title: Cerebrovascular and corticomotor function during progressive passive hyperthermia in humans.
abstract: The present study examined the integrative effects of passive heating on cerebral perfusion and alterations in central motor drive. Eight participants underwent passive hyperthermia [0.5°C increments in core temperature (Tc) from normothermia (37 ± 0.3°C) to their limit of thermal tolerance (T-LIM; 39.0 ± 0.4°C)]. Blood flow velocity in the middle cerebral artery (CBFv) and respiratory responses were measured continuously. Arterial blood gases and blood pressure were obtained intermittently. At baseline and each Tc level, supramaximal femoral nerve stimulation and transcranial magnetic stimulation (TMS) were performed to assess neuromuscular and cortical function, respectively. At T-LIM, measures were (in a randomized order) also made during a period of breathing 5% CO(2) gas to restore eucapnia (+5% CO(2)). Mean heating time was 179 ± 51 min, with each 0.5°C increment in Tc taking 40 ± 10 min. CBFv was reduced by ∼20% below baseline from +0.5°C until T-LIM. Maximal voluntary contraction (MVC) of the knee extensors was decreased at T-LIM (-9 ± 10%; P < 0.05), and cortical voluntary activation (VA), assessed by TMS, was decreased at +1.5°C and T-LIM by 11 ± 8 and 22 ± 23%, respectively (P < 0.05). Corticospinal excitability (measured as the EMG response produced by TMS) was unaltered. Reductions in cortical VA were related to changes in ventilation (Ve; R(2) = 0.76; P < 0.05) and partial pressure of end-tidal CO(2) (Pet(CO(2)); R(2) = 0.63; P < 0.05) and to changes in CBFv (R(2) = 0.61; P = 0.067). Interestingly, although CBFv was not fully restored, MVC and cortical VA were restored towards baseline values during inhalation of 5% CO(2). These results indicate that descending voluntary drive becomes progressively impaired as Tc is increased, presumably due, in part, to reductions in CBFv and to hyperthermia-induced hyperventilation and subsequent hypocapnia.
authors: Univ. of Brighton, Chelsea School, Denton Road, Eastbourne BN20 7SR, UK. e.z.ross@brighton.ac.uk
authors: Emma Z
authors: EZ
authors: Ross
authors: James D
authors: JD
authors: Cotter
authors: Luke
authors: L
authors: Wilson
authors: Jui-Lin
authors: JL
authors: Fan
authors: Samuel J E
authors: SJ
authors: Lucas
authors: Philip N
authors: PN
authors: Ainslie
mesh_terms: D000328:Adult; D001783:Blood Flow Velocity / Q000502:physiology; D001794:Blood Pressure / Q000502:physiology; D001831:Body Temperature / Q000502:physiology; D002245:Carbon Dioxide / Q000378:metabolism; D002560:Cerebrovascular Circulation / Q000502:physiology*; D004576:Electromyography / Q000379:methods; D005260:Female; D005267:Femoral Nerve / Q000503:physiopathology; D005334:Fever / Q000378:metabolism / Q000503:physiopathology*; D006801:Humans; D006985:Hyperventilation / Q000378:metabolism / Q000503:physiopathology; D016857:Hypocapnia / Q000378:metabolism / Q000503:physiopathology; D007719:Knee Joint / Q000378:metabolism / Q000503:physiopathology; D008297:Male; D020768:Middle Cerebral Artery / Q000378:metabolism / Q000503:physiopathology*; D009044:Motor Cortex / Q000378:metabolism / Q000503:physiopathology*; D018482:Muscle, Skeletal / Q000294:innervation / Q000378:metabolism / Q000503:physiopathology; D012119:Respiration; D050781:Transcranial Magnetic Stimulation / Q000379:methods
chemical_list: D002245:Carbon Dioxide