Viral Induced Wheeze

Question 1

What is viral induced wheeze?

Answer 1

Viral-induced wheeze describes is an acute wheezy illness caused by a viral infection.

Question 2

Briefly describe the pathophysiology of viral induced wheeze

Answer 2

Small children (typically under 3 years) have small airways. When these small airways encounter a virus (commonly RSV or rhinovirus) they develop a small amount of inflammation and oedema, swelling the walls of the airways and restricting the space for air to flow. This inflammation also triggers the smooth muscles of the airways to constrict, further narrowing the space in the airway.

This swelling and constriction of the airway caused by a virus has little noticeable effect on the larger airways of an older child or adult, however due to the small diameter of a child’s airway, the slight narrowing leads to a proportionally larger restriction in airflow. This is described by Poiseuille’s law, which states that flow rate is proportional to the radius of the tube to the power of four. Therefore, halving the diameter of the tube decreases flow rate by 16 fold.

Air flowing through these narrow airways causes a wheeze, and the restricted ventilation leads to respiratory distress. For some reason, certain children are much more prone to this airway swelling than others. There seems to be a hereditary element, so when assessing a wheezy child ask about a family history of viral-induced wheeze. These children are at higher risk of developing asthma in later life.

Question 3

How can viral induced wheeze and asthma be differentiated?

Answer 3

The distinction between a viral-induced wheeze and asthma is not definitive. Generally, typical features of viral-induced wheeze (as opposed to asthma) are:

• Presenting before 3 years of age
• No atopic history
• Only occurs during viral infections

Asthma can also be triggered by viral or bacterial infections, however it also has other triggers, such as exercise, cold weather, dust and strong emotions. Asthma is historically a clinical diagnosis, and the diagnosis is based on the presence of typical signs and symptoms along with variable and reversible airflow obstruction.

Question 4

How can viral induced wheeze present?

Answer 4

Evidence of a viral illness (fever, cough and coryzal symptoms) for 1-2 days preceding the onset of:

• Shortness of breath
• Signs of respiratory distress
• Expiratory wheeze throughout the chest

Question 5

Does viral induced wheeze or asthma present with a focal wheeze?

Answer 5

Neither viral-induced wheeze or asthma cause a focal wheeze. If you hear a focal wheeze be very cautious and investigate further for a focal airway obstruction such as an inhaled foreign body or tumour. These patients will require an urgent senior review.

Question 6

How is viral induced wheeze treated?

Answer 6

Management of viral-induced wheeze is the same as acute asthma in children.

Question 7

Briefly describe the management of acute viral induced wheeze or asthma

Answer 7

Staples of management in acute viral induced wheeze or asthma are:

• Supplementary oxygen if required (i.e. oxygen saturations less than 94% or working hard)
• Bronchodilators (e.g. salbutamol, ipratropium and magnesium sulphate)
• Steroids to reduce airway inflammation: prednisone (orally) or hydrocortisone (intravenous)
• Antibiotics only if a bacterial cause is suspected (e.g. amoxicillin or erythromycin)

Question 8

Briefly describe the step up bronchodilator routine

Answer 8

Bronchodilators are stepped up as required:

• Inhaled or nebulised salbutamol (a beta-2 agonist)
• Inhaled or nebulised ipratropium bromide (an anti-muscarinic)
• IV magnesium sulphate
• IV aminophylline

Question 9

Briefly describe the management of mild cases

Answer 9

Mild cases can be managed as an outpatient with regular salbutamol inhalers via a spacer (e.g. 4-6 puffs every 4 hours).

Question 10

Briefly describe the management of moderate to severe cases

Answer 10

Moderate to severe cases require a stepwise approach working upwards until control is achieved:

1. Salbutamol inhalers via a spacer device: starting with 10 puffs every 2 hours
2. Nebulisers with salbutamol / ipratropium bromide
3. Oral prednisone (e.g. 1mg per kg of body weight once a day for 3 days)
4. IV hydrocortisone
5. IV magnesium sulphate
6. IV salbutamol
7. IV aminophylline

If you haven’t got control by this point the situation is very serious. Call an anaesthetist and the intensive care unit. They may need intubation and ventilation. This call should be made earlier to give the best chance of successfully intubating them before the airway becomes too constricted.

Question 11

Once control is established, how is viral induced wheeze or asthma stepped down?

Answer 11

Once control is established: you can gradually work your way back down the ladder as they get better:

• Review the child prior to the next dose of their bronchodilator
• Look for evidence of cyanosis (central or peripheral), tracheal tug, subcostal recessions, hypoxia, tachypnoea or wheeze on auscultation
• If they look well, consider stepping down the number and frequency of the intervention
• A typical step down regime of inhaled salbutamol is 10 puffs 2 hourly then 10 puffs 4 hourly then 6 puffs 4 hourly then 4 puffs 6 hourly