CONCLUSION—AND UNANSWERED QUESTIONS SARS‐CoV‐2‐related inflammation and capillary damage may contribute synergistically to acute and long‐term COVID‐19 symptoms by interfering with blood and tissue oxygenation. Further studies of microvascular changes during COVID‐19—and especially subsequent capillary repair—are needed to understand capillary dysfunction’s impact on the acute and chronic health effects of the disease. If COVID‐19‐related capillary [endothelial] dysfunction persists in some organs, future attention to the vascular health in younger subjects with a history of COVID‐19 may be warranted. The COVID‐19 pandemic highlights a need to develop biomarkers of capillary function across organs in humans—and may increase our understanding of the microcirculation’s role in human health and disease.
Physiol Rep 2021 Feb;9(3):e14726. SARS CoV-2 related microvascular damage and symptoms during and after COVID-19: Consequences of capillary transit-time changes, tissue hypoxia and inflammation Leif Østergaard 1 2 Affiliations
1 Neuroradiology Research Unit, Section of Neuroradiology, Department of Radiology, Aarhus University Hospital, Aarhus, Denmark.
2 Center of Functionally Integrative Neuroscience, Department of Clinical Medicine, Aarhus University, Aarhus, Denmark.
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. Keywords: COVID-19; brain; capillary dysfunction; heart; hypoxemia; hypoxia; inflammation; long-term symptoms; lungs; microcirculation; muscle.