is often hypoechoic or even anechoic, so absence of visible clot does not exclude DVT; venous distension and low echogenicity only support acuity.

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Multi-Organ Shock Protocol: The RUSH Exam

The Rapid Ultrasound in Shock (RUSH) protocol provides a rapid, physiological evaluation of undifferentiated hypotension categorized into "The Pump", "The Tank", and "The Pipes":

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• Distributive (Septic) Shock Profile: Hyperdynamic LV contractility (end-systolic cavity obliteration), collapsing IVC (low central venous pressure), lung sliding with A-lines, and negative FAST. Septic cardiomyopathy can instead depress LV or RV function, so a normal-to-low ejection fraction does not exclude sepsis.
• Cardiogenic Shock Profile: Severely depressed LV systolic function (dilated LV, $\text{EPSS} > 15\text{ mm}$), plethoric non-collapsing IVC, diffuse bilateral B-lines (pulmonary edema), and negative FAST.
• Obstructive Shock Profile:
  • Pulmonary Embolism: Severely enlarged RV ($\text{RV/LV} > 1.0$), McConnell's sign, D-shaped septum in systole and diastole, plethoric IVC, normal lung sliding, and lower extremity DVT.
  • Cardiac Tamponade: Anechoic pericardial effusion, diastolic RV collapse, systolic RA collapse, and plethoric IVC.
  • Tension Pneumothorax: Complete absence of lung sliding, absent B-lines, stratosphere sign in M-mode, absent lung pulse, and plethoric IVC (impaired venous return). A lung point is often absent because the lung is fully collapsed; tension physiology is a clinical diagnosis that calls for immediate decompression.
• Hypovolemic / Hemorrhagic Shock Profile: Hyperdynamic LV, flat or completely collapsing IVC ("kissing walls"), bilateral lung sliding with normal A-lines, and positive FAST (free anechoic fluid in Morison's pouch, splenorenal recess, or retrovesical pouch) or leaking abdominal aortic aneurysm ($> 3\text{ cm}$).