Microcirculation: Physiology, Pathophysiology, and Clinical Application

Abstract

This paper briefly reviews the physiological components of the microcirculation, focusing on its function in homeostasis and its central function in the realization of oxygen transport to tissue cells. Its pivotal role in the understanding of circulatory compromise in states of shock and renal compromise is discussed. Our introduction of hand-held vital microscopes (HVM) to clinical medicine has revealed the importance of the microcirculation as a central target organ in states of critical illness and inadequate response to therapy. Technical and methodological developments have been made in hardware and in software including our recent introduction and validation of automatic analysis software called MicroTools, which now allows point-of-care use of HVM imaging at the bedside for instant availability of functional microcirculatory parameters needed for microcirculatory targeted resuscitation procedures to be a reality.

Keywords: Incident dark field imaging; Microcirculation; Sepsis; Shock; Sidestream dark field imaging; Tissue red blood cell perfusion.

Fig. 1

Microvascular anatomy. The microcirculation is the part of the vascular system and consists of the small vessels so-called arterioles, capillaries, and venules. The lymphatic capillaries carry the extravascular fluid into the venous system. The arterioles are surrounded by vascular smooth muscle cells responsible for the regulation of arteriole tone.

Fig. 2

Microvascular dysfunction and vascular endothelial damage. a The structure of a healthy microvessel is shown. EC and glycocalyx cover the lumen of the microvessel. The blood cells (leukocytes, RBC, thrombocytes) flow together with plasma inside the microvessels. b Microcirculatory damage can be caused by ischemia, reperfusion, inflammation, and hypoxia, resulting in endothelial and glycocalyx and RBC damage. Activation of leukocytes induces rolling, adhesion, and ultimately extravasation to the tissue, which further accelerates the inflammation. Decreased vascular permeability causes vascular leakage and edema formation. RBC, red blood cell; EC, endothelial cells.

Fig. 3

Condition of microcirculatory alterations associated with loss of hemodynamic coherence and reduced oxygen capacity of the tissues. Type 1: Heterogenous RBC flow caused by RBC and endothelial cell injury induced for example by sepsis results in RBC stagnant capillaries next to perfused capillaries resulting in microcirculatory shunts and a reduction of tissue oxygen extraction capacity. Type 2: A decrease in the oxygen-carrying potential of the microcirculation due to hemodilution induced anemia resulting from a low FCD. Type 3: A stasis in the RBC flow due to increased vascular resistance (R) [37] and/or elevated venous pressure (P) [38]. Type 4: Increased oxygen diffusion distances due to edema caused by capillary leak syndrome. Adapted from Ince [67].