Resting Muscle Sympathetic Nerve Activity Is Not Different in Symptomatic Concussed College Athletes versus Healthy Controls

Abstract

Heart rate variability, primarily an indicator of cardiac parasympathetic function, has been shown to be reduced at rest and during physiological stressors (e.g., aerobic exercise, handgrip exercise, and face cooling) following a concussion, which suggests altered autonomic function. Previous work in our lab has demonstrated attenuated increases in sympathetically‐mediated increases in blood pressure due to unaltered total peripheral resistance during face cooling and the cold pressor test in symptomatic concussed college athletes (CA) compared to healthy controls (HC). These responses suggest that CA have blunted sympathetic activation. It is unknown if a direct measure of sympathetic nerve activity (e.g., muscle sympathetic nerve activity (MSNA)) is altered at rest in CA compared to HC.PURPOSEWe tested the hypothesis that MSNA at rest would be lower in CA compared to HC.METHODSSeven CA (age: 19±2 y, 4 females, 4±2 days post‐injury) and seven HC (age: 21±3 y, 3 females) completed one study visit. Heart rate (3‐lead electrocardiogram), mean arterial pressure (photoplethosmography), and MSNA (microneurography) from the left radial nerve were continuously recorded at 1 kHz for 30 s following at least 20 minutes of supine rest. MSNA burst frequency, burst incidence, and total activity were assessed using commercially available software. Cardiac output was calculated as the product of heart rate and stroke volume. Total peripheral resistance was calculated as mean arterial pressure divided by cardiac output. We used unpaired t‐tests to determine if the main outcome variables were different between groups. Values are reported as mean ± SD.RESULTSResting heart rate (CA: 58±10 vs. HC: 61±12 bpm; P = 0.31), mean arterial pressure (CA: 95±9 vs. HC: 88±10 mmHg; P = 0.11), cardiac output (CA: 6±1 vs. HC: 7±3 L/min; P = 0.31), and total peripheral resistance (CA: 16±3 vs. HC: 16±11 mmHg/L/min; P = 0.45) were not different between CA and HC. Resting MSNA was not different between CA and HC for burst frequency (CA: 27±6 vs. HC: 29±9 beats/min; P = 0.33), burst incidence (CA: 47±10 vs. HC: 46±11 bursts/100 heartbeats; P = 0.44), or total activity (CA: 4883±1520 vs. HC: 6321±1689 AU/min; P = 0.07).CONCLUSIONThese preliminary data indicate that muscle sympathetic nerve burst frequency, burst incidence, and cardiovascular variables during resting conditions are not different in symptomatic concussed college athletes compared to healthy controls. Research is warranted to determine if muscle sympathetic nerve activity is different between symptomatic concussed colleges athletes and healthy controls during a sympathoexcitatory maneuver.Support or Funding InformationSupported by the National Center for Advancing Translational Sciences of the National Institutes of Health Pilot Grants Award Program under award Number UL1TR001412.