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Topic

Regional Cerebral Pulsatile Hemodynamics During Isocapnic and Poikilocapnic Hyperthermia

School of Exercise Science, Physical and Health Education

Date & location

• Monday, June 23, 2025
• 11:00 A.M.
• McKinnon Building, Room 155
• Hybrid

Examining Committee

Supervisory Committee

• Dr. Kurt Smith, School of Exercise Science, Physical and Health Education, ßÉßɱ¬ÁÏ (Supervisor)
• Dr. Sam Liu, School of Exercise Science, Physical and Health Education, UVic (Member)

External Examiner

• Dr. Trevor Day, Department of Biology, Mount Royal University

Chair of Oral Examination

• Dr. Kieka Mynhardt, Department of Mathematics and Statistics, UVic

Abstract

Hyperthermia reduces in cerebral blood flow yet, it’s unknown if it changes hemodynamic forces that negatively influence cerebrovascular function. This thesis aims to assess cerebrovascular hemodynamic buffering (damping factor; DFi) during poikilocapnic (HT) and isocapnic (HT-C) hyperthermia. A secondary aim is to assess the reproducibility of DFi during three with-in day measures (A1, A2, A3) and a between day measure (B1). We hypothesize that HT would reduce cerebral DFi, while HT-C would attenuate the reduction in DFi by limiting increases in resistance and that DFi is reproducible within and between days. Study 1: 10 males were passively heated +2°C from normothermia (BL). Study 2: 5 healthy participants laid supine for 3 measures on day 1 and 1 measure on day 2. Both studies blood flow through the internal carotid (ICA) and vertebral (VA) arteries was measured using vascular ultrasound. Blood velocity of the middle cerebral (MCA) and posterior cerebral (PCA) arteries was measured using transcranial ultrasound. DFi was calculated as the ratio of proximal to distal pulsatility index (PI): Anterior cerebral DFi=PIICA/PIMCA; Posterior cerebral DFi=PIVA/PIPCA. Study 1: Anterior DFi decreased in both HT (1.08±0.19 a.u; p=0.007) and HT-C (1.12±0.231 a.u; p=0.021) conditions from BL values (1.27±0.14 a.u). No changes were observed in the posterior DFi, p=0.116. Study 2: Anterior and posterior DFi had point estimates of 0.931 and 0.808 but had large %LoA outside the a priori of ≤21.6% (anterior) and ≤39.4% (posterior). Irrespective of P_aCO₂ clamping, both hyperthermic conditions reduced anterior DFi, suggesting other mechanisms are responsible for cerebral hemodynamic buffering. Posterior DFi did not change, suggesting preferential buffering of the hyperthermic posterior circulation (VA–PCA). DFi remains to be demonstrated as a reliable measure for assessing hemodynamic forces and was limited by the studies sample size.