title: Accelerated acidosis in response to variable fetal heart rate decelerations in chronically hypoxic ovine fetuses.
abstract: BACKGROUND
  Due to limitations of technology, clinicians are typically unable to determine if human fetuses are normoxic or moderately, chronically hypoxic. Risk factors for chronic hypoxia include fetal growth restriction, which is associated with an increased incidence of oligohydramnios and thus a risk for umbilical cord occlusion (UCO) and variable fetal heart rate (FHR) decelerations. At delivery, fetal growth restriction infants (<3rd percentile) have nearly twice the incidence of low Apgar scores and umbilical pH <7.0. Despite the risks of oligohydramnios and intermittent UCO, there is little understanding of the acid/base responses rates of chronically hypoxic fetuses to variable FHR decelerations as might occur during human labor.
  
  
  OBJECTIVE
  We sought to compare the increase in base deficit (BD) among chronically hypoxic as compared to normoxic ovine fetuses in response to simulated mild, moderate, and severe variable FHR decelerations.
  
  
  METHODS
  Near-term ovine fetuses were chronically prepared with brachial artery catheters and an inflatable umbilical cuff occluder. Following a recovery period, normoxic (n = 9) and spontaneously hypoxic (n = 5) fetuses were identified (arterial O2 saturation ≤55%). Both animal groups underwent graded, 1-minute occlusions every 2.5 minutes with 1 hour of mild (∼30 beats/min [bpm] decrease from baseline), 1 hour of moderate (∼60 bpm decrease from baseline), and up to 2 hours of severe (∼90 bpm decrease from baseline) variable FHR decelerations until fetal arterial pH reached 7.00, when occlusions were stopped.
  
  
  RESULTS
  Repetitive UCO resulted in development of acidosis (pH <7.0) in both groups. Hypoxic and normoxic fetuses demonstrated similar BD increases in response to both mild (0.39, interquartile range [IQR] 0.28-0.45 vs 0.26, IQR 0.01-0.30 mEq/L/10 min, P = .25) and severe (1.97, IQR 1.50-2.43 vs 1.51, IQR 0.97-2.45 mEq/L/10 min, P = .63) variable decelerations. However, moderate variable decelerations increased BD in hypoxic fetuses at 2.5 times the rate of normoxic fetuses (0.97, IQR 0.52-1.72 vs 0.39, IQR 0.23-0.47 mEq/L/10 min, P = .03). During the recovery period, hypoxic fetuses cleared BD slower than normoxic fetuses (0.08 ± 0.02 vs 0.12 ± 0.03 mEq/L/min, P = .02).
  
  
  CONCLUSIONS
  In comparison to normoxic fetuses, hypoxic fetuses can more rapidly progress to significant metabolic acidosis in response to moderate FHR variable decelerations, and more slowly recover with in utero resuscitation, likely a consequence of impaired placental function and fetal physiologic responses.
authors.affiliation: Department of Obstetrics and Gynecology, LA BioMed at Harbor-UCLA Medical Center, Torrance, CA.
authors.forename: Kevin E
authors.initials: KE
authors.lastname: Amaya
authors.affiliation: Department of Obstetrics and Gynecology, University Western Ontario, London, Ontario, Canada.
authors.forename: Brad
authors.initials: B
authors.lastname: Matushewski
authors.affiliation: Centre Hopitalier Universitaire Ste-Justine Research Center, Department of Obstetrics and Gynecology, Université de Montréal, Montreal, Quebec City, Canada.
authors.forename: L Daniel
authors.initials: LD
authors.lastname: Durosier
authors.affiliation: Centre Hopitalier Universitaire Ste-Justine Research Center, Department of Obstetrics and Gynecology, Université de Montréal, Montreal, Quebec City, Canada.
authors.forename: Martin G
authors.initials: MG
authors.lastname: Frasch
authors.affiliation: Department of Obstetrics and Gynecology, University Western Ontario, London, Ontario, Canada.
authors.forename: Bryan S
authors.initials: BS
authors.lastname: Richardson
authors.affiliation: Department of Obstetrics and Gynecology, LA BioMed at Harbor-UCLA Medical Center, Torrance, CA. Electronic address: mikeross@ucla.edu.
authors.forename: Michael G
authors.initials: MG
authors.lastname: Ross
mesh_terms: D000138:Acidosis / Q000378:metabolism*; D000818:Animals; D002908:Chronic Disease; D005260:Female; D005311:Fetal Hypoxia / Q000378:metabolism*; D005333:Fetus / Q000378:metabolism*; D006340:Heart Rate, Fetal; D006863:Hydrogen-Ion Concentration; D007743:Labor, Obstetric; D023421:Models, Animal; D011247:Pregnancy; D012756:Sheep; D014470:Umbilical Cord / Q000098:blood supply
keywords: acidosis; asphyxia; hypoxia