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Correlation of arterial fetal base deficit and lactate changes with severity of variable heart rate decelerations in the near-term ovine fetus

Published:October 29, 2012DOI:https://doi.org/10.1016/j.ajog.2012.10.883

      Objective

      Recent guidelines classify variable decelerations without detail as to degree of depth. We hypothesized that variable deceleration severity is highly correlated with fetal base deficit accumulation.

      Study Design

      Seven near-term fetal sheep underwent a series of graded umbilical cord occlusions resulting in mild (30 bpm decrease), moderate (60 bpm decrease), or severe (decrease of 90 bpm to baseline <70 bpm) variable decelerations at 2.5 minute intervals.

      Results

      Mild, moderate, and severe variable decelerations increased fetal base deficit (0.21 ± 0.03, 0.27 ± 0.03, and 0.54 ± 0.09 mEq/L per minute) in direct proportion to severity. During recovery, fetal base deficit cleared at 0.12 mEq/L per minute.

      Conclusion

      In this model, ovine fetuses can tolerate repetitive mild and moderate variable decelerations with minimal change in base deficit and lactate. In contrast, repetitive severe variable decelerations may result in significant base deficit increases, dependent on frequency. Modified guideline differentiation of mild/moderate vs severe variable decelerations may aid in the interpretation of fetal heart rate tracings and optimization of clinical management paradigms.

      Key words

      Intrapartum electronic fetal heart rate (FHR) monitoring (EFM) is used for assessment of fetal well-being in the vast majority of laboring patients in industrialized countries. Although debates continue regarding the relative merit of continuous EFM vs intermittent FHR auscultation, the majority of hospitals use continuous monitoring. A major goal of EFM is the reduction or, ideally, the elimination of intrapartum fetal asphyxia. However, despite widespread EFM use there has been little prospective evidence of its benefits. Initially, retrospective reports indicated significant improvements in perinatal mortality and newborn outcomes,
      • Paul R.H.
      • Hon E.H.
      Clinical fetal monitoring,V: effect on perinatal outcome.
      • Amato J.C.
      Fetal monitoring in a community hospital: a statistical analysis.
      • Gabert H.A.
      • Stenchever M.A.
      The results of a five-year study of continuous fetal monitoring on an obstetric service.
      • Yeh S.Y.
      • Diaz F.
      • Paul R.H.
      Ten-year experience of intrapartum fetal monitoring in Los Angeles County/University of Southern California Medical Center.
      although subsequent prospective studies have demonstrated little evidence of the clinical benefit of EFM.
      • Nelson K.B.
      • Dambrosia J.M.
      • Ting T.Y.
      • Grether J.K.
      Uncertain value of electronic fetal monitoring in predicting cerebral palsy.
      • Shy K.K.
      • Luthy D.A.
      • Bennett F.C.
      • et al.
      Effects of electronic fetal-heart-rate monitoring, as compared with periodic auscultation, on the neurologic development of premature infants.
      • Kuban K.C.
      • Leviton A.
      Cerebral palsy.
      • Graham E.M.
      • Petersen S.M.
      • Christo D.K.
      • Fox H.E.
      Intrapartum electronic fetal heart rate monitoring and the prevention of perinatal brain injury.
      • Winter S.
      • Autry A.
      • Boyle C.
      • Yeargin-Allsopp M.
      Trends in the prevalence of cerebral palsy in a population-based study.
      • Colditz P.B.
      • Henderson-Smart D.J.
      Electronic fetal heart rate monitoring during labour: does it prevent perinatal asphyxia and cerebral palsy?.
      • Alfirevic Z.
      • Devane D.
      • Gyte G.M.
      Continuous cardiotocography (CTG) as a form of electronic fetal monitoring (EFM) for fetal assessment during labour.
      For Editors' Commentary, see Contents
      To achieve the goal of a reduction in perinatal asphyxia and perhaps cerebral palsy, EFM assessment must be directed at prevention of severe metabolic acidosis at birth, defined by the American Congress of Obstetrics and Gynecologists and others as a base deficit (BD) equal to or greater than 12 mEq/L. However, current assessment of EFM heart rate patterns use broad categorizations of fetal well-being (reassuring, indeterminate, nonreassuring: Category 1, 2, 3). In contrast, there has been minimal codification of EFM patterns in regard to the degree of metabolic acidosis, associated with abnormal FHR patterns including that associated with FHR decelerations.
      The most common FHR deceleration associated with fetal hypoxia during labor is a variable deceleration,
      • Cibils L.A.
      Clinical significance of fetal heart rate patterns during labor, V: variable decelerations.
      likely a result of umbilical cord occlusion (UCO). In the early years of fetal monitoring, variable FHR deceleration classification was based on duration and depth of the deceleration.
      • Kubli F.W.
      • Hon E.H.
      • Khazin A.F.
      • Takemura H.
      Observations on heart rate and pH in the human fetus during labor.
      As defined by Kubli et al,
      • Kubli F.W.
      • Hon E.H.
      • Khazin A.F.
      • Takemura H.
      Observations on heart rate and pH in the human fetus during labor.
      marked (severe) variable FHR decelerations drop to below 70 beats per minute (bpm) and last at least 60 seconds, although mild/moderate variable decelerations do not achieve these depth/duration parameters. Human fetal acidosis is correlated with severe variable decelerations, though not with mild or moderate variable decelerations.
      • Kubli F.W.
      • Hon E.H.
      • Khazin A.F.
      • Takemura H.
      Observations on heart rate and pH in the human fetus during labor.
      Animal data confirm that severe variable decelerations occurring frequently may be less well tolerated by the fetus,
      • Richardson B.S.
      • Carmichael L.
      • Homan J.
      • Johnston L.
      • Gagnon R.
      Fetal cerebral, circulatory, and metabolic responses during heart rate decelerations with umbilical cord compression.
      • Westgate J.A.
      • Bennet L.
      • De Haan H.H.
      • Gunn A.J.
      Fetal heart rate overshoot during repeated umbilical cord occlusion in sheep.
      resulting in accumulation of metabolic acids. The NICHD Task Force on EFM in 2008 defined all variable decelerations as a single category,
      • Macones G.A.
      • Hankins G.D.
      • Spong C.Y.
      • Hauth J.
      • Moore T.
      The 2008 National Institute of Child Health and Human Development workshop report on electronic fetal monitoring: update on definitions, interpretation, and research guidelines.
      perhaps because of the lack of objective evidence of the impact of subcategories of variable decelerations.
      Our research team recently quantified the changes in BD and lactate in response to repetitive complete UCO in the near term ovine fetus, simulating severe variable decelerations.
      • Frasch M.G.
      • Mansano R.Z.
      • Gagnon R.
      • Richardson B.S.
      • Ross M.G.
      Measures of acidosis with repetitive umbilical cord occlusions leading to fetal asphyxia in the near-term ovine fetus.
      These studies demonstrated that BD increased at a rate of 0.56 mEq/L per minute of severe UCO. During recovery periods, between or after UCO, BD normalized at a rate of approximately 0.1 mEq/L per minute. In view of the frequency of mild and moderate variable decelerations occurring in labor, we sought to examine the changes in fetal BD in association with varying degrees of UCO to better characterize the impact of subcategories of partial UCO induced variable decelerations on the development of metabolic acidosis.

      Materials and Methods

      Surgical preparation

      Seven (7) near-term ovine fetuses (124 ± 1 days gestational age [GA], term = 145 days) of mixed breed were surgically instrumented. The anesthetic and surgical procedures and postoperative care of the animals have been previously described.
      • Frasch M.G.
      • Mansano R.Z.
      • Gagnon R.
      • Richardson B.S.
      • Ross M.G.
      Measures of acidosis with repetitive umbilical cord occlusions leading to fetal asphyxia in the near-term ovine fetus.
      Briefly, polyvinyl catheters were placed in the right and left brachiocephalic arteries, the cephalic vein, and the amniotic cavity. Stainless steel electrodes were sewn onto the fetal chest to monitor the electrocardiogram (EKG). An inflatable silicon rubber cuff (In Vivo Metric, Healdsburg, CA) was placed around the proximal portion of the umbilical cord and secured to the abdominal skin. Stainless steel electrodes were implanted biparietally on the dura for the recording of electrocortical activity (ECOG). A polyvinyl catheter was also placed in the maternal femoral vein. Postoperatively, animals were allowed 4 days to recover and were 128 ± 1 days GA on the first day of experimental study. Animal care was according to the guidelines of the Canadian Council on Animal Care and was approved by the University of Western Ontario Council on Animal Care.

      Experimental procedure

      Animals were studied over a 4- to 6-hour period. After a 1- to 2-hour baseline control period (to assure maternal and fetal well-being), mild, moderate, and severe series of repetitive UCOs were performed. UCOs were induced in all series by graduated inflation of the occluder cuff with a saline solution. Preliminary studies were performed to determine the amount of volume necessary to achieve mild, moderate, and severe variable decelerations. During the first hour following baseline, mild variable decelerations were performed with a partial UCO for 1 minute duration every 2.5 minutes, with the goal of decreasing FHR by ∼30 bpm, corresponding to an ∼50% reduction in umbilical blood flow.
      • Itskovitz J.
      • LaGamma E.F.
      • Rudolph A.M.
      Heart rate and blood pressure responses to umbilical cord compression in fetal lambs with special reference to the mechanism of variable deceleration.
      During the second hour, moderate variable decelerations were performed with an increased partial UCO for 1 minute duration every 2.5 minutes with the goal of decreasing FHR by ∼60 bpm, corresponding to an ∼75% reduction in umbilical blood flow.
      • Itskovitz J.
      • LaGamma E.F.
      • Rudolph A.M.
      Heart rate and blood pressure responses to umbilical cord compression in fetal lambs with special reference to the mechanism of variable deceleration.
      After the moderate variable decelerations, animals underwent severe variable decelerations with complete UCO for 1 minute duration every 2.5 minutes (Figure 1) until the targeted fetal arterial pH of less than 7.0 was detected, at which point the repetitive UCOs were terminated. All animals were allowed to recover after the last UCO.
      Figure thumbnail gr1
      FIGURE 1FHR deceleration pattern
      Ten minutes of representative severe variable FHR deceleration pattern (upper panel) in relation with UCP (lower panel).
      FHR, fetal heart rate; UCP, umbilical cord occluder pressure.
      Ross. Fetal base deficit changes with variable deceleration severity. Am J Obstet Gynecol 2013.
      Maternal venous blood samples were drawn at baseline and at completion of the UCO protocol. Fetal arterial blood samples were drawn at baseline, at the end of the first UCO of each series (mild, moderate, severe), and at 20-minute intervals (between UCOs) throughout each of the series, as well as at 1, 24, and 48 hours of recovery. For each UCO series blood gas sample, 0.7 mL of fetal blood was withdrawn, and 4 mL of fetal blood was withdrawn at baseline, at pH nadir less than 7.00, and at 1 hour and 48 hours of recovery. The amounts of blood withdrawn were documented for each fetus and replaced with an equivalent volume of maternal blood at the end of day 1.
      All blood samples were analyzed for blood gas values, pH, glucose, and lactate with an ABL-725 blood gas analyzer (Radiometer Medical, Copenhagen, Denmark) corrected to 39.0°C.
      After the 48-hour recovery blood sample, the ewe and the fetus were killed by an overdose of barbiturate and the location and function of the umbilical occluder was confirmed.

      Data acquisition and analysis

      Arterial and amniotic pressures were measured continuously through the baseline and repetitive UCO periods and during the first hour of the recovery period using Statham pressure Transducers (Gould Inc, Oxnard, CA). PowerLab system was used for data acquisition and analysis. Pressures, EKG, and ECOG were recorded and digitized at 1000 Hz for further study (results to be reported separately). FHR was triggered and calculated online from arterial pressure systolic peaks. True arterial blood pressure (ABP) was determined as the difference between instantaneous values of arterial blood and amniotic pressures.
      Averaged values of FHR and ABP were calculated from artifact-free baseline (a representative 10-minute interval), consecutive variable deceleration series (mild, moderate, severe), and the first hour of recovery. The values of individual consecutive nadirs of the FHR decelerations and corresponding ABP changes were determined for every fetus in each variable deceleration series and average values determined for every variable deceleration series.
      BD (mEq per liter) was calculated as follows
      • Siggaard-Andersen O.
      An acid-base chart for arterial blood with normal and pathophysiological reference areas.
      :
      BD=((0.02786×pCO2(10[pH6.1])+13.77×pH124.58)withpCO2measuredinmmHg.


      The rate of BD deterioration (mEq per liter per 10 minutes) was calculated for each period of mild, moderate, and severe variable deceleration and the first hour of recovery, as follows:
      Rate of BD deterioration = BD change/(time spent in period of variable deceleration or recovery). The rate of lactate deterioration was calculated similarly.
      We assessed the effect of a single 1 minute partial or complete UCO and associated variable deceleration on the change in BD within each UCO series as follows (exemplified here on assessing the absolute change in BD during a single variable deceleration within the mild variable deceleration series): Within a 10-minute period of mild variable decelerations, there were four 1 minute decelerations. Assuming the BD change during 1 minute of the variable deceleration is X, and the BD change during each minute of recovery (between the cord occlusions) is Y, the rate of BD deterioration per 10 minutes is defined as 4X − 6Y. We did the same for the moderate and severe variable deceleration series. We used the rate of BD recovery during the first hour of the recovery period to calculate Y. A series of similar equations was used for lactate. The lactate and BD changes were determined as differences between the respective values at the end of each variable deceleration series and the end of the preceding stage of the experiment.

      Statistical analysis

      Normal data distribution was tested using Kolmogorov-Smirnov test. Blood gas, pH, BD, and lactate measurements in response to repetitive UCOs and associated variable decelerations were compared with the corresponding baseline values; the respective recovery values were compared with the values at the end of the severe UCO series, and the differences in BD changes attributable to lactate after each UCO series were tested by 1-way repeated-measures analysis of variance (ANOVA) with Student-Newman-Keuls post hoc analysis.
      Differences in deterioration rates of BD and lactate as well as ABP and FHR measurements in response to cord occlusion were analyzed by Friedman repeated-measures ANOVA on ranks with Student-Newman-Keuls post hoc analysis.
      All values are expressed as means ± SEM. Statistical significance was assumed for P < .05. Pearson or Spearman correlation analysis was performed as appropriate, and R values are presented where P < .05 (SigmaStat; Systat Software, Inc, San Jose, CA).

      Results

      During the baseline period, maternal venous pH (7.36 ± 0.01), pO2 (50 ± 1.3 mm Hg), pCO2 (42 ± 1 mm Hg), blood glucose (2.94 ± 0.29 mmol/L), lactate (0.77 ± 0.06 mEq/L), and hematocrit (29.3 ± 1.2%), were within normal limits. Maternal parameters did not change significantly during the study protocol.
      Similarly, during the baseline period, fetal pH (7.36 ± 0.01), pO2 (23.4 ± 0.6 mm Hg), pCO2 (48.7 ± 0.8 mm Hg), blood glucose (1.2 ± 0.1 mmol/L), lactate (1.60 ± 0.11 mEq/L), hematocrit (32.0 ± 0.9%), and pH values were within the physiologic range.
      At the initiation of the study protocol, baseline mean FHR was 158 ± 6 bpm. The mean heart rate between UCO did not change throughout the mild, moderate, and severe variable deceleration protocols but increased (P < .001) by 7 bpm during the first hour of recovery. Fetal mean arterial blood pressure was 44 ± 2 mm Hg during the baseline period. In response to repetitive UCOs and associated variable decelerations, the mean pressure between UCO increased significantly during the moderate, severe, and 1 hour recovery periods (Figure 2, A and B).
      Figure thumbnail gr2
      FIGURE 2Fetal responses between UCO
      Mean (±SEM) FHR (bpm) (upper panel) and mean arterial blood pressure (mmHg) (lower panel) at baseline, between UCO of the mild, moderate, and severe periods, and during the 1 hour recovery. A, Mean arterial blood pressure (mm Hg) and B, at baseline, between UCO of the mild, moderate, and severe periods, and during the 1 hr recovery.
      ANOVA, analysis of variance; FHR, fetal heart rate; UCO, umbilical cord occlusion.
      *P < .05 vs baseline, tested with repeated measures ANOVA with Bonferroni correction.
      Ross. Fetal base deficit changes with variable deceleration severity. Am J Obstet Gynecol 2013.
      During the repetitive UCOs, FHR decreased in accord with our study design. Baseline, FHR (158 ± 6 bpm) decreased to a mean nadir of 129 ± 8 bpm during the mild variable decelerations, to 97 ± 6 bpm during the moderate variable decelerations, and to 63 ± 6. bpm during the severe variable decelerations (Figure 3, A). These values are consistent with definitions of mild, moderate, and severe variable decelerations as reported by Kubli et al.
      • Kubli F.W.
      • Hon E.H.
      • Khazin A.F.
      • Takemura H.
      Observations on heart rate and pH in the human fetus during labor.
      In response to UCO induced variable decelerations, the time to the FHR nadir increased from mild to moderate to severe (27 ± 2, 33 ± 2, 38 ± 3 seconds; Figure 3, B). Consistent with the fetal physiologic responses to UCO, fetal mean arterial blood pressure significantly increased at the time of the UCO-induced FHR nadir (Figure 3, C).
      Figure thumbnail gr3
      FIGURE 3Fetal responses at nadir
      Mean (±SEM) FHR nadir (bpm) (upper panel), time to FHR nadir (seconds) (middle panel) and arterial blood pressure at the FHR nadir (mmHg) (lower panel) during mild, moderate, and severe periods. A, Time to FHR nadir (seconds) and B, arterial blood pressure at the FHR nadir (mm Hg) C, during mild, moderate, and severe periods.
      ANOVA, analysis of variance; FHR, fetal heart rate.
      *P < .05 vs each other, tested with repeated measures ANOVA with Student-Newman-Keuls correction.
      Ross. Fetal base deficit changes with variable deceleration severity. Am J Obstet Gynecol 2013.
      Repetitive UCOs and associated variable decelerations, as studied, resulted in development of severe fetal acidosis (pH 7.36 ± 0.01 to 6.99 ± 0.01; BD −1.2 ± 0.7 to 14.6 ± 0.3 mEq/L) by study completion. However, BD increased in proportion with the severity of the variable decelerations. In response to 1 hour of mild variable decelerations BD did not change significantly (−1.2 ± 0.7 to 0.3 ± 0.8 mEq/L). In response to 1 hour moderate variable decelerations BD increased to 2.2 ± 1.0 mEq/L. With 1 hour of severe variable decelerations, BD increased to 9.5 ± 1.8 mEq/L with a further increase noted by the study completion.
      Mild, moderate and severe variable decelerations increased fetal BD (0.21 ± 0.03, 0.27 ± 0.03, and 0.54 ± 0.09 mEq/L per minute of UCO) and lactate (0.08 ± 0.03, 0.14 ± 0.04, and 0.23 ± 0.07 mEq/L per minute of UCO) in direct proportion to severity of the deceleration. During recovery, fetal BD cleared at 0.12 mEq/L per minute.

      Comment

      The results of the current study indicate that normoxic ovine fetuses near-term can tolerate repetitive mild and moderate variable decelerations (as defined in this study) with minimal changes in BD and lactate. In contrast, severe variable decelerations may result in marked BD increases, dependent on frequency.
      We used a model of chronically catheterized ovine fetuses that has been well studied. An ovine fetus approximates both the size of the human fetus and, as best can be determined, simulates the acid base changes in response to hypoxic stress.
      • Frasch M.G.
      • Mansano R.Z.
      • Gagnon R.
      • Richardson B.S.
      • Ross M.G.
      Measures of acidosis with repetitive umbilical cord occlusions leading to fetal asphyxia in the near-term ovine fetus.
      • Victory R.
      • Penava D.
      • Da S.O.
      • Natale R.
      • Richardson B.
      Umbilical cord pH and base excess values in relation to adverse outcome events for infants delivering at term.
      We used an inflatable cuff to induce mild, moderate, and severe variable decelerations resulting from umbilical cord occlusion. UCOs, induced at 2.5 minute intervals with a duration of 1 minute, simulated variable decelerations which may clinically occur during human labor. As reflected by basal maternal venous and fetal arterial blood values, both ewes and fetuses demonstrated normal physiologic parameters at the initiation of the study. Furthermore, the mean FHR and blood pressure were within normal ranges for near-term gestation.
      As per our study design, we sought to induce variable decelerations consistent with the definitions of mild, moderate, and severe variable decelerations as proposed by Kubli et al.
      • Kubli F.W.
      • Hon E.H.
      • Khazin A.F.
      • Takemura H.
      Observations on heart rate and pH in the human fetus during labor.
      Accordingly, the 30 bpm decrease associated with mild decelerations and 61 bpm decrease associated with moderate decelerations were consistent with our study objectives. Further, the decrease to a mean nadir of 63 bpm in response to complete UCO is consistent with severe variable decelerations. The decrease in heart rate in response to UCO occurs as a result of both baroreceptor and chemoreceptor mechanisms. Consistent with the contribution of a baroreceptor mediated response, ovine fetal mean arterial blood pressure significantly increased at the FHR nadir. Importantly, despite the rapid inflation of the cord occluder, the time to the FHR nadir was greater than 30 seconds in response to both moderate and severe UCO. The recent guidelines
      • Macones G.A.
      • Hankins G.D.
      • Spong C.Y.
      • Hauth J.
      • Moore T.
      The 2008 National Institute of Child Health and Human Development workshop report on electronic fetal monitoring: update on definitions, interpretation, and research guidelines.
      for FHR interpretation define variable decelerations as “an onset to nadir of less than 30 seconds.” Both the results in the current study, as well as experiential data from human FHR tracings, suggest that a 30-second time from onset to nadir may be too stringent for the criteria of a variable deceleration because of UCO.
      Despite the marked decrease in FHR during UCO, there was minimal change in mean FHR between the UCOs throughout the study. However, mean arterial blood pressure was significantly increased between UCOs for both the moderate and severe UCO series. During the recovery period, mean FHR increased significantly, likely reflecting a sustained fetal sympathetic response to the worsening acidosis in the prior cord occlusion period.
      As reflected in Figure 4, fetal BD did not change significantly during the mild UCO period, and increased only minimally (2.2 mEq/L) during the 60 minutes of moderate UCO. In contrast, severe variable decelerations resulted in a near linear increase in BD with end-experiment values exceeding 12 mEq/L in all fetuses. Although the design of the study included sequential series of cord occlusions with increasing severity, the very mild levels of base deficit reached at the end of the mild and moderate cord occlusion periods would not have a significant impact on the rate of development of acidosis in response to severe cord occlusion. We calculated the net change in fetal BD and lactate values in response to repetitive mild, moderate, and severe variable decelerations. Consistent with our previous report
      • Frasch M.G.
      • Mansano R.Z.
      • Gagnon R.
      • Richardson B.S.
      • Ross M.G.
      Measures of acidosis with repetitive umbilical cord occlusions leading to fetal asphyxia in the near-term ovine fetus.
      complete UCO resulting in a severe variable deceleration increased fetal BD by 0.54 mEq/L per minute of UCO. In comparison, partial UCO resulting in mild and moderate variable decelerations increased fetal BD by only 0.21 and 0.27 mEq/L per minute of actual UCO, respectively. As reflected in Figure 4, 10 minutes of severe variable decelerations occurring at a frequency of 1 every 2.5 minutes would result in the net increase in fetal BD of approximately 1.5 mEq/L, whereas there would be minimal net increase in BD in response to mild and moderate variable decelerations at this frequency. As expected, lactate levels increased in proportion to the degree of variable deceleration, with lactate representing approximately 50% of the BD value.
      • De Haan H.H.
      • Gunn A.J.
      • Gluckman P.D.
      Fetal heart rate changes do not reflect cardiovascular deterioration during brief repeated umbilical cord occlusions in near-term fetal lambs.
      Figure thumbnail gr4
      FIGURE 4Fetal base deficit at baseline, during mild, moderate, and severe UCO, and during recovery
      UCO, umbilical cord occlusion.
      Ross. Fetal base deficit changes with variable deceleration severity. Am J Obstet Gynecol 2013.
      Both the definitions of variable deceleration severity and the fetal buffering responses are similar between ovine fetuses in the current study and human fetuses. We used the Kubli et al
      • Kubli F.W.
      • Hon E.H.
      • Khazin A.F.
      • Takemura H.
      Observations on heart rate and pH in the human fetus during labor.
      definitions as a basis for our definitions of mild, moderate, and severe variable decelerations. Kubli's definitions of mild and moderate variable decelerations have multiple criteria, though they do use absolute cutoff values rather than change from baseline. We defined mild variable decelerations as a decrease of ∼30 bpm and achieved a mean decrease of 29 bpm (158 ± 6 to 129 ± 8 bpm), which is consistent with 1 of the 3 Kubli definitions of mild variable decelerations (ie, decrease of heart rate to level >80 bpm, irrespective of duration). Our severe variable was a complete cord occlusion, and resulted in a mean decrease of 95 bpm (to 63 ± 6 bpm), also meeting the Kubli definition of severe variable decelerations (ie, decrease to level <70 bpm, >60 seconds). We defined moderate variable decelerations as a decrease of ∼60 bpm, and achieved a mean decrease of 61 bpm. Although this would not precisely meet the Kubli definition of moderate variable, it is clearly midway between our mild and severe variable criteria. In regard to buffering, the body size of the ovine fetus is very similar to that of the human fetus, and it is believed that both species share similar physiology of buffering capabilities and systems, including bicarbonate, hemoglobin, and plasma proteins. Furthermore, the degree of acidosis induced in these studies, by models of cord occlusions that simulate that occurring in human labor (1 every 2.5 minutes), results in levels of acidosis similar to that seen in human pregnancies.
      The recent ACOG Bulletin
      ACOG Practice Bulletin No. 116 management of intrapartum fetal heart rate tracings.
      Management of Intrapartum Fetal Heart Rate Tracings did not discriminate among levels of variable decelerations. However, ACOG noted that recurrent variable decelerations, defined as occurring with ≥50% of contractions, that progress to a greater depth and longer duration are more indicative of impending fetal acidemia. The bulletin cited several references that related human acidemia with variable decelerations. Parer et al
      • Parer J.T.
      • King T.
      • Flanders S.
      • Fox M.
      • Kilpatrick S.J.
      Fetal acidemia and electronic fetal heart rate patterns: is there evidence of an association?.
      reported that there is a lack of data regarding the degree of fetal acidemia induced by various FHR patterns, but confirmed the positive relationship between the degree of acidemia and the depth of decelerations, with an increased proportion of acidemic newborns with severe vs mild decelerations. Kubli et al,
      • Kubli F.W.
      • Hon E.H.
      • Khazin A.F.
      • Takemura H.
      Observations on heart rate and pH in the human fetus during labor.
      who proposed the definitions of mild, moderate, and severe variable FHR decelerations used in the current paper, concluded that greater severity of deceleration was associated with increased fetal acidosis. Together, these papers indicate the lack of knowledge of the impact of varying degrees of variable decelerations because of cord compression on fetal levels of acidosis, and thus the primary reasoning behind our study. Notably, our results are consistent with the impressions from human studies, though now provide a quantitative level of BD that may be associated with repetitive variable FHR decelerations of varying degrees.
      These results provide a basis for the assessment of the degree of metabolic acidosis based on FHR monitoring. The normal fetus enters labor with a BD of approximately 2 mEq/L and after vaginal delivery the average umbilical artery BD is 5 mEq/L.
      • Ross M.G.
      • Gala R.
      Use of umbilical artery base excess: algorithm for the timing of hypoxic injury.
      One can use the current results to assess whether fetuses are at risk for rapid progression of metabolic acidosis and potential delivery beyond a threshold BD value of injury (12 mEq/L). These results further emphasize the value of categorizing variable FHR decelerations into at least 2 categories (eg, mild/moderate and severe) as the degree of deceleration reflects both the relative occlusion of the cord and is indicative of the rate of BD accumulation. Simplistically, severe variable decelerations (decrease to <70 bpm, lasting 60 seconds
      • Kubli F.W.
      • Hon E.H.
      • Khazin A.F.
      • Takemura H.
      Observations on heart rate and pH in the human fetus during labor.
      may be associated with a significant increase in the fetal metabolic acidosis, depending on the frequency of the deceleration, whereas mild/moderate variable decelerations are likely to be well tolerated by the fetus during labor. Furthermore, our findings indicate that the time from onset to FHR deceleration nadir may exceed 30 seconds. This suggests that one may consider modifying the criteria for the definition of variable FHR deceleration.

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