Vascular Resistance

Vascular resistance is defined as the resistance offered by the vessels to the flow of blood, which must be overcome by the blood, so as to ensure sound functioning of the circulatory system. Any kind of circulatory system problems and diseases often lead to a change in the vascular resistance. There are two main types of vascular resistance, which are: systemic or total peripheral vascular resistance and pulmonary vascular resistance. Vascular resistance is regulated by a number of factors, like platelet derived substances including serotonin, endothelium derived relaxing factor and adenosine to name a few. One more type is coronary vascular resistance, which is a complex type, as it depends on various factors, including metabolic, neurologic and endothelial factors. Given below are the details regarding this concept.

Factors Influencing Vascular Resistance

• The type of vascular arrangement, that is the exact arrangement pattern of the body's vasculature
• The type of blood present, that is the viscosity and thickness of blood, then whether the blood flow is laminar or turbulent in nature, as vascular resistance and blood flow are directly connected
• The size of the individual vessel, including its length and diameter
• Other forces acting on the blood vessel (gravity etc.)
• Presence of any vascular diseases, which causes blood circulation problems, like atherosclerosis, peripheral vascular disease etc. Certain diseases lead to vasoconstriction, that is, constriction of the blood vessels, which increases the vascular resistance, while some diseases lead to vasodilation, that is, dilation of blood vessels, which decreases the vascular resistance

Systemic Vascular Resistance

Systemic vascular resistance or total peripheral resistance is defined as the resistance offered to the blood flow by all the systemic vasculature, with the exception of the pulmonary vasculature. Peripheral resistance increases in cases of increased secretion of epinephrine and norepinephrine, as these cause vasoconstriction. This is an important concept that is often studied and used in physiology, to find out the exact cause and other factors affecting the heart and leading to heart diseases. The systemic vascular resistance is calculated in 'dyne.sec/cm⁵'. The normal SVR ranges between 900-1200dyne.sec/cm⁵. To calculate the vascular resistance due to the peripheral blood vessels, the following formula is used:

SVR = 80 x (MAP - MVP) / CO

Where,
SVR - Systemic vascular resistance
MAP - Mean arterial pressure, that is, the normal adult blood pressure in the arterioles of an individual
MVP - Mean venous pressure, that is, the average blood pressure in the veins in an individual
CO - Cardiac output, that is, the volume of blood pumped by a ventricle in a minute

Pulmonary Vascular Resistance

Pulmonary vascular resistance is defined as the resistance offered by the vasculature of the lungs. Normally, the pulmonary vascular resistance is much lower than systemic vascular resistance, the normal PVR being only around 155-255 dyne.sec/cm⁵. However, this PVR level is slightly the cause of its own undoing in the end, as the factors that govern PVR are slightly different from those that govern SVR. On most occasions, heart circulatory problems, directly affect the PVR. For example, with any changed communication between left and right sides of the heart, blood will get diverted to the areas of low resistance from areas of high resistance, which causes an increase in the pulmonary blood flow. This causes an increase in the PVR, thus, causing damage to the pulmonary vasculature, which may even become irreversible. To calculate the pulmonary vascular resistance, the following formula is used:

PVR = 80 x (MPAP - PCWP) / CO

Where,
PVR - Peripheral vascular resistance
MPAP - Mean pulmonary arterial pressure, that is, the pressure in the pulmonary arterioles which can be measured with the help of an echocardiography
PCWP - Pulmonary capillary wedge pressure, which is the pressure measured in a pulmonary artery distal to an occlusion of that artery, is measured by a pulmonary artery catheter
CO - Cardiac output

Vascular resistance is an important concept, that forms the basis of study and diagnosis of various cardiovascular diseases. Although this concept may seem a bit complicated initially, with time and practice, it will become easy to apply and use these formulas to find out vascular resistance.