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Concept
Vascular resistance
Summary
Vascular resistance is the resistance that must be overcome to push blood through the circulatory system and create blood flow. The resistance offered by the systemic circulation is known as the systemic vascular resistance (SVR) or may sometimes be called by the older term total peripheral resistance (TPR), while the resistance offered by the pulmonary circulation is known as the pulmonary vascular resistance (PVR). Systemic vascular resistance is used in calculations of blood pressure, blood flow, and cardiac function. Vasoconstriction (i.e., decrease in blood vessel diameter) increases SVR, whereas vasodilation (increase in diameter) decreases SVR.
Units for measuring vascular resistance are dyn·s·cm−5, pascal seconds per cubic metre (Pa·s/m3) or, for ease of deriving it by pressure (measured in mmHg) and cardiac output (measured in L/min), it can be given in mmHg·min/L. This is numerically equivalent to hybrid resistance units (HRU), also known as Wood units (in honor of Paul Wood, an early pioneer in the field), frequently used by pediatric cardiologists. The conversion between these units is:
The basic tenet of calculating resistance is that flow is equal to driving pressure divided by flow rate.
where
R is Resistance
ΔP is the change in pressure across the circulation loop (systemic / pulmonary) from its beginning (immediately after exiting the left ventricle / right ventricle) to its end (entering the right atrium / left atrium)
Q is the flow through the vasculature (when discussing SVR this is equal to cardiac output)
This is the hydraulic version of Ohm's law, V=IR (which can be restated as R=V/I), in which the pressure differential is analogous to the electrical voltage drop, flow is analogous to electric current, and vascular resistance is analogous to electrical resistance.
The systemic vascular resistance can therefore be calculated in units of dyn·s·cm−5 as
where mean arterial pressure is 2/3 of diastolic blood pressure plus 1/3 of systolic blood pressure [or Diastolic + 1/3(Systolic-Diastolic)].
Official source
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