title: Cardiovascular drift is related to reduced maximal oxygen uptake during heat stress.
abstract: OBJECTIVE
  This study investigated whether the progressive rise in heart rate (HR) and fall in stroke volume (SV) during prolonged, constant-rate, moderate-intensity exercise (cardiovascular drift, CVdrift) in a hot environment is associated with a reduction in VO(2max).
  
  
  METHODS
  CVdrift was measured in nine male cyclists between 15 and 45 min of cycling at 60% VO(2max) in 35 degrees C that was immediately followed by measurement of VO(2max). VO(2max) also was measured after 15 min of cycling on a separate day, so that any change in VO(2max) between 15 and 45 min could be associated with the CVdrift that occurred during that time interval. This protocol was performed under one condition in which fluid was ingested and there was no significant body weight change (0.3 +/- 0.4%), and under another in which no fluid was ingested and dehydration occurred (2.5 +/- 1%, P < 0.05).
  
  
  RESULTS
  Fluid ingestion did not affect CVdrift or change in VO(2max). A 12% increase in HR (151 +/- 9 vs 169 +/- 10 bpm, P < 0.05) and 16% decrease in SV (120 +/- 12 vs 101 +/- 10 mL.beat(-1), P < 0.05) between 15 and 45 min was accompanied by a 19% decrease in VO(2max) (4.4 +/- 0.6 vs 3.6 +/- 0.4 L.min(-1), P < 0.05) despite attainment of a higher maximal HR (P < 0.05) at 45 min (194 +/- 5 bpm) vs 15 min (191 +/- 5 bpm). Submaximal VO(2) increased only slightly over time, but VO(2max) increased from 63 +/- 5% at 15 min to 78 +/- 8% at 45 min (P < 0.05).
  
  
  CONCLUSIONS
  We conclude CVdrift during 45 min of exercise in the heat is associated with decreased VO(2max) and increased relative metabolic intensity. The results support the validity of using changes in HR to reflect changes in relative metabolic intensity during prolonged exercise in a hot environment in which CVdrift occurs.
authors.affiliation: Department of Exercise Science, Ramsey Center, 300 River Road, University of Georgia, Athens, GA 30602, USA. jwingo@uga.edu
authors.forename: Jonathan E
authors.initials: JE
authors.lastname: Wingo
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Lafrenz
authors.forename: Matthew S
authors.initials: MS
authors.lastname: Ganio
authors.forename: Gaylen L
authors.initials: GL
authors.lastname: Edwards
authors.forename: Kirk J
authors.initials: KJ
authors.lastname: Cureton
mesh_terms: D000328:Adult; D001642:Bicycling; D015415:Biomarkers / Q000097:blood; D001794:Blood Pressure; D002319:Cardiovascular System / Q000378:metabolism*; D017079:Exercise Tolerance; D006339:Heart Rate; D018882:Heat Stress Disorders / Q000378:metabolism / Q000503:physiopathology*; D006801:Humans; D019344:Lactic Acid / Q000378:metabolism; D008297:Male; D009200:Myocardial Contraction; D010101:Oxygen Consumption; D010807:Physical Endurance; D010953:Plasma Volume; D013318:Stroke Volume
chemical_list: D015415:Biomarkers; D019344:Lactic Acid