title: Red cell volume expansion at altitude: a meta-analysis and Monte Carlo simulation.
abstract: BACKGROUND
  Altitude acclimatization is associated with a rapid increase in hematocrit. The time course and the contribution of the red cell volume expansion are not clear. The purpose of the present meta-analysis was to explore how much altitude exposure is required to induce polycythemia in healthy lowlanders.
  
  
  METHODS
  A systematic review was performed of 66 published articles (including 447 volunteers) identified through literature search. We performed a mixed-model random-effects meta-analysis and a Monte Carlo simulation on the extracted data.
  
  
  RESULTS
  The following results were obtained in this study: 1) the red cell volume expansion for a given duration of exposure is dependent on altitude (P < 0.0001), that is, that the increase in red cell volume was accelerated at higher altitudes; and 2) the extent of the erythropoietic response depends on the initial red cell volume (P < 0.0001). It seems that exposure time must exceed 2 wk at an altitude of more than 4000 m to exert a statistically significant effect. At lower altitudes, longer exposure times are needed with altitudes lower than 3000 m not yielding an increase within 4 wk.
  
  
  CONCLUSIONS
  Red cell volume response to hypoxia is generally slow, although it accelerates with increasing altitude. This, in combination with a dependency on initial red cell volume, suggests that, for example, athletes may need to spend more time at altitude to see an effect on red cell volume than commonly recommended.
authors.affiliation: Zurich Center of Integrative Human Physiology, Institute of Physiology, Zurich, Switzerland. peter@prec.dk
authors.forename: Peter
authors.initials: P
authors.lastname: Rasmussen
authors.forename: Christoph
authors.initials: C
authors.lastname: Siebenmann
authors.forename: Víctor
authors.initials: V
authors.lastname: Díaz
authors.forename: Carsten
authors.initials: C
authors.lastname: Lundby
mesh_terms: D000064:Acclimatization / Q000502:physiology; D000531:Altitude; D004911:Erythrocyte Volume; D004912:Erythrocytes / Q000502:physiology*; D000860:Hypoxia / Q000150:complications / Q000503:physiopathology*; D009010:Monte Carlo Method; D011086:Polycythemia / Q000209:etiology / Q000503:physiopathology*; D013997:Time Factors