How and when umbilical cord gas analysis can justify your obstetric management

Illustration: Kimberly Martens for OBG Management

CASE 2: An infant with unusual umbilical artery values

An infant born via vacuum delivery for a prolonged second stage of labor had 1- and 5-minute Apgar scores of 8 and 9, respectively. Cord gas values were obtained, and analysis revealed that for the umbilical artery, the pH was 7.29; Pco₂, 20 mm Hg; and Po₂, 60 mm Hg. For the umbilical vein, the pH was 7.32; Pco₂, 38 mm Hg; and Po₂, 22 mm Hg.

The resident asked, “How is the Po₂ higher in the artery than in the vein?”

The curious Case 2 values suggest an air bubble

Although it is possible that the aberrant values in Case 2 could have resulted from switching the artery and vein samples, the pH is lower in the artery, and both the artery Po₂ and Pco₂ levels do not appear physiologic. The likely explanation for these values is that an air bubble was contained in the syringe. Since normal room air (21% O₂) has a Po₂ of 159 mm Hg and a Pco₂ of less than 1 mm Hg, exposure of cord blood gases to air bubbles will significantly increase the Po₂ and markedly reduce the Pco₂ values of the sample. Take care to avoid air bubbles in the syringes used to obtain samples for analysis.

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CASE 3: A vigorous baby with significant acidosis

A baby with 1- and 5-minute Apgar scores of 9 and 9 was delivered by cesarean and remained vigorous. Umbilical cord analysis revealed an umbilical artery pH level of 7.15, with normal Po₂ and Pco₂ values. What could be the explanation?

Was there a collection error in Case 3?

On occasion, a falsely low pH level and, thus, a falsely elevated BD may result from excessive heparin in the collection syringe. Heparin is acidotic and should be used only to coat the syringe. Although syringes in current use are often pre-heparinized, if one is drawing up heparin into the syringe, it should be coated and then fully expelled.

Umbilical cord sampling: Procedures and equipment

Many issues remain regarding the optimal storage of cord samples. Ideally, a doubly clamped section of the cord promptly should be sampled into glass syringes that can be placed on ice and rapidly measured for cord values.

Stability of umbilical cord samples within the cord is within 20 to 30 minutes. Delayed sampling of clamped cord sections generally has minimal effect on pH and Pco₂ values.¹¹ The BD does not change to a clinically significant degree over 15 to 30 minutes despite the cord specimen remaining at room temperature. However, one report demonstrated an increase in lactate and BD by 20 minutes under these conditions; this likely was a result of metabolism from endothelial or blood cells.¹² I therefore recommend that clamped cord be sampled as soon as is feasible and ideally not beyond 20 to 30 minutes.

Plastic syringes can introduce interference. Several studies have demonstrated that collection of samples in plastic may result in an increase in Po₂ values, likely due to the high room air Po₂ diffusing through the plastic to the blood sample.

Use glass, and “ice” the sample if necessary. Although it has been suggested that placing samples on ice minimizes metabolism, the cooled plastic may in fact be more susceptible to oxygen diffusion. Thus, unless samples will be analyzed promptly, it is best to use glass syringes on ice.^13,14

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What if the umbilical cord is torn?

Sometimes the umbilical cord is torn and discarded or cannot be accessed for other reasons. A sample can still be obtained, however, by aspirating the placental surface artery and vein vessels. Although there is some potential variance in pH, Po₂, and Pco₂ levels, the BD values of placental vessels have a high correlation with those of umbilical vessels and therefore can be used when the cord is not available.¹⁵

How do you obtain cord analysis when delaying cord clamping?

The American College of Obstetricians and Gynecologists (ACOG) now advises delayed cord clamping in term and preterm deliveries, which raises the question of how you obtain a blood sample in this setting. Importantly, ACOG recommends delayed cord clamping only in vigorous infants,¹⁶ whereas potentially compromised infants should be transferred rapidly for newborn care. Although several studies have demonstrated some variation in cord gas values with delayed cord clamping,^17–21 clamping after pulsation has ceased or after the recommended 30 to 60 seconds following birth results in minimal change in BD values. Thus, do not hesitate to perform delayed cord clamping in vigorous infants.

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