Exploring Alternative Methods to Reduce Milk Flow Rate From Infant Bottle Systems: Bottle Angle, Milk Volume, and Bottle Ventilation

Dysphagia is a commonly encountered deficit among medically complex infants in the acute care environment. Effects if left untreated pose significant short- and long-term health consequences. For example, dysphagia leads to aspiration in 86% of infants with congenital heart deficits (McGrattan, McGhee, et al., 2017), malnutrition in 42% of infants with hypoxic ischemic injuries (Martinez-Biarge et al., 2012), and delayed hospital discharge in 75% of preterm infants (Wang et al., 2004).

One of the most commonly used first-line interventions that clinicians employ to treat infant swallowing impairments is to restrict milk flow rate by providing feeds with a nipple that has a reduced orifice size. Research indicates this restriction limits how much milk is expressed per suck (Jackman, 2013; Mathew, 1991; Pados et al., 2015, 2019), thereby providing infants theoretical benefits in coordinating between breathing and swallowing for enhanced respiratory stability and airway protection (Al-Sayed et al., 1994; Mathew, 1991; Mathew et al., 1992; McGrattan, McFarland, et al., 2017; Pados et al., 2017; Scheel et al., 2005).

However, principles of fluid dynamics suggest there are other less commonly considered methods to modify milk flow rate from a bottle that do not require specialty bottle nipples. These include modifying the bottle's air and hydrostatic pressure. The bottle's internal air pressure is generated in the portion of the bottle where no milk—and only air—is present. This is initially equal to atmospheric pressure outside the bottle. Likewise, hydrostatic pressure is generated by the milk in the bottle and dependent on the milk's density, height, and gravitational acceleration. Just as a deep-sea diver experiences increasing hydrostatic pressure as they dive deeper below the surface, so does the pressure of milk with increasing height of milk above it. The flow of milk from bottle nipples is proportional to the net force generated by the combination of these pressures.

Illustration of the interplay between these pressures is most easily depicted by using an example of a traditional bottle setup where a caregiver fills an 8-oz unvented bottle with 6 oz of milk. At the start of the feed, when the 2-oz air space in the bottle where no milk is present is equal to atmospheric pressure outside the bottle, the only pressure being applied is that from milk at the nipple tip due to the height of the milk above it. However, once the bottle is inverted and milk is drained from the bottle, the volume of this air space will increase and cause a decrease in the bottle's air pressure due to the enlarging area if no venting system maintaining atmospheric pressure is in place (Lau et al., 2015; Lau & Schanler, 2000). This reduction in pressure places a drag on the milk in the bottle that theoretically causes a gradual reduction in milk flow as the feed progresses (Lau et al., 2015; Lau & Schanler, 2000).

Hydrostatic pressure also undergoes theoretical changes as the feed progresses due to its dependence on the height of the milk above the nipple tip. The orifice of an inverted bottle filled with 6 oz of milk will experience a greater hydrostatic pressure than one filled with 4 oz due to the greater fluid height above the nipple tip. This pressure has been postulated to cause milk to passively drip from bottles upon inversion regardless of infant sucking, causing potentially deleterious impacts on their ability to coordinate breathing and swallowing (Lau et al., 2015; Lau & Schanler, 2000).

Although these basic principles of fluid dynamics are well established and are not in question in the current investigation, there is a paucity of research testing the specifics of how these principles influence characteristics of how milk flows from a bottle nipple under the influence of common clinical variables. The objective of this investigation is to demonstrate how common clinical conditions including the presence of a bottle vent, alteration to bottle position, and alterations to the amount of milk placed in a bottle alter the aforementioned bottle pressures (atmospheric and hydrostatic) and, of greatest importance, milk flow.

Method

The impact of common clinical variables on bottle pressures and milk flow rate was evaluated under simulated conditions by fastening an inverted Dr. Brown's Natural Flow bottle equipped with a Level 1 bottle nipple to a retort stand (see Figure 1). Bottles were filled with varying volumes (1–4 oz) of Similac Advance formula based on the experimental condition. Powder formula was prepared at room temperature according to manufacturer instructions. The angle that the bottle was inverted ranged from completely inverted (90°) to partially inverted (45°) to horizontal (0°) according to the experimental condition (see Figure 2), with care to ensure the clamp holding the bottle was not overtightened to prevent bottle deformations that could alter the bottle's atmospheric pressure.

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Bottle pressures, including hydrostatic pressure and air pressure, were measured by threading Millar microtip pressure transducers through a prebored hole placed in the base of the bottle. The pressure transducer measuring hydrostatic pressure was positioned so the tip was located in the neck of the bottle just above where the venting system started and prohibited further insertion. The transducer measuring air pressure was positioned in the air space at the base of the inverted bottle (see Figure 1). A luer lock with a surrounding polymer seal was used to prevent any air leaks around the site of catheter insertion. All pressure signals were acquired with the BIOPAC AcqKnowledge system and calibrated to mm Hg using the Millar pressure control unit.

Milk flow rate was calculated by weighing the amount of milk that had freely dripped from the bottle without the addition of simulated suction pressures and converting it to volume using a standard 1 g = 1 ml conversion (Lau et al., 1997; Neville et al., 1988). Simulated sucking using a pressure pump was not used in this investigation as understanding how these pressures influence flow rate without the addition of external suction variables was desired, as well as the fact that understanding specifics of milk flow in the absence of sucking pressures has clinical relevance. Measurements for milk flow rate and bottle pressures were taken every 5 min for 20 min to simulate the typical duration of a bottle feed. Each condition was tested during 10 trials.

Specifications of each experimental testing condition are outlined as follows. When testing the effect of a bottle venting system on bottle pressures and flow rate, bottles were filled with 4 oz of formula that were completely inverted (90°) for flow testing while a Dr. Brown's venting system was in place or while removed pending the condition. As the majority of clinicians using the Dr. Brown's bottle system use it with the vent in place, the effect of bottle position on bottle pressures and milk flow rates was tested by filling bottles with 4 oz of formula with the vent system in place. Trials were completed while the bottle was positioned in three distinct positions verified with a protractor: completely inverted (90°), partially inverted (45°), and horizontal (0°). Of note, to ensure milk remained in the nipple tip during the testing of the horizontal position, the bottle was closely monitored with inversion within a 5° window. Lastly, as the majority of feeders hold the bottle in a partially inverted position (45°), the impact of milk volume was tested while the bottle was positioned partially inverted (45°) with the vent in place, with trials completed with each of the following milk volumes: 1, 2, 3, and 4 oz.

Statistical Analysis

The data from the 10 trials of each experimental condition were tested using repeated-measures analysis of variance, calculated using the R statistical software. Based on the experimental design isolating the variation on the factors of inquiry, namely, venting, angle, and volume, each was tested independently. Both the measured flow rate and the hydrostatic pressure were utilized as dependent variables in these models. In the first round of tests interrogating the effect of venting the bottles, the unvented bottles ceased flowing in the vast majority of cases, which nullified both the dependent variables in our tests (a flow rate of zero and a stable hydrostatic pressure). As a result, the statistical tests and results that we present below were calculated only on trials utilizing vented bottles.

Results

Bottle Venting Systems

Upon inversion of the unvented bottle, milk immediately began dripping with flow rate significantly slowing (p < .001) and, in the majority of cases (80%), completely ceasing throughout the 20-min testing period. The average time for milk flow to cease in cases where it did stop flowing was 7.8 ± 4.41 min. Examination of changes in hydrostatic and air pressure during this period revealed significant reductions in both bottle pressures (p < .01) corresponding to the reductions in milk flow (see Figure 3A). Specifically, as milk dripped out of the bottle throughout the first 5 min, hydrostatic pressure declined by an average of 8.3 ± 3.6 mm Hg, leading to corresponding reductions in bottle air pressure by an average of 9 ± 5.2 mm Hg.

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In contrast to the cessation of milk flow observed after 5 min in the unvented bottle, milk continued to drip from the vented bottle throughout the entire 20-min testing period in 90% of trials. Although air pressure remained stable throughout this time, only exhibiting an average change of 1 ± 1 mm Hg, hydrostatic pressure declined as milk dripped out of the bottle. This was characterized by an average reduction in hydrostatic pressure of 11.5 ± 16.3 mm Hg throughout the 20-min testing period. As this occurred, milk flow rate significantly declined by an average of 1.7 ± 1.5 ml/min as well (p < .001; see Figure 3B).

Bottle Position

Hydrostatic pressure increased an average of 7.3 mm Hg as the angle of bottle inversion increased from a horizontal position to a partially inverted position (increase of 2.5 mm Hg) and to a completely inverted position (increase of 4.8 mm Hg). These elevations in hydrostatic pressure had corresponding impacts on milk flow rate as calculated by significant elevations in milk flow rate as the bottle's angle of inversion increased from horizontal (1.1 ml/min) to partially inverted (2.43 ml/min) and to completely inverted (3.6 ml/min, p < .001; see Figure 4A).

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Milk Volume

Hydrostatic pressure increased an average of 1.4 ± 0.12 mm Hg per additional ounce of formula that was added to the bottle. Corresponding increases in milk flow rate were observed, increasing an average of 0.64 ml/min with each additional ounce of formula (p < .001; see Figure 4B).

Discussion

Reduction of milk flow rate through the use of a slower flowing nipple is one of the most commonly used clinical treatments for infants suffering from dysphagia. Results from our investigation demonstrate that flow rate modifications can also be achieved by optimizing more basic bottle characteristics. Specifically, our key findings indicate the following: (a) Milk flows from bottles upon inversion regardless of if an infant is sucking; (b) milk flow rate and hydrostatic pressure are lower when bottles are positioned in a more horizontal placement or when less milk is placed in the bottle; and (c) as milk drips out of unvented bottles, it causes a buildup of subatmospheric pressure that will eventually cease milk flow.

Our finding that milk flows from inverted bottles in the absence of subatmospheric “sucking” pressure holds tremendous clinical significance for methods of feeding medically fragile infants. These infants often exhibit prolonged suck burst breaks, during which time the bottle nipple is in their mouth but no sucking occurs (Gewolb et al., 2001; Mathew, 1988; McGrattan, McFarland, et al., 2017). This feeding pattern has been postulated to be a compensatory approach to enable the infant time to “catch their breath” in order to maintain systemic oxygenation following the respiratory inhibition that occurs during the suck burst (McGrattan, McFarland, et al., 2017). Our results suggest that despite the absence of sucking, milk is likely still dripping into their mouth if the bottle is being held in a traditional partially inverted position. This afferent stimulation to the oropharyngeal mucosa has the potential to inadvertently cut their suck burst break short by triggering a swallow response and may lead to hypoventilation during a feed despite the infant's self-pacing attempts. Holding the bottle in a horizontal position, where passive milk flow does not occur, may therefore enable the infant more control in timing the duration of their suck burst breaks that extends beyond what can be achieved by bottle nipple modifications alone. Future research examining how these principles translate to true infants feeding under real-world conditions is critical to understanding their clinical significance.

The demonstrated reduction in milk flow rate that occurs as a result of reducing the angle of bottle inversion and reducing the volume of milk that is placed in the bottle also have potential future clinical implications. Our results indicate milk flow was over 4 times faster when the bottle was held in the inverted position when compared to the horizontal position and over 50% faster when the bottle was filled with 4 oz of milk when compared to 1 oz. Though the clinical significance and therapeutic effects of these changes to flow rate require further clinical investigation, our results highlight an exciting potential for these variables to be utilized as alternative or supplementary treatment modalities. For example, although clinician and caregiver access to specialty bottle nipples that reduce milk flow is gaining international growth, it is far from a universally accessible treatment option. The ability to achieve beneficial feeding effects by strictly modifying the angle of bottle inversion or the volume of milk in the bottle would offer treatment where it is currently absent. Likewise, in some cases, the flow restrictions provided by even the slowest flow nipple are not sufficient to resolve a patient's deficits. It is plausible that further reductions in flow offered by using a slow flow nipple on a bottle filled with a very small volume of milk may offer sufficient restriction to allow safe therapeutic feeding. Lastly, the modification of milk flow rate by angle of bottle inversion offers a dynamic treatment option in which the clinician can increase or decrease the flow rate rapidly throughout the feed with slight adjustments to bottle positioning. This may have benefits for patients who require more flow restriction in the beginning of a feed and less as the feed progresses.

Though this investigation is the first to the authors' knowledge to systematically test how air and hydrostatic pressures influence how milk flows from a bottle, previous work led by Lau et al. have examined clinical implications of these variables in preterm infants. Lau et al. (2000) completed a case report on changes in hydrostatic pressure throughout the feed of a preterm infant evaluated at 52 weeks postmenstrual age using a traditional unvented bottle system (Lau & Schanler, 2000). This work, which utilized a single-pressure sensor positioned in the milk of the nipple tip, demonstrated the buildup of −32 mm Hg pressure as the infant sucked and expressed milk during the initial suck bursts of a feed, followed by an equalization of pressure once the bottle was removed from the oral cavity presumably caused by reductions in air pressure as milk is expressed (Lau & Schanler, 2000). Our results are consistent with these findings, as demonstrated by a reduction in the unvented bottle's air pressure by 9 mm Hg as milk passively flowed out during the first 5 min of inversion. This pressure became so great that it ceased milk flow completely within 5 min of inversion. In contrast, the buildup of pressure and corresponding cessation to milk flow did not occur when the vent was in the bottle. These findings relating to the impact of bottle venting systems have important clinical implications that should be considered by clinicians as they make assessment and treatment determinations. Two commonly observed impairments that limit medically fragile infants' abilities to consume full oral feeds are that they exert weak milk-extracting sucking pressures and that they fatigue with feeds (Lau et al., 2000). The buildup of pressure in unvented bottles, such as the disposable volufeeds often used in the neonatal intensive care unit or “drip free” bottle systems that reduce dripping by allowing subatmospheric pressure to build up, could exacerbate these deficits as the subatmospheric pressure theoretically requires the infant to exert greater force to overcome the pressure differential and express a bolus. Given preterm infant's suction pressures often do not exceed −50 mm Hg at even more advanced oral feeding stages, the buildup of these amounts of pressure in unvented systems is substantial (Lau et al., 2000). Furthermore, the constantly changing milk flow rate that likely occurs between periods of pressure generation and equalization likely requires the infant to constantly modulate their sucking, swallowing, and respiratory physiology—a task that may prove challenging for infants with immature neuromuscular systems. It is plausible that the use of a vented bottle that prevents this pressure buildup from occurring may therefore improve infant feeding performance. Examination of the impact of different vented and unvented bottle systems on infant feeding performance is warranted.

Other work examining these implications did so by testing the effect of a “self-paced” bottle system that was free of hydrostatic pressure and subatmospheric pressure effects (Lau et al., 2015; Lau & Schanler, 2000). Findings indicated preterm infants fed with the self-paced system consumed a greater proportion of their feed, did so at a faster rate, and had overall shorter feeding durations than those fed with a traditional unvented feeding system (Lau et al., 2015; Lau & Schanler, 2000). It is unclear if these beneficial treatment results can be achieved using clinically available vented bottle systems if the feeder solely maintains a horizontal bottle position to eliminate hydrostatic pressure as well.

Though this simulated feeding investigation provides a strong scientific foundation from which clinical investigations can be designed, its simulated nature imposes clear limitations to the clinical effects of these changes that have been demonstrated. The way the observed principles change under simulated or real sucking conditions is still largely unknown and warrants further investigation. Furthermore, this investigation strictly explored the impact of one type of bottle venting system on pressure and flow rate, and therefore, results cannot be generalized across all systems that may have different venting capabilities. Future research testing the effects of these differing systems is needed.

Conclusions

Reducing milk flow rate is one of the most widely used infant dysphagia interventions. This is most frequently achieved by making modifications to the bottle nipple. Results of this investigation reveal this can also be potentially achieved by making simple modifications to the feeding conditions that alter the milk's hydrostatic pressure. Reducing the angle of bottle inversion and filling the bottle with less milk are two easily employed strategies that reduce hydrostatic pressure, thereby reducing milk flow rate. Likewise, use of a vented bottle can reduce subatmospheric pressure buildup, which may offer a more consistent flow without increased energy expenditure in milk expression. Future research examining the clinical significance of these findings is critical in determining the optimal ways these potential interventions should be employed.

Data Availability Statement

The datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.

Author Notes