Microcirculation is the blood flow through the smallest vessels in the circulatory system, i.e. arterioles, venules, shunts and capillaries. Microcirculation exists in all tissues and organs except for the cornea. Laser Doppler perfusion monitoring (LDPM), laser speckle contrast analysis (LASCA) and transcutaneous oximetry (tcpO2) are well-known techniques for studying microcirculation.
The main functions of microcirculation are the transport of blood cells and substances such as oxygen to/from tissues (nutritive flow), to aid in blood pressure regulation and to act as a thermoregulator (mainly skin).
Microcirculatory blood flow can exhibit vasomotion, rhythmic oscillations in vascular tone caused by changes in smooth muscle constriction and dilation, which is controlled locally as well as systemically. The variations are usually 4-10 cycles per minute (cpm) and may vary with the temperature. Interestingly, vasomotion is significantly reduced in microneuropathy and problematic flaps.
The microcirculation also shows extreme dynamics. Under normal conditions, the blood perfusion can differ several thousand percent between a cold and warm fingertip. It also exhibits large spatial variations and may vary up to 100 percent in forearm skin if the measurement site is moved one millimeter. Blood perfusion measurements using laser Doppler will reflect these extreme dynamics and large spatial variations.
Parameters known to influence the microcirculation are:
- Temperature of the patient
- Systemic blood pressure
- General status (stress, food, medication, smoking, etc.)
- Mental activity
- Physical activity
- Arterial insufficiency (microcirculatory reserve)
“Microangiopathies are important per se in diabetes and collagen vascular disease. In other conditions, such as the broad spectrum of ischemic diseases and chronic venous incompetence, microvascular morphology and function are disturbed as a consequence of macrovascular damage. Microcirculation is the target section under both cases; the changes at this level decide whether a tissue survives or dies.”1
As a consequence of the large normal variations observed in the microcirculatory blood flow, provocations are often used to facilitate data interpretation. Provocations allow the user to look at the response to a certain provocation rather than just a basal value of microcirculatory flow. Commonly used provocations include:
- Heat—maximum dilation, tissue reserve capacity
- Occlusion—post-occlusive reactive hyperemia
- Drugs—patch tests, iontophoresis, injections
- Posture—veno-arterial reflex, leg elevation
- Cooling—Raynaud’s phenomena
- Electrical stimulation—c-fiber stimulation