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Re: Cord blood pHFrom: ainsron (ainsron@sbcglobal.net)Fri May 21 17:17:06 2004
It should be umbilical artery and pH <7.0 is indicative of possible fetal asphyxia. This table is from an article relating pH to seizure and neonatal encephalopathy. I don't have the name of the article off the top of my head, it's in a powerpoint presentation I'm preparing for a talk. pH* No. Seizures NND ICU 7.10-7.14 798 2 (0.3%) 3 (0.4%) 7 (0.9%) 7.05-7.09 290 0 0 4 (1.4%) 7.00-7.04 95 3 (3.2%) 2 (2.1%) 5 (5.3%) <7.00 87 9 (10.6%) 7 (8.2%) 17 (20.5%) This is the discussion from the article: As the limitations of the Apgar score in this regard became clear, there was hope that umbilical pH, a more objective if indirect measure of fetal oxygenation, would allow accurate prediction of permanent neurologic injury, either alone or in conjunction with the Apgar score. One team followed 925 infants from birth to 1 year of age. They found that 111 (12%) had a pH below 7.16 and 32 (3%) had a 5-minute Apgar score below 8. Only 12 of the 111 with acidemia had a low Apgar score and only 12 (41%) of 29 with low Apgar scores had acidemia; three had Apgar scores of 0 to 3, but only one was acidemic (pH 7.11); six had pH values below 7.00, but three of these had Apgar scores of 5 to 7. The authors examined the ability of a low Apgar score or umbilical artery acidemia at birth to identify the 42 infants who, at 1 year of age, had some disability possibly related to intrapartum asphyxia. The positive predictive value of a low Apgar score was 19%, while that of metabolic acidemia was 8%. Taken together, a low Apgar score and acidemia had a predictive value of only 27%. Very similar results were reported by another investigative team. The Apgar score was found to be a poor predictor of acidemia and acidemia, in turn, a poor predictor of the Apgar score; both, even taken together, were poor predictors of subsequent neurologic outcome. Why was this so? The reason some infants with a normal pH may have a depressed Apgar score is readily apparent when one considers the factors listed in Table 1 <http://obgyn.pdr.net/be_core/g/templates/issue/internallinks.jsp?filename=/ be_core/content/journals/g/data/1997/g6a/g6a080.html#G6A080-.-01> . Despite their low Apgar scores, such infants are not at risk for neurologic sequelae due to asphyxia. Whenever low Apgar scores are assigned, a careful investigation for any of these factors should be undertaken. The converse of this situation, the finding of a low umbilical artery pH with a normal Apgar score, is often explained by rapid accumulation of CO2, the so-called fetal respiratory acidosis. Although pH is a log function, accumulation of CO2 due to interruption of gas exchange (e.g., from cord occlusion) results in a fall in pH that can be roughly estimated by the linear equation of a 10-mm increase in pCO2 with a 0.1 drop in pH. The rate of change of fetal pH due to this mechanism can be very rapid during the course of labor. With repeated scalp sampling, it has been shown that a single deep variable deceleration can be associated with a rise in pCO2 of 10 mm. Such a change in pCO2 could result in a drop of pH from 7.25 to 7.15. Infants born with umbilical artery respiratory acidemia are not at marked risk for asphyxial neurologic sequelae. If umbilical pH is to be measured, the pCO2 should always be assessed as well. Less commonly, the finding of umbilical acidemia in a vigorous infant may be due to the effect of maternal acidemia on the fetal circulation. Ronald E. Ainsworth -----Original Message----- From: ob-gyn-l@obgyn.net [mailto:ob-gyn-l@obgyn.net] On Behalf Of Steve & Eryl Raymond Sent: Friday, May 21, 2004 11:59 AM To: Multiple recipients of list OB-GYN-L Subject: Cord blood pH We have been donated an Astrup machine which has fifty tests per cartridge. These cartridges expire quickly and up till now we have been using only about ten percent of the available tests. To make better use of the cartridges we want to get cord blood pH done on babies from caesars for fetal distress, but the paediatricians are saying that it must be done on the umbilical artery blood. What is the general experience of those who are doing these regularly regarding where the sample should be taken from, what pH level range is normal and is there a significant difference whether taken from artery or vein? Anybody care to give us the benefit of their experience? Steve Raymond
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