Human Milk: Composition, Clinical Benefits and Future Opportunities. Группа авторов
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Название: Human Milk: Composition, Clinical Benefits and Future Opportunities
Автор: Группа авторов
Издательство: Ingram
Жанр: Медицина
Серия: Nestlé Nutrition Institute Workshop Series
isbn: 9783318063417
isbn:
Within the baby’s mouth, creating the other side of the active pressure gradient, is (4) intense negative suction pressure created by the baby cyclically lowering the rear surface of its tongue. This is responsible for generating baseline suction pressure which both draws the breast into the baby’s mouth and retains it throughout the feed. This force is unstable, however, as any milk issuing from the breast into the oral cavity negates it, making it necessary for it to be reapplied in a cyclical manner throughout feeding.
These four principal forces are necessary and sufficient for a breast pump (electric or manual) to create adequate milk removal from the breast. The cyclical application of negative suction pressure at the nipple surface (aided by the three other factors) is adequate for sustaining milk collection from the breast.
The two unique features which the baby brings to the process are: (5) the compressive action of the baby’s jaws (and gums), and (6) PTMs applying retrograde waves of positive pressure to the underside of nipple surface. The peristaltic action of the baby’s tongue is obligate, playing the primary role in both milk transfer and expelling the milk bolus into the oro-pharynx for swallowing. The action of these two forces alone is not dissimilar to hand expression of the breast, which requires no negative suction pressure to remove milk. The fingers press into the breast at the base of the ducts, in a similar action to the baby’s jaws, and the opposed fingers are drawn towards the nipple end to express milk; this emulates the peristaltic action of the baby’s tongue. The role of the baby’s jaws should never be overlooked, as they effectively “gate” the release of milk, letting it enter the milk ducts, lying within the nipple/breast teat complex, in packaged bundles, rather than as a continuous outflow of milk from the breast.
Accordingly, there are two sets of forces – the first four alone are capable of milk extraction, and are employed specifically when a breast pump is used to extract milk. The next two forces alone are capable of milk expression from the breast, in the absence of the other forces. They are akin to “pump extraction” and “hand expression” and use entirely different modes of action, yet both are effective for expressing/extracting milk from the breast (just as “hand milking” and “machine milking” are equally effective with dairy animals). The essential beauty of mammalian suckling is that these separate forces combine in the baby’s mouth, and are most likely to be acting synergistically to remove milk in the most efficient way possible (the same is true of feeding by most dairy animals studied (sheep and goats) [15], and by pigs [16]).
Fig. 1. Example of output from sensors. Right: 60 s selection of the signals acquired during an experimental test. Left: zoom of a single burst and extracted parameters [17].
Two of these forces are well illustrated in a recent study by Grassi et al. [17] who fitted two sensors to a pacifier, one monitoring positive pressure from the jaws and gums, and one measuring negative suction pressure within the oral cavity. Their figure (Fig. 1) shows the negative suction profile and positive pressure profile superimposed on each other, illustrating the relative scale and timing of these pressure forces at play during sucking on a pacifier.
Negative pressure (–150 to –200 mbar) is some 3–4 times greater in intensity than positive pressure (50 mbar). Negative suction pressure starts being generated while the jaws are relaxed/open, thereby ensuring the “teat” (or nipple/breast complex) is drawn into the oral cavity. Positive pressure by the jaws causes the gums to clamp on the base of the “teat,” thereby retaining it in the mouth. Shortly after jaw pressure reaches its peak, and starts to decline, negative pressure begins being regenerated. This is caused by lowering of the rear surface of the tongue, which, itself, is the end phase of the peristaltic wave as it completes its traverse of the oral cavity.
This is a key demonstration that even when feeding on an artificial teat (pacifier), the same natural forces are at play, despite them no longer having the normal function they would during breastfeeding.
The final force (7) is localized drawing down of the tongue surface adjacent to the nipple tip [7], the existence of which, and its role in milk removal, has only been confirmed in the past decade [11]. Unlike PTMs, this action is not obligate, but appears more facultative or opportunistic, only being superimposed on PTMs for a proportion of the time spent sucking. These localized depressions of the tongue surface are deployed at particular times during most feeds. Nonetheless, while they are ubiquitous, they cannot exist in isolation from PTMs; recent evidence (below) indicates they are generated by the same process. Their effect is to produce increased or added suction pressure local to the nipple surface; in all likelihood, this facilitates or enhances milk extraction. The phrase extractive tongue depressions (ETDs) will be used to refer to these, in view of their assumed function.
While the appearance of ETDs is difficult to predict, they are regularly associated with times of high milk outflow from the nipple. This association between peak negative suction pressure and “peak milk flow” was first revealed by Geddes et al. [11], who proposed that ETDs play a predominant role in milk extraction (in very much the same way as a breast pump would). The ability to detect “peak milk flow” was visually based on the movement of “echogenic flecks” in the space just beyond the nipple tip (stated to be “milk fat”). My personal belief is that these visual markers of milk flow are in fact evidence of “stable cavitation” [18], caused by microbubbles of carbon dioxide being drawn out of solution by the high negative suction pressure, usually being reabsorbed before the milk bolus is swallowed (this theory has yet to be tested or confirmed).
Engineering-Based Approaches to Modelling Milk Removal from the Breast
The evidence gleaned from multiple imaging studies has since been expanded by employing engineering-based models of the milk duct structure of the breast, and the baby’s sucking pattern, in order to generate theoretical data on milk flow, for comparison with real clinical data.
The first substantive attempt to develop a mathematical model of milk extraction during breastfeeding was undertaken by Zoppou et al. [19]. Drawing on knowledge current at the time, they compared the action of a breast pump, which used a cyclic pattern of suction, with that of a baby using both suction and expression. Their theoretical model caused them to conclude that there was an optimal time during the suck cycle to apply a compressive force, which increased milk flow over that produced by suction alone. Given their conclusion, it is somewhat surprising that recent models have not included a compressive component.
Two recent studies of milk removal from the breast have adopted an engineering-based approach [20, 21]. These studies have sought to create mathematical models which simulate the dynamic relationship between СКАЧАТЬ