Structural and functional characteristics of the special regulatory devices in the peripheral pulmonary circulation in rabbits

The present study intended to describe in detail the several blood vessels harboring special regulatory devices in rabbit's pulmonary tissue using light and electron microscopy and immuno-histochemistry. Numerous throttle arteries were recorded within the adventitia of the segmental and sub-segmental bronchi and within pulmonary pleura. These arteries showed characteristic narrow or obliterated lumens and some of them bear longitudinal muscular intimal bolsters. For the first time, TEM revealed some structural modifications of the vascular endothelial cells of these arteries indicating that they become more activated to perform some additional functions. Arteriovenous anastomoses (AVAs) including direct shunt vessels and glomus organs were also recognized. Direct arteriovenous shunts appeared as small connecting devices communicating between small arteries and small veins while glomus organs consisted of the tortuous glomus vessels and the related afferent and efferent vessels. Several arteries and veins showing unique unusual structural characteristics were also described. For the first time, serotonin (5-HT) was strongly expressed in the vascular endothelium and muscle fibers of throttle arteries, in glomus cells of the glomus vessels, and in vascular endothelium of some veins and venules of special structure. The exact role of 5-HT is still unknown and further investigations are required to determine the types and distribution of 5-HT receptors present in these vascular devices. We concluded that these special vascular devices can play a critical role in controlling blood flow and pressure in the peripheral pulmonary circulation; however, the exact physiological mechanisms by which they work or are controlled remain unknown providing a ripe area for further investigation.

Mechanisms of Hypoxia in COVID-19 Patients: A Pathophysiologic Reflection

Coronavirus disease-2019 (COVID-19) causes severe hypoxemia which fulfills the criteria of acute respiratory distress syndrome (ARDS) but is not accompanied by typical features of the syndrome. The combination of factors including low P/F ratios, high A-a gradient, relatively preserved lung mechanics, and normal pulmonary pressures may imply a process occurring on the vascular side of the alveolar–capillary unit. The scant but rapidly evolving data available on the pathophysiology are seemingly conflicting, indicating the relative dominance of intrapulmonary shunting or dead space in different studies. In this hypothesis paper, we attempt to gather and explain these observations within a unified conceptual framework by invoking the relative contributions of microvascular thrombosis, along with two proposed vascular mechanisms of capillary flow redistribution and flow through intrapulmonary arteriovenous anastomoses (IPAVA).

How to cite this article: Nitsure M, Sarangi B, Shankar GH, Reddy VS, Walimbe A, Sharma V, et al. Mechanisms of Hypoxia in COVID-19 Patients: A Pathophysiologic Reflection. Indian J Crit Care Med 2020;24(10):967–970.