Newsletter

Sign up for our quarterly newsletter and get the newest articles from acutecaretesting.org

Printed from acutecaretesting.org

Article

June 2002

Reference range evaluation for cord blood gas parameters

In conjunction with Apgar scores and other parameters, umbilical-cord blood gas values are used to assess newborn respiratory status. 

Paired umbilical-cord gas venous and arterial samples were collected from 200 patients to establish reference ranges for blood gas/hemoximetry parameters. Umbilical-cord samples were collected by double clamping a segment of umbilical cord before the delivery of the placenta. 

Samples were analyzed in duplicate immediately upon arrival in the laboratory. An Apgar score of 7 at 5 minutes was used as the limiting criteria for an infant with normal respiratory status for inclusion of data in reference range analysis.

INTRODUCTION

Blood gas values obtained immediately after delivery from the umbilical-cord blood have been noted as a valuable means to assess the respiratory and metabolic status of the neonate. 

It is important to establish the correct etiologic diagnosis for any observed respiratory symptoms. “Not every cyanotic, rapidly breathing infant has respiratory distress syndrome or even respiratory disease. 

Hypovolemia, hyperviscosity (polycythemia), anemia, hypoglycemia, congenital heart disease, hypothermia, metabolic acidosis of any etiology, or even the effects of drugs or drug withdrawal may all mimic primary respiratory disorders” [1].

Evaluation of the status of the newborn is best accomplished by assessing multiple criteria. Because the Apgar scoring system is considered subjective, it is not always accurate in the prediction of asphyxia. 

Umbilical-cord acid-base balance is a more effective means of identifying hypoxia and acidemia (typically defined as an umbilical arterial pH of less than 7.20) [2]. Gilstrap et al [3] have suggested that the newborn is not considered high risk resulting from asphyxia until the umbilical arterial pH is < 7.00 with an Apgar score of < 3 at both 1 minute and at 5 minutes.

Apgar scores are defined by rating the following parameters with a score of 0, 1, 2, respectively for each parameter based on the following criteria:

Heart Rate: 

0, <100 bpm, >100 bpm.

Respiratory Effort: 

None, Irregular, Regular

Tone: 

Limp, Moving, Vigorous 

Reflex Irritability:

None, Responsive/Grimacing, Regular Spontaneous Crying

Color: 

Blue/Cyanotic, Peripheral Cyanosis, Pink

There is a lack of consensus as to when umbilical-cord blood gases should be performed. Some institutions perform them on all deliveries. Recommendations were suggested based upon a study performed by Johnson and Riley in 1993 [4]. 

It suggests that blood be sampled from an umbilical artery in the following situations: 1) all premature newborns, 2) the presence of meconium-stained amniotic fluid, 3) operative vaginal or abdominal deliveries for non-reassuring fetal heart-rate pattern, and 4) term newborns who are depressed at birth or who have a 5-minute Apgar score below 7 [2].

METHODS AND MATERIALS 

Umbilical-cord venous and arterial samples were collected before delivery of the placenta by double clamping of a 20 cm section of the umbilical cord. A minimum of 1.5 mL blood was collected into heparinized syringes for each venous and arterial cord sample. Samples were iced and immediately transported to the laboratory.

200 paired (arterial and venous) cord samples were collected and analyzed in duplicate for standard blood gas and hemoximetry parameters on the Radiometer Medical A/S ABL505/OSM3 immediately after arrival in the laboratory. All samples were analyzed within 30 minutes of collection and within 5 minutes of arrival in the laboratory.

A minimum 5-minute Apgar score of 7 obtained by the delivery team was used as the limiting criteria for indication of an active, vigorous infant with normal respiratory status. 198 paired samples were included in the statistical analysis, omitting 2 paired samples from infants with Apgar scores below 7. 

Data was then differentiated as to the type of delivery, spontaneous vaginal delivery (SVD), n=143 or cesarean section (CS), n= 55. A mean and SD were calculated for each parameter for both arterial and venous samples. 

Data were then analyzed by Kruskal-Wallis analysis of variance and the F-test to determine if there were statistical differences between values obtained for vaginal deliveries versus cesarean sections.

Analyte

 

Arterial



UCBG



Mean ± SD

Arterial



Reference



Interval

Venous



UCBG



Mean ± SD

Venous



Reference



Interval

 pH  

 

7.281  



± 0.070  

 7.14  - 7.42  

7.331  



± 0.056  

7.22  



- 7.44

pCO2 

mmHg

56  ± 11.1

34 - 78  

47  ± 8.3  

30  - 63

pO2

mmHg

21  ± 9.0 

3 - 40

28  ± 7.7  

12  - 43

cHCO3-

mmol/L

25  ± 2.2

21 - 29

24  ± 2.0

20  - 28

ctCO2

mmol/L

27  ± 2.4

22  - 32  

25  ± 2.2  

21  - 29

ABE 

mmol/L 

-2.4  ± 2.4 

-7  to +2 

-2.1  ± 2.1  

-6  to +2

ctHb

g/dL  

15.2  ± 1.8  

11.6 - 18.8  

15.2  ± 1.9  

11.4  



- 19.0

FO2Hb 

38.2  ± 21.0

0 - 80  

58.4  ± 17.8 

23  - 94

FCOHb

%  

3.1  ± 1.2  

0.7  - 5.5  

3.8  ± 1.8  

0.2  - 7.3

FMetHb

%  

0.94  ± 0.2 

0.5  - 1.4  

0.93  ± 0.2  

0.5  - 1.4

ctO2

8.1  ± 4.5  

0  - 17  

12.3  ± 3.9  

5  - 20

 

TABLE I: Reference Intervals for Arterial and Venous Umbilical Cord Blood Gases (UCBG). Mean and SD values for all arterial and venous blood gas parameters. Reference Intervals based on ± 2SD limits. Apgar scores > 7 at 5 minutes.

RESULTS

Table I indicates the reference intervals that were established for our population of patients at Duke University Hospital. 

Previous studies performed by Helwig et al [5] established the following data based on 15,073 deliveries (data from spontaneous vaginal deliveries, operative vaginal deliveries, and cesarean sections). 

Umbilical Artery (UA) ranges were established based upon the following achieved UA means and SDs: pH = 7.26 ± 0.07, pCO2 = 53 mmHg ± 10, pO2 = 17 mmHg ± 6, base excess (BE) = –4 mEq/L ± 3. Umbilical Vein (UV) achieved means and SDs were: pH = 7.34 ± 0.06, pCO2 = 41 mmHg ± 7, pO2 = 29 mmHg ± 7, and BE = –3 mEq/L ± 3.

Analyte

 

Mean



(CS)

Mean



(SVD)

Kruskal-Wallis



F-Test



Probability

 pH  

 

7.263 

7.288 

p = 0.0077 ** 

p = 0.0245*

pCO2 

mmHg

60.38 

53.69 

p = 0.0000 ***  

p = 0.0001***

pO2

mmHg

17.46 

22.22 

p = 0.0000 ***  

p = 0.0007 ***

ctHb

g/dL

15.09 

15.25 

Not Significant 

Not Significant

FO2Hb 

28.05 

42.15 

p = 0.0000 ***  

p = 0.0000 ***

FCOHb

%  

2.98  

3.12 

Not Significant  

Not Significant

FMetHb

%  

0.961 

0.938 

Not Significant  

Not Significant

ctO2

5.87 

8.93 

p = 0.0000 *** 

p = 0.0000 ***

 

 

TABLE II: Statistical Comparison for CS vs. SVD Delivery – Arterial Blood. Means of sample population for Cesarean Section (CS) and Spontaneous Vaginal Delivery (SVD) for arterial umbilical cord samples and the probability of significant differences in all measured blood gas and hemoximetry analytes. Compiled data evaluated with Kruskal-Wallis Analysis of Variance and F - Test Statistical Tests.

*    indicates     p < 0.05

**  indicates     p < 0.01.

***indicates     p < 0.001

 

Table II depicts the differences observed for all measured parameters for both CS and SVD deliveries on UA blood samples. Statistical differences were observed for all parameters except ctHb (concentration of total hemoglobin) in g/dL, fraction of carboxyhemoglobin (FCOHb), and fraction of methemoglobin (FMetHb) using the Kruskal-Wallis Analysis of Variance and F-Test.

Analyte

 

Mean



(CS)

Mean



(SVD)

Kruskal-Wallis



F-Test



Probability

 pH  

 

7.310  

7.339  

p = 0.0038 ** 

p = 0.0014**

pCO2 

mmHg

50.97 

 44.86

p = 0.0000 *** 

p = 0.0000***

pO2

mmHg

24.06 

28.97 

p = 0.0002 *** 

p = 0.0000 ***

ctHb

g/dL

15.03 

15.28 

Not Significant

Not Significant

FO2Hb 

49.88  

61.61

p = 0.0003 *** 

p = 0.0000 ***

FCOHb

%  

3.90 

3.73 

Not Significant

Not Significant

FMetHb

%  

0.957

0.916 

Not Significant 

Not Significant

ctO2

10.23 

 13.11

p = 0.0000 *** 

p = 0.0000 ***

 

TABLE III: Statistical Comparison for CS vs. SVD Delivery – Venous Blood. Means of sample population for Cesarean Section (CS) and Spontaneous Vaginal Delivery (SVD) for venous umbilical cord samples and the probability of significant differences in all measured blood gas and hemoximetry analytes. Compiled data evaluated with Kruskal-Wallis Analysis of Variance and F - Test Statistical Tests.

*    indicates     p < 0.05

**  indicates     p < 0.01.

***indicates     p < 0.001

Table III depicts the differences observed for all measured parameters for both CS and SVD deliveries on UV blood samples. Again, statistical differences were observed for all parameters except ctHb, FCOHb, and FMetHb using the Kruskal-Wallis Analysis of Variance and F-Test.

Our data agree, in general, with reported values for the basic gas parameters. The hemoximetry data presented here is unique. It is worthwhile to note that the fraction of oxyhemoglobin (FO2Hb) and the oxygen content (ctO2) are significantly different for SVD versus CS in both UA and UV samples.

DISCUSSION

In order to apply the results of umbilical-cord blood gases to the clinical condition of the newborn fetus, it is important to understand the basics of the fetal circulation in-utero. 

The placenta acts both as “lungs” and “kidneys” for the fetus by supplying oxygen and removing carbon dioxide and metabolites [2]. It provides efficient gas exchange as well as allows nutritional substances such as vitamins, glucose, free fatty acids, and electrolytes to pass between the two circulations without allowing the two circulations to mix. 

The blood in the intervillous space is supplied by spiral arteries which provide a pulsatile flow of maternal blood. The blood flows through the intervillous space to be drained from the placenta by endometrial veins. 

The villi are supplied with fetal blood by branches of the umbilical arteries. Blood flows through the capillaries of the villi, where the gas and nutrient exchange occurs, and is collected in the branches of the umbilical vein to be returned to the fetal circulation.

The supply of oxygen is dependent on 1) adequate maternal oxygenation, 2) blood flow to the placenta and adequate transfer across the placenta, 3) fetal oxygenation and delivery to fetal tissues. The removal of carbon dioxide depends on fetal blood flow to the placenta and transport across the placenta [2]. 

When the fetus is unable to remove carbon dioxide from its tissues and excrete it across the placenta, the pCO2 increases resulting in respiratory acidosis. Also, when the exchange of oxygen across the placenta from the mother to baby is inadequate, anaerobic metabolism occurs resulting in lactic acid production and the development of metabolic acidosis [6]. 

If the oxygen supply continues to drop, the physiological compensatory mechanisms also begin to fail and this results in a reduced cardiac output [7].

Umbilical venous cord blood gas values are similar to the maternal intervillous oxygen and acid-base status because the oxygen and carbon dioxide can equilibrate between these two compartments, whereas umbilical arterial cord blood gases represent the fetal status [8]. Umbilical venous blood has higher pH, pO2, base excess and lower pCO2 than umbilical arterial blood [9].

Over the years, investigators have noticed differences between umbilical arterial versus venous cord blood gas values and have tried to predict outcomes based upon them. 

Belai et al [6] noted a strong interrelationship between umbilical arterial and venous pCO2 concentrations in a cohort of infants born with severe acidemia (pH <7.0). When the difference in umbilical arteriovenous pCO2 exceeded 25 mmHg, there was a significant increase in the incidence of seizures, hypoxic-ischemic encephalopathy, cardiopulmonary and renal dysfunction. 

Egan et al [9] noted ΔpH discordancy (arteriovenous pH differences of >0.12) from 53 neonates. The ΔpH discordant group not only had lower arterial pH, but also had a higher venous pH. A fetus with a discordant pH was more likely to be acidemic than one without discordancy. 

Their data suggested that vaginal delivery, cord compression, and normal placental reserve were the most common factors associated with ΔpH discordancy. 

In a retrospective study, Johnson and Richards [10] noted that in abruptio placentae and in other cases of reduced maternal oxygen delivery to the placenta due to reduced maternal cardiac output, increased uterine arterial resistance, and maternal hypoxia, there was very little difference in pH and oxygen saturation between umbilical arterial versus venous cord blood gas samples. 

On the other hand, large umbilical vein-umbilical artery pH and oxygen saturation differences were noted with reduced fetal perfusion of the placenta such as is found with umbilical-cord prolapse. In cord prolapse, umbilical-cord blood flow is significantly reduced, thereby reducing the rate of oxygen delivery to the fetal tissues.

CONCLUSION

Umbilical-cord blood acid-base analysis provides an objective means to evaluate a newborn’s condition especially with regard to hypoxia and acidemia [11]. 

Specimens for analysis should generally be obtained from the umbilical artery, not the umbilical vein. The umbilical artery contains blood returning from the fetus to the placenta and thus should provide the most useful information on the acid-base of the fetus [12]. 

Studies have demonstrated that the collection of both arterial and venous umbilical-cord samples may further help to determine the pathogenesis of marked fetal acidosis [6,9,10]. 

In accordance with good laboratory practice, it is important for each institution to establish or verify the reference ranges used for interpretation of umbilical cord gas values. 

As newer instrumentation now allows the ability to measure, not only blood gas and hemoximetry parameters, but also electrolytes and metabolites on very small sample aliquots, it will be important to develop reference ranges for all parameters on cord blood samples. This then provides us with new diagnostic tools to help predict fetal outcomes.

Acknowledgments

I would like to recognize Stuart Shelton, William Herbert, MD and Frank Sedor, PhD., Duke University Hospital, Durham, NC. Without their efforts, this reference range study could not have been completed.

References
  1. Martin RJ, Sosenko I, Bancalari E. Respiratory Problems. In: Care of the High-Risk Neonate. 5th ed. Philadelphia, PA: WB Saunders, 2001: 243-76.
  2. Gilstrap III LC. Fetal Acid-Base Balance. In: Maternal-Fetal Medicine. 4th ed. Philadelphia, PA: WB Saunders, 1999: 331-40.
  3. Gilstrap LC, Leveno JK, Burns J, Williams ML, Little BB. Diagnosis of birth asphyxia on the basis of fetal pH, Apgar score, and newborn cerebral dysfunction. Am J Obstet Gynecol 1989; 161: 825-30.
  4. Johnson JWC, Riley W. Cord blood gas studies: A survey. Clin Obstet Gynecol 1993; 36: 99.
  5. Helwig JT, Parer JT, Kilpatrick SJ, Laros RK. Umbilical cord blood acid-base state: What is normal?. Am J Obstet Gynecol 1996; 174,6: 1807-14.
  6. Belai YI, Goodwin TM, Durand M, Greenspoon JS, Paul RH, Walther FJ. Umbilical arteriovenous pO2 and pCO2 differences and neonatal morbidity in term infants with severe acidosis. Am J Obstet Gynecol 1998; 178: 13-19.
  7. Block BS, Schlafer DH, Wentworth RA, Kreitzer LA, Nathanielsz PW. Intrauterine asphyxia and the breakdown of physiologic circulatory compensation in fetal sheep. Am J Obstet Gynecol 1990; 162: 1325-31.
  8. Fujikura T, Yoshida J. Blood gas analysis of placental and uterine blood flow during cesarean delivery. Obstet Gynecol 1996; 87: 133-36.
  9. Egan JFX, Vintzileos AM, Campbell WA, et al. Arteriovenous cord blood pH discordancy in a high-risk population and its clinical significance. J Matern Fetal Med 1992; 1: 39-44.
  10. Johnson JWC, Richards DS. The etiology of fetal acidosis as determined by umbilical cord acid-base studies. Am J Obstet Gynecol 1997; 177: 274-82.
  11. American College of Obstetricians and Gynecologists (ACOG): Umbilical artery blood acid-base analysis. Technical Bulletin No. 216, November 1995b.
  12. American College of Obstetricians and Gynecologists (ACOG) Committee on Obstetric Practice: Use and abuse of the Apgar score. No. 174, July 1996.
+ View more
References
  1. Martin RJ, Sosenko I, Bancalari E. Respiratory Problems. In: Care of the High-Risk Neonate. 5th ed. Philadelphia, PA: WB Saunders, 2001: 243-76.
  2. Gilstrap III LC. Fetal Acid-Base Balance. In: Maternal-Fetal Medicine. 4th ed. Philadelphia, PA: WB Saunders, 1999: 331-40.
  3. Gilstrap LC, Leveno JK, Burns J, Williams ML, Little BB. Diagnosis of birth asphyxia on the basis of fetal pH, Apgar score, and newborn cerebral dysfunction. Am J Obstet Gynecol 1989; 161: 825-30.
  4. Johnson JWC, Riley W. Cord blood gas studies: A survey. Clin Obstet Gynecol 1993; 36: 99.
  5. Helwig JT, Parer JT, Kilpatrick SJ, Laros RK. Umbilical cord blood acid-base state: What is normal?. Am J Obstet Gynecol 1996; 174,6: 1807-14.
  6. Belai YI, Goodwin TM, Durand M, Greenspoon JS, Paul RH, Walther FJ. Umbilical arteriovenous pO2 and pCO2 differences and neonatal morbidity in term infants with severe acidosis. Am J Obstet Gynecol 1998; 178: 13-19.
  7. Block BS, Schlafer DH, Wentworth RA, Kreitzer LA, Nathanielsz PW. Intrauterine asphyxia and the breakdown of physiologic circulatory compensation in fetal sheep. Am J Obstet Gynecol 1990; 162: 1325-31.
  8. Fujikura T, Yoshida J. Blood gas analysis of placental and uterine blood flow during cesarean delivery. Obstet Gynecol 1996; 87: 133-36.
  9. Egan JFX, Vintzileos AM, Campbell WA, et al. Arteriovenous cord blood pH discordancy in a high-risk population and its clinical significance. J Matern Fetal Med 1992; 1: 39-44.
  10. Johnson JWC, Richards DS. The etiology of fetal acidosis as determined by umbilical cord acid-base studies. Am J Obstet Gynecol 1997; 177: 274-82.
  11. American College of Obstetricians and Gynecologists (ACOG): Umbilical artery blood acid-base analysis. Technical Bulletin No. 216, November 1995b.
  12. American College of Obstetricians and Gynecologists (ACOG) Committee on Obstetric Practice: Use and abuse of the Apgar score. No. 174, July 1996.
Disclaimer

May contain information that is not supported by performance and intended use claims of Radiometer's products. See also Legal info.

Barbara L. Fouse Barbara L. Fouse

 

Technical Manager, 
Clinical Pediatric Laboratory 
Duke University Hospital 
Durham, NC 
USA

Articles by this author
Acutecaretesting handbook

Acute care testing handbook

Get the acute care testing handbook

Your practical guide to critical parameters in acute care testing. 

Download now
Obstetrical practice

Related webinar

Cord blood gas analysis in obstetrical practice

Webinar presented by Jan Stener Jørgensen, MD PhD, Head of Obstetrics and Professor of Clinical Obstetrics, University of Southern Denmark

Watch the webinar
Webinar on demand: Evolution of blood gas testing - Part 1

Related webinar

Evolution of blood gas testing Part 1

Presented by Ellis Jacobs, PhD, Assoc. Professor of Pathology, NYU School of Medicine.

Watch the webinar
Webinar: Evolution of blood gas testing - Part 2

Related webinar

Evolution of blood gas testing Part 2

Presented by Ellis Jacobs, PhD, Assoc. Professor of Pathology, NYU School of Medicine.

Watch the webinar
Preanalytical errors handbook

Blood gas Preanalytics app

Get the Blood gas Preanalytics app for your smartphone

This smartphone app focuses on the preanalytical phase of blood gas testing and what operators can do to avoid errors.

Download app

Sign up for the Acute Care Testing newsletter

Sign up
About this site About Radiometer Contact us Legal notice Privacy policy
This site uses cookies Read more