Hypercapnia in Orthostatic Hypotension

Overview

The Autonomic (or "automatic") Nervous System (ANS) regulates internal processes, including control of heart rate and blood pressure (BP). When someone stands, and gravity tries to pull blood away from the brain, the ANS works to maintain BP and brain blood flow. Neurogenic Orthostatic Hypotension (NOH) occurs when our "fight-or-flight" part ("sympathetic") of the ANS fails. BP can drop a lot when upright, reducing blood flow and oxygen delivery to the brain, and this can cause symptoms of light-headedness, nausea, and fainting. One solution to help counter the effects of NOH may be to increase sympathetic activity by breathing higher levels of carbon dioxide. In healthy volunteers, small increases in the amount of inhaled carbon dioxide has been shown to increase BP in the upright position, and this improves symptoms! The objectives of the current study are to apply carbon dioxide in patients with NOH and healthy controls to: (a) evaluate the effects of breathing carbon dioxide on BP and brain blood flow, and (b) determine if a device that increases carbon dioxide while standing will work as a new therapy

BACKGROUND: Regulation of tissue blood supply to vital organs such as the brain and heart is met in large part by local adjustment of the microvasculature (autoregulation) and autonomic nervous system control of the cardiovascular system. Neurogenic Orthostatic Hypotension (NOH) is a key example of when these systems fail. Patients experience a significant and persistent blood pressure (BP) drop (≥20/10 mmHg) in the upright position, resulting in cerebral hypoperfusion and symptoms of light-headedness, nausea, pre-syncope and even syncope. NOH and impaired cerebrovascular perfusion occur due to failure of the baroreflex to appropriately increase sympathetic outflow. A novel solution to counter the acute effects of NOH is to transiently increase sympathetic activity by stimulating the peripheral and central respiratory chemoreceptors with elevated Fractional Inspired (Fi)CO2. In healthy volunteers, elevated FiCO2 improves orthostatic tolerance and BP control during rapid postural transitions. Additionally, few have considered sex-difference effects on the chemoreflex-autonomic relationship. Existing evidence demonstrates an augmented sympathetic response to chemoreflex stimulation in postmenopausal women with observed vasoconstriction and increased BPs. These data indicate females may respond better to hypercapnia as a novel therapeutic intervention for NOH. Unfortunately, it may also highlight a predisposition for cardiovascular risk associated with supine hypertension. To better understand the mechanistic underpinnings of NOH in males and females, and to explore the use of elevated FiCO2 to treat it, researchers need a better way to monitor sympathetic activity and cerebrovascular perfusion. Functional Optical Coherence Tomography (fOCT) of the retinal and choroid vascular beds of the eye (an out crop of the brain) was recently developed in Calgary to allow physiological monitoring of these essential variables. In summary, elevated FiCO2 levels (hypercapnia) appear to improve BP responses to standing and orthostatic tolerance and may constitute an attractive therapy for NOH patients. This is a proof-of-concept study to evaluate hypercapnia as a novel therapeutic intervention to improve blood pressure and orthostatic tolerance in male and female patients with NOH. In addition, the investigators will aim to evaluate functional OCT as an advance, non-invasive tool to measure sympathetic and metabolic cerebrovascular control. OBJECTIVES: The aims of the current proposal are to apply hypercapnia during fOCT monitoring in male and female patients with NOH and healthy controls to: (a) evaluate and compare the effects of hypercapnia on cardiovascular and cerebrovascular responses to better understand basic chemoreflex and baroreflex physiology in male and female patients with NOH, (b) determine if a device that transiently increases FiCO2 in response to postural changes will have efficacy as a non-drug therapeutic and (c) evaluate fOCT as a novel advanced tool to measure sympathetic and metabolic components of cerebral autoregulation in patients with autonomic failure. METHODS: Male and female NOH patients (n=40) will be recruited from the Calgary Autonomic Clinic, along with sex and age-matched controls from the community. Participants will complete five Active Stand Tests during which they will be asked to target different end-tidal (ET) CO2 levels. OCT images will be captured throughout each test. Participants will complete the following breathing protocol during an active stand test: a) breathing normal room air (ETCO2 free to fluctuate), b) ETCO2 clamped at baseline, c) ETCO2 clamped at +5mmHg, d) ETCO2 clamped at +10mmHg, e) ETCO2 clamped at +10mmHg with ETO2 clamped at 50mmHg. Target ETCO2 levels will be achieved through a computerized gas delivery system. A rebreathing task to elicit hypercapnia and hypoxia (low oxygen) will be performed last. Each condition will be followed by a minimum 10-minute recovery period to ensure ETCO2 normalization. Hemodynamics (BP, HR and stroke volume) and orthostatic symptoms will be assessed throughout. Breath-by-breath data will include ETO2, ETCO2, respiration rate, tidal volume, and minute ventilation. OCT image analyses in the seated and standing position will measure choroid and retinal (surrogates for peripheral sympathetic activity and metabolic cerebral autoregulation, respectively) perfusion densities.