Regional cerebral pulsatile hemodynamics during isocapnic and poikilocapnic hyperthermia in young men

Abstract

Hyperthermia is known to induce hypocapnia-driven reductions in cerebral blood flow; however, it is unknown if it causes changes in hemodynamic pulsatility that negatively influence cerebrovascular function. This retrospective analysis aimed to assess cerebrovascular hemodynamic pulsatile buffering (damping factor; DFi) during poikilocapnic (HT) and isocapnic (HT-C) hyperthermia. We hypothesized that HT would reduce cerebral DFi, while HT-C would attenuate the reduction in DFi by limiting increases in resistance. Ten healthy males were passively heated +2°C from normothermia (BL). Blood flow through the internal carotid artery (ICA) and vertebral artery (VA) was measured using vascular ultrasound. Blood velocity through the middle cerebral artery (MCA) and the posterior cerebral artery (PCA) was measured using transcranial ultrasound. DFi was calculated as the ratio of proximal to distal pulsatility index (PI): Anterior cerebral DFi = PI_ICA/PI_MCA; Posterior cerebral DFi = PI_VA/PI_PCA. 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 posterior DFi, p = 0.116. Irrespective of P_aCO₂ clamping, both hyperthermic conditions reduced anterior DFi, suggesting other mechanisms are responsible for cerebrovascular hemodynamic buffering. Posterior DFi responses were not observed, suggesting preferential buffering of the hyperthermic posterior circulation (VA-PCA).

Keywords: carbon dioxide; cerebral blood flow; cerebral hemodynamics; hyperthermia; pulsatility.

FIGURE 1

Pulsatility index (PI) (a.u) of the MCA, PCA, ICA, and VA during BL, HT, and HT‐C. Pulsatility index (PI [a.u]) of the middle cerebral (MCA), posterior cerebral (PCA), internal carotid (ICA), and vertebral (VA) arteries was demonstrated on a violin plot during baseline (BL), poikilocapnic hyperthermia (HT) and isocapnic hyperthermia (HT‐C). The black line within the violin plots indicates the group median, and the light dotted line indicates the first and third quartiles. When there were significant differences between conditions (i.e., BL vs. HT, BL vs. HT‐C, or HT vs. HT‐C) p‐values were presented and pairwise bars were used to illustrate where differences lie.

FIGURE 2

Damping factor index (DFi) (a.u) of the anterior and posterior circulation during BL, HT, and HT‐C. The damping factor index (DFi) was presented for the anterior and posterior circulation during baseline (BL), poikilocapnic hyperthermia (HT), and isocapnic hyperthermia (HT‐C). The black line within the violin plots indicates the group median, and the light dotted line indicates the first and third quartiles. Any symbols (e.g., circles, squares, and triangles) representing individual data points below the thick dashed line that goes across the entire anterior and posterior graph indicate that the individual had increased transmission of pulsatile hemodynamic forces. When there were significant differences between conditions (i.e., BL vs. HT, BL vs. HT‐C, or HT vs. HT‐C) p‐values were presented, and pairwise bars were used to illustrate where differences lie.