c Early Human Development 42 (M’S) 67-71

Umbilical cord blood gas a~a~~s~s; a c liwkml of two simple methods of sampIe stooge

Philip Owen*a, Thomas A. Far&la, Wih&n §teynb uDepartment of Obstetrics and Gynaecology, h&tewds Hospitd and Me&al S&o/,

Duttake, scotkmd, UK bPerhtatal Mortality Research Unit. Department of obstetrics und Gymecology,

University of Stektbosch, Tygerberg, South Africa

Received 22 December 1994, revision received 23 February 1995; aaxpted 3 March 1995

Kcyworuk Blood-@ analysis; Umbilical pH

1. mmktbn

The limitations of the Apgar score [ I] in the ammmcnt of the neonate’s acid-base status at birth and as an index of subsequent morbidity (438) has mu&ed in m&r interest in wnbikal cord blood gas analysis. Cord mmpktg at birth is an objective estimate of the babies’ condition and is more pmdktivc of advise warolagical sequehe amongst those experiencing in- hypoxia [I I]. Fwtkmore, the presence of normal blood gas values effectively excludes intrapartum hypoxia as a

l Corresponding author, Tel.: +44 1382 60111; Fax: +44 1382 5666J7.

0378-3782/9%‘509.50 0 1995 Elsevkr Science Ireland Ltd. All rights mrvcd SSDI 0378-3782(95)01640-O

68 P. Owen et al. /Early Human Development 42 (1995) 67-71

cause of neonatal depression which has clinical implications at the time of delivery and possibly later medico-legal implications. For these and other reasons routine umbilical blood gas analysis at all deliveries has been proposed [9,19] and has recent- ly been adopted in our department.

The sampling and subsequent processing of umbilical vessel blood samples is a simple matter but during episodes of high clinical activity there may be a period of delay before the sample can be processed. Several methods of sample storage have been described to minimise time dependent changes including storage on ice [13]. The simplest and most convenient two techniques are to draw. the sample into a heparinized plastic syringe, leaving at room temperature or clamping a segment of cord with disposable umbilical cord clamps, subsequently processing a sample from the cord segment. Concern that such a delay may result in changes in blood gas values such that they no longer represent the situation at delivery prompted us to perform this study, comparing the validity of the two simplest methods of sample storage described above.

2. Materials and methods

Long segments of umbilical cord were immediately obtained from 28 otherwise unselected deliveries in our labour suite. A sample was drawn from the umbilical vein into a 2-d syringe containing 0.2 ml of heparin 1000 u/ml and capped. Blood gas analysis was performed from this sample on an ABL 55 blood gas analyser (Radiometer, Copenhagen), these results referred to as Time Zero. These initial anal- yses were always made within 4 mm of delivery. The remaining cord was divided into three 5-cm long segments with disposable umbilical cord clamps and kept at room temperature. Subsequent umbilical vein blood gas analysis was performed both from the original 2-ml sample, capped and stored at room temperature and also from a segment of cord via a series of l-ml syringes each containing 0.1 ml of heparin 1000 u/ml at 10, 20 and 30 min after the original sample at Time Zero. Results were obtained for pH, base deficit, Po2 and PCOZ. The paired t-test was used to compare the values obtained at 10, 20 and 30 min with the reference Time iero values. Statistical significance was taken at the P < 0.05 level.

3. Results

The mean values for pH, base deficit, PO* and PCO~ at lO-min intervals over 30 ruin for both syringe and cord segment samples are presented in Table 1. Significant changes are seen by 10 mm in the cord sampies whereas the syringe samples’ mean pH is significantly different from 20 min onwards. The base deficit is significantly greater in the cord segment samples at 10 mm but otherwise no significant changes are seen in either cord or syringe samples over 30 min. The non-linear changes in pH and base deficit seen in the umbilical cord most likely represent continuing meta- bolic activity within the umbilical vein. Values for Po2 show no significant changes over time in both syringe and cord samples. A significant rise in PCO~ values is seen in the syringe samples from 20 min but no significant changes in PCO, were seen in the cord segment samples.

P. Owen et al. /Early Human Development 42 (1995) 67-71 69

T&k1 Mean and standard deviation values for syringe samples at Time Zero, cord and syringe samples at IO, 2Oand3Omin

Time

Zero 10 min 20 min 30 min

PH Syringe 7.27 (0.05) 7.325 (0.06) Cord 7.317, (0.05)

BD (nlmd/l) Syringe 5.221 (2.7) 5.257 (2.4) Cord 6.064’ (3.2)

b Wa) syringe 3.59 (0.9) 3.6 (0.9) cord 3.63 (0.9)

PC% (kPa) Syringe 5.12 (0.9) 5.17 (0.8) Cord 5.06 (1.1)

BD, be. debit. *Significant difference at P < 0.05 (paired t-test).

7.32’ (0.07) 7.318. (0.06) 7.32* (0.06) 7.32 (0.06)

5.31 (2.4) 5.55 (2.4) 5.379 (2.4) 6.04 (2.8)

3.63 (0.9) 3.79 (1) 3.65 (I) 3.7 (1)

5.22, (0.8) 5.2; (0.84) 5.16 (I) 5.04 (I)

4.

Simplifyi any clinical task is more likely to result in its adoption into routine practice and eliminating the necessity for special storage requirements in the event of there being a delay in processing is clearly desirable, providing that the values obtained remain representative of the situation at birth.

Continuing glycolysis in leukocytes and diffusion of 02 and COz are princip&y respotible for the physidogical changes seen in cord blood va tlcassary [15]. Sykes and Molloy [17j described smab but changes in blood gas values from samples stored on ice in a re period but no significant changes in pH or poZ were seen from at room temperature for up to 1 h. Nhan et al. 1121, in ice concluded that blood gas changes could not be p Strickland et al. [16] described a linear pH decrease versus time in 105 arterial and venous samples stored in a syringe at room temperature for up to 2 h. Sirs6 Sharif et al. [14J described a mean fall in pH of 0.029 and a rise in base deficit of 1.85 mmol/l after 30 min in arterial samples kept in heparinized plastic syringes at room temperature. More recently, Chauhan et al. [3] have devised a math~ati~~ model for the retrospective, selective evaluation of acid-base status from a sample stored on ice for up to 60 h following delivery. Selective acid-base analysis is par- ticularly attractive when economical considerations are important ahhough we have found that the incorporation of blood-gas analysis into routine midwifery care requires minimal additional instruction and resources.

As an alternative to storing the sample in a syringe, a segment of umbihcal cord can be double clamped and subsequently sampled at a more convenient time. Hilger

70 P. Owen et al. /Early Human Development 42 (1995) 67-7/

et al. [6] found no significant changes in umbilical vein values for pH, Pco;! and Paz over 1 h when sampling from a cord still attached to its placenta but did not estimate base deficit. When sampling the umbilical artery from cord segments kept at room temperature, Duerbeck et al. [5] found no statistically significant changes in any of the blood gas values over 1 h. Their technique involved sampling with non- heparinized syringes which is not appropriate when using the analyser in our unit. A comparison between cord and syringe samples stored at room temperature has been made by Bartlett et al. [2] although base deficit was not estimated and the two small groups of samples were taken from different subjects. Since the rate of decrease of pH depends on the initial pH of the blood [12] then a true comparison necessitates the use of the same subjects in both sample groups.

Care is required when using heparin in the preparation of samples for blood gas analysis since high concentrations of heparin can affect the values obtained resulting in methodological errors and misinterpretation of results in the clinical setting [8,10]. For this reason, we were careful to use a weak solution of heparin and to maintain the same concentration in both syringe and cord segment samples.

The variety of results obtained from sampling different umbilical vessels and employing varying techniques reported by the authors of previous studies prompted us to perform this study, which we believe is the first to directly compare the reproducibility of pH, base deficit, Po2 and PCO* values against time for the two most convenient techniques of sample storage. The umbilical vein was chosen be- cause it is easier to sample reliably than the artery and as such is less likely to give rise to spurious results. Although arterial blood gas values will more closely reflect the metabolic condition of the fetus [19], venous pH performs equally as well in predicting neonatal neurological morbidity [7].

Despite demonstrating statistically significant changes in blood gas values over time, these are of a small magnitude and unlikely to be of clinical significance in a routine setting. However, when precision is important and delay in processing unavoidable, then keeping the sample in a syringe will result in values of pH and base deficit (the most important in clinical practice) most closely reflecting the values present at birth. Storage of samples in the cord is to be preferred when PCO, and POZ values are considered important since no significant changes are seen over 30 min. Whether this holds true for more acidotic samples obtained after complicated deliveries and for arterial specimens is the subject of further study.

Acluwwledgement

Dr Patrick Chien, Senior Registrar, Ninewells Hospital for statistical advice.

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