Body Temperature in Persons With Tetraplegia When Exposed to Cold

The ability to maintain normal body core temperature (Tcore = 98.6°F) is impaired in persons with tetraplegia. Despite the known challenges to the ability of persons with spinal cord injury (SCI) to maintain Tcore, and the effects of hypothermia to impair mental function in able-bodied (AB) persons, there has been no work to date addressing these issues in persons with tetraplegia.

The aim of this study is to determine if exposure of up to 2 hrs to cool temperatures (64°F) causes body core temperature to decrease in persons with tetraplegia and if that decrease is related to a decrease in mental performance.

After sitting in a cool (64°F) room for up to 2 hours the investigators hypotheses are:

Hypotheses (1): Tcore of most of the persons with tetraplegia will decline approximately 1.8°F (e.g., 98.6 to 96.8°F) while Tcore of controls will not decline at all; (2) Most of the persons with tetraplegia will show a decline in mental performance (memory or clear-headedness) while only some of AB controls will show a decline.

The second aim of this study is to determine if a 10 mg dose of an approved blood pressure raising medicine (midodrine hydrochloride) will (1) reduce the decrease in body core temperature and (2) prevent or delay the decline in mental performance in the group with tetraplegia compared to the exact same procedures performed on the day with no medicine (Visit 1) in the same group.

Hypotheses (3 \& 4): The changes in blood flow to the skin caused by taking a one-time dose of midodrine will lessen the decline in Tcore and prevent or delay the decline in mental performance compared to the changes in Tcore and mental performance during cool temperature exposure without midodrine in the group with tetraplegia.

This study will investigate the mechanisms contributing to the thermoregulatory fragility in persons with tetraplegia when exposed to cool ambient temperatures that are routinely encountered during activities of daily living (ADL). Subnormal body core temperatures and vulnerability to hypothermia (Tcore\<95°F) has been reported in veterans with tetraplegia upon exposure to relatively mild environmental temperatures. The impact that a drift in Tcore will be expected to have on cognitive performance, specifically working memory and executive function, will be demonstrated. These 2 areas of cognitive performance are vital for the ability to optimally care for one's self, which persons with higher cord lesions must excel at to ensure health, as well as to be able to attain the maximal degree of independence possible. Administration of an alpha agonist, midodrine hydrochloride, in an attempt to attenuate the drift in Tcore and prevent or delay the expected decline in cognitive performance to exposure to cool on cognitive function will be investigated as well.

Primary Specific Aim: To determine the change in: (1) Tcore and (2) cognitive performance (attention, working memory, processing speed, and executive function) in persons with tetraplegia after exposure to a cool environment (64°F) for up to 120 min in the seated position.

Primary Hypotheses:(1) 66% of persons with tetraplegia will demonstrate a decline of 1.8°F in Tcore while 0% of controls will demonstrate that same thermal decline; (2) 80% of persons with tetraplegia will have a decline of at least 1 T-score in Stroop Interference scores (executive functioning) while 30% of controls will demonstrate that same magnitude of decline.

Secondary Specific Aims: To determine the change in: (1) the average of distal skin temperatures, (2) metabolic rate, and (3) subjective rating of thermal sensitivity after exposure to 64°F in the seated position.

Secondary Hypotheses: Persons with tetraplegia will have less of a percent change in average distal skin temperatures and metabolic rate, and report lower thermal sensitivity ratings compared with AB.

Tertiary Specific Aim: To determine if a single, 10 mg dose of midodrine will (1) reduce the decrease in Tcore and (2) prevent or delay the decline in cognitive performance in the group with tetraplegia.

Tertiary Hypotheses: Because administration of a peripheral alpha-agonist will address the primary thermoregulatory impairment to cool temperature exposure in persons with tetraplegia-that is, lack of vasoconstriction, the midodrine-induced peripheral vasoconstriction will be anticipated to blunt the decrease in Tcore and prevent or delay the decline in cognitive performance compared to cool exposure without drug administration.

Preparation for Study Visits: The study visits will be separated by a minimum of 1 day and no more than 14 days. Subjects will wear minimal clothing (gym shorts, sports bra) during the study to maximize bare skin exposure to the cool temperature. Each subject will be asked to eat a light, standard meal 2 hours prior to their scheduled visit consisting of either a plain bagel or 2 pieces of toast. For each visit they will be asked to empty their bladders prior to arrival and again upon arrival, if needed.

Visit 1: Cold Ambient Challenge: Instrumentation: During Visit 1, all subjects will be transferred to a padded table for instrumentation, after which they will be transferred back to their own wheelchair or, for controls, to a provided wheelchair. All subjects will use a Roho seat cushion for air circulation consistency and decubiti prevention. A rectal probe will be placed 4 inches beyond the anal sphincter for core temperature measurement, and skin thermal sensors will be taped at 15 sites above and below the level of lesion for collection of skin temperatures. A mask will be placed over the subject's nose and mouth for measurement of exhaled gases from which resting metabolic rate will be calculated from analysis of expired gases (VO2) by a metabolic cart. Laser Doppler flowmetry (LDF) will be used to measure changes in microvascular perfusion by taping a doppler probe on the skin in the area of the ulnar styloid processes and medial malleoli bilaterally (wrists and ankles) to confirm vasoconstriction. A pulse oximeter will be placed on the left second digit to obtain blood oxygen saturation and heart rate (HR). An automated blood pressure cuff will be placed above the right elbow to measure brachial BP. An intravenous catheter will be placed in the right antecubital or nearby vein and secured for sequential blood collection for cortisol and norepinephrine.

Baseline Collection: At the end of the 30 minute acclimation period (81°F), a baseline (BL) collection of the following parameters will be performed for 15 minutes with Tcore, skin temperatures, and VO2 measured continuously; HR, BP, blood oxygen saturation, subjective measures of thermal sensitivity, and 5 minutes of LDF will be measured at 10 minute intervals. A venous blood draw will be collected once at baseline for norepinephrine and cortisol concentrations. At the end of the BL period, a cognitive performance battery will be administered.

Thermal Challenge: Following completion of the baseline period, subjects will be wheeled into an 18°C thermal chamber for 120 minutes or until Tcore ≤ 95°F. Tcore, skin temperatures, and VO2 will be continuously monitored to ensure subject safety throughout the protocol; brachial BP, HR, blood oxygen saturation, thermal sensitivity, and symptoms of hypothermia and autonomic dysreflexia will be assessed at 10 min intervals while LDF will be measured for 5 minutes every 20 minutes. Venous blood will be collected at 50 minute intervals. A decrease in Tcore to ≤ 95°F, or moderate subject discomfort, will result in termination of the protocol. The cognitive performance battery will be administered when Tcore has declined 1.8°F or is ≤ 95.9°F (in subjects with tetraplegia) or after 120 minutes of cold exposure (in both groups) on Visits 1 \& 2.

Visit 2: Cold Ambient Challenge with Midodrine: Visit 2 will be completed in subjects with tetraplegia who participated in Visit 1 and who had an impaired ability to maintain Tcore. Following completion of the BL period, subjects will be orally administered midodrine hydrochloride (10 mg tablet). Forty minutes after midodrine administration (for onset of drug effect), a second BL collection will be obtained, and subjects will be wheeled into the 64°F thermal chamber for 120 minutes or until Tcore ≤ 95°F. Data collection will follow the same schedule and be conducted in the seated position as in Visit 1. If brachial BP increases to 160/90 mmHg, the subject will be removed from the cool room and evaluated by Dr. William A. Bauman, who may consider the administration of labetalol (to lower BP), if deemed necessary.

Eligibility Criteria:
Inclusion Criteria:

* (1) Between 18 and 68 years of age;
* (2) Duration of injury ≥ 1 year; (2) Level of SCI C3-T1;
* (3) Euhydration (subjects will be instructed to avoid caffeine and alcohol, maintain normal salt and water intake, and avoid strenuous exercise for 24 hours prior to study)

Exclusion Criteria:

* (1) Known coronary heart, kidney, peripheral vascular or cerebral vascular disease;
* (2) High blood pressure;
* (3) Untreated thyroid disease;
* (4) Diabetes mellitus;
* (5) Acute illness or infection;
* (6) Dehydration;
* (7) Known allergies to midodrine hydrochloride;
* (8) Smoking;
* (9) Pregnancy