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Why assess cough?
Cough 🗣️💨 is important for ejecting foods and liquids from the lungs 🫁 to prevent asphyxiation and aspiration pneumonia. In this tutorial, we will discuss why and how to assess voluntary cough as a way to enhance your clinical swallow evaluation.
Evaluating cough is important three reasons when working with people with dysphagia:
First, impaired cough (dystussia), characterized by either blunted reflex cough sensation or a weak/ineffective cough motor response, is associated with dysphagia and aspiration in many patient populations. Therefore, assessing cough can improve the accuracy of identifying and predicting someone with dysphagia, making it a useful addition to your battery of dysphagia screening tasks.
Second, assessing cough provides a more holistic understanding of airway protection. In other words, we should try to understand how well a patient with dysphagia can clear aspirate material from the airway, in addition to understanding how frequently/severely they aspirate. People who aspirate but are able to clear the lower airway of foods and liquids through effective coughing should be managed clinically much differently than people who aspirate and are unable to clear the lower airway of aspirate material because of their ineffective coughs.
Third, if voluntary cough is assessed and found to be impaired, then it may also be an important and clinically feasible therapy target to include in a one's rehabilitation plan. For example, including exercises such as Expiratory Muscle Strength Training (EMST) or Voluntary Cough Skill Training (VCST) may be warranted if attempting to improve reflex and voluntary cough behaviors - see tutorials on these exercises.
General Overview
Two types of voluntary coughs can be assessed clinically: 'single' 🗣️💨 and 'sequential' 🗣️💨💨💨 . Reflex cough can also be assessed clinically, and while it is standard of practice to assess in many countries, it is only just now gaining popularity in the United States. There is no universally accepted standard set of instructions or pieces of equipment. Therefore, what I have outlined below is how I go about testing voluntary and reflex cough in clinical practice.
Clinically, I test both single and sequential voluntary cough as well as voluntary cough. For voluntary cough assessment, I prefer to test in the order of 'most natural' to 'least natural'. Therefore, I begin with sequential voluntary cough and then test single voluntary cough. For each type of voluntary cough, I complete a minimum of two trials. The first trial of each voluntary cough test is completed following a set of verbal instructions only. The second trial of each voluntary cough test is completed following a set of verbal instructions AND a clinical auditory-visual model (see below). After assessing voluntary cough, considering moving on to test reflex cough.
Time permitting, you may consider integrating more three trials of each with no cue to examine performance variability, and report both the maximum and median values.
Equipment for Reflex and Voluntary Cough Testing
As mentioned, there equipment for reflex and voluntary cough testing appears to vary from clinic to clinic. This is not ideal, since it can influence airflow measures and thus, the ability to compare measures between clinics, research studies, norms, etc. Below is a list of the equipment I use after having trialed many different pieces of equipment. I have no financial relationship with any of the products listed below. I use the pieces of equipment listed below because they are commercially available, relatively affordable, and can easily connect to each other.
Voluntary Cough Assessment Materials
Peak Flow Meter
Specifications:
The peak flow meter should have a round mouth piece, with an outer diameter of 22 mm
Example s:
PeakAir Peak Flow Meter; Manufacturer: Omron; Model Number: PF9940; available through Amazon (though I've found this peak flow meter frequently out of stock)
asmaPLAN Peak Flow Meter; Manufacturer: Vitalograph; available through MDSpiro (https://mdspiro.com/product/asmaplan-peak-flow-meter/)
Notes about asmaPLAN: this peak flow meter specifically says it can be used between patients with proper cleaning (which you should of course do, in addition to potentially adding respiratory filters)! This may be a nice option for hospitals who are more strict about single patient use devices.
Disposable Respiratory Bacteria Filter
Specifications:
One end of the bacteria filter should have an outer diameter of 25 mm and an inner diameter of 22 mm.
The other end of the bacteria filter should have an outer diameter of 22 mm and an inner diameter of 15 mm.
Example 1: AirLife Disposable Bacteria Filter; Manufacturer: Vyaire; REF: 001851; LOT: 0004197159
This is what I use and seems well-priced.
Example 2: ViroMax Viral & Bacterial Filter from A-M Systems.
I have not tested it yet to see if the connection is the exact same, but this is a reliable company and is worth exploring.
Disposable Air Cushion Mask
Specifications:
The connection port should have an inner diameter of 22 mm.
Masks typically come in adult small, adult medium, adult large, and adult extra large. Consider stocking up on medium and large.
Example 1: Manufacturer: Vacumed; LOT: 39531.
This is what I use, though I am seeking cheaper alternatives.
Reflex Cough Assessment Materials
Same as above, with the addition of the materials outlined below
Pulmomate Compressor Nebulize System (Manufacturer: DeVilbiss; Model: 4650D)
Nebulizing Kit
The kit should contain a nebulizing chamber, tubing, and a tee connector. The tee connector should have one of the horizontal connecting ends with an outer diameter of 22 mm, and the other horizontal connecting end with an inner diameter of 22 mm. The vertical connecting should connect to the nebulizing chamber.
Example 1: I purchase nebulizing kits manufactured by Roscoe Medical (REF: NEB-ROSKT), though others likely have the same specification.
Saline solution
100% pure grade citric acid
Borg CR10 category-ratio scale (used for Urge-to-Cough ratings)
Voluntary Cough Assessment
Instructions for Cough Assessment
Prior to starting the voluntary and reflex cough assessment, consider providing a brief overview of cough assessment to the patient using the following script:
"Now we are going to assess your cough function. This is important since it gives us a sense of how well you can cough foods, drinks, and saliva out of your lungs if you aspirate. This should take about 5 minutes to complete. For these coughing tasks, we will place a face mask around your nose and mouth. We want to try and prevent any air from escape around the sides of the mask, so we will push the face mask with a little bit of extra effort against your face. Feel free to match this pressure my pressing your face against the mask to help us prevent any air escape as you cough."
Then, move on to testing voluntary and reflex cough
Sequential Voluntary Cough Assessment
Trial 1:
Verbal Instruction:
“Cough as if something went down the wrong pipe, whatever that means to you.”
General Instructions:
The clinician should use one hand to hold the peak flow meter apparatus and use their other hand to support the back of the patient's head. The clinician should ensure they are not blocking the peak flow meter's moving needle as they hold the apparatus so that they don't affect the peak flow meter readings.
The clinician should then instruct the patient to breathe in. However, the clinician should not instruct the patient how to breath to take (e.g., a big breath). This is because we want to see what the patient does naturally. Instead, the purpose of the cue to breathe in is to facilitate proper timing when to let the patient breathe in with no face mask, and to place the face mask one during the cough trial.
While the patient inhales, anchor only the top of the face mask against the bridge of the nose while ensuring that the rest of the face mask is off the face so that the patient can breathe freely with no resistance from the face mask and filter. Once the patient appears to near the completion of their inhale, press the mask firmly against their their face around their nose and mouth prior to the onset of the cough.
When the cough trial is completed, look and feel to ensure there is minimal air escape around the face mask. There will likely be some air escape, but try to minimize air escape as much as possible, since this can affect airflow values.
When the patient completes the cough trial, make note of the cough peak flow rate, and also make note of auditory-perceptual outcomes (see below).
Trial 2: Verbal Instruction with Auditory-visual model:
Verbal Instruction:
“Cough as if something went down the wrong pipe. It might look something like this.”
General Instructions:
In addition to the general instructions outlined for trial 1, for trial 2, the clinician should model a three-cough epoch (i.e., one inhale followed by three expiratory coughs).
The clinician should be standardized and specific with their auditory-visual model. Specifically, the 3-cough epoch cough should be modeled by inhaling to approximately 80-90% vital capacity, and then producing three coughs. The first cough should ideally be slightly stronger than the second, and the second should be slightly stronger than the third. Furthermore, all coughs should be strong and crisp with minimal to no voicing.
Single Voluntary Cough Assessment
Trial 1: Verbal Instruction:
Verbal Instruction:
“Inhale as much air as possible, until you can't breathe in anymore, then cough hard once.”
(Note: the verbal instructions written here are different from those used in the current video - I recommend using the written instructions.)
General Instructions:
The clinician should use one hand to hold the peak flow meter apparatus and use their other hand to support the back of the patient's head. The clinician should ensure they are not blocking the peak flow meter's moving needle as they hold the apparatus so that they don't affect the peak flow meter readings.
The clinician should then instruct the patient to take a big breathe breathe in and cough hard once. Here, the clinician does want to instruct the patient how to inhale. This is because, for this 'less natural' single cough task, we want to put everyone on the same playing field by telling everyone to breathe in in the same way (ideally, high lung volume at cough initiation).
While the patient inhales, anchor only the top of the face mask against the bridge of the nose while ensuring that the rest of the face mask is off the face so that the patient can breathe freely with no resistance from the face mask and filter. Once the patient appears to near the completion of their inhale, press the mask firmly against their their face around their nose and mouth prior to the onset of the cough.
When the cough trial is completed, look and feel to ensure there is minimal air escape around the face mask. There will likely be some air escape, but try to minimize air escape as much as possible, since this can affect airflow values.
When the patient completes the cough trial, make note of the cough peak flow rate, and also make note of auditory-perceptual outcomes (see below).
Trial 2: Verbal Instruction with Auditory-visual model:
Verbal Instruction:
“Inhale as much air as possible, until you can't breathe in anymore, then cough hard once. It might look something like this.”
(Note: the verbal instructions written here are different from those used in the current video - I recommend using the written instructions.)
General Instructions:
In addition to the general instructions outlined for trial 1, for trial 2, the clinician should model a one-cough epoch (i.e., one breath in followed by one cough).
The clinician should model the 1-cough epoch by inhaling to approximately 90-100% vital capacity, and then producing one strong and crisp cough with minimal-to-no voicing.
Reflex Cough Testing for Hypotussic Cough/Dysphagia
Reflex Cough Testing for Hypotussic Cough Disorders
Verbal Instruction:
“We are going to give you some fog. The fog has citric acid in it. This may or may not make you cough. For this, we want you to breathe normally nice and relaxed in an out through you mouth and cough if you need to - whatever you would normally do.”
General Instructions:
Place the face mask firmly around the nose and mouth prior to turning on the compressor.
Wait for stable rest breathing. While the patient is breathing, continue to look at the facemask to ensure that the patient is breathing in and out through their mouth. If their mouth is closed and they are nose breathing, then instruct them to open their mouth and breathe through their mouth.
Turn the compressor on during just as they begin tidal exhalation (but not during inhalation). The idea is that all patients should be exposed to the nebulized citric acid at the onset of tidal inhalation, no later. This is because lung volume initiation significantly influences cough airflow, so we want to standardize when people are exposed to the cough stimulus.
Keep the compressor for a pre-determined length of timed based off the protocol you adopt (e.g., one single inhalation, three tidal breaths, 15 seconds, etc.) or until the patient coughs - whichever comes first. If the patient coughs, turn off the compressor, but keep the face mask pushed firmly against their face. The face mask should be on just the first cough epoch. Once the patient begins to inhale for a second cough epoch, immediately remove the face mask. The goal is to capture cough peak flow rate for the first cough epoch only.
After completing a trial, present the patient with the 0-10+ CR10 Urge-to-Cough scale. Have the patient indicate how strong their urge-to-cough was for that trial. They can pick any number, even using decimals, and even numbers above 10. I like to also have the patient indicate if they felt a 'tickle', 'burn', 'irritation', or 'something else', and to use that same 0-10+ scale to rate how strong that feeling was. If they coughed, then record the number of coughs and cough peak flow value. As described below, also make note of auditory-perceptual features of the cough.
For hypotussic cough disorders, you may be interested in seeing how sensitive the person is. If this is your primary interest, you'll likely want to select a couple different concentration levels to identify how sensitive the person is/is not.
Alternatively, you may be more interested in knowing how 'strong' their reflex cough is, when present. In this case, you'll want to select a concentration you think most people will cough to, even if they have reduced sensation. This is my current practice. As a result, I use a concentration of 0.8 mol/L of citric acid mixed with saline.
To develop a more detailed understanding of normative cough threshold data and reflex cough testing methods with citric acid, see below:
Monroe, M. D., Manco, K., Bennett, R., & Huckabee, M. L. (2014). Citric acid cough reflex test: establishing normative data. Speech, Language and Hearing, 17(4), 216-224.
Wallace, E., Hernandez, E. G., Ang, A., & Macrae, P. (2019). Quantifying test-retest variability of natural and suppressed citric acid cough thresholds and urge to cough ratings. Pulmonary Pharmacology & Therapeutics, 58, 101838.
Wallace, E., Hernandez, E. G., Ang, A., Hiew, S., & Macrae, P. (2019). A systematic review of methods of citric acid cough reflex testing. Pulmonary pharmacology & therapeutics, 58, 101827.
Reflex Cough Testing for Hypertussic Cough/Chronic Cough
Reflex Cough Testing for Hypertussic Cough Disorders (Chronic Cough)
The reflex cough testing procedure used when assessing people with chronic cough may likely differ to the type of reflex cough testing done when evaluating people with dysphagia.
Reflex cough testing can be used to assess sensory and motor function of cough when presenting a person with a cough stimulus. The equipment, techniques, and procedures used for reflex cough testing are very heterogeneous. Reflex cough testing outcomes will vary based in large part on the type of equipment, technique, and procedure. This is important to keep in mind if you are a clinician or research beginning to complete this type of exam.
Reflex cough testing can be used with a variety of types of patients. For example, you may complete reflex cough testing when evaluating swallowing in people with dysphagia to gain insight into how effectively they be able to detect aspirate material and produce a strong enough cough to clear aspirate material. Alternatively, you may complete reflex cough testing to assess laryngeal hypersensitivity in people with chronic cough.
Outcome measures that you may be interested in documenting may include, but are not limited to: reflex cough threshold (the lowest concentration to elicit a pre-specified cough respond); 0-10+ ratings of Urge-to-Cough; 0-10+ ratings of sensory magnitude estimation; number of coughs, and cough peak flow.
Verbal Instruction:
Natural Cough Threshold: “We are going to give you some fog which may or may not have citric acid in it. You may or may not feel it and it may or may not make you cough. For this, we want you to breathe in an out through you mouth and cough if you need to.”
Suppressed Cough Threshold: “Keep breathing in an out through you mouth, but this time, try not to cough.”
General Instructions:
Place the face mask firmly around the nose and mouth prior to turning on the compressor.
Wait for stable rest breathing. While the patient is breathing, continue to look at the facemask to ensure that the patient is breathing in and out through their mouth. If their mouth is closed and they are nose breathing, then instruct them to open their mouth and breathe through their mouth.
Turn the compressor on during just as they begin tidal exhalation (but not during inhalation). The idea is that all patients should be exposed to the nebulized citric acid at the onset of tidal inhalation, no later. This is because lung volume initiation significantly influences cough airflow, so we want to standardize when people are exposed to the cough stimulus.
Keep the compressor for a pre-determined length of timed based off the protocol you adopt (e.g., one single inhalation, three tidal breaths, 15 seconds, etc.) or until the patient coughs - whichever comes first. If the patient coughs, turn off the compressor, but keep the face mask pushed firmly against their face. The face mask should be on just the first cough epoch. Once the patient begins to inhale for a second cough epoch, immediately remove the face mask. The goal is to capture cough peak flow rate for the first cough epoch only.
After completing a trial, present the patient with the 0-10+ CR10 Urge-to-Cough scale. Have the patient indicate how strong their urge-to-cough was for that trial. They can pick any number, even using decimals, and even numbers above 10. I like to also have the patient indicate if they felt a 'tickle', 'burn', 'irritation', or 'something else', and to use that same 0-10+ scale to rate how strong that feeling was. If they coughed, then record the number of coughs and cough peak flow value. As described below, also make note of auditory-perceptual features of the cough.
For chronic cough, you'll likely begin at 0.0 mol/L of citric acid, and progressive to incrementally higher concentrations (0, 0.01, 0.02, 0.04, 0.06, 0.08, 0.10, 0.20, 0.40, 0.60, 0.80, 1.00). Begin at the lowest dose. If the patient doesn't does not cough on the first two trials, move up to the next concentration. If they cough on one of the first two trials, then do a third trial. Randomly intersperse saline trials to reduce likelihood of anticipation effect and to reduce likelihood of tachyphylaxis.
Stop once a patient produces a two-cough response on two out of three trials. This is considered their reflex cough threshold.
If doing NCT and SCT, then start with NCT. Once this threshold is determined, go down one concentration level and repeat as before, but with SCT instruction.
Notes about this video demonstration:
This video demonstration is of myself (James Curtis, PhD, SLP) facilitating a reflex cough testing chronic cough assessment in Allie Beam, MS, SLP. In this video, I complete both natural cough threshold (NCT) and suppressed cough threshold (SCT) using a continuous breathing technique with nebulized citric acid.
In this video, we present 2-3 trials of multiple different concentrations, moving progressively higher in concentration from 0.01 mol/L of citric acid up to 0.40 mol/L of citric acid, with saline (0.00 mol/L) randomly interspersed. We progress until we identify Allie's NCT, which for this procedure, was defined as the lowest concentration to elicit as a two-cough response on 2/3 trials. For Allie, their NCT was 0.40 mol/L - seen at 10 minutes 30 seconds in the video. At 11 minutes 36 seconds, we then proceed to assess SCT by beginning at one concentration below their NCT, and then moving progressively higher. For Allie, the SCT was also 0.40 mol/L.
For more information on reflex cough testing (not specific to chronic cough), consider the following select references:
Wallace, E., Hernandez, E. G., Ang, A., & Macrae, P. (2019). Quantifying test-retest variability of natural and suppressed citric acid cough thresholds and urge to cough ratings. Pulmonary Pharmacology & Therapeutics, 58, 101838.
Wallace, E., Hernandez, E. G., Ang, A., Hiew, S., & Macrae, P. (2019). A systematic review of methods of citric acid cough reflex testing. Pulmonary pharmacology & therapeutics, 58, 101827.
Monroe, M. D., Manco, K., Bennett, R., & Huckabee, M. L. (2014). Citric acid cough reflex test: establishing normative data. Speech, Language and Hearing, 17(4), 216-224
How might you interpret cough assessment outcomes?
Auditory-Perceptual Assessments of Cough
Auditory-perceptual assessments of cough involves listening 🗣️💨👂 for and documenting impressions related to subjective impressions of various perceptual cough descriptors.
Currently, I routinely report the following auditory-perceptual descriptors:
Perceived strength (WNL, mildly impaired, moderately impaired, severely impaired)
Perceived crispness (WNL, mildly impaired, moderately impaired, severely impaired)
Perceived amount of voicing (0-100%, defined as the percentage of time spent with voicing during the expulsive cough phase)
Type of expiratory effort (cough, throat clear, huff, vocalization/grunt, other)
Number of expiratory efforts
For more information on auditory-perceptual assessments of cough, consider the following training resources and research article:
Free training video: https://osf.io/ahfr4
Curtis, J. A., Borders, J. C., Dakin, A. E., & Troche, M. S. (2023). Auditory-Perceptual Assessments of Cough: Characterizing Rater Reliability and the Effects of a Standardized Training Protocol. Folia Phoniatrica et Logopaedica
Work is currently underway refining the definition of these descriptors and examining their relationship with acoustic and aerodynamic measures of cough and cough effectiveness seen during FEES
Instrumental Assessment of Cough using a Peak Flow Meter
Objective methods for assessing cough involve using instruments capable of measuring cough airflow 🗣️💨📏👩🔬. Handheld peak flow meters are one such instrument. Because they are cheap and portable, they are relatively feasible to use in clinical practice.
Peak flow meters measure the speed of air exhaled during breathing or cough. This peak velocity is measured in liters of air per minute (L/min). When peak velocity is measured for cough, it is referred to as cough Peak Flow Rate (PFR). This is sometimes also referred to as cough Peak Expiratory Flow Rate (PEFR). If using a digital peak flow measure, you may also be able to capture the amount of air exhaled during cough, which is referred to as Cough Expired Volume (CEV).
When completing voluntary and reflex cough, compare cough PFR to healthy norms and values associated with dysphagia & aspiration. More comprehensive norms need to be developed. The challenge with norms for cough is that airflow norms will vary based on cough task type (single voluntary cough, sequential voluntary cough, reflex cough) as well as the type of equipment used (amount of dead space, use of filter vs no filter, use of mouth piece vs facemask).
As a loose guideline, I consider PFR >300 L/min to be relatively normal for most people most of the time, 200-300 L/min to be potentially abnormal with a reduced ability to clear aspirate material from the airway during FEES (Borders & Troche, 2022), and <200 to be abnormal at risk of pneumonia.. See my below slide for some references and my general (informal) interpretation guide. Notbaly, these guidelines are likely more representative single voluntary cough function than anything else.
References:
Bianchi, C., Baiardi, P., Khirani, S., & Cantarella, G. (2012). Cough peak flow as a predictor of pulmonary morbidity in patients with dysphagia. American journal of physical medicine & rehabilitation, 91(9), 783-788.
Borders, J. C., & Troche, M. S. (2022). Voluntary cough effectiveness and airway clearance in neurodegenerative disease. Journal of Speech, Language, and Hearing Research, 65(2), 431-449.
Monroe, M. D., Manco, K., Bennett, R., & Huckabee, M. L. (2014). Citric acid cough reflex test: establishing normative data. Speech, Language and Hearing, 17(4), 216-224.
Curtis, J. A., Borders, J. C., Dakin, A. E., & Troche, M. S. (2023). Auditory-Perceptual Assessments of Cough: Characterizing Rater Reliability and the Effects of a Standardized Training Protocol. Folia Phoniatrica et Logopaedica
Curtis, J. A., & Troche, M. S. (2020). Handheld Cough Testing: A Novel Tool for Cough Assessment and Dysphagia Screening. Dysphagia, 1-8.
Ebihara, S., Saito, H., Kanda, A., Nakajoh, M., Takahashi, H., Arai, H., & Sasaki, H. (2003). Impaired efficacy of cough in patients with Parkinson disease. Chest, 124(3), 1009-1015.
Hammond, C. A. S., Goldstein, L. B., Horner, R. D., Ying, J., Gray, L., Gonzalez-Rothi, L., & Bolser, D. C. (2009). Predicting aspiration in patients with ischemic stroke: comparison of clinical signs and aerodynamic measures of voluntary cough. Chest, 135(3), 769-777.
Hammond, C. S., Goldstein, L. B., Zajac, D. J., Gray, L., Davenport, P. W., & Bolser, D. C. (2001). Assessment of aspiration risk in stroke patients with quantification of voluntary cough. Neurology, 56(4), 502-506.
Hegland, K. W., Okun, M. S., & Troche, M. S. (2014). Sequential voluntary cough and aspiration or aspiration risk in Parkinson’s disease. Lung, 192(4), 601-608.
Kimura, Y., Takahashi, M., Wada, F., & Hachisuka, K. (2013). Differences in the peak cough flow among stroke patients with and without dysphagia. Journal of UOEH, 35(1), 9-16.
Kulnik, S. T., Birring, S. S., Hodsoll, J., Moxham, J., Rafferty, G. F., & Kalra, L. (2016). Higher cough flow is associated with lower risk of pneumonia in acute stroke. Thorax, 71(5), 474-475.
Laciuga, H., Brandimore, A. E., Troche, M. S., & Hegland, K. W. (2016). Analysis of clinicians’ perceptual cough evaluation. Dysphagia, 31(4), 521-530.
Min, S. W., Oh, S. H., Kim, G. C., Sim, Y. J., Kim, D. K., & Jeong, H. J. (2018). Clinical Importance of Peak Cough Flow in Dysphagia Evaluation of Patients Diagnosed With Ischemic Stroke. Annals of Rehabilitation Medicine, 42(6), 798.
Monroe, M. D., Manco, K., Bennett, R., & Huckabee, M. L. (2014). Citric acid cough reflex test: establishing normative data. Speech, Language and Hearing, 17(4), 216-224.
Morice, A. H., Fontana, G. A., Belvisi, M. G., Birring, S. S., Chung, K. F., Dicpinigaitis, P. V., ... & Widdicombe, J. (2007). ERS guidelines on the assessment of cough. European respiratory journal, 29(6), 1256-1276.
Pitts, T., Bolser, D., Rosenbek, J., Troche, M., & Sapienza, C. (2008). Voluntary cough production and swallow dysfunction in Parkinson’s disease. Dysphagia, 23(3), 297-301.
Pitts, T., Troche, M., Mann, G., Rosenbek, J., Okun, M. S., & Sapienza, C. (2010). Using voluntary cough to detect penetration and aspiration during oropharyngeal swallowing in patients with Parkinson disease. Chest, 138(6), 1426-1431
Plowman, E. K., Watts, S. A., Robison, R., Tabor, L., Dion, C., Gaziano, J., ... & Gooch, C. (2016). Voluntary cough airflow differentiates safe versus unsafe swallowing in amyotrophic lateral sclerosis. Dysphagia, 31(3), 383-390.
Sakai, Y., Ohira, M., & Yokokawa, Y. (2020). Cough Strength Is an Indicator of Aspiration Risk When Restarting Food Intake in Elderly Subjects With Community-Acquired Pneumonia. Respiratory Care, 65(2), 169-176.
Sohn, D., Park, G. Y., Koo, H., Jang, Y., Han, Y., & Im, S. (2018). Determining peak cough flow cutoff values to predict aspiration pneumonia among patients with dysphagia using the citric acid reflexive cough test. Archives of physical medicine and rehabilitation, 99(12), 2532-2539.
Wallace, E., Hernandez, E. G., Ang, A., Hiew, S., & Macrae, P. (2019). A systematic review of methods of citric acid cough reflex testing. Pulmonary pharmacology & therapeutics, 58, 101827.
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