What is hyperemia?

Definition of Hyperemia

Hyperemia is the physiological process of increased blood flow to a tissue or organ, occurring either as a compensatory response to metabolic demand (active hyperemia) or following a period of ischemia (reactive hyperemia). 1

Types of Hyperemia

Reactive Hyperemia

• Reactive hyperemia is the compensatory increase in blood flow that occurs after a period of tissue ischemia, representing the body's response to oxygen debt and accumulation of metabolic waste products due to interruption of blood flow. 1, 2
• This response is measured as the magnitude of limb reperfusion following a brief period of ischemia induced by arterial occlusion, typically using a 5-minute brachial artery occlusion at 200 mmHg. 3, 4
• The hyperemic response can be quantified by measuring forearm blood flow, brachial artery diameter changes, or fingertip pulse-volume amplitude following release of arterial occlusion. 3

Active Hyperemia (Functional Hyperemia)

• Active hyperemia refers to the increased blood flow in an organ during a period of heightened metabolic activity, meeting the tissue's increased metabolic needs during function. 1, 5
• This process involves complex interactions between parenchymal cells and multiple classes of microvessels including capillaries, arterioles, and small arteries. 5

Physiological Mechanisms

Microcirculatory Control

• The microcirculation (arterioles, capillaries, and venules) is controlled partly by sympathetic vasoconstrictor impulses from the brain and partly by vasoactive substances secreted locally by endothelial cells. 1
• The most important local mediator is nitric oxide, which facilitates blood flow by causing relaxation of vascular smooth muscle. 1
• The hyperemic response involves local metabolic effects, propagated effects extending over long vascular segments, and flow-dependent vasodilation induced by local changes in blood flow. 5

Clinical Significance

Assessment of Vascular Function

• Reactive hyperemia serves as a noninvasive assessment of peripheral microvascular function and is a predictor of all-cause and cardiovascular morbidity and mortality. 4
• Blunted reactive hyperemia reflects impaired microvascular function and is associated with increased cardiovascular risk in patients with atherosclerosis. 2, 4
• Lower hyperemic flow velocity predicts cardiovascular events including cardiac death, myocardial infarction, and stroke in patients with peripheral arterial disease. 2

Measurement Techniques

• Multiple methodologies exist to assess reactive hyperemia, including brachial artery velocity by Doppler ultrasound, tissue reperfusion by near-infrared spectroscopy, venous occlusion plethysmography, peripheral artery tonometry, and laser speckle contrast imaging. 4, 6
• The reactive hyperemia index can be measured noninvasively using fingertip pulse-volume amplitude, calculated as the ratio of post-deflation to baseline pulse amplitude. 3

Pathological Implications

• Neural and endothelial control of blood flow are impaired by illness, predisposing patients with neurological or vascular disease to develop ischemic organ damage including pressure sores during intercurrent illness. 1
• Severe sepsis or trauma may cause irreversible microcirculatory dysfunction resulting in multi-organ failure and death. 1
• The hyperemic response to infection and trauma is vitally important for tissue survival and healing. 1