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Current base deficit is not a relevant marker of neonatal metabolic acidosis

Published:December 26, 2016DOI:https://doi.org/10.1016/j.ajog.2016.12.026
      To the Editors:
      The article by Clark et al
      • Clark S.L.
      • Hamilton E.
      • Garite T.J.
      • et al.
      The limits of electronic fetal heart rate monitoring in the prevention of neonatal metabolic acidemia.
      is a relevant attempt to assess the limits of electronic fetal heart rate monitoring to prevent neonatal metabolic acidosis (NMA), which is an intermediate biological marker of asphyxia and risk of neonatal encephalopathy. The challenge is to identify clinical information, biomarkers, and electrophysiological indicators that would best support clinical decision and better identify newborns who will benefit from therapeutic hypothermia to prevent postasphyxia cerebral damage. Although results from animal studies were promising, clinical research is still inconclusive when identifying biomarkers of asphyxia in human newborns,
      • Iwata O.
      • Iwata S.
      Filling the evidence gap: how can we improve the outcome of neonatal encephalopathy in the next 10 years?.
      most probably due to the lack of specificity of these biomarkers. At the present time a reliable biological marker of asphyxia harmful to the brain and organs is still to be found.
      Clark et al
      • Clark S.L.
      • Hamilton E.
      • Garite T.J.
      • et al.
      The limits of electronic fetal heart rate monitoring in the prevention of neonatal metabolic acidemia.
      used the base deficit without specifying if they used in vitro or in vivo base deficit, for which results diverge if acidemia is present. Moreover, when using base deficit in vivo, correction made to reach eucapnia when hypercapnia is found is based on adult pH and PCO2 values, which differ from newborn normal references. We have just completed a systematic critique of this topic
      • Racinet C.
      • Ouellet P.
      • Charles F.
      • Daboval T.
      Neonatal metabolic acidosis at birth: in search of a reliable marker.
      that has been positively received by several leading authors in this field. The current approach in clinical biochemical laboratories to calculate base deficit overestimates (by a factor of 2-4) the frequency of NMA (2.2% in Clark et al
      • Clark S.L.
      • Hamilton E.
      • Garite T.J.
      • et al.
      The limits of electronic fetal heart rate monitoring in the prevention of neonatal metabolic acidemia.
      groups vs 0.6% in ours).
      • Racinet C.
      • Peresse J.F.
      • Richalet G.
      • Corne C.
      • Ouellet P.
      Neonatal eucapnic pH at birth: application in a cohort of 5392 neonates.
      Aiming at identifying a reliable and predictive biomarker for newborn asphyxia, we proposed a new innovative approach–birth-related neonatal eucapnic pH–that is only representative of the metabolic component of the blood acidity.
      We hypothesize that the predictability of NMA is superior when electronic fetal heart rate monitoring is coupled with neonatal eucapnic pH than with base deficit. We would like to invite these authors to conduct a complementary study using neonatal eucapnic pH to verify our hypothesis. The neonatal eucapnic pH parameter is simple and easy to calculate using a tool available at the digital distribution platform Apple Store (App Store), and we are readily available to collaborate with any perinatologists interested in this new approach should they require additional information.

      References

        • Clark S.L.
        • Hamilton E.
        • Garite T.J.
        • et al.
        The limits of electronic fetal heart rate monitoring in the prevention of neonatal metabolic acidemia.
        Am J Obstet Gynecol. 2017; 216: 163.e1-163.e6
        • Iwata O.
        • Iwata S.
        Filling the evidence gap: how can we improve the outcome of neonatal encephalopathy in the next 10 years?.
        Brain Dev. 2011; 33: 221-228
        • Racinet C.
        • Ouellet P.
        • Charles F.
        • Daboval T.
        Neonatal metabolic acidosis at birth: in search of a reliable marker.
        Gynecol Obstet Fertil. 2016; 44: 357-362
        • Racinet C.
        • Peresse J.F.
        • Richalet G.
        • Corne C.
        • Ouellet P.
        Neonatal eucapnic pH at birth: application in a cohort of 5392 neonates.
        Gynecol Obstet Fertil. 2016; 44: 468-474

      Linked Article

      • The limits of electronic fetal heart rate monitoring in the prevention of neonatal metabolic acidemia
        American Journal of Obstetrics & GynecologyVol. 216Issue 2
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          Despite intensive efforts directed at initial training in fetal heart rate interpretation, continuing medical education, board certification/recertification, team training, and the development of specific protocols for the management of abnormal fetal heart rate patterns, the goals of consistently preventing hypoxia-induced fetal metabolic acidemia and neurologic injury remain elusive.
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      • Reply
        American Journal of Obstetrics & GynecologyVol. 216Issue 5
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          We appreciate the authors’ letter, which nicely articulates an important but often underappreciated fact: at very high levels, partial pressure of carbon dioxide (pCO2) has a significant impact on calculated base excess. Thus, in the presence of markedly elevated pCO2 values, the traditional distinction between short-term, clinically benign fetal respiratory acidemia and longer term, potentially important fetal metabolic acidemia cannot be made using base excess and pH alone. The elevations of pCO2 commonly seen in adult medicine are generally such that this effect is clinically insignificant.
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