Anesthesia for Catecholamine Excess

CATECHOLAMINE EXCESS

Clinical Manifestations

Pheochromocytoma is a catecholamine-secreting tumor that consists of cells originating from the embryonic neural crest. This tumor accounts for 0.1% of all cases of hypertension. Although the tumor is usually localized in a single adrenal gland, 10–15% are bilateral or extraadrenal. Approximately 10% of tumors are malignant. The cardinal mani-festations of pheochromocytoma are paroxysmal hypertension, headache, sweating, and palpitations. Unexpected intraoperative hypertension and tachy-cardia during manipulation of abdominal structures may occasionally be the first indications of an undi-agnosed pheochromocytoma. The pathophysiology, diagnosis, and treatment of these tumors require an understanding of catecholamine metabolism and of the pharmacology of adrenergic agonists and antag-onists.

Anesthetic Considerations

Preoperative assessment should focus on the adequacy of α-adrenergic blockade and volume replacement.  Specifically,  resting  arterial  blood pressure, orthostatic blood pressure and heart rate, ventricular ectopy, and electrocardiographic evi-dence of ischemia should be evaluated.

A decrease in plasma volume and red cell mass contributes to the severe chronic hypovolemia seen in these patients. The hematocrit may be normal or elevated, depending on the relative contribution of hypovolemia and anemia; thus neither hemato-crit nor hemoglobin concentration reliably defines the adequacy of intravenous volume. Preoperative α-adrenergic blockade with phenoxybenzamine(a noncompetitive inhibitor) helps correct the vol-ume deficit, in addition to correcting hypertension.

     Blockade should not be initiated prior to initia-tion of α blockade but may be added if there is a need to control heart rate and to reduce arrhythmias provoked by excess catecholamine concentrations. A drop in hematocrit should accompany the expan-sion of circulatory volume, sometimes unmasking an underlying anemia.

Potentially life-threatening variations in blood pressure—particularly during induction and manipulation of the tumor—indicate the useful-ness of invasive arterial pressure monitoring and of adequate intravenous access. Young patients with minimal or no heart disease may not need a cen-tral venous line. Patients with evidence of cardiac disease (or in whom cardiac disease is suspected) may benefit from having a central line (a convenient route of access for administering catecholamines, should they be required) and from intraoperative transesophageal echocardiography.

Intubation should not be attempted until a deep level of general anesthesia (possibly also including local anesthesia of the trachea) has been established. Intraoperative hypertension can be treated with phentolamine, nitroprusside, nicardip-ine, or clevidipine. Phentolamine specifically blocks α-adrenergic receptors and blocks the effects ofexcessive circulating catecholamines. Nitroprusside has a rapid onset of action, a short duration of action, and as a nitric oxide donor can be effective in cases where calcium channel blockers are ineffec-tive. Nicardipine and clevidipine are being used more frequently preoperatively and intraoperatively.

Drugs or techniques that indirectly stimulate or promote the release of catecholamines(eg, ephedrine, hypoventilation, or large bolus doses of ketamine), potentiate the arrhythmic effects of catecholamines (classically halothane), or consis-tently release histamine (eg, large doses of atracu-rium or morphine sulfate) are best avoided.

After ligation of the tumor’s venous supply, the primary problem frequently becomes hypotension from hypovolemia, persistent adrenergic blockade, and tolerance to the high levels of endogenous cat-echolamines that have been abruptly withdrawn. Appropriate fluid resuscitation should reflect surgical bleeding and other sources of fluid loss. Assessment of intravascular volume can be guided by echocardiographic assessment of left ventricular filling using transesophageal echocardiography or other noninvasive measures of cardiac output and stroke volume. Infusions of adrenergic agonists, such as phenylephrine or norepinephrine, often prove necessary. Postoperative hypertension is rare and may indicate the presence of unresected occult tumors.