Thermodilution measurement of human umbilical blood flow at delivery

Intiation of lung ventilation at birth

The newborn baby draws its first postnatal breath either during or within seconds of delivery. Within minutes, a regular breathing rhythm is established and this remains virtually continuous for the remainder of postnatal life. The mechanisms responsible for these sudden, dramatic and vital changes in the respiratory system at birth are only partially understood. Since fetuses make intermittent breathing movements long before birth, understanding the control of the fetal respiratory system may be essential to understanding the rapid onset of respiratory efforts at delivery. In this review the stimuli present at birth will be considered and, based on our current understanding of the fetal and neonatal respiratory control systems, those factors which are likely to play an important role in the initiation of lung ventilation at this time will be examined. Normal respiratory events in the early postnatal period will be concentrated on, but it is important to recognize that in some cases problems occur: a neonate may fail to initiate breathing efforts rapidly, or an apparently healthy premature neonate may suddenly stop breathing a few days after birth. In both cases, clinical intervention may be required to maintain adequate gas exchange and to prevent brain damage or death. Clearly, greater knowledge of respiratory control during this critical time of life would assist in the development of more appropriate and successful treatments for these life-threatening disorders.

Application of ultrasonic transit time flowmetry to the measurement of umbilical vein blood flow at caesarean section

OBJECTIVES

To determine the applicability of ultrasonic transit time flowmetry to the measurement at caesarean section of umbilical vein blood flow rate and to examine the relationship between flow rates and birthweight for gestational age.

DESIGN

Umbilical vein blood flow was measured at caesarean section using a transonic time flow probe on a loop of the umbilical cord in 33 appropriate and 21 small for gestational age fetuses.

RESULTS

The mean (SD) umbilical vein blood flow in the 54 fetuses was 78.4 (23.1) ml kg-1 min-1. There was a linear relation between umbilical vein blood flow measured by ultrasonic transit time flowmetry and birthweight (r = 0.63, P < 0.0001). The mean umbilical vein blood flow in appropriate for gestational age fetuses [90 (18) ml kg-1 min-1] was greater than that in the small for gestational age group [66 (23) ml kg-1 min-1], P < 0.04).

CONCLUSIONS

Umbilical vein blood flow measurements obtained by the ultrasonic transit time flowmetry technique are simple to perform and compare well with reported values obtained by the Doppler ultrasound technique (when vessel diameter is greater than 4 mm). Umbilical venous blood flow rate is significantly lower in small for gestational age fetuses.

Blood flow in obstetrics using Doppler ultrasound

We describe the history of obstetric blood flow research in the human fetal circulation (1978-1982) using both continuous wave (CW) Doppler and pulsed (PW) Doppler with linear array scanning. After the initial work (mainly in Ireland, Australia, Sweden, The Netherlands and the UK) it was recognized that CW Doppler velocity waveforms were of diagnostic significance and yielded as much useful clinical information as more complex PW duplex systems. PW duplex continues to be a useful research tool where accurate Doppler sampling or mean velocity measurement is required. The measurement of fetal Doppler velocity waveforms is clinically useful in the evaluation of the 'small-for-dates' fetus and abnormalities of cardiac anatomy or rhythm. Simple CW pencil probe study of the maternal utero-placental vasculature distinguishes normal from abnormal placentation and may be useful as a screening test to predict hypertensive disease of pregnancy.

Fetal and uteroplacental haemodynamics during short-term atenolol treatment of hypertension in pregnancy

Fetal circulation was studied by means of combined real-time and pulsed Doppler ultrasound in 14 women with pregnancy-associated hypertension before and during the first and third days of treatment with the beta 1-selective blocker, atenolol; in seven of the women the maternal uterine arcuate blood velocity waveform was also studied. Blood flow characteristics were normal both in the fetus and in the maternal arcuate artery, compared with those in uncomplicated pregnancies of corresponding gestational ages. Volume blood flow remained unchanged in the fetal descending aorta, and in the umbilical vein during atenolol treatment, whereas the pulsatility index increased in the fetal descending aorta and in the arcuate artery. This suggests that the peripheral vascular resistance, both on the maternal and fetal side of the placenta, increased during short-term antihypertensive treatment with atenolol.

Fetal central blood circulation in the third trimester of normal pregnancy--a longitudinal study. II. Aortic blood velocity waveform

Waveform of the maximum blood velocity recorded from the fetal descending thoracic and abdominal aorta was analyzed in a longitudinal study on 21 normal pregnancies from the 27th gestational week till term. Measurements of blood velocity were performed with combined real-time linear array and 2 MHz pulsed Doppler ultrasound technique every second week. The rising (RS) and the descending (DS) slopes did not change with gestational age in the thoracic descending aorta. The peak maximum velocity, the pulsatility index (PI) and the acceleration time percentage were constant during the last trimester of pregnancy, in both the thoracic and the abdominal aorta. In the former, the mean RS was 29.9 (S.D. 4.9), mean DS 5.3 (S.D. 0.9), mean peak velocity 115.6 cm X s-1 (S.D. 19.0), mean PI 1.96 (S.P. 0.31) and the mean acceleration time percentage 19.2% (S.D. 2.2). In the abdominal aorta, the mean RS was 25.7 (S.D. 5.6), mean DS 4.5 (S.D. 0.9), mean peak velocity 99.7 cm X s-1 (S.D. 18.8), mean PI 1.68 (S.D. 0.28) and the mean acceleration time percentage 19.1% (S.D. 2.2). The PI and the DS, considered to reflect mainly peripheral vascular resistance, were mutually related (r = 0.72). The acceleration time percentage, RS and PI did not show any relation to fetal heart rate or gestational age which justifies the use of gestational age-independent reference values for those parameters during the last 3 months of gestation, at least within the normal fetal heart rate range.

Ultrasonic assessment of cardiac function in the intact human fetus

Echocardiography may provide information concerning the structural development of the fetal human heart. The information available from such studies, when interpreted in light of existing knowledge of developmental cardiac physiology, may give insight concerning in utero cardiac pump function. The quantitation of cardiac structural growth in utero has been used to provide growth curves for the chambers of the fetal heart. Disparity in the ratio of right ventricular/left ventricular size (normally 1.0 to 1.2) may suggest acute ventricular failure and dilation. Electromechanical analysis using M-mode techniques may be used to analyze and monitor the treatment of fetal cardiac arrhythmias and provide systolic time interval analysis. Doppler waveform analysis provides information concerning directional flow and vascular impedance and Doppler flowmetry has promise for the measurement of fetal aortic and umbilical venous blood flows. Such studies have increased our understanding of fetal circulatory function and have practical implications for the establishment of fetal cardiac diagnosis and treatment programs.

Blood flow measurements in the fetal descending aorta: technique and clinics

A combination of two-dimensional and realtime pulsed-Doppler ultrasound provides a noninvasive method of measuring human fetal blood flow without side effects. By not altering the physiological conditions of the fetus, it minimizes external stimuli that might affect blood flow. However, due to the inaccessibility of the vessels under investigation, errors from the ultrasound technique arise and these are still being assessed. Studies of fetal blood flow suggest that the fetal circulation has a low peripheral resistance and that the increase in blood flow found with increasing gestational age is due predominantly to the increase in the actual dimensions of the fetal vasculature. Investigations in abnormal pregnancies, such as small-for-dates and those with cardiac arrhythmias have shown that the fetal cardiovascular system is capable of compensating efficiently to maintain normal physiological conditions, but only within the limits defined by the Frank Starling mechanism.

Umbilical venous flow in normal and complicated pregnancy

Using pulsed Doppler and B-mode ultrasonic techniques, umbilical venous flow has been measured for the first time under essentially normal physiological conditions. In normal pregnancies, the flow per unit fetal weight remains essentially constant at 110-120 ml/min/kg for most of the pregnancy. In pregnancies with complications, however, abnormally low or high flow values are frequently observed. Low flow values correlate strongly with retarded fetal growth, and with increased incidences of antenatal hypoxia, neonatal morbidity and neonatal death. In some circumstances high flow values suggest the presence of a compensatory mechanism. The results reported here suggest that umbilical flow can be used to separate all fetuses, whether growth retarded or not, into "low risk" and "high risk" groups with better sensitivity and accuracy than existing methods. In addition, low flow values have been measured an average of one week before growth retardation or fetal hypoxia were indicated by the conventional methods. A possible strategy for the diagnostic use of umbilical flow measurements is outlined.

Ultrasonic measurement of human fetal blood flow

Pulsed Doppler technique was combined with real-time B-mode ultrasonography for non-invasive measurement of human fetal blood flow in utero. The fetal vessel, e.g. aorta or umbilical vein, was identified in the real-time image and the real-time transducer put parallel to the vessel. The 2 MHz pulsed Doppler transducer insonated the vessel at a known angle, as the transducers were firmly attached to each other at an angle of 52 degrees. The Doppler instrument processed the Doppler shift signals and estimated the mean and maximum blood velocity. For calculation of blood flow, fetal vessel diameter must be measured. Three ultrasonic techniques (real-time B-mode, TM-mode, Time-distance recording) were applied for diameter measurements. The accuracy of the real-time B-mode technique was tested in vitro on 8 glass tubes with various diameters; the ultrasonically measured diameter was within +/- 0.4 mm of the diameter measured with vernier calipers. The reproducibility of diameter measurement was tested in vivo; the standard deviation of the difference between measurement was 0.15 mm for the fetal aorta and 0.20 mm for the umbilical vein. Fetal blood flow was measured in 38 normal late pregnancies. Blood flow in the thoracic part of the descending fetal aorta ws 185 ml min-1 kg-1 when based on mean blood velocity, and 261 ml min-1 kg-1 when based on maximum blood velocity. In the intra-abdominal part of the umbilical vein the mean blood flow was 115 ml min-1 kg-1.

Fetal umbilical venous flow measured in utero by pulsed Doppler and B-mode ultrasound. I. Normal pregnancies

Umbilical venous blood flow was measured with a pulsed Doppler unit, which was used in combination with a B-mode ultrasonic imaging system to permit location of the umbilical vein and measurement of its cross-sectional area. The accuracy and limitations of the method are discussed. Forty-seven normal fetuses with gestational ages ranging from 22 weeks to term were studied on a total of 61 occasions. Flow increased with gestational age until 36 weeks, was maximal between 37 and 38 weeks, then decreased during the last 2 weeks of pregnancy. Flow per unit of fetal weight was constant during pregnancy until 36 to 37 weeks, when a reduction occurred.

Measurement of human fetal blood flow

Real-time B-mode ultrasonography was combined with a pulsed Doppler ultrasound technique for transcutaneous measurement of human fetal blood flow in the aorta and intra-abdominal part of the umbilical vein. The target vessel was located and its diameter measured in the two-dimensional real-time image. The pulsed Doppler transducer was attached to the real-time transducer at a fixed angle. By processing the Doppler shift signals the instrument estimated the mean and maximum blood velocities and the integral under the velocity curves. This permitted calculation of the blood flow. The method was applied to 26 fetuses in normal late pregnancies. Mean blood flow in the descending part of the fetal aorta based on maximum velocity was 191 ml/kg/min. Mean flow in the intra-abdominal part of the umbilical vein was 110 ml/kg/min. This method of measurement is non-invasive and opens new perspectives in studying fetal haemodynamics.

Fetal blood velocity waveforms

In this paper a method is described for obtaining and characterizing fetal blood velocity waveforms. The signals were recorded with a range-gated Doppler instrument and characterized after spectral analysis. Preliminary observations indicate differences in the waveforms obtained during normal pregnancies compared with some complicated pregnancies.

Contribution of maternal circulation to blood-borne progesterone in the fetus. I. Studies on human subjects

Tritiated progesterone was infused intravenously at a constant rate into three pregnant volunteers in labor, for at least two hours before delivery. Blood samples from a maternal peripheral vein and from unbilical vessels were taken at birth. Comparison of the specific activities of progesterone in these samples indicated that 10 per cent or less of the hormone in fetal circulation is derived from transfer of maternally circulating progesterone. After consideration of reported values of umbilical vein blood flow at term and measured arteriovenous differences in concentrations of progesterone in umbilical vessels, the secretion rate of the placental hormone toward the fetus was estimated to be about 1/10 of the rate of secretion of progesterone toward the maternal circulation. About 1 per cent of the maternally circulating hormone was found to cross the placenta.

Non-invasive measurement of human fetal circulation using ultrasound: a new method

We combined two ultrasound techniques to develop a safe, non-invasive, transcutaneous method of observing the circulation in the umbilical arteries and vein in the fetus. The umbilical cord can be located by standard echo ultrasound procedures, and this information can be used to direct a Doppler ultrasound beam on to the vessels in the cord. The signals can be heard through audio headphones or recorded on a tape recorded and spectrum-analysed. The method was successful in each of 20 patients examined, whose pregnancies ranged from 12 to 40 weeks' gestational age, and was suitable for outpatient use. It should be useful in assessing such conditions as pre-eclampsia and intrauterine growth retardation.

Qualitative estimation of blood velocity changes in human umbilical arteries after delivery

Qualitative doppler shift ultrasound estimations of blood velocity were made from umbilical arteries of 17 neonates for 90 sec after delivery using a 10 MHz doppler instrument. The mean maximum doppler shift frequency at 10 sec was 1.5 kHz +/- 0.3 (SE) reducing to 0.5 +/- 0.1 kHz at 60 sec after delivery. This indicates that flow velocity cannot be quantified. The initial mean pulsatility index was 10.6 suggesting that either constrictions were occurring in the arteries by 10 sec after delivery or that the placenta has a high impedance or both.