The goal of this clinical trial is to determine if perfusion index (PI) can reliably predict changes in systolic blood pressure (SBP) in persons with spinal cord injury (SCI) during a cool temperature challenge. Participants with SCI can be 18-75 years of age, male or female, neurological level of injury C4-T12, ASIA Impairment Scale (AIS: A or B: motor complete) or gender and healthy age-matched able-bodied (AB) controls.

The main question this study aims to answer is: can PI reliably predict change in SBP during a cool temperature challenge? We hypothesize that PI can be used as a biomarker of SBP in persons with SCI.

Researchers will compare the relationship of 1) changes in PI and SBP and 2) relationship of changes in other vital signs to SBP, during the cool temperature challenge. The researchers aim to determine if PI can be used as a biomarker of SBP in persons with SCI during a cool temperature challenge (experimental group/arm). Able-bodied controls (control group/arm) will receive the same intervention (cool challenge) as the experimental group.

During the study, participants will be asked to wear only shorts and a T-shirt so that their skin can be exposed to the warm and cool temperatures of the 2 rooms.

We will put put on non-invasive sensors (taped or attached on the surface of your body) to measure PI, BP, and other vital signs. We will record these measurements while you are seated in a wheelchair for 10 minutes in a warm room and up to 90 minutes in a cool room.

Introduction/Background: Persons with spinal cord injury (SCI) may experience impaired control of several autonomic functions, including blood pressure (BP) fluctuations and impaired body temperature regulation in response to exercise, cold exposure, or a full bladder. Hypertension (high BP) is commonly episodic (comes and goes) and without clinical symptoms (you are unaware). Mechanisms for impaired BP regulation after SCI are not completely understood and contribute to the limited availability of reliable wearable devices for continuous BP monitoring in the community.

Purpose of the Study: By doing this research study, we hope to learn how perfusion index (PI) and the other vital signs we measure, respond to moving from the warm to cool room. We will use sophisticated analysis of the data collected, which will include artificial intelligence, to determine how perfusion index can predict changes in systolic blood pressure (SBP).

We have shown that PI can predict SBP (the greater of the 2 BP values, e.g., 120/80 mmHg) with high accuracy in animal studies of monkeys with SCI. Perfusion index can continuously measure (non-stop measuring) the blood flow in the skin using a pulse-oximeter (pulse oxygen meter) and other sensors on your skin. This new tool can help us to better understand why blood pressure is so unstable (goes quickly up and down) after SCI. The findings may also provide necessary information for developing new non-invasive and wearable devices to monitor blood pressure in the community, thereby allowing persons with SCI early detection, safety, and treatment of clinically silent high blood pressure episodes (episodes that you're not aware of).

Study Procedures: When you arrive, we will take your temperature with an ear thermometer to make sure you don't have a fever. If you are having a urinary tract infection (UTI), please call before you travel to the Center. Once at the Center, we will use about 30 minutes in a mildly warm room (75-80 deg F) to tape temperature sensors to your forehead (to measure your body temperature). We will put oxygen meters on your ear, finger and toe to measure perfusion index (PI: a ratio or percentage of blood flowing in your arteries compared to blood not flowing very much in your veins and tissues). We will place a device on another finger that measures beat-to-beat BP device (your BP of every heartbeat).

Once the instrument setup is complete, you will remain seated in your wheelchair (or we will provide you with one if you are an AB control) and we will take the above measures for 10 minutes. Then, we will move you into a cool room (64 deg F) for 90 minutes and continue the same measurements.

Impact: Our studies may provide the scientific community with a novel biomarker and tool to better understand mechanisms for unstable blood pressure control after SCI. The findings may also provide the scientific rationale for development of next-generation non-invasive and accurate and reliable wearable devices to monitor SBP in the community, thereby allowing persons with SCI early detection, safety, and treatment of clinically silent hypertensive episodes (autonomic dysreflexia: AD).

Study Aim: To determine the feasibility of PI as a novel non-invasive biomarker to predict SBP in persons with SCI during a cool temperature challenge.

Inclusion Criteria:

• You have had a spinal cord injury (SCI: C4-T12) for more than one year and the SCI was graded as ASIA Impairment Scale (AIS) A or B (unable to voluntarily move your legs)
• You can also be a gender and age-matched (±5 years) able-bodied (AB) control subject (similar gender and age to a subject with SCI).
• Subjects with SCI and AB controls must be between 18 and 75 years of age.
• If you are going to participate, you must avoid caffeine and alcohol, maintain normal salt and water intake, and avoid strenuous exercise for 24 hours prior to the study visit.

Exclusion Criteria:

• You have cardiovascular, kidney, or untreated thyroid disease
• You have a traumatic brain injury (TBI)
• You have diabetes mellitus
• You have an acute illness or infection
• You are dehydrated
• You smoke
• You are pregnant
• You have broken, inflamed, or otherwise fragile skin
• Your Body Mass Index (BMI) is more than 30 kg/m2

We have an approved Data Use Agreement (DUA) with Dr. Leif Havton of the VA Puget Sound Health Care System. Transferring data to our subaward site, the VA Puget Sound Health Care System, will allow Dr Leif Havton to perform analysis of the de-identified data, which will include artificial intelligence (AI)/machine learning (ML) to determine best surrogate biomarkers for blood pressure predictions.