Fetal Circulation Adaptation Following Birth PDF

Why is fetal circulation different? Fetal oxygen source: placenta (rather than lungs)  connects the maternal and fetal circulatory systems  the fetus receives oxygen (and nutrients) from the placenta and the placenta removes metabolic wastes and carbon dioxide from the fetus Temporary structures...

Why is fetal circulation different? Fetal oxygen source: placenta (rather than lungs)  connects the maternal and fetal circulatory systems  the fetus receives oxygen (and nutrients) from the placenta and the placenta removes metabolic wastes and carbon dioxide from the fetus Temporary structures in the fetal circulatory pathways allow the blood to bypass the lungs and pass directly to and from the placenta via the blood vessels in the umbilical cord By 3 weeks gestation, the fetal cardiovascular system has started to develop Essential adaption to ensure intrauterine survival 1 Temporary fetal circulation structures Ductus venosus - oxygenated blood is directed from the umbilical vein directly to the inferior vena cava Two hypogastric arteries - direct deoxygenated blood from the lower extremities back through the umbilical arteries to the placenta Foramen ovale - situated in the septum between the atria, through which blood flows to bypass the pulmonary circulation Ductus arteriosus - between the pulmonary arteries and the aorta, which further prevents blood flows into the pulmonary circulation Image reproduced from Coad et al 2020 2 Fetal Circulation Image reproduced from Coad et al 2020 DD/Month/ Professor/Dr: Topic title: 3 YYYY  Fetal blood The placenta is less efficient at respiration than mature lungs Fetal blood is modified to ensure adequate oxygenation: Larger red blood cells More red blood cells with higher hb content Shorter lifespan - fetal haemoglobin (HbF) lives for 90 days HbF carries oxygen at lower pressures Towards the end of pregnancy, HbF begins to be replaced by adult haemoglobin (HbA) Reproduced from: Anatomy and Physiology in Healthcare by Marshall et al. ISBN: 97819079042958 © Scion Publishing Ltd, 2017 4 Respiratory system in-utero At term, the acinar portion of the fetal lungs is well developed >25% of true alveoli are present The lungs hold ~25 ml/kg of pulmonary fluid From 10 weeks gestation, the fetus carries out breathing movements which may encourage muscle development, particularly the diaphragm Surfactant – a surface-active lipoprotein From 24 weeks gestation, secretion of surfactantby alveolar type II pneumocytes (increasing in amounts from 32 weeks onwards) Composed of phospholipids and protein molecules Coats the internal lining, reducing surface tension Allows easier initial lung expansion at birth and aids collapse and expansion as the lungs commence continuous use as an organ of gaseous exchange Reproduced from Rankin, 2017 DD/Month/ Professor/Dr: Topic title: 5 YYYY Neonatal adaptation at birth – onset of respirations Pulmonary lung fluid - onset of labour, hormonal surges cause secreting cells to switch from secretion to absorption; ~25% partially expelled during birth; remaining fluid is absorbed by the lymphatic and pulmonary vessels and returned into the baby’s blood stream Most neonates gasp within the first few seconds of birth and establish regular respirations within minutes Factors that initiate first breath and lung expansion - Compression of chest wall and immediate recoil during and after birth - Chemoreceptor stimulation by ↓ O2 and ↑ CO2 in blood - Sensory stimuli on the skin – touch, pressure, low environment temperature - Stimulation of the senses by light, noise and touch Inflation of a normal lung in a term neonate is completed within the first few breathes, and most alveoli are expanded within the first few hours, establishing a lung volume of ~25 ml/kg body weight Respiratory centre (in the medulla oblongata) - regulates respiratory effort to metabolic needs Reproduced from DD/Month/ Professor/Dr: Topic title: https://www.gettyimages.co.uk/photos 6 YYYY Neonatal adaptation at birth – establishment of neonatal circulation At birth, blood needs to fully perfuse the lungs so fetal vascular structures cease. May not be rapid or immediate – initiated in 60s but may not be fully completed for a few weeks Separation of the neonate from placental circulation – cessation of blood flow = collapse of the umbilical vein and arteries. The hypogastric arteries and ductus venosus gradually fibrose, giving rise to supporting ligaments Results in reduced blood flow to right atrium and systemic vascular resistance increases – improved venous and arterial returns to the heart and lungs. Increased pressure in left atrium causes foramen ovale to close. Initial equalising of the pressures in the two atria holds the flap of foramen ovale in position First breath –causes lung expansion and vasodilation of the pulmonary vessels & reduced pulmonary vascular resistance = increased blood flow to the lungs Amount of blood to ductus arteriosus decreases, and as oxygen tension in blood rises, ductus arteriosus constricts, eventually closing DD/Month/ Professor/Dr: Topic title: 7 YYYY Neonatal haematological changes Fetal-placental blood volume varies from ~110- 130ml/kg Circulating blood volume at term averages 85- 90ml/kg of body weight (Sinha et al 2012) Delayed vs immediate cord clamping.. Haemopoiesis (production of blood cells and platelets) proceeds at a steady pace and highly responsive Within the first week, HbF is gradually replaced by HbA Neonatal hb ranges 150-235 g/l and reduces to ~120g/l by 3 months Reproduced from Jennifer Mason Photography - Ann Arbor, Michigan Birth Photographer, Doula, Birth DD/Month/ Professor/Dr: Topic title: Film, Maternity Photographer 8 YYYY Delayed cord clamping Delaying cord clamping until after onset of breathing: allows placental blood to contribute to the volume of blood required during alveolar capillary bed expansion more gradual increase in systemic resistance Enabling: smoother CV transition to extra-uterine life physiological transfer of placental blood volume from to the baby – placental transfusion (~ 80-100ml of blood – including iron) DD/Month/ Professor/Dr: Topic title: 9 YYYY Assessment of the baby at birth – APGAR score APGAR Score can be useful in facilitating a structured assessment of extrauterine adaptation following birth – looking at the baby’s condition, and level of alertness Assessment made at one minute, 5 mins and 10 mins of age Quick and simple to carry out, however subjective and inter/intra-observer variability and classification based on white babies If the Apgar score is above 7, little intervention is required, but a baby with an Apgar score of