Introduction

Acute fetal distress (AFD) deserves a timely diagnosis since it is a metabolic alteration that leads to the development of hypoxia, hypercapnia, and acidosis, generating neurological alterations, multiple organ failure, and even intrauterine death. This condition is manifested by altered fetal heart rate, meconium amniotic fluid expulsion, sustained hypomotility, and intrauterine overactivity¹-⁵. Monitoring of patients with previous conditions, such as labor or prolonged expulsive disorders or labor induced with oxytocin or prostaglandins is essential, as they may be precipitating factors of AFD⁶, which is an indication for pregnancy resolution⁷,⁸.

The need for advanced neonatal resuscitation maneuvers is assessed at birth. At 1 and 5 min after birth, the Apgar test assesses the newborn (NB) and reports the adaptive response to the extrauterine environment⁹. There is an inverse relationship between the Apgar score and neonatal mortality: the lower the Apgar value with longer duration, the higher the probability of admission to the neonatal intensive care unit (NICU) and even death¹⁰. The Apgar score is fundamental for the initial clinical evaluation; however, its accuracy depends on the evaluator's skill and experience. Some NBs with hypoxia or acidemia may be overlooked with a normal Apgar score and suffer adverse consequences. Therefore, we should rely on gasometric values, especially lactate and pH, to establish the diagnosis, prognosis, and therapy¹¹.

Lactate and umbilical cord pH in NBs with a history of AFD contribute to predicting morbidity¹². Lactate is the product of anaerobic metabolism, its accumulation in the brain is associated with the presence of hypoxia, edema, and tissue necrosis¹³. The mean of lactate levels in healthy term NBs are 3.600 ± 1.833 mmol/L, in contrast to term NBs with hypoxia, with mean levels of 6.423 ± 2.335 mmol/L¹⁴. NBs that clinically progress to AFD show mean lactate levles of 7.05 ± 4.03 mmol/L¹⁵. Currently, umbilical artery lactate levels are known to be a more informative measure of term NB morbidity than pH¹³.

The pH is the result of acid-base balance¹⁶. Fetal acidemia in the umbilical cord has an umbilical artery pH cutoff value < 7.20 regardless of gestational age¹⁷,¹⁸. However, the American College of Obstetricians and Gynecologists and the National Institute for Health and Clinical Excellence recommend using arterial pH values < 7.00, which are clinically useful for identifying morbidity and mortality risk¹⁹,²⁰. Fetal acidemia is associated with NICU admission, hypoxic-ischemic encephalopathy (HIE), and respiratory distress syndrome²¹.

The measurement of lactate and pH may change according to the clamping time of the umbilical cord since it is stable for only 60 min after clamping to determine pH and gases²²,²³. Similarly, the umbilical cord is useful if kept in a heparinized syringe for the same time; after this period, the values tend to change²⁴. For this reason, this study aimed to determine lactate and pH values by umbilical cord arterial blood gasometry in NBs with a history of AFD at a secondary level of medical care in patients with or without neonatal morbidity.

Methods

We conducted a cross-sectional, descriptive, prospective, and homodemic study in a secondary-level hospital. We included term NBs (≥ 37-41 weeks of gestation), products of a single pregnancy, with perinatal risk factors for AFD such as hemorrhage in the third trimester, infections, hypertensive states of pregnancy, anemia, collagenopathies, drug intoxication, or obstetric comorbidity; uteroplacental factors such as cord abnormalities, placental abnormalities, altered uterine contractility, anatomical uterine abnormalities; and obstetric factors including amniotic fluid with meconium, cephalopelvic incompatibility, use of medications (oxytocin), abnormal fetal presentation, prolonged or accelerated labor, instrumental delivery or cesarean section, premature rupture of membranes, oligohydramnios or polyhydramnios, alterations in fetal heart rate (tachycardia > 180/bradycardia < 120), decreased maternal perception of fetal movements, intrauterine growth retardation, with a prenatal diagnosis of fetal distress²⁵. Pre-term or post-term NBs, multiple pregnancies, and those with congenital malformations were excluded from the study.

Before delivery and after birth, the neonatal care protocols were followed, which included the five basic measures of neonatal resuscitation, determination of Apgar at 1 min and 5 min, with double clamping of the umbilical cord. With a previously signed informed consent form by the mother, a sample was taken from the umbilical artery proximal to the NB and the gasometric analysis was performed within the first 30 min of obtaining the sample.

For our study population, neonatal morbidity was defined if the NB presented at least one of the following conditions: Apgar at 1-min ≤ 6, need for advanced neonatal resuscitation measures (bag-mask ventilation or chest compressions), or admission to the NICU²⁶-³⁰.

Data were analyzed with the statistical software (IBM SPSS Statistics v 20). Descriptive statistics were used for the general data of the mothers and NBs. Categorical variables were described as percentages, and quantitative variables were described as means and standard deviation; Student's t-test was used to compare the means of lactate and pH between NBs with or without morbidities.

Results

Sixty-six patients were recruited according to the inclusion criteria described; the perinatal characteristics are shown in table 1. The mean of maternal age remained within the optimal age group for gestation; most women have had multiple pregnancies. About half of the women were in the active labor phase, and 48.4% of the patients were in the latent labor phase induced with oxytocin. Approximately one third of the mothers showed at least one risk factor for developing AFD, with hypertensive disease of pregnancy being the most prevalent condition, followed by oligohydramnios.

Table 1 shows the neonatal characteristics starting with the mean weight and length, which were within the appropriate percentiles for the gestational age. Most of the NBs were female; 28.7% required advanced neonatal resuscitation maneuvers, with ventilatory assistance being more frequent than chest compressions; 27.2% obtained Apgar at 1-min ≤ 6; at 5 min, the vast majority recovered. Admission to the NICU was about one-eighth of the population. No NB mortality was observed in the study. However, 28.7% of the NBs presented morbidities, and 10.6% had direct risk factors for developing AFD, which determined the timing of their birth: 3% with the umbilical cord around the neck, 3% with dystocia at birth, 3% with intrauterine growth retardation, and 1.5% with macrosomia.

Table 2 shows the frequency of clinical signs used for prenatal diagnosis of AFD, such as bradycardia, tachycardia, and meconium in the amniotic fluid. The signs are listed in order of frequency, alone or simultaneously.

Regarding gas levels in the umbilical cord, the NBs who experienced neonatal morbidity showed higher lactate levels and lower pH levels, with values of 4.726 ± 1.401 and 7.293 ± 0.056, respectively. Conversely, the group with no associated neonatal health issues showed lower lactate levels of 2.240 ± 0.318 and slightly higher pH levels of 7.359 ± 0.022 (p < 0.05). The umbilical arterial lactate levels were higher and the pH levels were lower in NBs with no morbidities, as shown in table 3.

Discussion

AFD is a term used to determine abnormal variations in fetal cardiac patterns, decreased fetal movements, and the presence of meconium fluid that leads to progressive fetal hypoxia or acidemia secondary to poor fetal oxygenation³¹. In this study, NBs with prenatal history of AFD presented isolated bradycardia and meconium fluid as the most frequent clinical sign. These data were followed by tachycardia with or without meconium and, finally, the presence of meconium alone. The results were similar to those reported by Yaipén et al.⁷, where the diagnosis of most of their patients was established by fetal heart rate alterations, followed by the presence of meconium. These clinical signs occur in response to hypoxia, which triggers fetal compensatory mechanisms: a decrease in heart rate; a reduction in oxygen consumption secondary to the cessation of non-essential functions, such as gross body movements; redistribution of cardiac output to perfuse essential organs, such as heart, brain, and adrenal glands; and predominance of anaerobic metabolism⁶,³². All of these mechanisms affect umbilical gases, and the effects of AFD can lead to increased perinatal morbidity and mortality, highlighting complications such as encephalopathy, seizures, cerebral palsy, neurodevelopmental delay, and death¹⁹,³³.

As for the perinatal characteristics, of our study, more than half of the mothers had already had other pregnancies. Zhang et al.³⁴. found that the duration of labor in primigravidae was longer than in multigravidae, which is explained by the slower cervical dilatation of 4 to 6 cm, increasing the state of hypoxia in the uterus. One-third of our patients required induction - conduction of labor with oxytocin due to lack of progression. Einikyte et al. reported oxytocin as one of the maternal and pregnancy resolution factors influencing blood gas status in NBs, as oxytocin itself increases arterial lactate values¹⁴.

One or more maternal factors accompanied the AFD. One-third of our population showed hypertensive disease of pregnancy, with the highest incidence, followed by oligohydramnios, fetal hypomotility, intrauterine growth restriction, premature rupture of membranes, prolonged delivery, polysystole, presentation dystocia, umbilical cord around the neck and fetal macrosomia, consistent with the studies of Sabol and Caughey and Ulloa-Ricardez et al., respectively³⁵,³⁶.

Regarding the characteristics of NBs with a history of AFD, most were within the appropriate percentiles for gestational age, weight, and length. In NBs with clinical signs compatible with the disease, in addition to an Apgar score ≤ 6 points, AFD diagnosis was corroborated by lactate and pH altered values. As in the study by Carpio and Romero the risk of obtaining a pH < 7.20 coupled with amniotic fluid with meconium or 1-min Apgar ≤ 6 points was higher compared to neonates with clear fluid at delivery, and in those who obtained 1-min Apgar ≥ 7 points³⁷.

Regarding the characteristics of the umbilical cord gases, we identified that the average lactate value was lower, and the pH value higher than those reported by other authors. This difference could be attributed to an early diagnosis and timely treatment of AFD in our population, thus reducing neonatal morbidity, despite the alteration of its gasometric values¹³,¹⁵,³⁸.

In our study, arterial lactate levels were higher in NBs with morbidity than those with no morbidity data despite having a history of AFD. This finding is similar to Natesan's findings, where higher lactate concentrations correlated with poor neonatal outcomes and prognosis³⁹.

In clinical practice, pH is more commonly used than lactate as an indicator of neonatal status. It is important to note that pH and base excess require more blood and more complex equipment than lactate measurement. Furthermore, lactate can be measured using a portable device, requiring less blood, which makes it more affordable and maintenance-friendly. Some authors have stated that the combined measurement of pH and lactate does not represent a better prediction of abnormal results than each method separately⁴⁰,⁴¹, generating an increase in the number of abnormal results. If combined tests are performed, a high lactate value indicates action, even if the pH is normal. Cases of severely depressed NBs have been reported with this combination of findings⁴².

As these studies were not performed in the same population group, the combined evaluation of these measurements in a homodemic group is interesting. Therefore, the diagnosis of AFD should be corroborated by complementary methods, such as continuous fetal electrocardiogram recording or computer-assisted electronic fetal monitoring, fetal pulse oximetry, or fetal scalp sampling with immediate blood gas or lactate determination⁴³.

Among the limitations of our study were that no information was available on the remaining arterial blood gas parameters, no follow-up of gasometric changes was possible, NBs with HIE could not be identified, and no NBs were known to have died in the NICU.

Neonatal care requires highly technical and costly interventions; we believe that immediate gains in reducing neonatal morbidity will come mainly through implementing simple, practical, and proven interventions on a broader scale during the prenatal, intrapartum, and postpartum periods. By better understanding traditional practices that rely on scientific evidence communication strategies, we can change practices to improve neonatal care, particularly at birth. In events such as hypoxia, AFD, perinatal asphyxia, and infections, this understanding offers the most significant potential to improve the survival and health of future generations of infants and children.

In summary, an increase in umbilical cord lactate values of approximately 25% was observed in the group of term NBs with a history of AFD and associated neonatal morbidity. Besides pH decreased 1% without reaching acidemia levels. These findings contrasted with the group of term NBs with a history of AFD but no associated neonatal morbidity.