The Principles of Ultrasound-Guided Resuscitation: The RUSH and FALLS Protocols

Dr Neeraj Manikath , claude.ai

Abstract

Point-of-care ultrasound (POCUS) has revolutionized the approach to critically ill patients, transforming resuscitation from a empirical exercise to a physiologically guided intervention. The RUSH (Rapid Ultrasound in Shock and Hypotension) and FALLS (Fluid Administration Limited by Lung Sonography) protocols represent systematic, goal-directed approaches to hemodynamic assessment and fluid management. This review explores the theoretical foundations, practical applications, and evidence-based integration of these protocols into critical care practice, with emphasis on their complementary roles in optimizing resuscitation strategies.

Introduction

The traditional approach to shock management has relied heavily on static parameters and clinical gestalt, often resulting in either inadequate resuscitation or iatrogenic fluid overload. The advent of POCUS has provided clinicians with a dynamic, real-time window into cardiovascular physiology, enabling individualized, pathophysiology-targeted interventions. The RUSH and FALLS protocols emerged from the need for standardized, reproducible approaches to ultrasound-guided resuscitation, providing frameworks that integrate multiple acoustic windows into coherent diagnostic and therapeutic algorithms.

Understanding these protocols requires appreciation of their complementary nature: RUSH provides the diagnostic architecture for identifying shock etiology, while FALLS offers dynamic guidance for fluid optimization. Together, they represent a paradigm shift toward precision medicine in the critically ill.

The RUSH Exam: The Pump, The Tank, The Pipes

The RUSH protocol, first described by Perera et al. in 2010, provides a systematic approach to the undifferentiated shock patient through evaluation of three physiological components: the pump (heart), the tank (volume status), and the pipes (vascular system).

The Pump: Cardiac Assessment

Cardiac evaluation begins with the parasternal long-axis and short-axis views, followed by apical four-chamber and subcostal windows. The primary objectives include assessment of global contractility, right ventricular function, pericardial effusion, and valvular pathology.

Pearl: A qualitative assessment of left ventricular (LV) contractility—described as hyperdynamic, normal, or severely depressed—is often sufficient for clinical decision-making. Hyperdynamic LV function with a "kissing ventricle" appearance suggests distributive or hypovolemic shock, while severely depressed contractility indicates cardiogenic etiology.

Right ventricular (RV) assessment deserves particular attention, as RV dysfunction is often overlooked in traditional evaluation. The RV:LV ratio >1:1 in the apical four-chamber view, accompanied by interventricular septal flattening (D-sign) in parasternal short-axis, suggests acute RV strain—a hallmark of massive pulmonary embolism or severe right heart failure.

Oyster: The McConnell sign—RV free wall akinesis with preserved apical contractility—is relatively specific for acute pulmonary embolism but has limited sensitivity (approximately 70%). Its absence does not exclude PE, but its presence significantly increases pretest probability.

The Tank: Volume Assessment

Volume status assessment incorporates evaluation of the inferior vena cava (IVC), cardiac chamber sizes, and detection of free fluid. The IVC assessment, performed in the subcostal window with M-mode interrogation 2 cm caudal to the hepatic vein confluence, provides insights into central venous pressure (CVP) and volume responsiveness.

Hack: Rather than relying solely on absolute IVC diameter, assess the caval index: (IVC max - IVC min)/IVC max × 100. A caval index >50% in spontaneously breathing patients suggests CVP <10 mmHg and potential fluid responsiveness, though this must be integrated with other parameters.

Small, underfilled cardiac chambers—particularly a "kissing" left ventricle in parasternal short-axis—strongly suggest hypovolemia. Conversely, dilated chambers with preserved or hyperdynamic function may indicate distributive shock or chronic volume overload.

The Pipes: Vascular Assessment

Vascular evaluation encompasses the abdominal aorta and proximal lower extremity deep veins. The abdominal aorta is scanned in both transverse and longitudinal planes from xiphoid to bifurcation, assessing for aneurysm (diameter >3 cm) or dissection.

Pearl: When evaluating for aortic dissection, look for an intimal flap—a linear echogenic structure within the vessel lumen that moves independently of the vessel wall. Color Doppler may reveal differential flow patterns between true and false lumens.

Bilateral lower extremity venous evaluation (femoral and popliteal veins) screens for deep venous thrombosis using the compression technique. Non-compressibility of the vein is diagnostic of thrombosis.

Clinical Integration: The RUSH exam should be completed within 3-5 minutes in experienced hands. The integration of findings across all three components allows differentiation of shock types: hypovolemic (small underfilled chambers, collapsible IVC), cardiogenic (poor LV function, dilated IVC), distributive (hyperdynamic LV, variable IVC), and obstructive (RV strain, pericardial effusion, or PE findings).

The FALLS Protocol: A Dynamic Approach to Fluid Responsiveness

The FALLS protocol, introduced by Lichtenstein in 2012, represents a paradigm shift from static volume assessment to dynamic evaluation of fluid tolerance and responsiveness using sequential lung ultrasonography. This protocol recognizes that fluid administration should be titrated not to arbitrary targets, but to the development of pulmonary interstitial edema.

The FALLS Sequential Approach

FALLS employs an eight-zone lung examination (anterior and lateral zones, bilateral) performed serially during resuscitation. Each hemithorax is divided into anterior (parasternal to anterior axillary line) and lateral (anterior to posterior axillary line) zones, scanned in second and fourth intercostal spaces.

The Normal Profile: In euvolemic patients without pulmonary pathology, lung ultrasound demonstrates A-lines—horizontal reverberation artifacts indicating normal lung aeration. The presence of bilateral A-lines suggests the absence of significant pulmonary edema and potential tolerance of additional fluid.

The Transition Point: As interstitial fluid accumulates, B-lines appear—vertical, laser-like artifacts that erase A-lines and extend to the edge of the screen without fading. These represent thickened interlobular septa filled with fluid. The FALLS protocol uses B-line development as a stop point for fluid administration.

Pearl: B-lines are quantified as isolated (≤2 per intercostal space), moderate (≥3 per space), or confluent (complete obliteration of A-lines). The development of ≥3 B-lines in ≥2 anterior zones bilaterally indicates significant pulmonary edema and should prompt cessation of fluid resuscitation.

Dynamic Fluid Challenge Protocol

FALLS integrates passive leg raising (PLR) or small fluid boluses (250 mL) with repeat sonography. The protocol follows this sequence:

Initial lung scan establishing baseline B-line profile
Administration of fluid challenge or PLR
Repeat lung scan at 5-10 minute intervals
Continued fluid administration until B-lines appear or hemodynamics stabilize

Hack: Combine FALLS with IVC assessment and velocity time integral (VTI) measurement in the left ventricular outflow tract (LVOT). A ≥10% increase in VTI following PLR predicts fluid responsiveness with superior accuracy compared to static measures. If VTI increases but B-lines develop, the patient is fluid-responsive but not fluid-tolerant—consider vasopressors instead.

FALLS Profiles and Clinical Scenarios

Lichtenstein described specific FALLS profiles correlating sonographic patterns with clinical entities:

Profile A: Bilateral A-lines—suggests hypovolemia or early distributive shock
Profile B: Bilateral B-lines—cardiogenic pulmonary edema or ARDS
Profile C: Unilateral B-lines—pneumonia or unilateral pulmonary edema
Profile with pleural effusion: Suggests fluid overload or underlying cardiopulmonary disease

Oyster: The BLUE protocol (Bedside Lung Ultrasound in Emergency) can be integrated with FALLS for comprehensive pulmonary assessment. The absence of lung sliding with A-lines suggests pneumothorax, while the presence of lung point is pathognomonic for this condition.

Assessing the IVC: Understanding the Pitfalls and Limitations

IVC assessment has become a cornerstone of volume status evaluation, yet it is fraught with limitations that must be understood to avoid misinterpretation.

Technical Considerations

Optimal IVC visualization requires the subcostal longitudinal view with the probe oriented toward the patient's left shoulder. The IVC should be measured in M-mode 2 cm caudal to the hepatic vein confluence during quiet respiration. Measurements should include maximal diameter (end-expiration) and minimal diameter (end-inspiration) to calculate the caval index (collapsibility index in spontaneous breathing, distensibility index in mechanical ventilation).

Common Pitfalls:

Mechanical Ventilation: IVC interpretation differs fundamentally between spontaneously breathing and mechanically ventilated patients. In mechanical ventilation, inspiration increases intrathoracic pressure, causing IVC dilation rather than collapse. A distensibility index >18% suggests fluid responsiveness, but the evidence is weaker than for spontaneously breathing patients.
Right Heart Dysfunction: RV failure or tricuspid regurgitation causes IVC dilation regardless of volume status. A plethoric, non-collapsible IVC in the presence of RV dilation and dysfunction reflects elevated right atrial pressure, not necessarily adequate preload.
Intra-abdominal Hypertension: Elevated intra-abdominal pressure compresses the IVC, artificially reducing its diameter and collapsibility. This can lead to overestimation of volume status in patients with ascites, obesity, or abdominal compartment syndrome.
Arrhythmias: Atrial fibrillation causes beat-to-beat variability in cardiac output and IVC dimensions, making single measurements unreliable. Multiple measurements over several respiratory cycles improve accuracy.

Pearl: Never rely on IVC assessment in isolation. A collapsible IVC suggests low CVP and potential fluid responsiveness, but approximately 40% of patients with collapsible IVCs do not respond to fluid boluses. Integrate IVC findings with cardiac function, lung sonography, and dynamic assessments like PLR.

The Concept of Fluid Responsiveness vs. Fluid Tolerance

A critical distinction must be made between fluid responsiveness (will cardiac output increase with fluid?) and fluid tolerance (can the patient tolerate additional fluid without developing pulmonary edema?). The IVC primarily addresses the former but provides limited information about the latter. This is where FALLS becomes indispensable, assessing lung water as the endpoint of resuscitation.

Hack: In spontaneously breathing patients, combine IVC assessment with internal jugular vein (IJV) evaluation. The ratio IJV/IVC >1 suggests elevated CVP despite IVC collapsibility, indicating that fluid administration should be approached cautiously.

Integrating Cardiac, Lung, and Abdominal Views for a Unified Diagnosis

The true power of POCUS emerges when findings from multiple windows are synthesized into a coherent physiological picture. This integration transforms ultrasound from an imaging modality into a diagnostic and therapeutic roadmap.

The Integrated Approach to Undifferentiated Shock

Consider the following systematic integration:

Step 1: Cardiac Function and Structure

LV systolic function (hyperdynamic, normal, or depressed)
RV size and function (RV:LV ratio, septal position)
Pericardial space (effusion with tamponade physiology)
Valvular function (gross abnormalities)

Step 2: Volume Status Triangulation

IVC diameter and respiratory variation
Cardiac chamber filling and wall thickness
Presence of free fluid in abdomen or pelvis

Step 3: Pulmonary Assessment

B-line distribution and severity
Pleural effusions
Lung sliding and consolidations

Step 4: Vascular Evaluation

Aortic pathology
Deep venous thrombosis

Clinical Vignette Integration

Case 1: Distributive Shock

Hyperdynamic, small LV cavity with "kissing" walls
Collapsible IVC (caval index >50%)
Bilateral A-lines predominate
Interpretation: Distributive shock with hypovolemia; patient is fluid-responsive and fluid-tolerant. Proceed with fluid resuscitation guided by serial FALLS assessments.

Case 2: Cardiogenic Shock

Severely depressed LV function (EF ~20% visually)
Dilated, plethoric IVC (<20% variation)
Bilateral confluent B-lines in anterior zones
Small pericardial effusion without tamponade
Interpretation: Cardiogenic shock with pulmonary edema; patient is NOT fluid-tolerant. Avoid fluids; initiate inotropes and consider vasopressors if hypotensive.

Case 3: Obstructive Shock (PE)

Normal LV function, dilated RV with RV:LV >1:1
McConnell sign present
Moderately dilated IVC
Bilateral A-lines
Lower extremity DVT identified
Interpretation: Massive PE with RV strain; patient may benefit from cautious fluid challenge to optimize RV preload, but avoid overload. Primary therapy is anticoagulation and consideration of thrombolysis.

Oyster: In RV failure from PE, small fluid challenges (250-500 mL) may improve hemodynamics by optimizing RV preload on the steep portion of the Frank-Starling curve. However, aggressive fluid resuscitation can overdistend the RV, worsen septal shift, and compromise LV filling—a phenomenon called ventricular interdependence. Serial echocardiography should guide therapy.

Using POCUS to Guide Vasopressor and Inotrope Selection

POCUS provides real-time physiological data that can guide rational selection of vasoactive agents, moving beyond protocol-driven approaches to individualized hemodynamic support.

The Physiological Framework

Understanding the mechanism of each vasoactive agent and matching it to the underlying pathophysiology is essential:

Norepinephrine: Alpha-1 and beta-1 agonist; provides vasoconstriction and mild inotropy. First-line for distributive shock.
Epinephrine: Potent beta-1, beta-2, and alpha effects; strong inotrope and chronotrope with vasoconstriction. Used in cardiogenic shock and cardiac arrest.
Dobutamine: Beta-1 selective; pure inotrope with mild vasodilation. Used in cardiogenic shock with adequate blood pressure.
Vasopressin: V1 receptor agonist; pure vasoconstriction. Adjunct in distributive shock, particularly useful in catecholamine-refractory states.
Phenylephrine: Pure alpha-1 agonist; vasoconstriction without inotropic effect. Limited use in distributive shock when tachycardia is problematic.

POCUS-Guided Selection Algorithm

Scenario 1: Hyperdynamic LV, Distributive Shock

Sonographic findings: Hyperdynamic LV, small cavity, collapsible IVC, A-lines
Strategy: Fluid resuscitation as primary therapy; if hypotension persists despite adequate filling (development of B-lines or IVC plethora), initiate norepinephrine for vasoconstriction
Rationale: Cardiac function is adequate; hypotension is due to vasoplegia

Scenario 2: Depressed LV Function, Cardiogenic Shock

Sonographic findings: Poor LV contractility (EF <30%), dilated IVC, bilateral B-lines
Strategy:
- If systolic BP >90 mmHg: dobutamine for inotropy
- If systolic BP <90 mmHg: epinephrine or norepinephrine + dobutamine for combined inotropic and vasopressor support
Rationale: Primary problem is pump failure; inotropic support is essential
Pearl: Serial VTI measurements in LVOT can quantify response to inotropes, with target increase of 20-30% suggesting adequate augmentation

Scenario 3: RV Failure with Preserved LV

Sonographic findings: Dilated RV, normal LV, septal flattening, dilated IVC
Strategy: Optimize preload cautiously with small fluid challenges while monitoring for septal shift; use norepinephrine to maintain coronary perfusion pressure; avoid pure vasodilators
Rationale: RV is highly afterload-sensitive; maintain systemic pressure to ensure RV perfusion while avoiding RV overdistension

Scenario 4: Mixed Shock (Septic with Myocardial Depression)

Sonographic findings: Moderately depressed LV function, some B-lines, variable IVC
Strategy: Balanced approach with norepinephrine for vasopressor support; consider adding dobutamine if cardiac output remains low despite adequate MAP
Rationale: Combination of distributive and cardiogenic components requires both vasoconstriction and inotropic support

Hack: Use POCUS to titrate vasoactive agents, not just initiate them. Serial cardiac windows every 30-60 minutes during the first 6 hours of resuscitation allow real-time assessment of response. Look for:

Improvement in LV contractility (increasing EF)
Reduction in cardiac chamber size (suggesting improved forward flow)
Reduction in B-lines (decreasing pulmonary edema)
Normalization of IVC variation (suggesting improved volume status)

Advanced Technique: VTI as a Surrogate for Cardiac Output

The velocity time integral measured in the LVOT provides a surrogate for stroke volume. Combined with heart rate, this allows calculation of cardiac output:

Cardiac Output = VTI × LVOT CSA × HR

Where LVOT cross-sectional area (CSA) = π × (LVOT diameter/2)²

Pearl: Even without calculating absolute cardiac output, serial VTI measurements track relative changes in stroke volume. A >15% increase in VTI following intervention (fluid, vasopressor adjustment, or inotrope initiation) confirms hemodynamic improvement.

Practical Pearls for Integration into Clinical Practice

Standardize Your Scanning Protocol: Consistency improves both speed and accuracy. Develop a systematic approach: cardiac → IVC → lungs → abdomen → vessels.
Document and Compare: Image archiving allows comparison over time. Serial studies are more valuable than single assessments.
Beware of Overconfidence: POCUS findings should complement, not replace, clinical assessment and other monitoring modalities. Integration with clinical context is paramount.
Training and Competency: Adequate training is essential. Studies suggest 25-50 supervised examinations are required for basic competency in POCUS applications.
Team Communication: Develop a shared language for communicating findings. Describe what you see (dilated RV, confluent B-lines) rather than jumping to conclusions, allowing the team to integrate findings collaboratively.

Limitations and Future Directions

While POCUS has transformed critical care, important limitations must be acknowledged. Body habitus, subcutaneous emphysema, and patient positioning can compromise image quality. Operator dependency remains a significant challenge, with inter-rater reliability varying across applications. Furthermore, the evidence base, while growing, still contains gaps regarding specific clinical outcomes.

Future directions include artificial intelligence-assisted interpretation, automated measurement tools, and integration with other monitoring modalities (invasive hemodynamics, biomarkers). Wearable ultrasound devices and handheld systems continue to improve accessibility and workflow integration.

Conclusion

The RUSH and FALLS protocols represent more than systematic scanning algorithms—they embody a physiologically rational approach to resuscitation that replaces empiricism with precision. By integrating cardiac function assessment, volume status evaluation, pulmonary fluid tolerance, and vascular pathology into unified diagnostic frameworks, these protocols enable individualized, goal-directed therapy.

The skilled clinician recognizes that POCUS is not merely a diagnostic tool but a therapeutic guide, allowing real-time titration of fluids and vasoactive agents to patient-specific physiology. As technology advances and evidence accumulates, ultrasound-guided resuscitation will continue to evolve, but the fundamental principles—systematic evaluation, physiological reasoning, and integration of multiple parameters—will remain central to optimal critical care practice.

Final Pearl: The best POCUS examination is the one that changes management. Before scanning, ask yourself: "What question am I trying to answer, and how will the answer alter my therapeutic approach?" This question-driven approach ensures that ultrasound serves as a tool for clinical decision-making rather than an end in itself.

References

Perera P, Mailhot T, Riley D, Mandavia D. The RUSH exam: Rapid Ultrasound in SHock in the evaluation of the critically ill. Emerg Med Clin North Am. 2010;28(1):29-56.
Lichtenstein DA. FALLS-protocol: Lung ultrasound in hemodynamic assessment of shock. Heart Lung Vessels. 2013;5(3):142-147.
Marik PE, Lemson J. Fluid responsiveness: an evolution of our understanding. Br J Anaesth. 2014;112(4):617-620.
Lichtenstein DA, Mezière GA. Relevance of lung ultrasound in the diagnosis of acute respiratory failure: the BLUE protocol. Chest. 2008;134(1):117-125.
Vieillard-Baron A, Millington SJ, Sanfilippo F, et al. A decade of progress in critical care echocardiography: a narrative review. Intensive Care Med. 2019;45(6):770-788.
Volpicelli G, Elbarbary M, Blaivas M, et al. International evidence-based recommendations for point-of-care lung ultrasound. Intensive Care Med. 2012;38(4):577-591.
Monnet X, Teboul JL. Passive leg raising: five rules, not a drop of fluid! Crit Care. 2015;19:18.
Atkinson PR, McAuley DJ, Kendall RJ, et al. Abdominal and Cardiac Evaluation with Sonography in Shock (ACES): an approach by emergency physicians for the use of ultrasound in patients with undifferentiated hypotension. Emerg Med J. 2009;26(2):87-91.
Jardin F, Dubourg O, Bourdarias JP. Echocardiographic pattern of acute cor pulmonale. Chest. 1997;111(1):209-217.
Mayo PH, Narasimhan M, Koenig S. Critical care transesophageal echocardiography. Chest. 2015;148(5):1323-1332.
Malbrain ML, Marik PE, Witters I, et al. Fluid overload, de-resuscitation, and outcomes in critically ill or injured patients: a systematic review with suggestions for clinical practice. Anaesthesiol Intensive Ther. 2014;46(5):361-380.
Weingart SD, Levitan RM. Preoxygenation and prevention of desaturation during emergency airway management. Ann Emerg Med. 2012;59(3):165-175.
Via G, Hussain A, Wells M, et al. International evidence-based recommendations for focused cardiac ultrasound. J Am Soc Echocardiogr. 2014;27(7):683.e1-683.e33.
Teboul JL, Monnet X, Chemla D, Michard F. Arterial pulse pressure variation with mechanical ventilation. Am J Respir Crit Care Med. 2019;199(1):22-31.
Levitov A, Frankel HL, Blaivas M, et al. Guidelines for the appropriate use of bedside general and cardiac ultrasonography in the evaluation of critically ill patients-part II: cardiac ultrasonography. Crit Care Med. 2016;44(6):1206-1227.

Word Count: 3,947 words

learningmedicine

Friday, October 31, 2025

Principles of Ultrasound-Guided Resuscitation