Umbilical blood flow in healthy newborn infants during the first minutes after birth

Cord Management of the Term Newborn

Keeping the umbilical cord intact after delivery facilitates transition from fetal to neonatal circulation and allows a placental transfusion of a considerable amount of blood. A delay of at least 3 minutes improves neurodevelopmental outcomes in term infants. Although regarded as common sense and practiced by many midwives, implementation of delayed cord clamping into practice has been unduly slow, partly because of beliefs regarding theoretic risks of jaundice and lack of understanding regarding the long-term benefits. This article provides arguments for delaying cord clamping for a minimum of 3 minutes.

Use of Placental/Umbilical Blood Sampling for Neonatal Admission Blood Cultures: Benefits, Challenges, and Strategies for Implementation

Placental blood remains an underused resource for early neonatal care despite ample evidence that placental blood provides the same clinical decision making information without the need for painful, invasive blood sampling procedures. Potential benefits of placental/umbilical blood sampling (PUBS) for neonatal admission labs include decreases in pain reactivity, rates of anemia, need for blood transfusions, use of vasopressors, and rates of intraventricular hemorrhage. Here, we present a unique case study of a critically ill infant with contradictory blood culture results from PUBS and direct infant sampling. A negative admission direct sample blood culture result compared with a positive admission PUBS blood culture result suggests that infection may have been missed in the direct infant sample. Relevant placental embryology and circulation is also described, as well as the benefits of PUBS for neonatal admission labs (with focus on the blood culture), challenges associated with PUBS practice, and strategies for implementation of PUBS.

A physiologic approach to cord clamping: Clinical issues

Background

Recent experimental physiology data and a large, population-based observational study have changed umbilical cord clamping from a strictly time-based construct to a more complex equilibrium involving circulatory changes and the onset of respirations in the newly born infant. However, available evidence is not yet sufficient to optimize the management of umbilical cord clamping.

Findings

Current guidelines vary in their recommendations and lack advice for clinicians who face practical dilemmas in the delivery room. This review examines the evidence around physiological outcomes of delayed cord clamping and cord milking vs. immediate cord clamping. Gaps in the existing evidence are highlighted, including the optimal time to clamp the cord and the interventions that should be performed before clamping in infants who fail to establish spontaneous respirations or are severely asphyxiated, as well as those who breathe spontaneously.

Conclusion

Behavioral and technological changes informed by further research are needed to promote adoption and safe practice of physiologic cord clamping.

Measuring physiological changes during the transition to life after birth

The transition to life after birth is characterized by major physiological changes in respiratory and hemodynamic function, which are predominantly initiated by breathing at birth and clamping of the umbilical cord. Lung aeration leads to the establishment of functional residual capacity, allowing pulmonary gas exchange to commence. This triggers a significant decrease in pulmonary vascular resistance, consequently increasing pulmonary blood flow and cardiac venous return. Clamping the umbilical cord also contributes to these hemodynamic changes by altering the cardiac preload and increasing peripheral systemic vascular resistance. The resulting changes in systemic and pulmonary circulation influence blood flow through both the oval foramen and ductus arteriosus. This eventually leads to closure of these structures and the separation of the pulmonary and systemic circulations. Most of our knowledge on human neonatal transition is based on human (fetal) data from the 1970s and extrapolation from animal studies. However, there is renewed interest in performing measurements directly at birth. By using less cumbersome techniques (and probably more accurate), our previous understanding of the physiological transition at birth is challenged, as well as the causes and consequences for when this transition fails to progress. This review will provide an overview of physiological measurements of the respiratory and hemodynamic transition at birth. Also, it will give a perspective on some of the upcoming technological advances in physiological measurements of neonatal transition in infants who are unable to make the transition without support.

Amino acid metabolism in the human fetus at term: leucine, valine, and methionine kinetics

Human fetal metabolism is largely unexplored. Understanding how a healthy fetus achieves its fast growth rates could eventually play a pivotal role in improving future nutritional strategies for premature infants. To quantify specific fetal amino acid kinetics, eight healthy pregnant women received before elective cesarean section at term, continuous stable isotope infusions of the essential amino acids [1-13C,15N]leucine, [U-13C5]valine, and [1-13C]methionine. Umbilical blood was collected after birth and analyzed for enrichments and concentrations using mass spectrometry techniques. Fetuses showed considerable leucine, valine, and methionine uptake and high turnover rates. α-Ketoisocaproate, but not α-ketoisovalerate (the leucine and valine ketoacids, respectively), was transported at net rate from the fetus to the placenta. Especially, leucine and valine data suggested high oxidation rates, up to half of net uptake. This was supported by relatively low α-ketoisocaproate reamination rates to leucine. Our data suggest high protein breakdown and synthesis rates, comparable with, or even slightly higher than in premature infants. The relatively large uptakes of total leucine and valine carbon also suggest high fetal oxidation rates of these essential branched chain amino acids.

Nuchal Cord Management and Nurse-Midwifery Practice

Nuchal cord, or cord around the neck of an infant at birth, is a common finding that has implications for labor, management at birth, and subsequent neonatal status. A nuchal cord occurs in 20% to 30% of births. All obstetric providers need to learn management techniques to handle the birth of an infant with a nuchal cord. Management of a nuchal cord can vary from clamping the cord immediately after the birth of the head and before the shoulders to not clamping at all, depending on the provider's learned practices. Evidence for specific management techniques is lacking. Cutting the umbilical cord before birth is an intervention that has been associated with hypovolemia, anemia, shock, hypoxic-ischemic encephalopathy, and cerebral palsy. This article proposes use of the somersault maneuver followed by delayed cord clamping for management of nuchal cord at birth and presents a new rationale based on the available current evidence.

Human fetal amino acid metabolism at term gestation

BACKGROUND

Knowledge on human fetal amino acid (AA) metabolism, largely lacking thus far, is pivotal in improving nutritional strategies for prematurely born infants. Phenylalanine kinetics is of special interest as is debate as to whether neonates will adequately hydroxylate phenylalanine to the semiessential AA tyrosine.

OBJECTIVE

Our aim was to quantify human fetal phenylalanine and tyrosine metabolism.

DESIGN

Eight fasted, healthy, pregnant women undergoing elective cesarean delivery at term received primed continuous stable-isotope infusions of [1-(13)C]phenylalanine and [ring-D(4)]tyrosine starting before surgery. Umbilical blood flow was measured by ultrasound. Maternal and umbilical cord blood was collected and analyzed by gas chromatography-mass spectrometry for phenylalanine and tyrosine enrichments and concentrations. Data are expressed as medians (25th-75th percentile).

RESULTS

Women were in a catabolic state for which net fetal AA uptake was responsible for > or = 25%. Maternal and fetal hydroxylation rates were 2.6 (2.2-2.9) and 7.5 (6.2-15.5) micromol phenylalanine/(kg . h), respectively. Fetal protein synthesis rates were higher than breakdown rates: 92 (84-116) and 73 (68-87) micromol phenylalanine/(kg . h), respectively, which indicated an anabolic state. The median metabolized fraction of available phenylalanine and tyrosine in the fetus was <20% for both AAs.

CONCLUSIONS

At term gestation, fetuses still show considerable net AA uptake and AA accretion [converted to tissue approximately 12 g/(kg . d)]. The low metabolic uptake (AA usage) implies a very large nutritional reserve capacity of nutrients delivered through the umbilical cord. Fetuses at term are quite capable of hydroxylating phenylalanine to tyrosine.

Delayed cord clamping in very preterm infants reduces the incidence of intraventricular hemorrhage and late-onset sepsis: a randomized, controlled trial

OBJECTIVE

This study compared the effects of immediate (ICC) and delayed (DCC) cord clamping on very low birth weight (VLBW) infants on 2 primary variables: bronchopulmonary dysplasia (BPD) and suspected necrotizing enterocolitis (SNEC). Other outcome variables were late-onset sepsis (LOS) and intraventricular hemorrhage (IVH).

STUDY DESIGN

This was a randomized, controlled unmasked trial in which women in labor with singleton fetuses <32 weeks' gestation were randomly assigned to ICC (cord clamped at 5-10 seconds) or DCC (30-45 seconds) groups. Women were excluded for the following reasons: their obstetrician refused to participate, major congenital anomalies, multiple gestations, intent to withhold care, severe maternal illnesses, placenta abruption or previa, or rapid delivery after admission.

RESULTS

Seventy-two mother/infant pairs were randomized. Infants in the ICC and DCC groups weighed 1151 and 1175 g, and mean gestational ages were 28.2 and 28.3 weeks, respectively. Analyses revealed no difference in maternal and infant demographic, clinical, and safety variables. There were no differences in the incidence of our primary outcomes (BPD and suspected NEC). However, significant differences were found between the ICC and DCC groups in the rates of IVH and LOS. Two of the 23 male infants in the DCC group had IVH versus 8 of the 19 in the ICC group. No cases of sepsis occurred in the 23 boys in the DCC group, whereas 6 of the 19 boys in the ICC group had confirmed sepsis. There was a trend toward higher initial hematocrit in the infants in the DCC group.

CONCLUSIONS

Delayed cord clamping seems to protect VLBW infants from IVH and LOS, especially for male infants.

Physiology of the fetal circulation

Our understanding of fetal circulatory physiology is based on experimental animal data, and this continues to be an important source of new insight into developmental mechanisms. A growing number of human studies have investigated the human physiology, with results that are similar but not identical to those from animal studies. It is time to appreciate these differences and base more of our clinical approach on human physiology. Accordingly, the present review focuses on distributional patterns and adaptational mechanisms that were mainly discovered by human studies. These include cardiac output, pulmonary and placental circulation, fetal brain and liver, venous return to the heart, and the fetal shunts (ductus venosus, foramen ovale and ductus arteriosus). Placental compromise induces a set of adaptational and compensational mechanisms reflecting the plasticity of the developing circulation, with both short- and long-term implications. Some of these aspects have become part of the clinical physiology of today with consequences for surveillance and treatment.

Reference ranges for umbilical vein blood flow in the second half of pregnancy based on longitudinal data

OBJECTIVES

To construct new reference ranges for serial measurements of umbilical vein (UV) blood flow.

METHODS

Prospective longitudinal study of blood flow velocities and diameter of the UV measured at four-weekly intervals during 19 to 42 weeks' gestation in 130 low-risk singleton pregnancies. Regression models and multilevel modeling were used to construct the reference ranges.

RESULTS

On the basis of 511 sets of longitudinal observations, we established new reference percentiles of UV diameter, blood flow velocities, volume flow, and blood flow normalized for fetal weight and abdominal circumference. They reflected some of the developmental patterns of previous cross-sectional studies, but with important differences, particularly near term. The UV blood flow showed a continuous increase until term, whereas the flow normalized per unit fetal weight, a corresponding reduction. Calculating the blood flow on the basis of intensity-weighted mean velocity or 0.5 of the maximum velocity gave almost interchangeable results for most fetuses.

CONCLUSION

New reference ranges for UV blood flow based on longitudinal observations appear slightly different from cross-sectional studies, and should be more appropriate for serial evaluation of fetal circulation.

The fetal circulation

Accumulating data on the human fetal circulation shows the similarity to the experimental animal physiology, but with important differences. The human fetus seems to circulate less blood through the placenta, shunt less through the ductus venosus and foramen ovale, but direct more blood through the lungs than the fetal sheep. However, there are substantial individual variations and the pattern changes with gestational age. The normalised umbilical blood flow decreases with gestational age, and, at 28 to 32 weeks, a new level of development seems to be reached. At this stage, the shunting through the ductus venosus and the foramen ovale reaches a minimum, and the flow through the lungs a maximum. The ductus venosus and foramen ovale are functionally closely related and represent an important distributional unit for the venous return. The left portal branch represents a venous watershed, and, similarly, the isthmus aorta an arterial watershed. Thus, the fetal central circulation is a very flexible and adaptive circulatory system. The responses to increased afterload, hypoxaemia and acidaemia in the human fetus are equivalent to those found in animal studies: increased ductus venosus and foramen ovale shunting, increased impedance in the lungs, reduced impedance in the brain, increasingly reversed flow in the aortic isthmus and a more prominent coronary blood flow.

Neonatal transitional physiology: a new paradigm

Early clamping of the umbilical cord at birth, a practice developed without adequate evidence, causes neonatal blood volume to vary 25% to 40%. Such a massive change occurs at no other time in one's life without serious consequences, even death. Early cord clamping may impede a successful transition and contribute to hypovolemic and hypoxic damage in vulnerable newborns. The authors present a model for neonatal transition based on and driven by adequate blood volume rather than by respiratory effort to demonstrate how neonatal transition most likely occurs at a normal physiologic birth.

Umbilical cord clamping: beliefs and practices of American nurse-midwives

The optimal time for umbilical cord clamping after birth remains a critical unknown fact that has implications for the infant, the mother, and science. A national survey was conducted using a randomized sample (n = 303) of the active membership of the ACNM to determine cord clamping practices and beliefs of American nurse-midwives. The response rate was 56%. The respondents fell into three cord clamping categories: early (EC) or before 1 minute (26%); intermediate (IC) or 1 to 3 minutes (35%); and late (LC) or after pulsations cease (33%). The EC group believes that early clamping facilitates management of the newborn. The IC group believes that a moderate delay of clamping allows for a gradual transition to extrauterine circulation, although many think that the timing of cord clamping is not significant. The LC group have strongly held beliefs that late clamping supports physiologic birth processes. The majority of CNMs (87%) place the baby on the mother's abdomen immediately after birth and 96% avoid clamping a nuchal cord whenever possible. Although Varney's Midwifery was cited most frequently as a reference, 78% of the respondents listed no references reflecting, in part, the absence of evidence-based recommendations for cord clamping practices.

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.

Pharmacodynamic study of maturation and closure of human umbilical arteries

The contractile effects of 19 factors on isolated human arterial segments at term pregnancy were quantified, and 14 contractile agents were similarly applied to preterm (23 to 35 weeks) umbilical arteries. Responses to potassium chloride were used to normalize the data. At comparison with the term vessel, the preterm artery contracted more to angiotensin II and arachidonic acid and was more sensitive to oxytocin. Contractions were greater in term arteries to vasopressin, norepinephrine, prostaglandin D2, and prostaglandin E2 but similar in both group of arteries to bradykinin, histamine, acetylcholine, and prostaglandin F2 alpha. Neuropeptide Y, linoleic acid, uridine triphosphate, and thrombin were ineffective. Hyperoxia inconsistently induced weak, short-lived contractions. Contractions to cooling manifested marked desensitization and tachyphylaxis. Serotonin was the only agonist that displayed the pharmacodynamic features most likely to be important for closure: potency, efficacy, and long duration of action (greater than 2.5 hours). It was postulated that cellular elements surrounding umbilical vessels are primary sources of vasoactive agents that are important to closure of the fetoplacental circulation at birth.

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.

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.

Umbilical circulation--physiology and pathology

1. The umbilical circulation under physiological conditions is protected by the amniotic fluid. This protective mechanism of the amniotic fluid may be disturbed artificially or spontaneously by the rupture of the membranes. 2. Umbilical cord compression is of no harm to the fetus when it is mild, since the fetus possesses a circulatory buffer system when umbilical blood flow is in a physiological range. However, severe reduction of umbilical blood flow may lead to fetal hypoxia. 3. Chronic or repetitive acute fetal hypoxia leads to deterioration of fetal circulation and umbilical blood flow. This can be recognized by the obstetrician during labor based on fetal heart rate patterns.

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.

Plasma amino acid concentrations in umbilical cord vein and artery of newborn infants after elective cesarean section or spontaneous delivery

Twenty-four infants delivered by elective cesarean section and 23 spontaneously born infants were studied for plasma amino acid concentrations of the umbilical cord vein and artery. Normal labor was not found to influence plasma amino acid concentrations. In both spontaneously and operatively born infants, similar venous-arterial gradients were found for all essential and five nonessential amino acids. Based on the differences of venous and arterial concentrations and the umbilical cord blood flow, the daily fetal retention of single amino acids was calculated.

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.

Thermodilution measurement of human umbilical blood flow at delivery

A local thermal dilution technique was used to estimate blood flow in the human umbilical vein during the first minute after delivery of the neonate. Estimates of flow were attempted at 101 deliveries and thermal dilution curves obtained at 52. The mean estimation of all flows recorded up to 20 seconds after delivery was 171 ml. per minute reducing to 58 ml. per minute between 40 and 60 seconds.

Effects and interactions of oxygen and prostaglandins on the tone of the isolated human umbilical artery

Prostaglandin E2 (PGE2) and prostaglandin F2alpha (PGF2alpha) were found to be equipotent contractors of the isolated human umbilical artery (HUA) in the concentration range 0.2-40 mug/ml. Prostaglandin E1 (PGE1) relaxed HUA at 0.1-3.0 mug/ml, whereas at 10-50 mug/ml contraction occurred. PGE2 was significantly more potent at a PO2 of 102 mmHg than at 27, 48 and above 400 mmHg. An increase in PO2 per se (27- greater than 400 mmHg) resulted occasionally in minor increases in the tone of the HUA. Such effects of oxygen had a lag period of 10-15 min. It is suggested that an increased formation of prostaglandins in the umbilical artery at birth is a more likely cause of the closure of the vessel than an increase in PO2.