Systemic Vascular Resistance

The arrangement of the blood vessels throughout the body can be categorized into two type of circuits: systemic circulation and pulmonary circulation. The vascular resistance offered by the peripheral vasculature in the systemic circulation is referred to as systemic vascular resistance (SVR). Total vascular resistance (TPR) is another reference for SVR. As the term suggests, total vascular resistance is the total of all resistance offered by the peripheral vasculature included in the systemic circulation. It should not be confused with pulmonary vascular resistance (PVR) which offers resistance by the lung's vasculature. The factors like changes in blood vessel diameter and blood viscosity which affect vascular resistance in vascular beds determine SVR. So, now you have the answer for what is systemic vascular resistance, how it is important for us to know about it. Then let us move on to the formula, normal value and index of SVR.

Formula for Calculation

The relationship between the variables for calculating it is the same as the relationship of variables in electrical circuit defined in Ohm's law, resistance = pressure/ flow. SVR is measured from difference between the mean arterial pressure and central venous pressure divided by cardiac output flow. To convert the resultant value into dyne/s/cm^-5, the result is multiplied with 79.9.

SVR Formula

SVR = (MAP - CVP) / CO x 79.9 CVP is normally 0 mm Hg, so in this case the calculation becomes: SVR = MAP / CO x 79.9

Where,
MAP = Mean Arterial Pressure,
CVP = Central Venous Pressure,
CO = Cardiac Output

Some physicians divide the difference between the mean arterial pressure and central venous pressure with cardiac index instead of cardiac output. Notwithstanding the logic of rectifying for surface area of the body, this is not a commonly followed process. The result is said to be SVR and not systemic vascular resistance index.

The contraction of blood vessels due to vessel's muscular wall contraction restricts the flow of blood, thus, increases the vascular resistance. This process is called vasoconstriction. The process opposite to this is called vasodilation where the blood vessels widen due to the relaxation of muscular walls of the vessel, resulting in increased blood circulation as the vascular resistance decreases. Vasoconstriction increases SVR and vasodilation decreases SVR. Its normal value after calculation is between 900 to 1,400 dyne/s/cm^-5.

Doctors use SVR value to estimate afterload. Afterload is the resistance offered to ventricular ejection, which is important for determining ventricular function. The ventricular muscle fiber tension is directly proportional to intracavitary ventricular radius and pressure. For this reason, the dilated ventricles bear more afterload as compared to the normal level in the same level of aortic resistance. Total vascular resistance or SVR also known as systolic contraction can be used for categorizing shock. Shock syndromes are classified on the basis of circulation of blood and the involvement of forces into two categories: high SVR and low SVR. Following are the types of shock in both the categories.

High SVR	Low SVR
Cardiogenic Shock Hypovolemic Obstructive	Sepsis (Blood Infection) Metabolic Dysfunction Toxic Syndromes Neurogenic Endocrine Syndrome

So, to calculate its exact measurement, you need to know the mean pressure and flow of blood in the complete systemic circulation, but it might not give an accurate picture of the regional differences on the basis of vascular resistance.