From a Barky Cough to a Squeaky Voice: Heliox for Croup
The Case
Figure 1: Chest X-ray showing inflammatory changes consistent with a viral respiratory infection. Case courtesy of Dr Luke Danaher, Radiopaedia.org. From the case rID: 16821
A 22 month old otherwise healthy female presents to the ED with respiratory distress. Patient has had cough and congestion for two days. She was seen in the ER the day prior for a fever to 105F. A chest x-ray at that time showed inflammatory changes but no obvious pneumonia. The patient had no increased work of breathing and was thus discharged with antipyretics with initial improvement in symptoms. The patient now presents in respiratory distress with a barky cough and stridor consistent with croup.
The patient was given three treatments of nebulized racemic epinephrine, IV dexamethasone, and a normal saline bolus with mild improvement in her symptoms, however, she still had significant intercostal retractions and increased work of breathing. She was started on heliox (80% helium, 20% O2) via high-flow nasal cannula with near total improvement in her retractions. She was admitted to the PICU where she remained on heliox overnight and received dexamethasone every 6 hours. The next day she was weaned form heliox and discharged home.
Discussion
Laryngotracheitis, or croup, is inflammation of the larynx and trachea. It is most frequently caused by parainfluenza type 1 virus and most commonly seen in children 6 – 36 months of age. Inflammation causes narrowing of the airway which leads to increased resistance to air movement, favoring turbulent versus laminar flow of air into the lungs. Its clinical course typically starts with congestion, coryza, and fever which progresses to a barky cough, hoarseness, and stridor over 12-48 hours. The cough usually resolves after 3 days with the other symptoms lasting up to a week. The main treatments for croup are steroids and nebulized racemic epinephrine. In severe cases, patients may need intubation to secure the airway while steroids take effect.
In an attempt to support the patient while waiting for steroids to take effect, clinicians may try a helium / oxygen gas mixture (Heliox) to improve oxygen delivery to the lungs and decrease work of breathing. Helium is a low density, inert gas, which is insoluble in human tissue, non-reactive with cell membranes, and not known to have any negative effects on lung tissue.
Physics
The Reynolds number (Re) is used to determine if air flow is turbulent or dynamic.
Re = [p x d x V] / [u]
Where:
- p = gas density
- d = tube diameter
- V = gas velocity
- u = gas viscosity
In general, for a straight non-branching tube, a Re < 2,000 correlates with predominately laminar flow whereas a Re > 4,000 correlates with a predominately turbulent flow. Air and helium have similar viscosities however helium is ~1/10 the density of air thus significantly decreasing the Re.
Figure 2: (a) Pattern of laminar flow as seen with heliox mixture, (b) Pattern of turbulent flow seen with a more viscous mixture such as atmospheric air alone.
Evidence
While in theory heliox should ease work of breathing, there is little evidence to support a significant clinical benefit in croup. Cochrane did a review in 2013 to determine the efficacy of heliox in croup. Unfortunately, they were only able to find 3 randomized control trials with a total of 91 participants that met inclusion criteria. In one of the studies the investigators compared heliox to humidified oxygen and found no difference in Westley score at 20 minutes. The Westley score is a clinical tool used to quantify the severity of croup by assessing level of consciousness, cyanosis, stridor, air entry, and retractions. That study, however, only had 15 subjects, the patients had mild disease, and they were not given any other treatment, such as steroids or racemic epinephrine. In another study they looked at the difference between heliox and intermittent saline administration (placebo) vs humidified oxygen with up to two doses racemic epinephrine. This was also a small study with only 29 participants. Both the heliox and racemic epinephrine groups had improvements in their croup scores; however, there was no significant difference in croup score, oxygen saturation, respiratory rate, or heart rate between the groups. This suggests heliox is equally as effective as racemic epinephrine but doesn’t address the benefit of heliox as an adjunct therapy.
Conclusion
While heliox theoretically should decrease work of breathing in croup, the evidence is lacking. That being said, in the case presented above, heliox appeared to help avoid intubation. In addition to the need for more studies addressing the overall benefit of using heliox in treating croup, there are many other factors to analyze. In the studies mentioned above, heliox was administered via facemask. In our case, we administered it via high flow nasal canula. Does the route of administration make a significant difference? Also, does heliox affect the distribution of nebulized medications delivered? An additional factor to consider is, while heliox seems benign and a good tool to provide time for steroids to take action, will taking the time to trial heliox on a patient who ultimately needs intubation, make for a more dangerous intubation, in a patient with a narrower airway, and less respiratory reserve?
Faculty Reviewer: Jane Preotle, MD
References
UpToDate: Croup: Clinical features, evaluation, and diagnosis
UpToDate: Croup: Pharmacologic and supportive interventions
UpToDate: Physiology and clinical use of heliox
Terregino CA, Nairn SJ, Chansky ME, Kass JE. The effect of heliox on croup: a pilot study. Acad Emerg Med. 1998 Nov;5(11):1130-3.
Weber JE, Chudnofsky CR, Younger JG, Larkin GL, Boczar M, Wilkerson MD, Zuriekat GY, Nolan B, Eicke DM. A randomized comparison of helium-oxygen mixture (Heliox) and racemic epinephrine for the treatment of moderate to severe croup. Pediatrics. 2001 Jun;107(6):E96.
Moraa I, Sturman N, McGuire T, van Driel ML. Heliox for croup in children. Cochrane Database Syst Rev. 2013 Dec 7;(12):CD006822. doi: 10.1002/14651858.CD006822.pub4.