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After about 1 week of age, pulmonary arterial blood pressure
is normally under 25/12 mm Hg, and mean pressure is under
18 mm Hg.1,2 Any higher pressure is regarded as pulmonary
arterial hypertension, although slight increases are usually not
clinically important.
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To evaluate the causes of pulmonary arterial hypertension, consider
the concept of vascular resistance (R), which is, by definition,
the ratio of the pressure drop across a vascular bed to the blood
flow through it. For the pulmonary circulation, the pressure drop
is from the pulmonary artery (Pa) to the pulmonary veins (Pv) and
is normally 4 to 10 mm Hg at rest. The flow is the pulmonary blood
flow (Qp), which at all ages is about 3.5 L/min for each
square meter of body surface area. Thus, resting pulmonary vascular resistance
is about 1 to 3 mm Hg/L/min/m2.
On exercise in the normal subject, pulmonary blood flow can increase
three- to fourfold, with little rise in pulmonary arterial pressure; the
resistance falls to one third or one quarter of its resting value
because the small resistance vessels dilate and new vessels are
recruited.
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The resistance equation Rp = (Pa – Pv)/Qp
can be rearranged as Pa = RpQp + Pv. Thus, pulmonary
arterial pressure (Pa) rises with an increase in pulmonary vascular
resistance, pulmonary blood flow, or pulmonary venous pressure,
provided that a rise in one variable does not alter the others.
An increase in flow normally causes a fall in pulmonary vascular
resistance that tends to prevent a rise in pulmonary arterial pressure. Thus,
pulmonary arterial pressures are usually normal or only minimally
elevated with exercise or with the large pulmonary flows found with large
atrial septal defects. Conversely, the high pulmonary arterial pressures
seen with large ventricular septal defects (which equalize pressures between
the 2 ventricles) indicate the failure of pulmonary vascular resistance
to fall when pulmonary blood flow increases, so that increases in both
Qp and R are responsible for the pulmonary arterial hypertension.
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If pulmonary venous pressure rises (eg, because of total anomalous
pulmonary venous connection with obstruction, mitral stenosis, or
left ventricular failure), pulmonary arterial pressure rises by
about the same amount or more if these patients also have an increased pulmonary
vascular resistance secondary to vascular narrowing from vasoconstriction
or pulmonary edema.
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To analyze pulmonary vascular resistance in more detail, consider
the relationship found by Poiseuille for the resistance offered
by a straight glass tube to the steady flow of a Newtonian liquid
(eg, water) through it: Resistance equals pressure drop across the
tube divided by the flow through it, and the value of this ratio
(the resistance) = (8/π) (l/r4)
(η). That is, resistance depends on a constant
(8/π), the geometric factor of tube length
(l) divided by the fourth power of the radius (r),
and the viscosity (η). Although pulsatile blood
flow ...