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In her paper, Gardiner (1) describes “the most dramatic changes in loading occur at birth when there is a sudden increase in distal impedance associated with removal of the placental circulation, a six fold increase in pulmonary blood flow leading to a rise in left at...
In her paper, Gardiner (1) describes “the most dramatic changes in loading occur at birth when there is a sudden increase in distal impedance associated with removal of the placental circulation, a six fold increase in pulmonary blood flow leading to a rise in left atrial pressure and reversal or cessation of flow through the oval foramen, and a more gradual closure of the arterial duct. This new serial arrangement of the circulation demands that both ventricles are suddenly able to receive and eject the entire cardiac output that was previously shared between them.” (our emphases). She is correct in describing the effect on the newborn heart of the usual transition from fetal to adult pattern circulation that occurs in the majority of hospital births in the UK at present. However,, this is NOT a description of the natural or physiological fetal to neonatal transition but of a ‘routine’ and possibly unjustified intervention. After early cord clamping, there is indeed a sudden removal of the placental circulation leading to a sudden increase in distal impedance. By contrast, in a physiological birth the first event is for the newborn baby to initiate ventilation. This results in the marked fall in the impedance of the pulmonary circulation. Most of the output of the right ventricle is now directed into the pulmonary artery and leads to a reduced flow through the arterial duct. None of these changes are sudden although they are largely completed within a few minutes of birth. Oxygenated blood now returns to the left ventricle and is directed into the aorta and umbilical arteries. The high oxygen level of this blood together with substances released from the newly expanded lungs constricts the umbilical artery. The systemic blood pressure, which would otherwise fall with the reduced contribution from the arterial duct, is thus maintained by a slow closure of the umbilical and placental circulation. As the placental circulation closes down and the pulmonary circulation opens up there need be no significant change in the afterload of either ventricle.
The new serial arrangements of the heart require both sides of the heart to pump out precisely the same volume. Until this happens the shunts of the arterial duct and the oval duct allow compensation of any slight differences in flow. In the term fetus the output of the right ventricle is slightly higher (250ml/minute/kg) than the left (200mls/min/kg). With the shunts in place the circulations work in parallel and allow the outputs to change without any marked changes in tissue flow. In the fetus the cardiac output is therefore described as the total output from both ventricles as the combined cardiac output (CCO). However, in the adult circulation, with the two sides of the heart working in series, the output from the left is the same as the output from the right. The cardiac output is now described as the output from one ventricle and in the newborn will be about 225mls/min/kg. There is no sudden change and certainly nothing like the doubling as suggested.
The “sudden” changes associated with removal of the placental circulation described in Gardiner's paper are the result of applying a cord clamp to a functional placental circulation; an effect which is exaggerated if the clamp is applied before the baby breathes and the pulmonary circulation has become well established. Applying the clamp throws a large afterload onto both ventricles and it is unnecessary. As the author points out, it may result in ventricular damage (among other effects) from which the recovery is incomplete. Immediate cord clamping is standard practice in most obstetric units. Its lack of apparent harm in the majority of neonates has prevented recognition of the evidence base that it does cause injury in both term (2) and preterm babies.(3)
Fetal and neonatal cardiology or cardiothoracic specialists need to be aware that midwives, obstetricians and neonatologists may require specific guidance about the management of the third stage of labour, particularly when a baby has an identified cardiac anomaly that might benefit from a gradual placental transfusion (or no placental transfusion). Plans for the management of the third stage should be clearly made and documented in maternal notes in advance.
David J R Hutchon FRCOG,
Consultant Obsttetrician and Gynaecologist,
Darlington. DL3 6HX
Dr Susan Bewley MA MD FRCOG,
Consultant Obstetrician/ Maternal Fetal Medicine
Guy's & St Thomas' NHS Foundation Trust
Women's Services, 10th Floor North Wing
St Thomas' Hospital
Westminster Bridge Rd
London SE1 7NH
1. Gardiner HM. Response of the heart to changes in load: from hyperplasia to heart failure. Heart 2005 91(7) 871-873
2 McDonald SJ, Middleton P Effect of timing of umbilical cord clamping of term infants on maternal and neonatal outcomes. Cochrane Database of Systematic Reviews 2008, Issue 2. Art. No.: CD004074. DOI: 10.1002/14651858.CD004074.pub2
3. Rabe H, Reynolds G, Diaz-Rossello J. A Systematic Review and Meta-Analysis of a Brief Delay in Clamping the Umbilical Cord of Preterm Infants. Neonatology 2008;93:138-144