Human Milk: Composition, Clinical Benefits and Future Opportunities. Группа авторов

the nipple surface is the force responsible? The answer is likely to be an emphatic “No.” Any level of suction pressure applied outside the nipple surface (if this exceeds the positive milk pressure created by the mother’s MER), is likely to cause collapse of the teat openings. While suction can be transmitted through a fixed aperture, and propagated back along a rigid tube, this cannot occur in the flexible, collapsible milk duct system of the breast. Nipple duct opening, therefore, cannot be achieved from outside the nipple surface.

      Instead, this can only be achieved from within, by increasing intra-ductal pressure. This is precisely what the peristaltic tongue movements do. Having captured milk within the milk ducts held in the oral cavity, the peristaltic wave of compression squeezes this milk towards the nipple end; the resulting rise in intra-ductal pressure forces the milk duct ends open. Only when this has happened, might extra-ductal pressure (added intra-oral suction from an ETD) be capable of enhancing either the rate of milk extraction, or the net volume of milk transferred during that suck. The mechanism by which added suction is likely to achieve this is by extending the suck duration, potentially achieving more effective emptying of the ducts.

      From this perspective, not only are peristaltic tongue movements (PTMs) the obligate, primary tongue movement, present throughout active sucking, they also appear to be the primary mechanism by which milk is forced towards the duct openings, and out into the baby’s mouth. It may be deduced from this that the efficiency of such a mechanism will depend on the surface area of the nipple-breast “teat” complex lying against the baby’s tongue. In addition, the wider the baby’s mouth is flanged, the better will be its apposition to the breast; resulting in a greater mouthful of breast tissue being taken by the baby. Both these key features will be enhanced by maximising the “positioning” and “attachment” of the baby at the breast.

      A Final Piece of Evidence

One final piece of unique evidence comes from a historical study, not from any recent research. In the 1980s, Alan Lucas, then based at the John Radcliffe Hospital in Oxford, came up with the novel idea of measuring milk transfer from a mother to her baby directly, by placing a flow transducer between them, housed in the tip of a latex nipple shield [25]. The research team (Bio-Engineering Unit) developed a Doppler ultrasound flow transducer which insonated an area of parallel milk flow, created as breast milk passed through the transducer body. This technique provides completely unique views of instantaneous milk transfer during suckling [26]; I have been able to revisit a proportion of the original milk flow traces, undertaking some fresh analysis of them, in an attempt to resolve some the issues emerging from the “revised suckling physiology” above.

If, as Geddes et al. [11] assert, added suction (ETDs) is the predominant force in milk removal from the breast, then one would be likely to observe a “mid-suck” peak in milk flow, with relatively little milk flow either side of this. In practice, this is not the case – peak milk flow is invariably seen early in the suck cycle (first 20%), tailing off towards the end (Fig. 9). In many sucks, following an early high amplitude flow, a later more attenuated phase of irregular milk flow may be observed; this is most conspicuous in sucks of longer duration. These sucks are most likely to be those which include an ETD, which were shown by Eishima [7] to extend the suck duration.

      More commonly, the two phases of milk flow grade into each other, so first we have a high-amplitude, short-duration flow, followed by a lower-amplitude, longer-duration flow. The net contribution each of these makes to milk transfer may be the same, although it is important to remember that the secondary peak of milk flow may be absent in a large proportion of sucks. This novel source of information about milk flow during suckling suggests that baseline suction and PTMs are uniquely responsible for initiating and maintaining milk flow on each and every suck. When ETDs are superimposed on the incipient rhythm, they appear to enhance milk flow, mainly by sustaining it over a longer duration.

Fig. 9. Section of a milk flow trace obtained with a Doppler ultrasound flow transducer. Suck duration is shown in the box around each one, so, in terms of whether they are long (L) or short (S), this series of seven sucks is L-S-S-S-L-S-S.

      This unique insight into the process of milk transfer has been provided, not by new engineering-based models, but by a much earlier piece of research. Nonetheless, we have only recently been able to explain fully the complex shape of the milk flow profile in light of the evidence that both PTMs and ETDs coexist during breastfeeding, demonstrating that the baby both suckles and sucks milk from the breast.

      Clinical Implications of the “Revised Suckling Physiology”

Based on the evidence presented above, it is reassuring to learn that the standard tenets of good breastfeeding technique remain as true today as when they were first proposed [27, 28]. Enhancing the Positioning and Attachment of the baby at the breast will have the specific benefit of maximizing mouth:breast apposition. The greater the mouthful of breast tissue the baby takes, the further it will extend into the oral cavity, so providing a greater opportunity for the dorsum of the baby’s tongue to compress the underside of the teat. Explicitly, this will maximize the baby’s ability to amplify intraductal pressure during the suck, thereby boosting milk expression. Only when the duct ends have been opened by pressure from within, will added suction (ETDs) be capable of enhancing milk flow. No intervention has yet been identified which allows the level of suction that the baby produces to be modified (although the faster the rate of milk flow, the less suction pressure is likely to be applied). Accordingly, the basic tenets of Positioning and Attachment would apply here also.

      Disclosure Statement

      M.W.W. was remunerated for his participation in this workshop; his honorarium offsetting, in part, the time invested in preparation of this paper. The views expressed within it are entirely his own, and do not reflect those of any commercial sponsor.

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