Umbilical cord blood gas analysis at delivery

https://doi.org/10.1053/ob.1996.v175.a74401Get rights and content

Abstract

Fetal acid-base status is the end point that all antepartum and intrapartum surveillance tools are designed to assess, either directly or indirectly. Umbilical arterial blood pH is normal in 80% of depressed newborns, thereby proving the absence of birth asphyxia both in the delivery room and in the courtroom. The technique, normal ranges, and risks, benefits and cost analysis of routine versus selected umbilical blood sampling are considered. The American College of Obstetricians and Gynecologists recommends performing umbilical blood sampling for acid-base analysis in selected deliveries. However, in our opinion routine umbilical blood sampling at all deliveries has merit. (Am J Obstet Gynecol 1996;175:517-22.)

Section snippets

TECHNIQUE

Immediately after delivery a segment of umbilical cord is doubly clamped. Delayed clamping of the umbilical cord may result in significant decreases in the arterial blood pH and increases in arterial blood PCO2 and base deficit.6Prepackaged syringes containing dried heparin are inexpensive and provide a consistent preparation and amount of heparin, thus minimizing error. Drawing liquid heparin may result in a spurious metabolic acidemia if too much heparin or not enough blood is drawn into the

NORMAL VALUES

Depending on how normality is defined and on what population is studied, reported normal ranges for umbilical cord blood gas values may vary (Table I).[1], [10], [11], [12], [13], [14], [15], [16], [17], [18]Delivery by cesarean section without labor results in values more closely approximating adult blood gases (higher pH, PO2, base excess, and bicarbonate and lower PCO2).19Likewise, nulliparous parturients tend to have a lower pH range than did multiparas, which is related to the difference

WHICH UMBILICAL VESSEL SHOULD BE SAMPLED?

Umbilical arterial blood most accurately reflects fetal status because it has circulated throughout the fetus. In contrast, the umbilical vein carries blood from the placenta back to the fetus. Thus umbilical venous blood gas measurements better reflect maternal acid-base status and placental function. Therefore, if only one vessel is selected, it should be the umbilical artery rather than the vein. It is not helpful to obtain an isolated umbilical venous blood pH or gas because it may be

pH ALONE OR A COMPLETE BLOOD GAS ANALYSIS?

There may be some advantage in determining the PCO2, bicarbonate, and base deficit. Paradoxically, the least valuable parameter in evaluating fetal oxygenation is the PO2. This is explained by the characteristics of the fetal oxygen dissociation curve and by the variable affinity of oxygen to hemoglobin, which is affected by pH (Bohr effect). It has been suggested that a severe acidemia in term gestations, classified as metabolic or mixed, imparts a more concerning prognosis than a pure

BENEFITS OF SELECTIVE SAMPLING

Some of the benefits of selective sampling are depicted in Table II. Depending on how ″selective” the physician or institution is, there is some cost benefit. Although there is a large variance in charges among hospitals for blood gas analysis, the actual cost is only in the range of $3.50 to $5.00 per sample.9At St. Luke's Hospital of Kansas City the charge for a pH is $44.00 and a ″complete blood gas” is $58.00 and at the University of Utah the charge for a ″complete blood gas” including

DRAWBACKS OF SELECTIVE SAMPLING

Unfortunately, the civil system in the United States often seems irrational and unpredictable when perinatal injury resulting from medical negligence is alleged. Large settlements and awards, in addition to anxiety, depression, and lost time are common experiences for today's obstetric staff, especially in ″damaged baby” allegations. This often occurs in spite of convincing scientific proof that institutions or health care providers did not cause or contribute to the damage. Thus, whenever

BENEFITS OF ROUTINE SAMPLING

Some potential benefits of routine sampling are listed in Table III. It is generally accepted by the medical community that the majority of children with major congenial neurologic deficits did not sustain the insult in the peripartum period.24Much to the advantage of the plaintiffs' attorneys, medical record coders use the International Classification of Diseases codes, which erroneously define asphyxia by the Apgar score. Moreover, it uses only the 1-minute Apgar score! The code defines mild

DISADVANTAGES OF ROUTINE SAMPLING

Potential disadvantages of routine sampling are listed in Table III. One of the concerns regarding universal sampling is the finding of an abnormal cord pH in the completely normal vigorous newborn. Approximately 1% of vigorous newborns may have umbilical arterial blood acidemia (<7.10) at birth.1It is generally believed that cord arterial blood acidemia has no clinical significance in the vigorous newborn. Nonetheless, some have expressed concern that it could pose a potential medicolegal

POTENTIAL FUTURE APPLICATIONS OF BLOOD GAS ANALYSIS

New methods of intrapartum fetal surveillance are being developed to improve specificity in diagnosing fetal compromise. Currently, multicenter randomized clinical trials are being performed to evaluate the safety and efficacy of intrapartum fetal oxygen saturation monitoring.26If fetal pulse oximetry proves to be clinically useful, blood gas analysis with cooximetry may be helpful in correlating electronic fetal monitoring and pulse oximetry with acid-base condition at delivery.27

In summary,

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    The results of multiple regression analysis revealed that oxidative stress in the fetal circulation was independent of the time of delivery. The values reported in our study for the umbilical cord blood acid–base status are in general agreement with those of other studies [20]. There are contradictory findings regarding the influence of the absence or presence of labor at the time of birth on the concentration of isoenzymes of SOD.

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From the Departments of Obstetrics and Gynecology, Saint Luke's Hospital of Kansas City and the University of Missouri at Kansas City,a the University of Utah School of Medicine,b the University of Texas at Houston,c and the State University of New York at Stony Brook.d

Supported by the Andrew Nicely Memorial Fund through the Saint Luke's Foundation of Kansas City, Missouri.

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