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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.


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.


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.


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.


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 ...

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