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Technical Briefs |
1
Depts. of Obstet. and Gynecol. and Neonatol., Kaplan Hosp., 76100 Rehovot, Israel (affiliated with Hadassah-Hebrew Univ. School of Med., Jerusalem);
a author for correspondence: fax 972-8-9411944, e-mail blick{at}netvision.net.il
Determination of cord blood gases and pH is recommended in all neonates with low Apgar scores to distinguish metabolic acidosis from hypoxemia or from other causes that might result in low Apgar scores (1). Although the metabolic acidosis found in cord blood is a poor predictor of long-term neurological injury (2), assessment of umbilical cord blood gas is helpful to exclude intrapartum or birth events that cause acidosis and serves as legal evidence against any alleged association with poor outcome (3).
Textbook recommendations for postpartum umbilical cord blood sampling include immediate transport of the blood in a heparin-containing syringe placed in a plastic sack containing crushed ice (4). Several studies have previously questioned the utility of this method (5)(6)(7)(8)(9)(10). Sato and Saling (5) evaluated pH values after only 30 min and at every hour up to 7 h in 30 blood samples stored at room temperature and in 30 different samples stored in a refrigerator. They concluded that if fetal blood has to be stored for >50 min, it must be kept refrigerated to inhibit autoxidation. Hilger et al. (6) evaluated the sequential changes in gases and pH in blood taken from the umbilical vein or from a superficial placental vein at 15-min intervals and at room temperature only. They found that as long as blood was taken from the cord vein, the gases and pH were not affected by as long as 1 h delay in sampling. Pel and Trefferes (7) and Sykes and Molloy (8) used blood samples collected into heparin-containing syringes and stored refrigerated for as long as 6 h and compared this with blood collected from an umbilical cord segment left at room temperature. The conflicting results of the latter studies were attributed to differences in the dose of heparin in the collecting syringes. Strickland et al. (9) evaluated pH and PCO2 in cord blood stored at room temperature for variable intervals after delivery and concluded that blood drawn for determination of pH and PCO2 can be kept at room temperature for up to 30 min. Duerbeck et al. (10) used blood taken in non-heparin-containing syringes and stored at room temperature; they reported that during the 60 min after delivery, there was no significant change in the tested parameters.
Obviously, the different methodologies used in previous studies obviate a true comparison between the results. As quoted, some studies used a rather small sample size that perhaps does not have enough statistical power to demonstrate a difference. Moreover, only a few studies demonstrated reproducibility of measurements of their samples (9) and instead relied on the precision quoted by the manufacturer (10).
Because the validity of umbilical artery gas measurements has obvious importance for both clinical and medicolegal aspects, we undertook a standardized evaluation of the effect of time and temperature on umbilical cord blood gases and pH.
The effect of time and temperature on blood gas determination was studied in a series of 50 random cases. Blood (6 mL) drawn from the umbilical artery within 2 min of cord clamping was immediately transferred to six 2-mL plastic syringes that had been flushed with a 1000 units/mL heparin solution (each syringe contained <0.1 mL of residual solution). Residual air was ejected and the needle was capped. The three pairs of syringes made up the three study groups. The blood in two syringes was immediately analyzed; two syringes were kept in room temperature (20–24 °C); and two were kept in the refrigerator (4 °C), to be analyzed after 1 h. The double sampling was used to assess the reproducibility of the measurements, and the average value of the two samples was used to assess the effect of time (immediate assessment vs 1 h) and temperature (room temperature vs refrigeration).
Blood PO2, PCO2, and pH were determined by the Radiometer ABL 228, which measures these analytes directly. The data were analyzed by True Epistat statistic software. We used Student's paired t-test to examine four null hypotheses; P <0.05 was considered significant.
The first null hypothesis was that there was a difference between the
two samples of each group, caused by the methodology of blood sampling
and analysis. The data shown in Table 1
, however, indicate no significant differences between the two
samples; therefore, the rejection of the null hypothesis implies high
reproducibility of the method. The second null hypothesis suggested a
difference between samples tested immediately and those tested after
storage for 1 h in the refrigerator, caused by the effect of time
and temperature. The data shown in Table 1
indicate no significant
difference between the mean values of all analytes tested in both
groups. The third null hypothesis suggested a difference between
samples examined immediately and after storage of 1 h at room
temperature, caused by the effect of time. However, the data shown
reject this hypothesis and suggest that a period of 1 h has no
effect on the analytes tested. The fourth null hypothesis was that
temperature had an effect on the test results. The data shown in Table 1
also reject this hypothesis and suggest that temperature alone does
not affect the tested variables.
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There are almost no arguments against the value of umbilical artery blood gases analysis taken postpartum in cases indicated by ante- and intrapartum events. Because blood is a living tissue, it has been argued that metabolic changes will continue at room temperature, therefore making it necessary to examine the blood immediately or to minimize the metabolic changes by reducing the temperature (5). We carried out the present study to examine the validity of blood gas measurements under the different situations that may occur in a busy practice, where shortage of staff and equipment, compounded with a difficult delivery, may sometimes prevent immediate or appropriate storage/shipment of the blood sample to the laboratory. Our data indicate that a time of 1 h, alone or in combination with temperature (with or without refrigeration), does not significantly affect the results for pH, PCO2, and PO2 of arterial cord blood.
Our conclusions were similar to those found in previous studies. However, our study examined the whole range of blood gas parameters, in comparison with only selected parameters in other studies. Moreover, we used standardized sampling, i.e., blood taken into heparin-containing plastic syringes directly from the umbilical cord, whereas other studies examined blood taken with or without heparin, directly or indirectly from the cord or from a cut segment. Finally, our study is among the few that assessed reproducibility. All in all, our study seems to be among the most complete of its kind.
The results we found provide two important clinical conclusions. First, the clinical laboratory should accept an umbilical cord blood sample within 1 h after storage at room temperature. Second, although uniformity of sampling (using minimally preheparinized syringes) and shipment procedures are to be preferred, the clinician working in the busy delivery room where no one is available to optimally handle the blood sample can be reassured that if the blood sample is taken immediately, he or she does not need to arrange for immediate shipment or for special storage. As long as the blood sample is examined within 1 h, the results are still valid for both clinical and medicolegal purposes.
References
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