CLUE Protocol – Cardiopulmonary US

Brandon Fainstad, MD

  1. Learn a systematic cardiopulmonary exam by ultrasound for evaluating a patient with hypoxemia or shock.
  2. Review proper image acquisition technique and interpretation for basic cardiac and lung ultrasound.

This post assumes a basic skill in obtaining standard cardiac views.  Click here for an additional review from on how to obtain cardiac views.

The CLUE protocol is a quick and rough assessment of cardiac function, pleural disease, lung pathology, and fluid responsiveness/overload in a patient with hypoxemia or shock.  It is performed in as little as 60 seconds with 5 basic views moving against the flow of blood:

Left Ventricle (LV) → Left Atrium (LA) → Lungs → Right Ventricle (RV)/Pericardium → Inferior Vena Cava (IVC)

1. Parasternal long axis (PSLA)

Assess general LV systolic function (normal, moderate or severely reduced EF) and wall motion abnormalities based on thickening and shortening of the LV walls.  To calculate the E-point septal separation (EPSS), align your m-mode spike through the intraventricular septum (IVS) and tip of the mitral valve leaflet.  The EPSS is the shortest distance from the IVS to leaflet.  In general, the smaller the EPSS, the greater the ejection fraction.

EPSS < 7 mm normal
EPSS > 10 mm reduced ejection fraction

** both aortic regurgitation and mitral stenosis will falsely increase EPSS, suggesting a falsely low ejection fraction **

An enlarged left atrium (diameter of LA > Aortic outflow track) suggests acutely or chronically elevated left atrial pressures.

Click here For an additional review on how to obtain a PSLA and assess cardiac function

2. Anterior lung apices

Quick intro to lung ultrasound – US waves do not penetrate air, rather they are reflected or dispersed.  Thus, healthy, well-aerated lungs are poorly visualized by ultrasound.  Instead, we rely primarily on ultrasound artifacts to provide information about the lung. 

Lung sliding = use a linear or high-frequency probe positioned on the anterior chest with two rib spaces in view.  The bright line between the two rib spaces is the pleural line.  “Lung sliding” is the twinkling or “marching ants” along this line, due to the visceral and parietal layers sliding against one another during respiration.  Lack of sliding is highly sensitive for pneumothorax but there are multiple issues that will prevent lung sliding: pneumothorax, pleural effusion, pleural inflammation and apnea.   To confirm it is a pneumothorax, identify the transition point.

A-lines = horizontal lines repeated at regular intervals.   A-lines represent the parietal-visceral pleural interface which is as far as the US waves can penetrate before being reflected by pleural air (pneumothorax) or aerated lung (healthy lung).   These waves reverberate back and forth between the probe and pleura, making repeated artificial lines at multiples of the distance fr m the probe to the pleura.

Obtained with permission from, courtesy of Dr. Jacob Avila

B lines = Three or more vertical rays that 1. move with respiration, 2. obliterate A-lines, 3. extend to the bottom of the screen, 4. in 2 or more regions on either side of the chest.   They represent increased density in the alveoli (water, puss or blood) or interstitium (fluid or scaring) where US waves are able to penetrate and ricochet between adjacent low density-high density interfaces

Obtained with permission from, courtesy of Dr. Jacob Avila

Review of lung ultrasound: 
– If there is no lung sliding and A-lines – there is a pneumothorax or inflammation of the pleura preventing sliding.
– If there is lung sliding and A-lines – the lung is normal in that particular window
– If there are B lines – pathology either in the alveoli or interstitium

3. Lateral lung bases

Visualize the diaphragm in the middle of the screen and assess for fluid around the lung (pleural effusion) or hepatization/consolidation of the lung (pneumonia).

4. Subcostal cardiac view – Compare relative sizes of R and L ventricles and pericardium.
5. IVC
– If the IVC is >2.5cm and collapses by <50% this suggests elevated right heart pressures from volume overload, tamponade, pulmonary embolus or pulmonary hypertension.
– If the IVC is <1.5cm and collapses by >50% with respirations this suggests volume responsiveness in a hypotensive patient (this is most validated in ventilated and paralyzed patients)

How to obtain:

Figure 1. CLUE with normal findings (Upper) and abnormal findings (Lower) for each view (see text for “quick-look” diagnostic criteria). Probe positions are seen within insets (black bars). Parasternal long-axis view (Left) shown in mid-diastole and demonstrates LV systolic dysfunction and LA enlargement. Longitudinal subcostal view (Center) shown in end-inspiration and demonstrates IVC. Lung apical view (Right) shown at end-expiration and demonstrates 3 normal horizontal reverberation artifacts compared to ULC examination with 3 vertical linear artifacts.
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Example video of CLUE:

Cases – thanks to Cameron Bass and Amy Morris


Kimura, Bruce J., et al. “Cardiopulmonary Limited Ultrasound Examination for ‘Quick-Look’ Bedside Application.” The American Journal of Cardiology, vol. 108, no. 4, 2011, pp. 586–590.

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