Vascular resistance is the resistance that must be overcome for blood to flow through the circulatory system. The resistance offered by the systemic circulation is known as the systemic vascular resistance or may sometimes be called by another term total peripheral resistance, while the resistance caused by the pulmonary circulation is known as the pulmonary vascular resistance. Vasoconstriction (i.e., decrease in the diameter of arteries and arterioles) increases resistance, whereas vasodilation (increase in diameter) decreases resistance. Blood flow and cardiac output are related to blood pressure and inversely related to vascular resistance.
Measurement The measurement of vascular resistance is challenging in most situations. The standard method is by the use of a Pulmonary artery catheter. This is common in ICU settings but impractical in most other settings.
Units for measuring 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:
1 mmHg ⋅ min L ( HRUs ) = 8 MPa ⋅ s m 3 = 80 dyn ⋅ sec cm 5 {\displaystyle 1\,{\frac {{\text{mmHg}}\cdot {\text{min}}}{\text{ L }}}({\text{HRUs}})=8\,{\frac {{\text{MPa}}\cdot {\text{s}}}{{\text{m}}^{3}}}=80\,{\frac {{\text{dyn}}\cdot {\text{sec}}}{{\text{cm}}^{5}}}}
Calculation In the hydraulic version of Ohm's law, sometimes called Ohm’s law of fluid flow, vascular resistance is analogous to electrical resistance, the pressure difference is analogous to the electrical voltage difference, and volumetric flow is analogous to electric current flow:
R = Δ P / Q {\displaystyle R=\Delta P/Q}
where
R is resistance ΔP is the difference 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)
Systemic vascular resistance The SVR can therefore be calculated in units of dyn·s·cm−5 as
80 ⋅ ( m e a n a r t e r i a l p r e s s u r e − m e a n r i g h t a t r i a l p r e s s u r e ) c a r d i a c o u t p u t {\displaystyle {\frac {80\cdot (\mathrm {mean\ arterial\ pressure} -\mathrm {mean\ right\ atrial\ pressure} )}{\mathrm {cardiac\ output} }}}
where the pressures are measured in mmHg and the cardiac output is measured in units of litres per minute (L/min). Mean arterial pressure is the cycle average of blood pressure and is commonly approximated as 2 x diastolic blood pressure + systolic blood pressure/3 [or diastolic blood pressure + 1/3(systolic blood pressure - diastolic blood pressure)]. Mean right atrial pressure or central venous pressure, is usually very low (normally around 4mmHg), and as a result, it is frequently disregarded. As an example: if systolic blood pressure = 120 mmHg, diastolic blood pressure = 80 mmHg, right atrial mean pressure = 3 mmHg and cardiac output = 5 L/min, Then mean arterial pressure = 2 x diastolic pressure + systolic pressure/3 = 93.3 mmHg, and SVR = (93 - 3) / 5 = 18 Wood units, or equivalently 1440 dyn·s/cm5. It is difficult to measure or monitor SVR in most locations outside the ICU. An invasive catheter is necessary. SVR, BP and CO are related to each other but only BP is easily measured. In the typical situation at the bedside we have an equation with three variables, one known, that is the BP and two unknown, CO and SVR. For this reason the BP is frequently used as a practical but somewhat inadequate definition of shock or the state of blood flow.
Pulmonary vascular resistance The PVR can be calculated similarly (in units of dyn·s·cm−5 ) as:
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