Delaying cord clamping until respiration is well established is a physiologically pleasing approach, and avoids the dramatic decrease in left ventricular preload, simultaneously with an increase in afterload that occurs with early clamping. But does delaying clamping during positive pressure ventilation have the same physiologic benefits? I had previously thought that the decrease in intra-thoracic pressure associated with an inspiratory effort might increase placental venous return, and that therefore positive pressure ventilation with the cord intact might not have the same benefits. That is probably wrong, although the differential effects of delayed cord clamping with spontaneous respiration and assisted ventilation are not clear to me.
What is clear is that positive pressure ventilation decreases pulmonary vascular resistance, although surprisingly, we don’t really understand the mechanism. Stuart Hooper’s group has done much of this work and in one fascinating study (Lang JA, et al. Increase in pulmonary blood flow at birth: role of oxygen and lung aeration. J Physiol. 2016;594(5):1389-98), they showed that positive pressure ventilation of one lung with nitrogen, causes improved lung perfusion, of BOTH lungs. This was a study in near term fetal rabbits who were instrumented during partial cesarean delivery with the cord intact, but the actual procedures and images were taken after cutting the cord. So it tell us about the physiology of PVR reduction during positive pressure ventilation, but not about other aspects of delayed clamping and ventilation.
The fetal rabbit kits were ventilated unilaterally in the right lung with nitrogen or air or oxygen, then unilaterally with air (1LV2) then the tube was pulled back to ventilate both lungs with air. This is one selected image from the publication, showing the number of vessels that were seen in each lung, and that ventilating the right lung increased perfusion of both lungs, ventilating the right lung with oxygen increased perfusion further, especially of the right lung.
When you are doing physiologic studies in animals it is difficult to ensure that the animals make respiratory efforts reliably at the right moment, so most studies are about positive pressure ventilation. I guess in some ways it is less important for the future of delayed cord clamping what happens during spontaneous respiration, as it has become the standard of care to clamp the cord after at least one minute, if the baby is breathing. The responses to clamping before or after initiating PPV are of more relevance for the decision that we are still considering, whether we should routinely initiate PPV prior to cord clamping in depressed babies. The recent studies have not suggested any reason to me why we should clamp before PPV, if that is technically, logistically possible. In cases of an increased risk of needing PPV, I think the recent trial from Melbourne shows that it is not too difficult to get organized to do this. See the comment on my previous post from Doug Blank. (I’ve never linked to a comment before, hope that works).
I was thinking, based on the physiology, and the BabyDUCC trial, that we should all prepare to perform the initial steps of resuscitation during “natural cord management”. But hang on, what about an alternative… cord milking? (see next post!)