Vasopressin response to hyperosmotic stimulus: blood pressure effect in normal subjects and patients with impaired sympathetic system

Profound hypotension in a tetraplegic patient following angiotensin-converting enzyme inhibitor lisinopril. Case report

We present the case of a 60 year old C6 complete tetraplegic patient who developed profound hypotension following initiation of the angiotensin-converting enzyme inhibitor lisinopril to control blood pressure. Other causes of hypotension, such as myocardial infarction and sepsis was ruled out. Inhibition of the renin-angiotensin-aldosterone system was the probable cause of hypotension. This case demonstrates the critical importance of the renin-angiotensin-aldosterone axis in the maintenance of blood pressure in tetraplegic patients, who may lack input from the brain to sympathetic neurons, and therefore have increased reliance on the renin-angiotensin-aldosterone axis for the maintenance of blood pressure. Angiotensin-converting enzyme inhibitors should be avoided in tetraplegic patients, unless other treatment modalities are ineffective.

Human autonomic rhythms: vagal cardiac mechanisms in tetraplegic subjects

1. We studied eight young men (age range: 20-37 years) with chronic, clinically complete high cervical spinal cord injuries and ten age-matched healthy men to determine how interruption of connections between the central nervous system and spinal sympathetic motoneurones affects autonomic cardiovascular control. 2. Baseline diastolic pressures and R-R intervals (heart periods) were similar in the two groups. Slopes of R-R interval responses to brief neck pressure changes were significantly lower in tetraplegic than in healthy subjects, but slopes of R-R interval responses to steady-state arterial pressure reductions and increases were comparable. Plasma noradrenaline levels did not change significantly during steady-state arterial pressure reductions in tetraplegic patients, but rose sharply in healthy subjects. The range of arterial pressure and R-R interval responses to vasoactive drugs (nitroprusside and phenylephrine) was significantly greater in tetraplegic than healthy subjects. 3. Resting R-R interval spectral power at respiratory and low frequencies was similar in the two groups. During infusions of vasoactive drugs, low-frequency R-R interval spectral power was directly proportional to arterial pressure in tetraplegic patients, but was unrelated to arterial pressure in healthy subjects. Vagolytic doses of atropine nearly abolished both low- and respiratory-frequency R-R interval spectral power in both groups. 4. Our conclusions are as follows. First, since tetraplegic patients have significant levels of low-frequency arterial pressure and R-R interval spectral power, human Mayer arterial pressure waves may result from mechanisms that do not involve stimulation of spinal sympathetic motoneurones by brainstem neurones. Second, since in tetraplegic patients, low-frequency R-R interval spectral power is proportional to arterial pressure, it is likely to be mediated by a baroreflex mechanism. Third, since low-frequency R-R interval rhythms were nearly abolished by atropine in both tetraplegic and healthy subjects, these rhythms reflect in an important way rhythmic firing of vagal cardiac motoneurones.

Norepinephrine and epinephrine responses to postural stimulus in orthostatic hypotension due to brainstem ischemic lesions

Norepinephrine (NE) and epinephrine (E) responses to upright posture were investigated in 25 patients with orthostatic hypotension due to brainstem ischemic lesions and in 25 control subjects. In controls the postural stimulus induced constantly a rise in NE urinary excretion and a reduction in E excretion, while in patients with orthostatic hypotension it caused a depression in NE urinary excretion and a rise in E urinary excretion; the last alteration was noticed in all but one patient. The E discharge induced by posture in patients with orthostatic hypotension may be involved in the reduction of vascular peripheral resistance and then in postural fall of blood pressure displayed by such patients.

Effect of arginine vasopressin on coronary and systemic hemodynamics in man

Arginine vasopressin is a potent constrictor of isolated arterial segments in vitro. However, it is disputed whether, in vivo, arginine vasopressin acts as either a systemic or coronary vasoconstrictor. The purpose of this study was to determine the effect of arginine vasopressin on coronary and systemic vascular tone in man. Six patients undergoing routine diagnostic cardiac catheterization were studied. At angiography, all patients were found to have severe coronary artery disease. Cardiac and systemic pressures and coronary blood flow as well as serum arginine vasopressin and osmolality levels were measured before and after administration of hypertonic contrast dye and then following intravenous injection of an analog inhibitor of arginine vasopressin. At baseline, serum arginine vasopressin levels were within the normal range in all patients, but, following infusion of contrast dye, rose above physiological levels in 5 patients. In these latter patients, there were no significant changes in coronary and systemic hemodynamics between baseline (condition 1), following administration of contrast dye (condition 2) and 10 minutes after injection of arginine vasopressin inhibitor (condition 3). Mean coronary sinus blood flow (ml/min) and mean coronary vascular resistance (dyne.sec.cm-5) were 184 +/- 49 and 41,235 +/- 8275 during condition 1, 204 +/- 62 and 39,442 +/- 8781 during condition 2, and 192 +/- 75 and 44,930 +/- 11455 during condition 3. Mean arterial pressure (mm Hg) and mean systemic vascular resistance (dyne.sec.cm-5) during conditions 1, 2 and 3 were 105 +/- 12 and 1454 +/- 174, 114 +/- 21 and 1432 +/- 210, and 114 +/- 15 and 1436 +/- 147, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Vasopressin in end-stage renal disease: relationship to salt, catecholamines and renin activity

The blood pressure changes and behavior of vasoactive hormones after various stimuli were studied in eighteen patients with end-stage renal disease maintained on chronic hemodialysis. Group A patients (n = 9) were not subject to intra- or post-dialysis hypotensive episodes, and Group B (n = 9) frequently had such episodes. A 500 ml hypertonic saline infusion produced no change in blood pressure in either group, despite significant rise of vasopressin levels in both. Plasma renin activity levels were similar and became appropriately suppressed by the infusion in both groups, whereas norepinephrine rose significantly only in Group A, but not Group B where it was already higher at baseline. The regular dialysis session produced, as expected, a significantly more profound hypotensive effect in Group B, but was accompanied in both groups by decrease in vasopressin and increase in plasma renin activity. Norepinephrine change differed in the two groups: it decreased in Group A as expected from its capacity to be dialyzed, but rose in several hypotensive patients in Group B, indicating appropriate response to baroreceptor stimulation and leading to an unchanged average. These findings suggest that dialysis-induced hypotensive episodes are not necessarily associated with autonomic neuropathy or with abnormal patterns of vasopressor hormone response to stimuli. They also shed new light on the factors regulating vasopressin secretion under these circumstances, since they indicate that the osmoreceptor and/or sodium-sensitive receptor may be a more dominant mechanism in the regulation of vasopressin release than the volumetric mechanism responding to fluid volume changes.