Baroreflex failure

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Perhaps the most dramatic form of neurogenic hypertension is baroreflex failure (6,7). Baroreflex failure occurs when afferent IX and X cranial nerves are lost (8). Although unilateral loss occasionally causes altered heart rate regulation (9,10), it is more typically tolerated from a cardiovascular standpoint. On the other hand, bilateral loss results acutely in an important syndrome of accelerated hypertension (11) and even encephalopathy (12). Baroreflex failure should be diagnosed when a patient initially presumed to have pheochromocytoma is found to have no tumor. Blood pressures in this condition can be among the highest found in human subjects. We have seen an acute pressure of 320/160 mmHg in a patient with baroreflex failure who was asymptomatic except for headache. In such patients, systolic blood pressures (SBP) of 260-300 mmHg are commonly observed during stress, requiring urgent admission to an intensive care unit for blood pressure control. Such pressures are most likely to be seen after acute baroreflex disruption by injury or by surgery (13). However, over succeeding days and weeks, there is a moderation in the hypertension, and the episodes of normal or low blood pressures punctuate the elevated blood pressures. Eventually, some patients with baroreflex failure develop orthostatic hypotension.

Patients with baroreflex failure are exquisitely sensitive to the effects of a-2 agonists to lower blood pressure. There is often a 50-mmHg fall in systolic blood pressure within 1 h of oral administration of 0.1 mg of clonidine in this disorder (14). Patients with baroreflex failure also often have excessive elevated blood pressures in response to the cold pressor test (6). Benzodiazepines, which promote GABA transmission, also reduce hypertension in these individuals. It is likely that this model of neurogenic hypertension is the most susceptible to interventions involving biofeedback, though this has not been systematically tested. Over a period, many patients with baroreflex failure learn on their own how best to control their pressure by avoiding stressful situations and thoughts if they sense their blood pressure is rising.

The selective baroreflex failure (Jordan syndrome), a special disorder, has been described in ref. (15). These patients differ from others in having retained at least some efferent control of heart rate by the vagus. Thus, in times of sedation or during sleep, profound bradycardia may occur. Pauses of 10 or 20 s in heartbeat have been noted (malignant vagotonia), causing patients to awaken with confusion and headache. The unusual regimen of cardiac pacemaker, guanethidine, and fludrocortisone has proven helpful to these patients.

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