Dilated blood vessels in the lung may explain low oxygen lev
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A new pilot study published in the American Journal of Respiratory and Critical Care Medicine suggests that COVID-19 is causing significant dilation of the blood vessels of the lung, specifically the capillaries. This vasodilation is contributing to the very low oxygen levels seen in COVID-19 respiratory failure and also helps explain why the disease behaves differently than classic ARDS.

In classical ARDS, pulmonary inflammation leads to leaky pulmonary blood vessels that flood the lungs with fluid, making the lungs stiff and impairing oxygenation. Many patients with COVID-19 pneumonia demonstrate severe hypoxemia that is markedly out of proportion to the degree of lung stiffness. This disconnect between gas exchange and lung mechanics in COVID-19 pneumonia has raised the question of whether the mechanisms of hypoxemia in COVID-19 differ from those in classical ARDS.

Researchers were initially assessing cerebral blood flow in mechanically ventilated COVID-19 patients with altered mental status to look for, among other things, abnormalities consistent with stroke. They used a robotic transcranial Doppler (TCD), the Lucid Robotic System by NovaSignal, to perform a "bubble study," which is a non-invasive and painless ultrasound technique.

In the pilot study, 18 mechanically ventilated patients with severe COVID-19 pneumonia underwent TCD with bubble study. Fifteen out of the 18 (83 percent) patients had detectable microbubbles, indicating the presence of abnormally dilated pulmonary blood vessels. The number of microbubbles detected by the TCD correlated with the severity of hypoxemia, indicating that the pulmonary vasodilations may explain the disproportionate hypoxemia seen in many patients with COVID-19 pneumonia.

Previous studies have demonstrated that only 26 percent of patients with classical ARDS have microbubbles during a bubble study; furthermore, the number of these microbubbles does not correlate with the severity of hypoxemia, implying that pulmonary vascular dilations are not a major mechanism of hypoxemia in classical ARDS.

Source: https://www.atsjournals.org/doi/abs/10.1164/rccm.202006-2219LE
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