Background
The Apgar score is universally used for fetal assessment at the time of birth, whereas, the collection of fetal cord blood gases is performed commonly in high-risk situations or in the setting of Apgar scores of <7, which is a less standardized approach. It has been well-established that neonatal acidemia at the time of delivery can result in significant neonatal morbidity and death. Because of this association, knowledge of the fetal acid-base status and detection of acidemia at the time of delivery can serve as a sensitive and useful component in the assessment of a neonate’s risk. Umbilical cord blood gas analysis is an accurate and validated tool for the assessment of neonatal acidemia at the time of delivery. Because the collection of fetal cord blood gases is not a standardized practice, it is possible that, with such a varied approach, some cases of neonatal acidemia are not detected, particularly in the setting of reassuring Apgar scores.
Objective
In a setting of universally obtained cord blood gases, we sought to identify the rates of acidemia and associated factors in neonates with 5-minute Apgar scores of ≥7.
Study Design
This retrospective cohort study identified all term, singleton, nonanomolous neonates with 5-minute Apgar scores of ≥7. The incidence of umbilical artery pH ≤7.0 or ≤7.1 and base excess ≤–12 mmol/L or ≤–10 mmol/L were examined overall and in association with obstetric complications and adverse neonatal outcomes. Chi-squared tests were used to compare proportions, and multivariable logistic regression was used to control for potential confounders.
Results
In this cohort, the incidence of an umbilical artery pH of ≤7.0 was 0.5%, of a pH ≤7.1 was 3.4%, of a base excess ≤–12 mmol/L was 1.4%, and of ≤–10 mmol/L was 4.0%. Rates of neonatal acidemia were greater in the setting of meconium (4.3% vs 3.2%; P <.001), placental abruption (13.2% vs 3.4%; P <.001), and cesarean deliveries (5.8% vs 2.8%; P <.001), despite normal 5-minute Apgar scores. Additionally, umbilical artery pH ≤7.0 was associated with an increased risk of respiratory distress syndrome (adjusted odds ratio, 6.5; 95% confidence interval, 2.9–14.3) and neonatal intensive care unit admission (adjusted odds ratio, 10.8; 95% confidence interval, 6.8–17.4). Base excess of ≤–12 mmol/L was also associated with an increased risk of neonatal sepsis (adjusted odds ratio, 4.7; 95% confidence interval, 1.9–12.1). Finally, when examined together, neonates with both a pH of ≤7.0 and base excess of ≤–12 mmol/L continued to demonstrate an increased risk of neonatal intensive care unit admission and respiratory distress syndrome, with adjusted odds ratios of 9.6 and 6.0, respectively. This risk persisted in neonates with a pH of ≤7.1 and base excess of ≤–10 mmol/L as well, with adjusted odds ratios of 4.5 and 1.1, respectively.
Conclusion
Because neonates with reassuring Apgar scores have a residual risk of neonatal acidemia that is associated with higher rates of adverse outcomes, the potential utility of obtaining universal cord blood gases should be further investigated.
Fetal and subsequent neonatal acidemia is associated with multiorgan dysfunction, hypoxic ischemic encephalopathy, seizures, cerebral palsy, long-term neurologic deficits, and neonatal death. Because of this association, knowledge of the fetal acid-base status and detection of acidemia at the time of delivery can serve as a sensitive and useful component in the assessment of a neonate’s risk of morbidity and death. Umbilical cord blood gas analysis is an accurate and validated tool for the assessment of neonatal acidemia at time of delivery.
The components of umbilical cord blood gas, which most commonly are used as a means of capturing neonates who are at risk for adverse outcomes, are pH and base excess. Studies have demonstrated an increased risk of neonatal morbidity when umbilical artery cord pH is ≤7.0. Recent studies have also demonstrated that even moderate degrees of fetal acidemia (pH threshold of ≤7.10) may place neonates at risks for adverse outcomes. Base excess is an additional threshold value that is used to indicate the severity and duration of neonatal acidemia. At the time of delivery, base excess levels of ≤–12 mmol/L (10%) and ≤–16 mmol/L (40%) are associated with moderate-to-severe newborn infant complications.
Although pH and base excess are useful tools in the prediction of adverse outcomes, universal umbilical cord blood gas analysis at the time of delivery is not a routine practice. The American College of Obstetricians and Gynecologists recommends the use of selective umbilical cord blood sampling. Common thresholds for obtaining neonatal cord blood gases are a 5-minute Apgar score of <7 and in patients who are at high risk for neonatal asphyxia (ie, cord prolapse, placental abruption). Because the collection of fetal cord blood gases is not a standardized practice, there is great variation in collection practice among institutions and among providers within the same institution. It is possible that, with such a varied approach, some cases of neonatal acidemia are not detected, particularly in the setting of reassuring Apgar scores. Additionally, it is unclear whether cases of neonatal acidemia with a normal Apgar score by 5 minutes are clinically important.
The objective of our study was to assess the rate of neonatal acidemia that occurs in neonates with a 5-minute Apgar score of ≥7. Additionally, we sought to determine factors that are associated with an increased risk of neonatal acidemia in this setting and their associated outcomes.
Materials and Methods
This is a retrospective cohort study of all nonanomalous, term, singleton neonates who were born at Moffitt-Long Hospital from 1990 until 2009 with a 5-minute Apgar score of ≥7 (N=26,669) in a setting where routine collection of cord blood gases was attempted in every delivery. Deliveries were excluded if the 5-minute Apgar score was <7 (n=873) or if a cord blood gas was not obtained, was inadequate, or was only a venous sample (n=3385). Approval from the institutional review board at Oregon Health & Science University was obtained. All diagnoses were made by the providing clinicians. Detailed information on all deliveries during the study time frame were abstracted from the medical records by trained abstractors and recorded in an electronic database. The abstracted data were reviewed monthly by a Neonatologist and Maternal Fetal Medicine specialist to ensure accuracy.
Within the cohort of deliveries with a 5-minute Apgar score of ≥7, the incidence of deliveries with an umbilical artery pH ≤7.0, pH ≤7.1, base excess ≤–12 mmol/L, and base excess ≤–10 mmol/L were then identified. To identify risk factors for abnormal cord blood gases in this setting, these groups were then examined in a variety of maternal subgroups that included mode of delivery and those women with placental abruption, presence of meconium, shoulder dystocia, and preexisting maternal conditions such as gestational diabetes mellitus, chronic hypertension, and preeclampsia.
Next, several neonatal outcomes were compared between acidemic and nonacidemic neonates who had 5-minute Apgar scores of ≥7. Neonatal outcomes that were examined included neonatal intensive care unit (NICU) admission, meconium aspiration syndrome (MAS), respiratory distress syndrome (RDS), and neonatal sepsis.
Chi-squared tests were used to compare proportions and multivariable logistic regression was used to control for potential confounders. The potential confounding variables that were examined were race/ethnicity, maternal age, parity, insurance status, gestational diabetes mellitus, chronic hypertension, and preeclampsia. A probability value of <.05 was considered statistically significant. Multivariable analyses are presented as odds ratios with 95% confidence intervals (CIs).
Results
Of the 26,669 deliveries that met the inclusion criteria, the overall incidence of an umbilical artery pH ≤7.0 was 0.5% (n=133), of umbilical artery cord pH ≤7.1 was 3.4% (n=906), of base excess ≤–12 mmol/L was 1.4% (n=373), and of base excess ≤–10 mmol/L was 4.0% (n=1067). Maternal characteristics between those with and without a neonate with an umbilical artery pH of ≤7.1 were similar with respect to insurance status and rates of gestational diabetes mellitus and chronic hypertension but differed in regards to maternal age, parity, and rates of preeclampsia ( Table 1 ). Women who delivered neonates with a pH of ≤7.1 were more likely to be nulliparous and >35 years old and to have preeclampsia.
| Demographic | pH, % | |
|---|---|---|
| >7.1 | ≤7.1 | |
| Parity a | ||
| Nulliparous | 96.9 | 3.1 |
| Multiparous | 96.2 | 3.8 |
| Age, y b | ||
| <35 | 96.8 | 3.2 |
| >35 | 95.7 | 4.3 |
| Race b | ||
| White | 96.0 | 4.0 |
| Black | 96.4 | 3.6 |
| Hispanic | 96.3 | 3.7 |
| Asian | 97.6 | 2.4 |
| Other | 98 | 2 |
| Insurance type | ||
| Public | 96.6 | 3.4 |
| Private | 96.9 | 3.1 |
| Gestational diabetes mellitus | ||
| Yes | 96.6 | 3.4 |
| No | 96.0 | 4.0 |
| Chronic hypertension | ||
| Yes | 96.6 | 3.4 |
| No | 96.2 | 3.8 |
| Preeclampsia b | ||
| Yes | 96.6 | 3.4 |
| No | 94.6 | 3.4 |
The incidence of abnormal cord blood gases was compared with obstetric complications that included placental abruption, presence of meconium, shoulder dystocia, mode of delivery, maternal gestational diabetes mellitus, chronic hypertension, and preeclampsia. Rates of neonatal acidemia (pH ≤7.1), despite a normal 5-minute Apgar score, were greater in the setting of meconium (4.3% vs 3.2%; P <.001), placental abruption (13.2% vs 3.4%; P <.001), cesarean deliveries (5.8% vs 2.8%; P <.001), and pregnancies that were complicated by preeclampsia (6.3% vs 3.9%; P <.001; Table 2 ).
| Characteristic | pH ≤7.0, % | Base excess ≤–12 mmol/L, % | pH ≤7.1, % | Base excess ≤–10 mmol/L, % |
|---|---|---|---|---|
| Meconium | ||||
| Yes | 0.7 a | 2.0 b | 4.3 b | 5.2 b |
| No | 0.4 | 1.2 | 3.2 | 3.7 |
| Abruption | ||||
| Yes | 3.6 b | 3.7 | 13.2 b | 8.6 a |
| No | 0.5 | 1.4 | 3.4 | 4.0 |
| Mode of delivery | ||||
| Cesarean | 1.0 b | 1.7 a | 5.8 b | 4.6 a |
| Vaginal | 0.3 | 1.3 | 2.8 | 3.9 |
| Shoulder dystocia | ||||
| Yes | 0.5 | 1.5 | 3.6 | 4.2 |
| No | 0.3 | 0.6 | 3.4 | 2.8 |
| Operative delivery | ||||
| Yes | 0.5 | 1.8 a | 4.3 b | 5.4 |
| No | 0.4 | 1.3 | 3.3 | 3.8 |
| Gestational diabetes mellitus | ||||
| Yes | 1.1 | 1.3 | 4.8 | 3.4 |
| No | 0.8 | 1.7 | 4.0 | 4.5 |
| Chronic hypertension | ||||
| Yes | 0.5 | 1.4 | 4.5 | 4.9 |
| No | 0.8 | 1.7 | 4.0 | 4.5 |
| Preeclampsia | ||||
| Yes | 1.6 a | 2.8 a | 6.3 b | 7.8 b |
| No | 0.7 | 1.7 | 3.9 | 4.4 2 a |
With regard to neonatal outcomes, a cord blood gas with pH of ≤7.0 and ≤7.1 was associated with increased rates of RDS, MAS, and NICU admissions. Similarly, a cord blood gas base excess of ≤–10 mmol/L or ≤–12 mmol/L was also associated with these outcomes in addition to increased rates of neonatal sepsis ( Table 3 ).
| Cord blood gas component | Meconium aspiration syndrome, % | Respiratory distress syndrome, % | Sepsis, % | Neonatal intensive care unit admission, % |
|---|---|---|---|---|
| pH | ||||
| ≤7.1 | 1.7 a | 4.7 b | 0.4 | 11.4 b |
| >7.1 | 0.7 a | 1.0 b | 0.2 | 4.3 b |
| ≤7.0 | 1.9 a | 7.6 b | 0 | 28.9 b |
| >7.0 | 0.7 a | 1.1 b | 0.2 | 4.4 b |
| Base excess, mmol/L | ||||
| >–10 | 0.6 b | 1.1 a | 0.2 b | 4.4 b |
| ≤–10 | 1.7 b | 2.0 a | 1.0 b | 8.0 b |
| >–12 | 0.7 b | 1.1 a | 0.2 b | 4.5 b |
| ≤–12 | 2.6 b | 2.9 a | 1.4 b | 9.7 b |
Multivariable regression analyses were performed to control for potential confounding variables that included parity, maternal age, maternal race/ethnicity, insurance type, chronic hypertension, gestational diabetes mellitus, and preeclampsia. After we controlled for these variables, we determined that neonates with a pH of ≤7.0, despite an Apgar of >7 at 5-minutes, had a statistically significant increased risk of RDS and NICU admission with an adjusted odds ratio (aOR) of 6.5 (95% CI, 2.9–14.3) and 10.8 (95% CI, 6.8–17.4) respectively ( Table 4 ). Similarly, in neonates with a pH ≤7.1 and normal Apgar scores, there were also statistically significant increases in rates of MAS, RDS, and NICU admissions ( Table 4 ).
| Outcomes | Adjusted odds ratio | 95% Confidence interval | P value |
|---|---|---|---|
| pH ≤7.0 | |||
| Meconium aspiration syndrome | 1.47 | 0.20–10.72 | .699 |
| Respiratory distress syndrome | 6.47 | 2.93–14.28 | <.001 c |
| Neonatal intensive care unit admission | 10.84 | 6.76–17.38 | <.001 c |
| pH ≤7.1 | |||
| Meconium aspiration syndrome | 2.43 | 1.30–4.53 | .005 b |
| Respiratory distress syndrome | 4.60 | 3.10–6.84 | <.001 c |
| Sepsis | 1.67 | 0.60–4.65 | .328 |
| Neonatal intensive care unit admission | 3.68 | 2.81–4.82 | <.001 c |
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