Lower Body Muscle Pre-activation in Initial Orthostatic Hypotension

IOH is a form of orthostatic intolerance defined by a transient decrease in systolic blood pressure (SBP) by 40mmHg or diastolic blood pressure (DBP) by 20mmHg within the first 15-30 seconds of assuming a standing position from a supine or seated position. IOH commonly occurs in teenagers and young adults and is associated with presyncope symptoms such as nausea, light-headedness, and blurred vision and may sometimes lead to full syncope. These symptoms are a consequence of cerebral hypoperfusion caused by a rapid reduction in BP and are often present only during an active stand, and not with a passive tilt-table test. This suggests that a muscle activation reflex may play an important role underlying IOH. Identifying the refractory period of leg muscle activation will allow future investigators to accurately determine a common baseline seated period to allow hemodynamic recovery in patients after a stand. This will allow investigators to observe the physiological effects of repeated stands without any interference from a prior stand.

Upon standing there is a large shift of blood (approximately 500 mL) towards the lower extremities and abdomen. With this shift there is a decrease in venous return which results in a drop in BP. Subsequently, the decrease in BP activates the baroreceptors to increase sympathetic activity and reduce parasympathetic activity, which functions to restore BP by increasing heart rate (HR) and total peripheral resistance (TPR).

The underlying physiological mechanism behind the large decrease in blood pressure upon standing in IOH patients has not yet been confirmed. The most widely accepted explanation at this time involves rapid vasodilation localized to the contracting leg muscles during a stand. Research in both human and animal models from the past couple of decades have shown that skeletal muscle resistance vessels can dilate rapidly in response to, as well as in proportion to, a singular brief muscle contraction. This effect may be exaggerated in IOH patients, which could explain the decrease in TPR in IOH patients upon a stand and thus their subsequent drop in blood pressure. Current treatments of IOH include non-pharmacological interventions and physical counter-maneuvers such as standing up slowly or sitting up first before fully standing.

Physical counter-measures function to oppose the gravitational pull experienced on systemic circulation during a stand. During a stand, a large percentage of circulating blood shifts towards the lower extremities and abdomen resulting in blood pooling. Gravitational venous pooling can be minimized and opposed by performing a physical countermeasure such as a lower-body muscle contraction to translocate blood centrally. Sustained tensing of the lower-body muscles prevents further peripheral pooling in the legs and abdomen.

The investigators hypothesize that pre-activation of lower body muscles prior to standing, will attenuate the drop in SBP seen in IOH patients upon a stand, and alleviate symptoms of IOH, compared to an active stand without muscle pre-activation. The study participant will complete 14 active stands in total within a single day. The first 8 stands will be in an attempt to define the refractoriness of leg muscle activation. The next 4 stands will involve stress tests prior to standing to isolate the roles of sympathetic activity vs. simple muscle contraction. The final 2 stands will involve physical counter-maneuvers intended to mitigate the large drop in BP seen in IOH patients upon a stand.

The first 8 stands will be performed from a seated position. They will begin at a 20 minute seated baseline before the first stand, then in a random order perform stands after baseline durations of 30 sec, 2 min, 3 min, 4 min, 5 min, 7 min, and 10 min.

The next 4 stands will also be performed from a seated position. The interventions that will be performed in a random order are: (1) free stand with no intervention, (2) Serial 7's mental arithmetic stress test, (3) Cold Pressor stress test, (4) functional electrical stimulation.

The final 2 stands will be performed from a seated position as well. The interventions that will be performed in a random order are: (1) supine knee raises and tensing prior to standing and (2) leg crossing and tensing after standing. The entire study will take approximately 3-4 hours.

The study participant will be instrumented in a fasting state and on an empty bladder. The investigators will apply skin electrodes to continuously monitor heart rate and record an ECG. BP will be monitored continuously using finger volume clamp photoplethysmography and calibrated with intermittent brachial cuff measurements. From the continuous BP waveform, the investigators can obtain an estimate of stroke volume, cardiac output, and systemic vascular resistance (Modelflow). Oxygen saturation will be assessed from a finger probe. Middle cerebral blood flow velocity will be assessed using transcranial doppler (TCD).