Physiolgical transition ?
We congratulate the authors on this study of neonatal transitional circulation performed so quickly after birth. The authors state that the ductal flow ratio reported in their study reflects pulmonary and haemodynamic transition and can be used to monitor neonatal transition in healthy infants. The implication is that their study describes a physiological transition in healthy term infants, but we question that this is the case.
Delivery by elective caesarean section is not a physiological birth but it does permit the neonate an atraumatic birth. We are particularly concerned that the transition may have been disrupted by the timing of cord clamping which was between 30 and 60 seconds. While this is considered delayed cord clamping by some, most guidelines recommend a minimum of 60 seconds have elapsed before the circulation is interrupted by cord clamping. The WHO advises 3 minutes.(1) Not all of the babies in the series had established respiration before clamping and cord clamping before the onset of respiration has a marked effect on cardiac output.(2) It is therefore questionable that the mean and range of results published represents a normal transition in healthy infants.
The normal fetal circulation is well described with the two ventricles pumping in parallel, the right ventricular output being significantly more than the left and the flow across two shunts, right to left in the ductus arteriousus and right to left across the foramen ovale. The normal neonatal circulation is also well described with equal outputs by both ventricles and closed shunts. A closer look at the results suggest some anomalous flow not readily compatible with the end result of the neonatal circulation. For example at ten minutes the mean right ventricular output is 343 mls/kg/min and mean left ventricular output is 212 mls/kg/min and the DA flow is 8mls/kg/min left to right. (ratio R to L =0.9). Where does the right ventricle get the 343 output as only 212 - 8 are reaching the systemic circulation and returning to the right atrium. Also if the pulmonary blood flow is 343+8 = 351, the excess of 141 must be spilling left to right across the foramen ovale. Reverse flow in the foramen ovale is described but has not been quantified. We question that these flows are evidence that this heart is now close to transitioning to a parallel ventricular pattern with equal outputs. Reverse flow across the foramen ovale of 141 is contributing to over 40% of the right ventricular output. This blood has just been through the pulmonary circulation and is oxygenated but is now returning to the right ventricle to be pumped through the pulmonary circulaton once more. Such flow is completely inefficient. Can the authors explain this anomaly in physiogical terms or could this finding in fact be a demonstration that the early clamping at under 60 seconds sometimes before the onset of respiration has indeed disrupted the transitional circulation ?
Reference 1. World Health Organization (WHO), US Agency for International Development (US AID), Maternal and Child Health Integrated Program (MCHIP). (2013). Delayed cord clamping of the umbilical cord to reduce infant anaemia. Updated 2013. Available from: http://www.mchip.net/node/1562 (Accessed June 12, 2014). 2. Bhatt S, Alison BJ,Wallace EM, Crossley KJ, Gill AW, Kluckow M, et al. Delaying cord clamping until ventilation onset improves cardiovascular function at birth in preterm lambs. J Physiol 2013 591(Pt 8): 2113-26.
Conflict of Interest:
I am a co-inventer of the LifeStart trolley