Abstract
Corona virus disease 2019 (COVID‐19) causes symptoms from multiple organs after infection by severe acute respiratory syndrome corona virus 2 (SARS CoV‐2). They range from early, low blood oxygen levels (hypoxemia) without breathlessness (“silent hypoxia”), delirium, rashes, and loss of smell (anosmia), to persisting chest pain, muscle weakness and ‐pain, fatigue, confusion, memory problems and difficulty to concentrate (“brain fog”), mood changes, and unexpected onset of hypertension or diabetes. SARS CoV‐2 affects the microcirculation, causing endothelial cell swelling and damage (endotheliitis), microscopic blood clots (microthrombosis), capillary congestion, and damage to pericytes that are integral to capillary integrity and barrier function, tissue repair (angiogenesis), and scar formation. Similar to other instances of critical illness, COVID‐19 is also associated with elevated cytokine levels in the systemic circulation. This review examines how capillary damage and inflammation may contribute to these acute and persisting COVID‐19 symptoms by interfering with blood and tissue oxygenation and with brain function. Undetectable by current diagnostic methods, capillary flow disturbances limit oxygen diffusion exchange in lungs and tissue and may therefore cause hypoxemia and tissue hypoxia. The review analyzes the combined effects of COVID‐19‐related capillary damage, pre‐existing microvascular changes, and upstream vascular tone on tissue oxygenation in key organs. It identifies a vicious cycle, as infection‐ and hypoxia‐related inflammation cause capillary function to deteriorate, which in turn accelerates hypoxia‐related inflammation and tissue damage. Finally, the review addresses the effects of low oxygen and high cytokine levels in brain tissue on neurotransmitter synthesis and mood. Methods to assess capillary functions in human organs and therapeutic means to protect capillary functions and stimulate capillary bed repair may prove important for the individualized management of COVID‐19 patients and targeted rehabilitation strategies.
Highlights
Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections have swept across continents, claiming over 1.5 million lives (The Johns Hopkins Coronavirus Resource Center (CRC), 2020)
This review focuses on the effects of break-downs in the factors that ensure the crucial, parallel increase in PS and blood flow, after biophysical models have predicted that capillary occlusions and severe capillary flow disturbances can limit tissue oxygenation to the same extent as symptomatic reductions in blood flow in brain, heart, and kidneys (Østergaard et al, 2013, 2014, 2015)
TNF-α and other inflammatory factors are known to damage the glycocalyx and impair endothelial functions (Liu et al, 2020; Zhang et al, 2020). This immune response is likely to contribute to capillary dysfunction and reduced tO2 in affected tissue, where cellular oxygen sensors, in turn, activate hypoxia-inducible transcription factor (HIF) and the transcription of genes that help tissue adapt to hypoxia (Eltzschig & Carmeliet, 2011)
Summary
Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections have swept across continents, claiming over 1.5 million lives (The Johns Hopkins Coronavirus Resource Center (CRC), 2020). Brain, capillary dysfunction, COVID-19, heart, hypoxemia, hypoxia, inflammation, long-term symptoms, lungs, microcirculation, muscle
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