title: Blunted cutaneous vasoconstriction and increased frequency of presyncope during an orthostatic challenge under moderate heat stress in the morning.
abstract: OBJECTIVE
  In normothermia, the tolerance time to presyncope during an orthostatic challenge is shortened during the early morning. Heat stress reduces tolerance to presyncope and the degree of cutaneous vasoconstriction prior to presyncope. However, whether these changes show diurnal variations remains unknown. Therefore, we examined diurnal changes in orthostatic tolerance and cutaneous vascular conductance (CVC) during an orthostatic challenge under moderate heat stress.
  
  
  METHODS
  Each lower body negative pressure (LBNP) under normothermia and whole body heat stress was applied for 7 min or until the appearance of presyncopal symptoms in 16 males at both 08:00 (a.m.) and 17:00 hours (p.m.). Measurements included internal and skin temperatures, forearm skin blood flow, arterial pressure, and heart rate. CVC was calculated as skin blood flow/mean arterial pressure, normalized to CVC prior to LBNP and expressed as %CVC.
  
  
  RESULTS
  The average tolerance time in eight subjects exhibiting presyncopal symptoms due to LBNP and moderate heat stress was significantly shorter in the a.m. than in the p.m. (3.7 ± 0.8 versus 6.7 ± 0.3 min, respectively; P = 0.005). Neither %CVC during LBNP in these subjects under moderate heat stress nor normothermia were significantly decreased in the a.m. (P > 0.05, respectively).
  
  
  CONCLUSIONS
  These findings indicate an orthostatic challenge even during moderate heat stress that led to an increase in the frequency of presyncope, especially in the morning. The reduction in tolerance was accompanied by blunted cutaneous vasoconstriction prior to presyncope. Hence, diurnal changes in cutaneous vascular responses during combined orthostatic and heat stresses should contribute, at least partly, to heat-induced orthostatic intolerance in the morning.
authors.affiliation: Division of Hygiene, Department of Social Medicine, Nihon University School of Medicine, 30-1 Oyaguchi-Kamicho, Itabashi-ku, Tokyo, 173-8610, Japan, aoki.ken@nihon-u.ac.jp.
authors.forename: Ken
authors.initials: K
authors.lastname: Aoki
authors.forename: Yojiro
authors.initials: Y
authors.lastname: Ogawa
authors.forename: Ken-ichi
authors.initials: K
authors.lastname: Iwasaki
mesh_terms: D000328:Adult; D062186:Arterial Pressure / Q000502:physiology; D018882:Heat Stress Disorders / Q000503:physiopathology*; D006358:Hot Temperature; D006801:Humans; D008150:Lower Body Negative Pressure / Q000379:methods; D008297:Male; D054971:Orthostatic Intolerance / Q000503:physiopathology*; D012039:Regional Blood Flow / Q000502:physiology; D012867:Skin / Q000503:physiopathology*; D012881:Skin Temperature / Q000502:physiology; D013575:Syncope / Q000503:physiopathology*; D014655:Vascular Resistance / Q000502:physiology; D014661:Vasoconstriction / Q000502:physiology*; D055815:Young Adult