Pulmonary arterial hypertension

Published on 13/02/2015 by admin

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Pulmonary arterial hypertension

Barry A. Harrison, MD

Pulmonary arterial hypertension (PAH) is an uncommon disease entity manifested by vasoconstriction of the arterioles within the lung, thickening of the vessel wall, and thrombosis in situ that leads to an increase in pulmonary vascular resistance (PVR). The right ventricle is not capable of handling a large pressure load, so, over time, the right ventricle fails, and if PAH is untreated, patients usually die within approximately 3 years after diagnosis. Men and women of all ages can develop the disease, but it is more common in women between the ages of 20 and 40. The initial symptoms of the disease are easy fatigability, dyspnea on exertion, and dizziness. As the disease progresses, the patient experiences shortness of breath at rest, palpitations, and chest pain. As the right ventricle begins to fail, the patient may develop syncope and signs of right ventricular failure, which are evidence of severe disease.

Diagnosis

In a patient suspected of having PAH, a right-sided heart catheterization is performed to measure pulmonary arterial pressures, allowing the calculation of the PVR, which is equal to the decrease in pressure across the pulmonary vascular system (mean pulmonary arterial pressure [mPAP] minus the pulmonary arterial occlusion pressure [PAOP]—an indirect measure of left atrial pressure) divided by the cardiac output (CO):

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The normal resting mPAP value is 14 ± 3.3 mm Hg; the World Health Organization defines PAH as a sustained elevation of mPAP to more than 25 mm Hg at rest or to more than 30 mm Hg with exercise, with a mean PAOP or left ventricular end-diastolic pressure of less than 15 mm Hg. The World Health Organization classifies PAH as primary or secondary, with recognition that considerable overlap exists between groups, and further divides secondary PAH into left-sided heart disease, PAH from hypoxemia, PAH from pulmonary thrombotic disease, and PAH from miscellaneous causes (Box 228-1). The distinguishing feature between PAH caused by left-sided heart disease and other causes of PAH is that the latter diagnoses require a PVR greater than 3 Wood units (1 Wood unit = 80 dyn·sec−1·cm−5). If left atrial (PAOP) pressure is high (e.g., in patients with mitral valve insufficiency), then PVR is unlikely to be greater than 3 Wood units, and the PAH is secondary to left-sided heart disease.

Box 228-1   Clinical Classification of Pulmonary Arterial Hypertension

Treatment

Once a diagnosis of PAH is made, treatment first focuses on correcting any secondary causes of the disease; these treatments include, for example, supplemental O2 to treat hypoxemia, surgery to correct mitral insufficiency, and pulmonary thromboendarterectomy to treat chronic thromboembolic disease. Ultimately, however, most patients with PAH will require pharmacologic therapy to treat the vasoconstriction, smooth muscle cell and endothelial cell proliferation, and prothrombotic state.

Pharmacologic treatment

Currently, four classes of drugs are used to treat patients with primary PAH, and these drugs are often used in combination. Calcium channel blockers are relatively inexpensive and, with the exception of verapamil (the use of which is associated with too much negative inotropy), can be used if the patient responds with at least a 10% decrease in mPAP.

Studies have demonstrated an association between PAH and decreased endogenous prostacyclin synthase; prostacyclin is a potent endogenous pulmonary vasodilator. Several prostacyclin analogs are therefore used to treat PAH. Epoprostenol (prostacyclin) is given by continuous intravenous infusion, whereas iloprost (a prostacyclin analogue) is delivered by an aerosol route.

Phosphodiesterase inhibitors, especially sildenafil, have a role in the treatment of PAH by inhibiting the breakdown of cyclic guanosine monophosphate, which, in turn, leads to relaxation in smooth muscle cells within the vascular intima, resulting in vasodilation.

Endothelin receptor antagonists are a newer class of drugs that block endothelin receptors, decreasing the vasoconstrictive and vascular remodeling effects of endothelin-1. Bosentan, an oral preparation, increases exercise capacity and delays disease progression but, unfortunately, is hepatotoxic, is teratogenic, and causes anemia.

Anesthetic management of the patient with pulmonary arterial hypertension

As mentioned earlier, without treatment, an early death is not uncommon; however, with better awareness, improved diagnosis, and therapy, patients with PAH are surviving much longer and are more likely to present for surgery for unrelated reasons. Because of their underlying condition, these patients have significant perioperative morbidity and mortality rates; one study found a perioperative mortality rate of 7%.

Preoperative evaluation

When evaluating a patient with PAH, an easy maxim to remember is that the worse the patient’s dyspnea at rest, the more severe the PAH. A history of syncope is a harbinger of a poor outcome. The patient likewise should be evaluated for symptoms and signs of right-sided heart failure. A right atrial pressure of more than 20 mm Hg, pericardial effusion, and a cardiac output of less than 2 L·min−1·m−2 have each been associated with adverse perioperative outcomes. Patients with significant dyspnea and evidence of right-sided heart failure who present for elective operations but who have not undergone any other diagnostic testing should probably be seen by a cardiologist prior to anesthesia and surgery.

All medications that the patient is taking to treat PAH should be continued throughout the perioperative period, especially any of the prostacyclin analogs, because abrupt discontinuation can result in severe rebound pulmonary hypertension.

Intraoperative management

Pulse oximetry and end-tidal CO2 monitoring are considered essential monitors by the American Society of Anesthesiologists and are particularly important for patients with PAH. Consideration should be given to placing an arterial cannula and a pulmonary artery catheter and even to using transesophageal echocardiography in patients with PAH who are undergoing complex operations in which the risk of hemorrhage is high, large fluid shifts are anticipated, or the patient is positioned other than supine.

Both regional and general anesthetic techniques have been used successfully to anesthetize patients with PAH. More important than the technique used is the avoidance or prompt treatment of hypoxia, hypercapnia, acidosis, hypothermia, and pain because all increase PVR and right ventricular afterload. A decreased right ventricular stroke volume leads to decreased left ventricular preload, which is further compromised by the septal displacement from the enlarged right ventricle into the left ventricle. If left ventricular stroke volume decreases (and along with it, aortic root perfusion pressure), coronary blood flow to the right ventricle is decreased, further compromising the performance of the right ventricle. Goals for these patients then include maintaining normal sinus rhythm, with a heart rate of approximately 80 to 90 beats/min, to optimize cardiac output. Right ventricular function is sensitive to both intravascular volume depletion and excess; therefore, fluids should be administered slowly and in small volumes, with a goal of maintaining a central venous pressure of 12 mm Hg or less (Box 228-2).

Box 228-2   Suggested Management Strategies for Acute Decompensation of Pulmonary Artery Hypertension and Right-Sided Heart Failure

Administer O2 to correct hypoxemia and decrease PVR.

Decrease PVR by correcting hypercapnia, acidosis, and hypothermia.

Manage systemic hemodynamics.

Manage pulmonary hemodynamics.

Pharmacologic management

Intravascular inodilators—milrinone (load 50 μg/kg, infuse 0.25-0.75 μg/kg/min) and dobutamine (2-5 μg·kg−1·min−1)*—will increase right ventricular contractility and decrease mPAP. Systemic hypotension may result, requiring the use of vasopressors.

Inhaled pulmonary artery vasodilators

Mechanical devices

*Levosimendan is an inodilator that is not currently approved by the Food and Drug Administration in the United States.

CVP, Central venous pressure; mPAP, mean pulmonary arterial pressure; PVR, pulmonary vascular resistance.

Intravenously administered anesthesia agents (e.g., propofol) should be carefully titrated to avoid precipitous decreases in systemic arterial pressure. Etomidate may be a better choice for the induction of anesthesia in patients with PAH. Nitrous oxide should be avoided because of its pulmonary vasoconstrictive properties. α-Adrenergic agents, which also increase mPAP, should be used with caution, if at all, in these patients. If allowed to breathe spontaneously, patients who have a laryngeal mask airway in place, and who have received any of a number of drugs that shift the CO2 response curve to the right, have a significant risk of developing hypercapnia and hypoxia, which will worsen the PAH. The use of mechanical ventilation is neither indicated nor contraindicated. High tidal volumes and peak airway pressures increase mean airway pressure, thereby increasing PVR; however, extremely low tidal volumes will also increase PVR. High peak end-expiratory pressures (>10 cm H2O) will also increase PVR.

Neuraxial anesthesia may be indicated, especially for procedures below the level of the umbilicus, but it must be borne in mind that prostacyclin analogs have an inhibitory effect on platelet aggregation. Neuraxial anesthesia and analgesia can result in significant systemic vasodilation, which can result in decreased coronary artery perfusion pressure to the right ventricle and in decreased preload to the right ventricle.

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