UltraRounds: Contrast Lung Ultrasound to detect Microthrombi

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The Pre-brief

The novel coronavirus has truly brought to light the utility of bedside ultrasound in a big way. But as we highlight below, not just noncontrast ultrasound but contrast enhanced ultrasound (CEUS).

Introduction to CEUS

A methodology introduced over 25 years ago, contrast ultrasound involves the administration of IV macrobubble or nanobubble agents; these are most commonly used to image the liver, kidneys, and bladder. The goal of these agents are to aid dynamic visualization of blood flow in macro and microvasculature. The way this is achieved is physics- the concept of compressibility of gases and how it creates alternating pressure waves whereas tissue does not produce this same oscillation.

In practice “first generation and second agents” exist but the main distinction is that most second generation agents have replaced first generation agents. Second generation agents are those with low soluble gases versus air. Second generation agents have replaced former agents because they are easier to use and more effective. The actual contrast agent in and CEUS can be injected as a bolus (study discussed below) or as a steady IV infusion (primarily cardiac imaging). In some cases, the principle is to look for enhancement (ex. Neovascularization of a tumor) and others non-enhancement (ex. Occluding thrombus).

Key Questions:

  • What is the medium of CEUS in this case? Small transpulmonarily stable microbubbles of sulfur hexafluoride about the same size as RBCs are introduced intravenously. Regions where these bubbles do not easily cross the capillary bed are considered avascular and therefore do not enhance. 
  • What is the potential utility of CEUS in COVID19? Embolic consolidations described as peripheral subpleural consolidation with no or little enhancement on CEUS have been detected as embolic regions with pulmonary infarction in subsequent histology.
  • What concern would this address? Elevated acute phase reactants and D-dimers have been linked to increased mortality, and this would increase detection of peripheral pulmonary infarction in acute illness.

COVID 19 CASE: UTILITY OF IMAGING

CEUS can be integrated in the evaluation for pulmonary emboli in the setting of COVID. As more cases of COVID19 related thrombosis emerge, this tool could be another asset to the detection of microthrombi. 

Special thank you to Dr. Adrian Wong for the use of these images and the case. (cited below)

The Debrief

  • CEUS should be considered in the setting of a high D-dimer given associated mortality of COVID19
  • Detection on peripheral subpleural consolidation could be a sign of peripheral infarction further evaluated with CEUS.

References

  1. Ignee, A., Atkinson, N. S., Schuessler, G., & Dietrich, C. F. (2016). Ultrasound contrast agents. Endoscopic ultrasound, 5(6), 355–362. https://doi.org/10.4103/2303-9027.193594

  2. Tee, A., Wong, A., Yusuf, G.T. et al. Contrast-enhanced ultrasound (CEUS) of the lung reveals multiple areas of microthrombi in a COVID-19 patient. Intensive Care Med 46, 1660–1662 (2020). https://doi.org/10.1007/s00134-020-06085-4

  3. Trenker C, Apitzsch JC, Pastor S et al (2017) Detection of peripheral embolic consolidations using contrast-enhanced ultrasonography in patients with no evidence of pulmonary embolism on computed tomography: A pilot study. J Clin Ultrasound JCU 45(9):575–579. https://doi.org/10.1002/jcu.22511

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