Blood pressure

Blood pressure does not only depend on intravascular volume. Ohm's law (voltage = current x resistance) has been adapted to demonstrate that blood pressure is the product of cardiac output (CO) and systemic vascular resistance (SVR):

Blood pressure and flow are not the same thing. A low blood pressure does not necessarily mean a low cardiac output - it could be due to a low systemic vascular resistance, or both.

SVR may be thought of as the resistance against which the heart pumps. Vasoconstriction increases SVR whereas vasodilatation reduces it.

Mean arterial pressure (MAP) is the average arterial blood pressure throughout the cardiac cycle. This is produced by integrating a pressure signal for the duration of one cycle. A rough calculation is:

(2 x diastolic) + systolic pressure 3

Many critical care clinicians prefer using MAP because it is less liable to error owing to measuring techniques; it represents the mean pressure available for perfusing vital organs and is often used in studies.

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