42: Pulmonary Hypertension

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CHAPTER 42 Pulmonary Hypertension

James Duke, MD, MBA

1 Define pulmonary hypertension

Although there is no widely accepted range for normal pulmonary blood pressures, pulmonary hypertension (PH) exists when the pulmonary systolic pressure exceeds 25 mm Hg in the presence of normal cardiac output (CO) and pulmonary capillary wedge pressure. During exercise PH is considered a pulmonary artery pressure (PAP) greater than 30 mm Hg.

2 List conditions that produce pulmonary hypertension

PH is said to be idiopathic if it is found in the absence of secondary causes such as pulmonary disease (congenital or parenchymal), cardiac disease (e.g., shunts, mitral stenosis, left ventricular failure), thromboembolic or obliterative pulmonary vascular disease, connective tissue disease (especially the scleroderma family), exogenous vasoconstrictive substances (e.g., the appetite suppressant flufenamine), or portal hypertension. Although rare in the general population, at-risk populations include those with connective tissue disease and human immunodeficiency virus. Idiopathic PH was formally called primary PH, is a diagnosis of exclusion, affects one to two per million populations, sometimes has a familial link, and is found more in women than in men. PAPs tend to be higher in idiopathic PH when compared to secondary causes of PH.

3 Discuss the pathophysiology and natural history of pulmonary hypertension

Endothelial cell injury leads to imbalances between vasodilator and vasopressor molecules. Reductions in the endogenous vasodilators nitric oxide (NO) and prostacyclin (PGI₂) are noted, whereas the vasoconstrictors thromboxane and endothelin are increased. But vasoconstriction appears to be only part of the answer because thrombosis, inflammation, free radical generation, and smooth muscle hyperplasia are also common features noted in PH. Indeed, vascular remodeling is a prominent feature of PH.

The pulmonary circulation has high flow and low resistance. Changes in CO, airway pressure, and gravity affect the pulmonary circulation more than the systemic circulation. The right ventricle is thin walled and accommodates changes in volume better than changes in pressure. To accommodate increases in flow such as during exercise, unopened vessels are recruited, patent vessels distended, and pulmonary vascular resistance (PVR) may decrease. Such normal adaptive mechanisms can accommodate threefold to fivefold increases in flow without significant increases in PAPs.

Early in the evolution of PH, the pressure overload results in hypertrophy of the right ventricle without significant changes in CO or RV filling pressures either at rest or during exercise. As the disease progresses, the vessel walls thicken and smooth muscle cells hypertrophy and increase in number. Vessels become less distensible, and the actual cross-sectional area of the pulmonary circulation decreases. Initially with exercise CO eventually declines despite modest increases in right ventricular (RV) end-diastolic pressures (RVEDP). Mechanisms for enhancing contractility are few for the right ventricle. In time, RV failure (RV ejection fraction below 45%) ensues, and the patient is symptomatic even at rest. RV myocardial blood flow becomes compromised, and tricuspid regurgitation develops secondary to right ventricle distention, further increasing RVEDP and worsening failure. In addition, left ventricular (LV) diastolic function may deteriorate, and LV filling may be compromised by excessive septal incursion into the left ventricle, with a resultant decrease in CO.

Without treatment PH is universally fatal, with a mean survival of 2.8 years. One-, 3, and 5-year survival rates are 68%, 48%, and 34%, respectively. The mortality rate during pregnancy is 30% to 50%, and most experts recommend early termination of pregnancy in these women should pregnancy occur.

4 What is the blood supply to the right ventricle?

It depends on whether the right or left coronary artery is dominant. Usually the right and left anterior descending coronary arteries supply the septum and portions of the RV free wall. Normally the RV free wall is perfused during systole and diastole. RV performance is directly related to systemic pressure during PH.

5 How is pulmonary vascular resistance calculated and what are normal values?

where LAP is left atrial pressure. CO is often substituted for pulmonary blood flow, although intracardiac and other shunts make these unequal. Normal PVR is 1.1 to 1.4 Woods units or about 90 to 120 dynes/sec/cm⁻⁵. (A Woods units is 240 dynes/sec/cm⁻⁵.) A PVR greater than 300 dynes/sec/cm⁻⁵ is indicative of PH.

6 What are some electrocardiographic and radiologic features of the disease?

The electrocardiogram commonly shows right axis deviation, RV hypertrophy (tall R waves in V₁-V₃), RV strain (T-wave inversion in V₁-V₃), S wave in V₆, and enlarged P waves in II, III, and aVF. Although atrial fibrillation is rare, its presence should be viewed with concern because the contribution of atrial kick to ventricular filling is unavailable.

Radiologic abnormalities have been observed in over 90% of cases. Abnormalities on chest radiographs suggestive of PH include prominence of the right ventricle and the hilar pulmonary artery trunk, rapid tapering of vascular markings, a hyperlucent lung periphery, and decrease in the retrosternal air space.

8 Discuss the observed abnormalities on pulmonary function testing

Pulmonary function tests demonstrate mild restrictive defects secondary to the effects of the noncompliant pulmonary vasculature. Arterial blood gases reveal varying degrees of hypoxemia.

9 What additional diagnostic tests are available for evaluating pulmonary hypertension? What results may be expected?

An echocardiogram is an excellent noninvasive method for following progression of the disease. Echocardiographic features of PH include enlarged RV dimension, small LV dimension, thickened interventricular septum, systolic mitral valve prolapse, and abnormal septal motion. Determination of PAPs and PVR by pulsed Doppler echocardiography correlates well with values determined at cardiac catheterization.

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