Postural changes in plasma renin activity and responses to vasoactive drugs in a case of Shy-Drager syndrome

Gastric distension attenuates the hypotensive effect of intraduodenal glucose in healthy older subjects

Postprandial hypotension occurs frequently, and current management is suboptimal. Recent studies suggest that the magnitude of the fall in postprandial blood pressure (BP) may be attenuated by gastric distension. The aim of this study was to determine the effect of gastric distension on the hypotensive response to intraduodenal (ID) glucose. Eight healthy subjects (5 males, 3 females, aged 65-76 years) received an ID infusion of either 1) 50 g glucose in 300 ml saline (ID glucose) over 60 min (t=0-60 min), 2) 50 g glucose in 300 ml saline over 60 min and intragastric (4) infusion of 500 ml water between t=7-10 min (IG water and ID glucose), or 3) ID saline (0.9%) infusion over 60 min and IG infusion of 500 ml water (IG water and ID saline) all followed by ID saline infusion for another 60 min (t=60-120 min) on three separate days. BP and heart rate (HR) were measured. Gastric emptying (GE) of the IG water was quantified by two-dimensional ultrasonography. Between t=0-60 min, systolic and diastolic BP was greater (P<0.05 for both) with IG water and ID saline compared with IG water and ID glucose, and less (P<0.05 for both) with ID glucose compared with IG water and ID glucose. These effects were evident at relatively low IG volumes (approximately 300 ml). GE was faster with IG water and ID saline when compared with IG water and ID glucose. We conclude that, in healthy older subjects, IG administration of water markedly attenuates the hypotensive response to ID glucose, presumably as a result of gastric distension.

The effect of intraduodenal glucose on muscle sympathetic nerve activity in healthy young and older subjects

OBJECTIVE

The cardiovascular response to a meal is modulated by gastric distension and the interaction of nutrients, particularly carbohydrate, within the small intestine. We tested the hypothesis that the depressor effect of small intestinal glucose is greater in older than in young subjects, because the reflex increase in muscle sympathetic nerve activity (MSNA) is blunted by age.

METHODS

The effects of intraduodenal glucose infusion (IDGI) on blood pressure, heart rate and MSNA were evaluated in eight healthy young subjects (4 women; mean age +/- SEM: 28.8 +/- 3.4 years), eight healthy elderly (4 women; 75.3 +/- 1.6 years) and in two patients with symptomatic postprandial hypotension (PPH), one young (21 years), and one old (90 years).

RESULTS

In both young and elderly healthy subjects, IDGI decreased blood pressure (P < 0.05), but the fall in systolic blood pressure was greater in the older subjects (-17.0 +/- 4.1 vs. -6.5 +/- 1.6 mmHg, P < 0.03). MSNA increased similarly, after infusion in both young (9.0 +/- 3.4 bursts/min) and elderly (7.8 +/- 1.0 bursts/min) subjects. Baroreflex sensitivity for number of sympathetic bursts was attenuated in the elderly (P < 0.03). The increase in burst area in the young patient with PPH was attenuated (18 vs. 63% in the healthy young group).

INTERPRETATION

The fall in BP induced by IDGI was greater in healthy elderly compared to healthy young subjects. The reason for this is unclear, as they have similar increases in MSNA.

Postprandial hypotension in response to duodenal glucose delivery in healthy older subjects

Postprandial hypotension occurs frequently in older people and may lead to syncope and falls. Some recent studies suggest that the magnitude of the postprandial fall in blood pressure (BP) is influenced by the rate of gastric emptying. The aim of this study was, therefore, to determine whether the fall in blood pressure induced by intraduodenal glucose is influenced by the rate of nutrient delivery into the small intestine, bypassing the effects of gastric emptying. Eight healthy elderly subjects (four male and four female, age 70.3 +/- 3.4 years) were studied on two separate days, in double-blind, randomised order. Glucose was infused intraduodenally at a rate of either 1 or 3 kcal min(-1), for 60 min, (0-60 min) followed by 0.9 % saline for a further 60 min (60-120 min). Blood pressure and heart rate were recorded at baseline and every 3 min during the study. Blood glucose and plasma insulin were also determined. Only the 3 kcal min(-1) infusion caused a significant fall in systolic (P < 0.001) and diastolic (P < 0.0001) blood pressure and an increase in the heart rate (P < 0.0001). The rises in blood glucose (P < 0.01) and plasma insulin (P < 0.05) concentrations were greater during the 3 kcal min(-1) infusion. We conclude that in healthy older subjects, the magnitude of the fall in blood pressure and increase in heart rate induced by intraduodenal glucose infusion is dependent on the rate of nutrient delivery into the small intestine. These results may have relevance to the treatment of postprandial hypotension.

Pure Autonomic Failure: complex abnormalities in the neural mechanisms regulating the cardiovascular system

The aim of this study was to evaluate the sympatho-vagal interaction modulating cardiovascular function and the possible impairment of baroreceptor sensitivity in patients affected by Pure Autonomic Failure (PAF). We studied 4 patients affected by PAF and 7 controls at rest and during different levels (45 degrees, 60 degrees, 90 degrees) of head-up tilt. On a different day all subjects underwent i.v. administration of phenylephrine at dosages adequate to enhance systolic blood pressure by about 20 mmHg both at rest and during 45 degrees head-up tilt. Finally, 1.5 mg atropine was infused intravenously only in the patients. Spectral analysis of RR interval and systolic arterial pressure (SAP) variabilities provided markers of sympathetic (low-frequency oscillations, about 0.1 Hz, LFRR) and vagal (high-frequency oscillations, about 0.25 Hz, HFRR) modulations of heart period and of sympathetic vasomotor activity (low-frequency oscillations of SAP variability, LFSAP). Baroreceptor mechanisms were quantified by means of the index alpha (calculated from the square root of the ratio between the powers of HF components of RR interval and SAP variabilities) and of the phenylephrine RR-SAP slope. Patients affected by PAF were characterized by a drastic decrease in total power of RR variability and by the absence of LFRR and LFSAP components. Moreover, HFRR, although largely predominant in its relative value, was also markedly reduced in its absolute value. Finally, the baroreceptive mechanisms appeared to be heavily impaired. In conclusion, PAF patients seem to be characterized by a complex alteration of neural mechanisms, which in addition to the signs of a sympathetic denervation include an impairment, at least functional, of the vagal modulation of heart rate.

Abnormality of superior mesenteric artery blood flow responses in human sympathetic failure

1. Systemic and regional haemodynamic responses, including superior mesenteric artery blood flow, were measured during stimuli which increase sympatho-neural activity in age-matched normal subjects (controls) and in two groups of patients with sympathetic failure (pure autonomic failure and multiple system atrophy). The stimuli included the pressor tests (mental arithmetic, cutaneous cold and isometric exercise) and head-up tilt. 2. In the controls, the blood pressure did not rise in some during mental arithmetic, but rose in all subjects during cutaneous cold and isometric exercise and was maintained during head-up tilt. In sympathetic failure patients, blood pressure was unchanged during each pressor test and fell during head-up tilt. 3. In the controls, superior mesenteric artery blood flow did not fall significantly during mental arithmetic but fell (with a corresponding rise in calculated superior mesenteric artery vascular resistance), during cutaneous cold, isometric exercise and head-up tilt. In sympathetic failure patients, there were no changes in superior mesenteric artery blood flow and vascular resistance during the pressor tests and head-up tilt. 4. There were no changes in cardiac index or forearm blood flow during each pressor test in both controls and patients. Cardiac index fell and forearm vascular resistance rose during head-up tilt in the controls only. 5. In conclusion, active constriction of the superior mesenteric artery occurs in normal subjects during sympatho-neural activation induced by stimuli such as cutaneous cold, isometric exercise and head-up tilt. This does not occur in patients with sympathetic failure and probably contributes to postural hypotension, emphasizing the role of the splanchnic vascular bed in overall blood pressure control. This study confirms the necessity of integrity of sympathetic pathways in the neural control of the splanchnic vascular bed.

Effect of xamoterol in Shy-Drager syndrome

BACKGROUND

Xamoterol, a cardioselective beta 1-adrenoceptor partial agonist, has been reported to be effective on postural hypotension. We investigated the effect of xamoterol in five patients with Shy-Drager syndrome (SDS) in relation to their prevailing sympathetic nerve activity and sensitivity of beta-adrenoceptors and the change in circadian variation of blood pressure.

METHODS AND RESULTS

Ambulatory blood pressure over 24 hours was monitored by noninvasive sphygmomanometer (model 5200, Spacelab). Plasma norepinephrine levels of SDS patients were significantly lower than that of normal subjects (n = 5) both at rest (54 +/- 15 versus 178 +/- 83 pg/ml) and after 10-minute standing (74 +/- 24 versus 318 +/- 143 pg/ml). Infusion of isoproterenol (0.02 micrograms/kg/min) produced a mild rise of systolic blood pressure and tachycardia in normal subjects but resulted in marked hypotension and tachycardia in SDS subjects. After xamoterol administration (200 mg b.i.d.), systolic blood pressure and heart rate were significantly increased in the averages during the day; however, increases were more pronounced at night. In two of the five patients, the improvement in dizziness was large enough to enable them to increase their daily activities.

CONCLUSIONS

Our observations suggest that 1) beta 1-selective, high intrinsic sympathomimetic activity of xamoterol increases blood pressure and heart rate in patients with SDS as a consequence of their prevailing beta 1-adrenoceptor hypersensitive state, and 2) blood pressure monitoring over 24 hours appears to have important advantages in evaluating the therapeutic effects on postural hypotension.