The Effect of Chin-Down Position and Bolus Volume on Swallow-Induced Respiratory Measures in Young Healthy Adults
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Though head positioning strategies are widely used in the rehabilitation of swallowing dysfunction, little is yet known about its effect on the respiratory-swallow interaction. We hypothesised that a chin-down positioning would alter the respiratory behaviour in healthy young individuals. In a within-group study, we compared the characteristics of nasal airflow and its coordination with swallow events before, after, and during the swallow of liquid in a group of 30 young healthy individuals. The measures were obtained with simultaneous recording of surface electromyography, nasal airflow, and swallow sound recording during spontaneous drinking of 5, 15, and 20 ml water. Duration of swallowing apnoea (SAD) and the slope (Spre & Spost), duration (Dpre & Dpost), and direction (Expiration/Inspiration) of respiratory phases surrounding the apnoea were obtained. The statistical comparisons revealed that chin-down position significantly prolonged breathing arrest (SAD) and pharyngeal transit time (event interval) during swallow. The Dpost of large volume swallows were shorter indicating a quick resumption of breathing, indicating high post-swallow respiratory demand. The other statistically significant differences in various event intervals suggested the influence of chin-down position and bolus volume on the oral and pharyngeal transit time. Also, there was an increase in the incidence of inspiratory swallows in the chin-down position with large bolus volumes. We conclude that the chin-down position can provide longer airway decoupling but may increase the respiratory demand during swallows. Use of smaller bolus volumes should be recommended for chin-down swallows in persons with respiratory distress and dysphagia.
KeywordsHead- tilt Flexion Airway Apnea Deglutition Breath*
The pharyngeal phase of swallowing transports the cohesive bolus from the oral cavity to the oesophagus with precisely timed and coordinated motor sequences. As the bolus enters the oro-pharynx [1, 2], and the breathing activity is paused for a short period of time leading to the swallow-induced respiratory apnoea (SA). The physiology associated with this apnoeic period is speculated to be an important component of the airway protection mechanism that prevents the lower respiratory system from contamination and its consequences. In typical respiro-deglutometry recordings, SA is identified as a straight line of 1–2 s between regular breathing cycles indicating an arrest of breathing activity. The duration of SA was found to be a possible clinical indicator of atypical swallowing in elderly and in persons with respiratory inefficiency [3, 4, 5]. The phenomenon has long been subjected to studies for its normative values [6, 7] and influencing factors [3, 6, 8, 9, 10] but provided inconclusive results. This expanded the scientific quest for more consistent measures of typical respiratory coordination among the breathing phases and cycles surrounding the apnoea [8, 9, 11], and also its time coordinates [12, 13, 14].
Previous reports have extensively researched on the changes in apnoea duration and characteristics of respiratory phases surrounding it, as a function of whole-body position [8, 15, 16], and bolus characteristics [7, 10]. Apnoea duration was found to vary with volume [6, 7, 10, 17] but not with consistency [6, 17], but vice versa was also reported in few studies [7, 9, 18]. The duration of respiratory cycles surrounding the SA has also been probed [8, 9]. However, the rate and slope of these cycles have received less attention. With the upcoming interest in the efficacy of respiratory re-training in rehabilitation of dysphagia [19, 20], the rate and rhythm of breathing before and after swallow may be an influential measure.
Among the various compensatory strategies in rehabilitation of dysphagia, the chin-down position and bolus modifications have been popular for the immediate changes that these strategies bring in the swallow function [21, 22, 23, 24]. It is important that the respiratory function associated with these strategies are also known before clinical decision-making. The chin-down position has received considerable attention in deglutition literature for the changes it brings in typical oral [25, 26, 27] and pharyngeal [22, 28, 29, 30] function. Combined whole and head position strategy significantly altered the duration of SA, characteristics of breathing cycles before and after SA, and also the coordination of respiratory-swallow events  while only the whole body position did not [8, 31, 32]. This suggested that head position does alter the swallow-induced respiratory behaviours but lacks an in-depth understanding of its patterns. Therefore, this study was taken up to compare the swallow-induced respiratory behaviours in neutral and chin-down position while healthy young individuals swallowed varied bolus volumes. We hypothesised that the chin-down position would significantly affect the respiratory measures before, during, and/or after the swallow and also possibly alter the coordination. Specifically, the study compared (1) duration of swallow-induced respiratory apnoea, (2) characteristics, duration, and slopes of respiratory cycles surrounding apnoea, and (3) interval between onset and offset of respiratory and swallow events across neutral and chin-down position during healthy swallow of 5, 15, and 20 ml water. In addition, the study also compared the variation in these measures across bolus volumes in each head position.
A single group design was used to compare the effect of head position and bolus volumes on swallow-induced respiratory behaviours. The methodology of this study was approved by the research review board and bio-behavioural ethics committee of the host institute. Written consent in line with the Declaration of Helinski was obtained from all the participants prior to the recording of instrumental data. A total of 30 healthy volunteers (15 males, 15 females) of 20–40 years (mean age 29.04 ± 5.58 years) participated in this study. All participants had clinically normal cognitive, linguistic, neurological, muscular, respiratory, and oro-motor skills. None of them had undergone oro-pharyngo-laryngeal surgery and none showed symptoms of swallowing dysfunction for their daily nutrition and hydration intake. All participants had their body mass index (BMI) within the range of 19–29 for excluding the possibility of an undiagnosed respiratory inefficiency related to obesity [33, 34]. Also, chronic smokers and individuals with blocked nose sensation were excluded from the study.
The sEMG sensor was a single, disposable, and dry disk/round electrode of 5.72 cm diameter. Each disk array consisted of three inbuilt silver electrodes arranged in a triangular pattern at a distance of 2 cm from each other. Out of these three electrodes, two acted as active, and one served as ground electrode during data acquisition. The adhesive side of the electrode array was placed on the sub-mental muscles located on the inferior side of the mandible. The triangular sensor array was then connected to the Swallowing Signals Lab 7120 using the manufacturer supplied standard crocodile connectors with white clips for active and black clips for ground electrode. The potentials during the swallow activity were obtained from the sub-mental muscles at a sampling rate of 500 Hz and bandpass filtered (50–250 Hz) and rectified by the Swallowing Signals Lab 7120 interfaced with the Digital Swallowing Workstation. The inbuilt algorithm calculated the difference in the electromyographic potentials between the active and reference electrodes and was displayed on screen as sEMG waveform.
The respiratory tracings before, during, and after the swallow event were recorded with nasal module using a calibrated standard adult size (7 ft) transparent cannula. The nasal cannula had a combination of pressure transducer and a thermistor that are inbuilt in the Swallowing Signals Lab 7120. The two openings of the cannula were placed into the nares of the participant to let respiratory airflow through the cannula to the pressure and thermistor combination. This arrangement deduced the directionality as well as pressure generated with the airflow. The other end was connected to the Swallowing Signals Lab 7120 which process the pressure and direction data and display as bi-directional respiratory tracings across time. The recordings were obtained at a sampling rate of 250 Hz, and the recording window was synchronised with the other modules.
The swallow sound recordings were obtained during each swallow with the acoustic module using a high-response stethoscopic microphone placed on the thyroid lamina, and the other end connected to the Swallowing Signals Lab 7120. Throughout the data acquisition, the microphone was secured in place with a Velcro strap around the neck. The synchronised swallowing sound was recorded at a sampling rate of 22,000 Hz, bandpass filtered and acoustically represented in the form of amplitude waveform in the recording window.
Three cups each of 5-ml, 15-ml and 20-ml-thin liquid (drinking water) was used to obtain the swallowing associated respiratory data. The bolus volumes were randomised across and within the participants.
The participant was seated comfortably and explained about the procedure of the study. They were told that the study would record the ‘movements’ that occur during swallowing in two different head positions. The two positions were demonstrated and the participants were asked to perform trial dry swallows in both the head position. This was done to acclimatise the participant to the recording settings. The investigator ensured the accurate placement of sensors as well as the working condition of the instrumental set-up during these trial swallows. All participants were asked to drink the entire volume of water in one swallow from any of the cups placed in front of them and swallow when ready. All swallows were performed in one head position before repeating the bolus volumes in the second position.
Participants were seated comfortably on a cushioned chair with headrest and were provided with written and oral instructions regarding the procedure. All were blinded to the purpose of the study but were informed that they were required to drink few sips of water in two head positions, one after the other. After ensuring comprehension of instructions, the sensors (sEMG, calibrated nasal cannula, and auscultation steth microphone) were placed on the sub-mental muscles, nares, and thyroid lamina and secured in position.
The head position and bolus volume was randomised across participants while bolus volume was randomised within participants as well. All participants were instructed to drink the water from the three cups (5 ml, 15 ml, and 20 ml of water). The participants were not cued or timed for their swallows so that the most natural swallow performance could be obtained for study. Recording of the data started before the presentation of boluses till completion of the nine random volume swallows (3 trials × 3 volumes) in one head position. Swallows were detected from the respiratory and acoustic module with presence of apnoea and swallow sound respectively. The event was confirmed with their close approximation to sEMG spike. Data was tagged, saved, and analysed for measures of respiration and event intervals.
Operational definition of the outcome measures obtained from the respiratory and swallowing data
Swallow apnoea duration (SAD)
Duration of the black straight line between or within a respiratory cycle
Duration of preceding respiratory phase (Dpre)
The duration between the point of onset of apnoea to the point of change of trace colour preceding the apnoea.
Duration of following respiratory phase (Dpost)
The duration between offset of apnoea to point of change of trace colour following the apnoea.
Slope of preceding respiratory phase (Spre)
The slope of the respiratory trace included for Dpre.
Slope of following respiratory phase (Spost)
The slope of the respiratory trace included for Dpost
Respiratory phase interrupted (Phasei)
The direction of breathing identified with the colour of the trace preceding the apnoea. This was categorised as expiration (green) or inspiration (red) before swallow.
Respiratory phase resumed (Phaser)
The direction of breathing identified with the colour of the trace following the apnoea. This was categorised as expiration (green) or inspiration (red) after swallow.
Onset of sEMG to onset of apnoea (EMGiRAi)
Time interval between the onset of sEMG spike (EMG0) in the sEMG module to onset of apnoea in the nasal module (NC0).
Onset of apnoea to onset of swallow sound (RAiSSi)
Time interval between the onset of apnoea (NC0) in the nasal module to onset of swallow sound in the acoustic module (CA0).
Onset of apnoea to end of swallow sound (RAiSSt)
Time interval between the onset of apnoea in the nasal module (NC0) to end of swallow sound in the acoustic module (CA1).
Offset of apnoea to end of swallow sound (RAtSSt)
Time interval between offset of apnoea in nasal module (NC1) to offset of swallow sound in the acoustic module (CA1).
Offset of apnoea to end of sEMG (RAtEMGt)
Time interval between end of apnoea in nasal module (NC1) to end of EMG spike in sEMG module (EMG1).
Recorded data from four randomly participants (72 swallows) were given to another speech-language pathologist trained in the operation of the equipment, and with 1-year experience in swallowing rehabilitation. This judge was blinded towards the participant details and independently analysed the swallows for outcome measures as described in Table 1. A strong inter-judge agreement was obtained for the respiratory measures and event intervals (Cronbach’s alpha, α = 0.793).
Mean (standard deviation) of swallow-induced respiratory measures and event intervals in healthy young individuals
While SAD, Dpre, and RAiSSt increased in magnitude, the Dpost, Spost, and RAiSSi decreased with the chin-down position. Other measures showed irregular trends. An apparent trend could also be observed in the Spre measure with the slope increasing with volume in the neutral position. The event intervals RAiSSi, RAiSSt, and RAtEMGt also suggested volume dependency in the neutral position. The interval between onset of apnoea and swallow sound increased and between the offset of apnoea and EMG decreased with volume in this position. When the head position was changed to chin-down, Dpre, EMGiRAi, RAtSSt, and RAtEMGt showed a possible trend. Duration of the respiratory phase before swallow and the event intervals increased with volume in the chin-down position. The Kolmogorov-Smirnov test of normality revealed that the data was heavy-tailed, and therefore, non-parametric comparisons were run for all further statistical testing of hypotheses at the 0.05 level of significance.
The Friedman test for related samples revealed significant difference in the RAiSSi event interval (χ2 = 15.025, p = 0.010) and marginal differences in SAD (χ2 = 11.263, p = 0.046), Dpost (χ2 = 11.556, p = 0.041), and EMGiRAi (χ2 = 11.174, p = 0.048). Other respiratory and event intervals failed to show a significant difference in repeated measure analysis. The measures that showed marginal and significant differences were subjected to pair-wise comparisons using Wilcoxon’s Sign Rank test across position and bolus volumes.
Comparison across Head Position
Across Bolus Volume
Percentage occurrence of inspiratory and expiratory phase before and after the swallow-induced respiratory apnoea
Neutral head position
Respiratory phase (aEx/bIx)
Percentage of time the phase preceded a swallow (%)
Percentage of time the phase followed a swallow (%)
Percentage of time the phase preceded a swallow (%)
Percentage of time the phase followed a swallow (%)
Swallow demands respiration to be briefly paused for its efficient execution. Before this arrest is executed, the human body needs to analyse the status quo at multiple interacting systems so that the respiratory demands are satisfied during the apnoeic interval. The information regarding the respiratory demands may be integrated into the reflexive motor sequence of swallow for its safe execution. The random variables at the moment of swallow may have led to the high inter-subject variability in the respiratory-swallow measures included in this study. Prevailing airway-deglutition literature with inconclusive evidences in physiology, function, and sequence of swallowing events [35, 36, 37, 38] supports this opinion of variability.
The primary aim of this study was to compare the swallow-induced respiratory measures and its coordination as a function of head position. Segizbaeva, Pogodin, Lavrova, Balykin and Aleksandrova (2011) analysed the respiratory function with head-down posture during the non-deglutition period and found that this position increased the laryngeal resistance therefore lengthening the respiratory rate and time. This study made a similar attempt but included deglutition episodes, and found prolonged swallow-induced respiratory apnoea with chin-down position with the differences reaching statistical significance in higher bolus volumes. Increased duration of swallow apnoea may be a functional consequence of altered anatomical dimensions of pharyngeal structures with chin-down [23, 39] and may be one of the factors that effectively augment airway decoupling in persons with dysphagia [40, 41]. However, this finding implies that this posture may not be effective in persons with respiratory insufficiency, and associated swallowing difficulties.
Comparison of duration of the respiratory phase after swallow apnoea revealed interesting findings. The decrease in Dpost measure from 5 to 15-ml liquid swallow suggests a quick resumption of breathing in neutral and chin-down position. However, with further increase in volume from 15 to 20 ml, the duration of the respiratory phase post-swallow increased but was not statistically significant. A quick resumption of breathing may indicate the greater respiratory demand following the prolonged SAD with 15-ml liquid swallows compared with 5-ml swallows. This further supports the use of bolus volume modification strategies and use of small volume of thin-liquid bolus in persons with swallowing and/or respiratory difficulties. An insignificant increase in the post-swallow phase with increase in bolus volume from 15 to 20 ml may be a precautionary mechanism by which the healthy body maintains high lung volume for longer duration to elicit a cough reflex under the circumstances of a possible penetration/aspiration in large bolus volume. This inference is supported by earlier reports of Hegland, Huber, Pitts, and Sapienza (2009) and McFarland, Martin-Harris, Fortin, Humphries, Hill, and Armeson (2016) that found higher lung volume at initiation of thin-liquid swallows [42, 43]. This protective mechanism is operational irrespective of head position, as suggested by the findings of this study.
Meaningful interpretations of the respire-deglutometric event intervals are possible only if one understands the significance of each event in this assessment procedure. Perlman et al. (2005) studied the co-occurrence of respirodeglutometric and videofluroscopic events during swallow of liquid barium by healthy individuals . According to them, the onset of EMG occurred prior to onset of respiratory apnoea and in majority, the bolus tail is in the oral cavity at the point of cessation of breathing. Similarly, swallow sound has been correlated in time with passage of bolus through the hypo-pharynx, including pyriform sinus and upper oesophageal sphincter (UES) [1, 44]. Passage of bolus through the UES marks the end of oro-pharyngeal swallowing followed by return of pharyngeal structures to its resting position [45, 46]. The interval between EMG and apnoea onset during thin-liquid swallows, therefore, could be approximated to the oral transit time (OTT) and between onset of apnoea and offset of swallow sound to pharyngeal transit time (PTT). The present study as well as the existing reports is in consensus that the OTT is volume dependent [47, 48]. The position was found to affect OTT but not PTT during solid swallows  but this study found both position and volume to significantly influence the PTT. The estimated PTT (RAiSSt) was longer in chin-down compared with neutral position, and prolongation was greatest for higher bolus volumes supporting the belief that chin-down position could help in slowing down the bolus flow velocity.
In line with many previous findings, swallow was preferred to be placed in the expiratory phase (Ex/Ex) or at the end of an inspiratory phase (Ix/Ex) of respiration while a very small percentage of swallows tend to proceed with inspiratory cycle at higher volumes in neutral head position [8, 50]. With chin-down position, a larger number of swallows are placed in an inspiratory cycle and incidence increased with volume. Therefore, use of this head position in persons with airway compromise and reduced respiratory capacity should only be with caution. A controlled volume of thin-liquid swallows may be more advantageous to young persons with dysphagia.
To conclude, the study found a significant effect of head position and bolus volume on respiratory-swallow coordination as visualised in respirodeglutometric events. Chin-down position increased the duration of swallow apnoea, and incidence of inspiratory swallows in young healthy individuals. Bolus volume also influenced the duration of apnoea, and the speed of respiratory resumption post-swallow. The time interval between onsets of sEMG to apnoea (approximately the OTT) was volume dependent, and the interval between onset of apnoea to offset of swallow sound (approximately the PTT) was both volume and head position-dependent. A quick resumption of breathing after higher thin-liquid bolus volume swallows may be a consequence of prolonged respiratory apnoea, and increase in respiratory demand towards the end of the swallow. The study strongly suggests recommendation of small bolus swallows in chin-down position for clinical purposes. Also, in persons with respiratory insufficiency, use of chin-down position may increase the respiratory demand post-swallow. In clinical decision making for management of dysphagia, these findings may be beneficial to establish safe airway decoupling during swallow.
This study aimed at investigating the respiratory-swallow relationship as a function of head position and bolus volume in healthy swallows using respiro-deglutometry. The thirty healthy individuals included in this study randomly performed 5-ml-, 15-ml-, and 20-ml-thin-liquid swallows in neutral and chin-down position. Analysis of respiratory measures before, during, and after swallow suggested that chin-down position increased the duration of swallow apnea and prolonged the pharyngeal transit time. Large bolus volume swallows prolonged the duration of respiratory apnea, oral transit time, and also the pharyngeal transit time. The findings also found a quick resumption of breathing post-swallow of larger bolus volumes compared with 5-ml swallows. The study suggests that head position and bolus volume significantly alter the respiratory-swallow coordination in young healthy individuals during thin-liquid swallows.
We thank the Director for providing us all the necessary resources for conducting this study. We also thank our junior research fellows for their enthusiasm and participation in reliability checking and proofreading of the report.
Both GK and SPG have contributed significantly to conceptualisation, data collection, data analysis, interpretation, and report preparation.
Compliance with Ethical Standards
Conflict of Interest
The authors declare that they have no conflict of interest.
The study complied with the guidelines of Bio-behavioural Research Committee, University of Mysore and was approved by the Research Advisory Council and also the Ethical Clearance Committee of the host institution.
All the participants provided written informed consent for participation in the study.
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