1 Academic Departments of Obstetrics and Gynaecology, and 2 Anaesthetics, University College London (UCL), London and 3 Department of Anatomy, University of Cambridge, Cambridge, UK
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Abstract |
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Key words: acidbase balance/gases/in-vivo measurements/placenta/pregnancy
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Introduction |
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The aim of most of the early studies was to find amniotic fluid parameters that would reflect intrauterine fetal acidbase balance and could be used as an indicator of fetal asphyxia in utero (Vasika and Hutchinson, 1964; Johnson and Ojo, 1967
; Corson and Bolognese, 1968
). Often, until multipurpose automatic analysers were developed, only one parameter could be investigated at a time. We have used a multiparameter sensor and a monitor which can simultaneously measure in-vivo pH, carbon dioxide partial pressure (PCO2), oxygen partial pressure (PO2), bicarbonate concentration (HCO3), base excess, and oxygen saturation (O2Sat) and compared these data with those obtained by means of aspiration.
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Materials and methods |
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The Paratrend 7 (Diametrics Medical Inc., St Paul, MN, USA) is a monitoring system with automated sensor calibration. The Paratrend sensor (Figure 1) is a single, compact, 0.5 mm multiparameter sensor that combines electrochemical technology (Clark electrode) for PO2 measurement and fibre-optic technology to monitor pH and PCO2 (Clutton-Brock et al., 1994
; Hoffman et al., 1996
). Changes in hydrogen ion concentration produce colour changes in phenol red, which are detected by the pH fibre-optic elements. The CO2 sensor includes an ion impermeable barrier that excludes the movement of hydrogen ions but allows movement of CO2, which alters the local pH, producing a colour change in phenol red. The sensor also contains a thermocouple for determining temperature, and all measured gas and pH variables are corrected for temperature. The Paratrend monitoring system incorporates an alarm to inform the operator if the sensor is damaged.
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In each case, an 18 gauge needle was first introduced under ultrasound guidance into the amniotic cavity, near the placental edge. Amniotic fluid was aspirated in a 2 ml gas-tight syringe and immediately measured on the IRMA analyser. A calibrated multiparameter sensor was subsequently inserted through the needle into the amniotic cavity and measurements were started after 150 s. Gas and acidbase values were monitored for 5 min. The needle and the sensor were then removed from the amniotic cavity and reinserted within the decidua of the placental bed, in its central part, near the uterine muscle. The needle and sensor were kept in exactly the same position as assessed by continuous ultrasound guidance. Similar measurements were obtained as above. After 5 min the sensor was removed and 2 ml of uterine blood were aspirated through the needle and immediately analysed on the IRMA analyser.
The data were analysed with a biomedical processing statistics package (Statgraphics, Manugistics, Rockville, MA, USA). Standardized kurtosis was used to determine whether the samples derived from a normal distribution; as they were normally distributed, the data are presented as the mean and SEM. The difference in mean gas and acidbase values obtained with the sensors and with the cartridges was tested using Student's t-test. Results were considered statistically significant at P < 0.05.
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Results |
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Eleven series of matched measurements were available for evaluation. These measurements are presented and compared in Table I. The PO2 of the amniotic cavity and the PO2 and O2Sat in the placental bed measured by the cartridges were significantly higher than with the sensors. Significantly higher pH (P < 0.005) and lower PCO2 were also found in the placental bed with the cartridge system as compared with the Paratrend sensor system. The mean maximum variation in measurements over the 5 min of monitoring was 0.5 and 1% for pH, 2 and 6% for PCO2, and 8 and 10% for PO2 in the amniotic cavity and placental bed respectively.
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Discussion |
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There is little information on in-vivo fetal tissue or fluid gas measurements in the literature. The PO2 of amniotic fluid has been measured at term by inserting a PO2 needle electrode into the amniotic cavity (Vasika and Hutchinson, 1964). More recently, we have measured placental and endometrial PO2 in the first trimester of pregnancy using an umbilical artery polarographic electrode introduced transcervically into the uterine cavity under ultrasound guidance (Rodesch et al., 1992
). Using these electrodes, a wider range of PO2 measurements was obtained than in the present study.
The multipurpose Paratrend sensor has been validated and used mainly for continuous intra-arterial monitoring in intensive care units (Venkatesh et al., 1994). Recently this sensor has been evaluated in direct tissue measurements in order to provide information on the adequacy of brain tissue oxygenation in neurological patients at risk of ischaemia (Hoffman et al., 1996
). In human pregnancy, the adequacy of placental perfusion and fetal oxygenation has been indirectly evaluated using ultrasound Doppler investigation (Bower et al., 1993
) and directly by fetal blood sampling (Soothill et al., 1986
).
Measurement of amniotic or placental PO2, PCO2, pH, and bicarbonate may allow better assessment of substrate delivery, clearance, and metabolism than has hitherto been possible with any other method. However, due to the size of the sensor we do not anticipate that it will find many applications in fetal monitoring in utero. For practical reasons, we have evaluated this instrument early in pregnancy when there is limited direct access to the fetal circulation. A shorter and thinner version (Neotrend; Diametrics Medical, Inc.) of the original sensor, which can be introduced via an umbilical artery catheter, is currently being evaluated in neonatology. This new sensor, which can be inserted through a 20-gauge needle, could be placed in the placenta and eventually in late gestation into the umbilical vein, where it can remain in place for several hours and be used to monitor continuing pregnancies. This may improve the management of fetal hypoxia and our understanding of the pathophysiology of placental-related disorders of pregnancy, such as fetal growth restriction and pregnancy-induced hypertension. As a research tool, we are currently investigating other applications using an animal model.
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Acknowledgments |
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Notes |
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References |
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Submitted on June 2, 1999; accepted on August 13, 1999.