Ultrasound in the intensive care unit is a reasonable gold standard to detect many causes of respiratory failure with a sensitivity and specificity ranging from 90% to 100%. The BLUE-protocol (Bedside Lung Ultrasound in Emergency) was developed at university-affiliated teaching-hospital ICUs in France led by Lichtenstein et al, and is now widely used as a fast and easy tool in the differential diagnosis of acute respiratory failure [1].
The BLUE-protocol combines certain ultrasound signs (lung sliding, A lines, B lines, and PLAPS) with location, associated with results of a sequential venous analysis (thrombosed versus free veins) to give a clinical diagnosis and etiology of acute dyspnea and respiratory failure.
As mentioned in a previous blog, lung sliding is the shimmering movement of the visceral and parietal pleural surface along with respiration and indicates the lack of air (pneumothorax) between the two layers of the pleura. A-lines are horizontal artifacts result from reverberation of the ultrasound beams off the pleura. They occur at equal intervals and indicate dry interlobular septa (normal lung). B-lines are discrete laser-like (comet-tail) vertical hyperechoic artifacts extending from the pleura to the edge of the screen and indicate a thickened interlobular septa (pulmonary edema or pneumonia). PLAPS stands for a PosteroLateral Alveolar and/or Pleural Syndrome representing lung consolidation/collapse and/or pleural effusion.
The combination of these signs results in seven different profiles:
A-profile combines anterior lung-sliding with A-lines bilaterally.
A’-profile when A-lines are present bilaterally with abolished lung-sliding.
B-profile associates anterior lung-sliding with lung-rockets bilaterally.
B’-profile is B-profile with abolished lung-sliding.
C-profile indicates anterior lung consolidation (any size and number) or a thickened, irregular pleural line.
The A/B profile is a half A-profile at one side, a half B-profile at another.
The PLAPS-profile indicates a PosteroLateral Alveolar and/or Pleural Syndrome.
Pulmonary edema, pulmonary embolism, pneumonia, chronic obstructive pulmonary disease, asthma, and pneumothorax yield specific profile. The presence of B-profile on the ultrasound in a dyspneic patient indicates pulmonary edema with a sensitivity of 97% and specificity of 95%. Ultrasound has a low sensitivity in the diagnosis of pneumonia when individual profiles are found (B-profile, A/B profile, C profile, or A-profile with PLAPS and free veins). However, when multiple profiles are present, sensitivity goes up to 89% and specificity up to 94%. Ultrasound with A’-profile and lung point is very specific (100%) and sensitive (88%) in the diagnosis of pneumothorax. The presence of thrombosed veins with A-profile indicates pulmonary embolism with 81% sensitivity and 99% specificity. Finally, the nude profile (A-profile with free veins) in a dyspneic patient is and indication of COPD or asthma with a sensitivity of 89% and specificity of 97% [1}.
Reference
Lichtenstein DA, Mezière GA. Relevance of lung ultrasound in the diagnosis of acute respiratory failure: the BLUE protocol. Chest. 2008 Jul;134(1):117-25. doi: 10.1378/chest.07-2800. Epub 2008 Apr 10. Erratum in: Chest. 2013 Aug;144(2):721. PMID: 18403664; PMCID: PMC3734893.