Approach to Pulmonary Vascular Disease

David Poch

INTRODUCTION: THE PULMONARY VASCULAR SYSTEM

Under normal conditions, the pulmonary vascular bed is a low resistance and high compliance circuit that accommodates the entire cardiac output at pressures that are 20% to 25% of systemic pressures. Normal pulmonary artery pressures are as follows: for systolic, 15 to 30 mmHg; for diastolic, 4 to 12 mmHg; and for mean, 9 to 18 mmHg. Even during exercise, when the cardiac output increases by 2- to 4-fold, the normal pulmonary vascular bed can accommodate increased flow with only modest increases in pressure and minimal effects on pulmonary vascular resistance.

Disruption of the normal pulmonary vascular bed can occur as a result of a primary vasculopathy of the pulmonary vessels, diseases of the lung parenchyma, thromboembolic disease, and pulmonary venous hypertension secondary to left heart dysfunction. Identification of the anatomic location of the diseased portion of the pulmonary vascular bed is paramount in the evaluation of pulmonary vascular disease. Diseases that involve the precapillary pulmonary arterial bed have a very different natural history and treatment strategy as compared to primary parenchymal lung disorders and diseases affecting the postcapillary, pulmonary venous compartment.

Pulmonary hypertension (PH) is present when the mean pulmonary artery pressure is greater than 25 mmHg. Pulmonary arterial hypertension (PAH) is present when the mean pulmonary artery pressure is greater than 25 mmHg and the pulmonary capillary wedge pressure or left ventricular end-diastolic pressure is less than or equal to 15 mmHg. The pressure gradient from the mean pulmonary artery pressure to the pulmonary capillary wedge pressure is the transpulmonary gradient; elevation of the gradient distinguishes PAH from other causes of elevated pulmonary pressures. Despite the fact that PAH is defined clinically by elevated pressures, it is truly a disease of increased pulmonary vascular resistance. Pulmonary vascular resistance is equal to the transpulmonary gradient divided by cardiac output. It is the pulmonary vascular resistance that determines disease severity. Normal pulmonary vascular resistance is less than 3 WU, and if there is significant PAH, the pulmonary vascular resistance must be higher than this. The right ventricle, unlike the left ventricle, is ill-equipped to maintain cardiac output against an increased afterload. As destruction of the precapillary vessels progresses, pulmonary vascular resistance increases and the right ventricle fails. A patient may experience dyspnea, chest pain, fluid retention, and syncope.

While increased pulmonary vascular resistance and right heart failure account for the majority of morbidity and mortality in PAH, alterations in ventilation and perfusion matching and impairments in oxygen diffusion also play a role. Increased dead space ventilation, particularly in patients with chronic thromboembolic PH, may account for symptoms of dyspnea that seem to exceed what alterations in hemodynamics alone might predict.

DETECTION OF PULMONARY HYPERTENSION

The majority of PH is due to heart diseases associated with elevated pulmonary arterial pressures due to increased pulmonary venous pressures (e.g., congestive heart failure, valvular heart disease) and to parenchymal lung disease (e.g., chronic obstructive pulmonary disease and interstitial lung disease). PAH is rare and has a prevalence of 15 to 40 cases per million. Before treatment can be properly planned, it is essential to identify the etiology of elevated pulmonary arterial pressure because pulmonary vasodilators used to treat PAH are not helpful in most other causes of PH.

The most common presenting symptom in PAH is dyspnea (>85%) which is often initially attributed to more common cardiopulmonary disorders leading to a sometimes problematic delay in diagnosis. Data from multicenter PAH registries indicate that more than 20% of patients report symptoms for more than 2 years before the correct diagnosis is made. Other symptoms that may help suggest the diagnosis of PAH include fatigue (26%), chest pain (22%), presyncope/syncope (17%), edema (20%), and palpitations (12%).

Cardiac exam findings consistent with PAH include elevated jugular venous distention, a right ventricular heave, a loud P2, a fixed split S2, a right sided S3, and a tricuspid regurgitation murmur. The absence of physical examination findings of left heart disease is important to a diagnosis of PAH. The presence of pulmonary edema or pleural effusions should raise suspicion for left heart disease or, in the correct setting, rare conditions such as pulmonary veno-occlusive disease. Exam findings consistent with alternative causes of dyspnea such as obstructive lung disease or sleep disordered breathing should prompt further evaluation.

EPIDEMIOLOGY OF PULMONARY VASCULAR DISEASE

The current World Health Organization classification of PH (see Table 22-1) provides an excellent outline of populations at risk for developing PAH. The idiopathic form of the disease (IPAH) accounts for nearly half of the prevalent PAH in recent published registries; it affects women more frequently than men (1.7:1) and presents most often in the third decade of life.

TABLE 22-1

Revised WHO Classification of PH17

Basis for Current Classification Schema of Pulmonary Hypertension

Group I

1. Pulmonary arterial hypertension (PAH)

1.1. Idiopathic (IPAH)

1.2. Familial (FPAH)

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