Systemic vascular resistance mainly consists of arteriolar constriction in the entire systemic circulation, and is calculated by dividing the difference of arterial and venous pressure with cardiac output. Read the following article to gain more information about this subject.
The arrangement of blood vessels throughout the body can be categorized into two types of circuits: systemic circulation and pulmonary circulation. The vascular resistance offered by 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, it is the total of all resistances offered by the peripheral vasculature, included in systemic circulation. It should not be confused with pulmonary vascular resistance (PVR), which offers resistance by the lung’s vasculature. Factors like changes in blood vessel diameter and blood viscosity, which affect vascular resistance in vascular beds determine SVR.
Formula for Calculation
The relationship between the variables for calculating SVR is the same as the relationship of variables in electrical circuit defined in Ohm’s law: resistance = pressure/ flow. SVR is measured from the 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 = (MAP – CVP) / CO x 79.9
CVP normally is 0 mm Hg. Hence, in this case, the calculation becomes:
SVR = MAP / CO x 79.9
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 restricts the flow of blood, resulting in increase of the vascular resistance. This process is called vasoconstriction. The process opposite to this is called vasodilation, where the blood vessels widen due to relaxation of muscular walls of the vessel, leading to increased blood circulation, as the vascular resistance decreases. Vasoconstriction and vasodilation increase and decrease SVR, respectively. Its normal value after calculation is between 900 to 1,400 dyne/s/cm-5.
Doctors use SVR value to estimate afterload, which is the resistance offered to ventricular ejection; it is important for determining ventricular functions. 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 stage of aortic resistance. Total vascular resistance or SVR, also known as systolic contraction, can be used for categorizing shock. On the basis of circulation of blood and the involvement of forces, shock syndromes are classified into two categories: high SVR and low SVR. Following are the types of shock in both the categories.
|High SVR||Low SVR|
Thus, to calculate its exact measurement, you need to know the mean pressure and blood flow in the complete systemic circulation, but it might not give an accurate picture of the regional differences on the basis of vascular resistance.