Sunday, December 2, 2012

Blood Pressure

Blood Pressure Basics

When you get your blood pressure measured there are two numbers:

  • Systolic blood pressure (SBP) is the higher number and it measures the pressure in arteries following a contraction of the heart, when the pulse of blood reaches the cuff
  • Diastolic blood pressure (DBP) is the lower number and it measures the pressure in arteries at baseline.  This value reflects how well blood travels from arteries to capillaries and is returned by veins.

From systolic and diastolic you can get two more values:

  • Pulse pressure (PP) is the difference between systolic and diastolic.  It reflects the stroke volume of the heart (the amount of blood pumped each heart beat) and the elasticity of the arteries
          PP = SBP – DBP
  • Mean arterial pressure (MAP) is the average pressure in the arteries, which at 60-70 beats per minute can be estimated by one third systolic and two thirds diastolic or diastolic plus one third pulse pressure, whichever is easiest.  (As heart rate increases mean arterial pressure will increase because there is more pulses from the heart muscle contracting, but the equations assume a normal heart rate)
          MAP = ⅓SBP + ⅔DBP
          MAP = DBP + ⅓PP 

The classification of blood pressure is as follows: 

Systolic (mmHg)
Diastolic (mmHg)
Hypotension
<90
<60
Normal
90-119
60-79
Prehypertension
120-139
80-89
Stage 1 Hypertension
140-159
90-99
Stage 2 Hypertension
≥160
≥100
Isolated Systolic Hypertension
≥140
≤90

* Blood pressure measurements only measure the pressure in arteries.  Blood pressure in veins is much lower (<20 mmHg) 

Factors That Affect Blood Pressure 

Mean arterial pressure is proportional to cardiac output multiplied by the resistance in the arteries (MAP CO x R).  Cardiac output is equal to the stroke volume of each heart beat multiplied by the number of heart beats per minute (CO = SV x HR).  A factor that influences stroke volume is the overall blood volume, which is the idea behind reducing salt.  The resistance in the arteries is proportional to length multiplied by viscosity divided by radius to the power of four* (R Lμ/r4), but viscosity and length don’t really vary much.  The last main factor on blood pressure is arterial elasticity which affects systolic and pulse pressure 

So we have four main factors that could affect blood pressure: cardiac output, blood volume, arterial width/peripheral resistance and arterial stiffness. 

* The body often regulates blood pressure by contracting the smooth muscles of the arteries (vasoconstriction), which decreases the radius, or by relaxing them (vasodilation), which increases the radius.

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