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Vascular resistance
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Vascular resistance
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.
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 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:
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:
where
The SVR can therefore be calculated in units of dyn·s·cm−5 as
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.
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Vascular resistance
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.
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 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:
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:
where
The SVR can therefore be calculated in units of dyn·s·cm−5 as
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.