Human Milk: Composition, Clinical Benefits and Future Opportunities. Группа авторов
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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 suction generated in the baby’s oral cavity, and the milk-filled duct system of the breast [20, 21]; both studies use data recorded directly during breastfeeding. They are elegant, complex, and sophisticated, although it can prove difficult to evaluate them fully, in order to determine if any of the assumptions made might create erroneous conclusions. Modelling of the milk duct system of the breast is complex and will be bypassed in the discussion below, assuming them to be essentially accurate – that of Mortazavi et al. [21] is claimed to be more elaborate.
Elad et al. [20] have made some expansive claims about their work, but critical analysis of their study suggests some shortcomings. One noteworthy beneficial feature of their study is that their data analysis process allows them to use the hard palate as a register for movements of the tongue (Fig. 2). A relative weakness, in contrast, is that their data are derived from review of a relatively small selection of ultrasound images.
In brief, their methodology is as follows: 5–8 points are digitized on the hard palate (this is a manual process), to which a smooth curve is fitted by interpolation (red line in their Fig. 1A); the same process is applied to the tongue surface (green line). A set of tracings of the hard palate is collected for 4–6 suck cycles, which are then aligned with reference to the Hard-Soft Palate Junction, so that movement of the tongue relative to the hard palate can be visualized (identified as Fig 1A in their figure (Fig. 2A)).
Superimposed on this image (their Fig. 1C (Fig. 2A)) is a set of 28 equally spaced radiating lines (referred to as “polar coordinates,” numbered 1–27 in the figure), which radiate out from the scan head to above the hard and soft palate. The movement of the tongue surface is then plotted along every 5th or 6th polar coordinate, enabling the time lag, relative to the preceding focal coordinate to be visualized.
One apparent limitation of their approach is that 28 polar coordinates do not fully encompass the whole of the oral cavity. This might be regarded as a trivial issue, but the full passage of a suck across the oral cavity determines the overall suck duration, so that more lines would be required, up to at least 36, in order to embrace a full suckling action, including the prepharyngeal phase of swallowing.
Evaluating movement of the tongue surface relative to the hard palate, across four focal polar coordinates – #8, #13, #17, and #22 (illustrated), shows evidence of a phase shift between these separated lines (their Fig. 1E (Fig. 2B)).
In marked contrast, the time shift between ALL of the first 8 polar coordinates is evaluated and no phase shift is seen between individual lines. Based on their Fig. 1D (reproduced in my Fig. 2C), they assert that there is no phase shift between the lines, indicating that the “anterior tongue moves as a rigid body … ruling out the hypothesis of a peristaltic squeezing of the nipple” [20].
Personally, this line of argument appears misleading to me. Certainly, movement along the first three coordinates closest to the mandible (#1–3) is likely to be determined largely by up/down movement of the jaw, but beyond this point, there is evidence of propagation of a peristaltic wave from as early as polar coordinate #4, right through to #28.
Fig. 2. A Figure taken from Elad et al. [20] – a full description is contained in the text. B, C After Elad et al. [10], highlighted enlargement of their their Fig. 1C–E.
Fig. 3. Frame of ultrasound recording showing three user-applied rectangles, in which movements can be automatically tracked and compared.
The study by Monaci and Woolridge [22] merits discussion in this context, as it adopted a similar approach, but used signal processing techniques to analyze ultrasound records in real-time, thereby generating fully objective, automated results. They arbitrarily divided the oral cavity into three, spatially separated, non-overlapping sectors, equating to: (1) the anterior sector of the oral cavity, including nipple and front part of tongue (excluding lower jaw); (2) the middle sector of the oral cavity, comprising the mid-surface of the tongue and the space at the tip of the nipple in which milk accumulates; and (3) the posterior sector, comprising the oropharynx, where swallowing can be detected (Fig. 3).
These