The Venous Distension Reflex and Orthostatic Hypertension
The Venous Distension Reflex and Orthostatic Hypertension
This research is being done to find out whether distension of veins in legs will cause a rise
 in blood pressure (orthostatic hypertension).
In humans, two-thirds of the blood volume is contained within the venous vasculature. Because
 of this, changes in peripheral blood volume and alterations in the mechanical properties of
 peripheral veins can greatly impact cardiac filling, cardiac output and blood pressure (BP)
 responses to physiologic stress. Work from our laboratory over the past seven years has shown
 that local upper limb venous distension via volume infusion into an occluded arm (i.e. volume
 infusion model) or applying negative pressure to an occluded leg (limb suction experimental
 model) leads to an acute and dramatic increase in Muscle Sympathetic Nerve Activity (MSNA; an
 index of sympathetic activity directed to skeletal muscle) and BP in humans. Since this
 venous distension reflex (VDR) differs physiologically from the other reflex systems, and
 since this system may play a critical role in orthostatic BP control, we believe that it is a
 significant area of study.

 The investigators speculate that VDR from lower limbs contributes to the autonomic adjustment
 to orthostatic stress. To examine the VDR in lower limbs, an arterial occlusion cuff on the
 mid-thigh was inflated (250 mm mercury; Hg). Then, limb suction (-100 mmHg) was applied
 ~10-15 cm below the level of arterial occlusion (i.e. below the knee). MSNA was measured in
 the opposite control limb. When suction was applied below the level of arterial occlusion
 (i.e. occlusion + suction), both MSNA and mean arterial BP (MAP) increased. In control
 trials, arterial occlusion without limb suction (i.e. occlusion alone) did not increase MSNA.
 Plethysmographic data showed calf circumference increased without detectable arterial
 pulsations. Pilot data suggest that the fluid shifts from the occluded but non-depressurized
 zone of the limb (i.e. between the cuff and knee) into the occluded and depressurized region
 of the limb within the tank. Thus, the results suggest that the VDR was engaged with this
 limb suction experimental model.

 These experimental models "selectively" alter peripheral venous volume as the investigators
 measure sympathetic reflex responses. This approach is innovative and allows examination of a
 previously overlooked autonomic reflex in conscious humans. If these studies confirm the
 hypotheses, the obtained data would challenge the present teaching regarding how the
 sympathetic nervous system is engaged in humans during postural stress.

 In ~2.5-10% of the population, BP rises as the person stands. This has been termed
 orthostatic hypertension, and is different from the "normal" sustained BP response when a
 person stands. The incidence of orthostatic hypertension may increase with aging (~2.4% for
 45-64 years old and ~8.7% for >70 years old). Orthostatic hypertension is a risk factor for
 the development of stroke, left ventricular hypertrophy, and chronic kidney disease. It is
 unclear why BP rises with standing in some individuals. Some investigators have speculated
 that orthostatic hypertension is due to exaggerated baroreceptor withdrawal with standing.
 Others have speculated that this response is due to an increase in sympathetic output. It has
 been noted that some patients with orthostatic hypertension have increased venous pooling in
 their lower legs. Based these data, the investigators postulate that heightened engagement of
 the VDR reflexly increases MSNA and also serves to reset the aortic baroreflex. In this
 protocol, the investigators will determine if the MSNA response to leg suction is heightened
 in the individuals with elevated standing BP, and examine if the baroreflex is altered in
 these individuals. The investigators will also examine if external pressure on lower limbs,
 which limits the venous pooling in the lower limbs, will attenuate the increase in BP during
 standing in those individuals.
Inclusion Criteria:

 -  Are capable of giving informed consent

 -  Are of any race or ethnicity

 -  Are fluent in written and spoken English

 -  Whose history and physical exam did not uncover any exclusion criteria

 -  Are 18 years of age

 For subjects with elevated standing BP

 -  Systolic BP (SBP) during standing is at least 10 mmHg > the supine SBP

 -  Patients with a diagnosis of stage I hypertension without other chronic diseases may
 be INCLUDED

 For normal subjects without elevated standing BP

 -  The change in SBP by standing is within ± 5 mmHg from the supine SBP

 -  Matched gender, similar age and BMI (within 10%) to participants with elevated
 standing BP

 -  Free of acute or chronic medical conditions

 Exclusion Criteria:

 -  Age < 18 years of age

 -  Are a pregnant or nursing woman

 -  Are a prisoner or institutionalized individual or unable to consent

 -  Have chronic diseases (e.g. heart, lung, neuromuscular disease, or cancer) other than
 stage I hypertension

 -  Have orthostatic hypotension or a history of syncope

 -  Current smoker

 -  History of blood clots