Elucidating mechanisms of attenuated skin vasodilation during passive heat stress in persons with spinal cord injury

Abstract

Objective

Persons with spinal cord injury (SCI) are unable to efficiently dissipate heat via thermoregulatory vasodilation as efficiently as able-bodied persons during whole body passive heat stress (PHS). Skin blood flow (SkBF) is controlled by dual sympathetic vasomotor systems: noradrenergic vasoconstrictor (VC) nerves and cholinergic vasodilator (VD) nerves. Thus, impaired vasodilation could result from inappropriate increases in noradrenergic VC tone that compete with cholinergic vasodilation or diminished cholinergic tone. To address this issue, we used bretylium (BR) which selectively blocks neural release of norepinephrine, thereby reducing noradrenergic VC tone. If impaired vasodilation during PHS is due to inappropriate increase in VC tone, BR treatment will improve SkBF responses during PHS.

Design

Prospective interventional trial.

Setting

laboratory.

Participants

22 veterans with SCI.

Interventions

Skin surface areas with previously defined intact vs. impaired thermoregulatory vasodilation were treated with BR iontophoresis with a nearby untreated site serving as control/CON. Participants underwent PHS until core temperature rose 1°C.

Outcome Measures

Laser doppler flowmeters measured SkBF over BR and CON sites in areas with impaired and intact thermoregulatory vasodilation. Cutaneous vascular conductance (CVC) was calculated for all sites. Peak-PHS CVC was normalized to baseline (BL): (CVC peak-PHS/CVC BL) to quantify SkBF change.

Results

CVC rise in BR sites was significantly less than CON sites in areas with intact (P = 0.03) and impaired (P = 0.04) thermoregulatory vasodilation.

Conclusion

Cutaneous blockade of neural release of noradrenergic neurotransmitters affecting vasoconstriction did not enhance thermoregulatory vasodilation during PHS in persons with SCI; rather BR attenuated the response. Cutaneous blockade of neural release of noradrenergic neurotransmitters affecting vasoconstriction did not restore cutaneous active vasodilation during PHS in persons with SCI.

Keywords:

• Spinal cord injury
• active vasodilation
• bretylium
• passive heat stress

Disclaimer statements

Contributors None.

Funding This study was funded by a Career Development Award (1 IK2 RX001805-01A2) from the Veteran's Health Administration Rehabilitation and Research Development Office.

Conflicts of interest Authors have no conflict of interests to declare.

Acknowledgements

This study was supported with the facilities and resources at the South Texas Veteran’s Health Care System, San Antonio, TX

Data availability statement

The data that support the findings of this study are available from the corresponding author, [MT], upon request.

Ethics approvals

All human protocols were approved by the university affiliated Institutional Review Board and therefore were performed in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki and its later amendments. Veterans Affairs local Research Development office approved all protocols in this study. All participants provided written informed consent to participate.