Viral Induced Wheeze -- Case Study

Source: VIW.pptx (38 slides) Author: Sammy Reed Date: January 2025 Format: Respiratory IST Case Study Learning Level: Primarily Intermediate/Advanced

Key Learning Points

• VIW pathophysiology and epidemiology
• Ventilation concepts: peak pressures, plateau pressures, auto-PEEP
• Capnometry and capnography interpretation
• Medications used in acute wheeze (aminophylline, IV magnesium, ketamine)
• Physiotherapy interventions including DNase nebuliser
• Clinical reasoning through a complex PICU case

1. Case Study — Clinical Narrative

Presenting Complaint (HPC)

Past Medical History (PMH)

Learning Level: Foundation/Intermediate

2. Definition and Pathophysiology of VIW

What Is Viral Induced Wheeze?

• Caused by viruses e.g. RSV, rhinovirus, metapneumovirus, bocavirus
• Results in wheeze and hyperproduction of sputum
• Children have narrower airways — more susceptible
• Largely affects children between 6 months and 5 years
• More episodes of wheeze = more likely to develop asthma

Speaker Notes: Difficult to diagnose asthma in under-5s: difficulty in differentiation from bronchiolitis, asthma medications do not work well in this age group, pulmonary function testing cannot be completed (NIH).

Epidemiology and Risk Factors

Learning Level: Foundation

3. Post-Intubation Assessment

Initial Blood Gas (Post Intubation)

Speaker Notes: Lactate at the time of admission can be valuable in helping identify paediatric patients at greater risk for inpatient mortality. Normal lactate: 0.5—2 (arterial).

Learning Level: Intermediate/Advanced

Initial Physiotherapy Assessment

Ventilator Settings

Clinical Findings

Learning Level: Advanced

4. Ventilation Concepts

Peak Pressures

• Maximum inspiratory pressure
• Need to overcome airway and alveolar resistance
• Affected by compliance and resistance

Speaker Notes: Individually, the smaller airways have much higher resistance than larger airways such as the trachea. However, the significant downstream branching of the airways means there are many smaller airways in parallel. This reduces the total resistance to airflow. Due to the vast number of bronchioles present within the lungs running in parallel, the highest total resistance is actually in the trachea and larger bronchi.

• Resistance = change in pressure divided by flow
• This is the total sum of resistance in the “patient circuit” — from the tubes connecting the patient to the ventilator, to the bronchi, the chest wall, the lung parenchyma, the distended abdomen, the bronchopleural fistula. This is the net product of all these factors.
• Resistance is only a meaningful concept while there is flow. No flow means there is nothing to resist, and therefore whatever pressure one measures in the absence of flow is generated purely by the elastic components of the circuit — mainly, the patient’s lungs.
• Compliance = volume divided by change in pressure

Learning Level: Intermediate

Plateau Pressures

• Pressure applied to small airways and alveoli
• Measured via an inspiratory hold for 3—5 seconds (at end of inspiration)
• At this point, flow = 0
• The hold allows for redistribution of gases
• Influenced only by compliance, not resistance

Speaker Notes:

• Inspiratory pause — no flow — therefore no resistance — so only has compliance to deal with
• Will cause redistribution of pressure across the lungs
• Shows you your alveolar pressure (i.e. the actual pressure going into alveoli rather than PIP)
• During this inspiratory pause, there is loss of resistance due to flow throughout the airways, and there is a redistribution of pressure across the lung, which results in a total loss of elastic energy stored in the airways, lung tissue and chest wall tissue
• Therefore really don’t want above 30

Learning Level: Intermediate/Advanced

Interpreting Peak vs Plateau Pressures

In this case: High peak (35) with low plateau (21) = resistance issue (consistent with bronchospasm/VIW)

Speaker Notes — Ventilation Strategy for VIW:

• FiO2: Lowest required to achieve SpO2 of 90—92%
• Tidal volume: Small, protective 5—7 ml/kg
• Respiratory rate: Slow, 10—12 breaths per minute (or even less)
• Use a long expiratory time, with I:E ratio 1:3 or 1:4
• Use a volume-controlled mode, or any other mode with a square flow waveform (i.e. constant flow) — this decreases the peak airway pressure
• Reset the pressure limits (i.e. ignore high peak airway pressures) as not a true representation of PIP
• Use heavy sedation
• Use neuromuscular blockade
• Use minimal PEEP when the patient is paralysed, and titrate PEEP to work of triggering once the patient is breathing spontaneously
• Keep the Pplat below 25 cmH2O to prevent dynamic hyperinflation
• Permissive hypercapnia can be tolerated as long as the patient remains adequately oxygenated

Learning Level: Advanced

Auto/Intrinsic PEEP

• Alveoli continually not fully emptying on expiration
• Will lead to dynamic hyperinflation
• Causes: e.g. bronchospasm, secretions

Measuring Auto-PEEP: Expiratory Hold

• 3—5 second expiratory hold on ventilator
• Will give you a total PEEP value
• This should equal the set PEEP
• If total PEEP > set PEEP = auto-PEEP is present

Case Example

Learning Level: Intermediate/Advanced

5. Capnometry and Capnography

Credit: Penny Wilcox

Capnometry (The Number)

• Non-invasive, continuous measurement of CO2
• Measures the concentration (partial pressure) of end-expiratory CO2 in exhaled air
• Reduces need for blood gases (if correlates)
• Accurately records respiratory rate

Speaker Notes: CO2 is a waste product of metabolism. Diffuses 20x more easily than O2 as it is more soluble.

Capnography (The Waveform)

• Graphical representation of the level of exhaled CO2
• Each waveform represents one respiratory cycle
• Good objective marker of respiratory status

Learning Level: Intermediate

Normal Capnogram — Phases

Speaker Notes — Capnogram Components:

• A = Baseline — beginning of expiration should be at 0
• B = Transitional Part — represents mixing of dead space and alveolar gas
• C = The Alpha Angle — represents the change to alveolar gas
• D = The Alveolar Part — represents the change to alveolar gas
• E = The End-tidal CO2 value
• F = The Beta Angle — represents the change to inspiration
• G = Inspiration — curve shows rapid decrease in CO2 concentration

What to Look for on Capnography

1. Baseline starting at 0
2. Height (EtCO2 value)
3. Frequency (respiratory rate)
4. Shape of the waveform

Abnormal Capnography Patterns

Decreasing / Loss of EtCO2

Increasing EtCO2

Speaker Notes: Higher CO2 leads to higher HR leads to higher cardiac output.

Bronchospasm / Obstruction of Expiration

• “Shark fin” pattern on capnography — characteristic of airway obstruction/bronchospasm

CO2 Rebreathing

• Increasing EtCO2
• Inspiratory waveform won’t reach baseline of 0
• Caused by inadequate expiratory time

Speaker Notes: Shallow breathing, faulty expiratory valve, inadequate inspiratory flow.

Learning Level: Advanced

6. Medications

Aminophylline

Learning Level: Intermediate

IV Magnesium

Speaker Notes: Acetylcholine (parasympathetic system) contracts smooth muscle. Calcium initiates smooth muscle contraction.

Literature review note (2026): Evidence for IV MgSO4 in the VIW age group (6 months to 5 years) is actually negative. The Pruikkonen 2018 randomised double-blind trial in young children with virus-induced wheezing found IV MgSO4 to be ineffective (p=0.594). In contrast, the Cochrane review (Griffiths & Kew 2016, CD011050) found IV MgSO4 effective for reducing hospital admissions in children >2 years with moderate-severe asthma. The case patient described here is 21 months old and the literature does not support IV MgSO4 as an evidence-based pharmacological adjunct in this age group. This material should be reviewed by the clinical author. References: Pruikkonen H et al. Eur Respir J. 2018;51(2):1701579. DOI: 10.1183/13993003.01579-2017. Griffiths B, Kew KM. Cochrane Database Syst Rev. 2016;(4):CD011050. DOI: 10.1002/14651858.CD011050.pub2.

Learning Level: Intermediate

Ketamine

Learning Level: Intermediate

7. Physiotherapy Assessment and Interventions

Pharmacological Adjuncts Available

• Atrovent nebuliser
• Salbutamol nebuliser
• Aminophylline
• Magnesium

Speaker Notes (Salbutamol): New evidence — include consideration of salbutamol nebuliser use.

Physiotherapy Techniques

Learning Level: Advanced

Manual Decompression

Speaker Notes: Insufficient evidence base but anecdotally of benefit with patients with gas trapping / severe hyperinflation in acute severe asthma.

Theory: Manual compression of the chest wall to decrease air-trapping of the alveoli to allow for improved tidal volumes.

A patient with status asthmaticus in respiratory failure on mechanical ventilation usually has a significant amount of air trapping that results in intrinsic PEEP, which may be worsened by maintaining PEEP during exhalation.

Learning Level: Advanced

DNase Nebuliser

Learning Level: Advanced

Evidence for DNase

Study 1: Hollander et al. (2020)

“Use of DNase in PICU: current literature and a national cross-sectional survey”

• Systematic review — 1 RCT identified
• DNase (instilled) vs 0.9% saline
• Population: children post cardiac surgery
• Result: Reduced mechanical ventilation by 1 day
• Observational studies showed: improved atelectasis, CXR scores

Study 2: Vettleson et al. (2023)

“DNase in mechanically ventilated children with bronchiolitis”

• Single centre retrospective review
• Population: bronchiolitis admission to PICU with mechanical ventilation
• Excluded if received >48 hours post mechanical ventilation
• Dose: BD DNase 2.5 mg, median duration 6 days
• Results: DNase group had:
  • Longer mechanical ventilation by 33 hours
  • Improved Oxygen Saturation Index (OSI)
  • Longer length of stay: 2 days on PICU and in hospital

Learning Level: Advanced

8. Key Learning Points (Summary)

1. Utilise your additional markers — plateau pressure, capnography, flow loops
2. Use clinical reasoning — integrate all available information
3. Medical management alongside physiotherapy — understand the medications being used
4. Things may not be right first time — be prepared to adjust your approach

Learning Level: Intermediate/Advanced

9. References

Ventilation and Pressure

1. Differentiating Peak and Plateau Pressures — CriticalCareNow
2. The normal capnograph waveform | Deranged Physiology
3. Plateau Pressure - an overview | ScienceDirect Topics
4. Inspiratory pause, I:E ratio and inspiratory rise time | Deranged Physiology
5. Auto-PEEP: how to detect and how to prevent — a review - PubMed (nih.gov)
6. How I Teach Auto-PEEP: Applying the Physiology of Expiration | ATS Scholar (atsjournals.org)
7. Flow, volume, pressure, resistance and compliance | Deranged Physiology
8. Clinical review: Mechanical ventilation in severe asthma - PMC (nih.gov)

VIW and Asthma

9. Viral-Induced Wheeze and Asthma Development - PMC (nih.gov)
10. Oxford University Hospitals — Paediatric Wheeze guideline: 13916Pwheeze.pdf (ouh.nhs.uk)

Medications

11. Intravenous aminophylline for acute severe asthma in children over two years receiving inhaled bronchodilators - PMC (nih.gov)
12. Ketamine versus aminophylline for acute asthma in children: A randomized, controlled trial - PMC (nih.gov)
13. Treating acute severe asthma attacks in children: using aminophylline | European Respiratory Society (ersnet.org)
14. Aminophylline | Drugs | BNF | NICE
15. Intravenous magnesium sulfate for acute wheezing in young children: a randomised double-blind trial | European Respiratory Society (ersnet.org)
16. Magnesium sulfate | Drugs | BNF | NICE
17. Role of Intravenous Magnesium in the Management of Moderate to Severe Exacerbation of Asthma: A Literature Review - PMC (nih.gov)
18. Ketamine in status asthmaticus: A review - PMC (nih.gov)
19. Ketamine | Deranged Physiology

DNase

20. Nebulised deoxyribonuclease for viral bronchiolitis in children younger than 24 months - PMC (nih.gov)
21. Dornase alfa in mechanically ventilated children with bronchiolitis: A retrospective cohort study (wiley.com)
22. Use of dornase alfa in the paediatric intensive care unit: current literature and a national cross-sectional survey - PMC (nih.gov)
23. Recombinant Human Deoxyribonuclease Shortens Ventilation Time in Young, Mechanically Ventilated Children (wiley.com)