5.6 Asthma, Cystic Fibrosis & Foreign-Body Aspiration

5.6 Asthma, Cystic Fibrosis & Foreign-Body Aspiration

Pediatric Asthma Exacerbation & Status Asthmaticus

Asthma is a chronic inflammatory disorder characterized by bronchial hyperreactivity and episodic, reversible airflow obstruction. Severe acute exacerbation unresponsive to initial bronchodilator therapy is termed status asthmaticus.

Pathophysiological Triad

1. Acute Bronchospasm: Contraction of hypertrophied bronchial smooth muscle triggered by allergens, viral respiratory infections, cold air, or exercise.
2. Airway Wall Edema: Microvascular leakage, inflammatory cellular infiltration (eosinophils, mast cells, T-lymphocytes), and mucosal swelling.
3. Intraluminal Mucus Hypersecretion: Tenacious, viscous mucus plugging of small conducting airways.

The Blood Gas Progression in Acute Asthma

Understanding arterial blood gas (ABG) evolution in acute asthma is paramount on the NPS specialty examination:

+-------------------------------------------------------------------------------------------------------------+
|                               BLOOD GAS EVOLUTION IN PEDIATRIC ASTHMA                                       |
+---------------------+-----------------------+---------------------------------------------------------------+
| Stage               | ABG Pattern           | Clinical Correlation & Mechanistic Interpretation             |
+---------------------+-----------------------+---------------------------------------------------------------+
| Early / Mild        | Low PaO2, Low PaCO2,  | Hyperventilation: hypoxemia drives tachypnea. Excessive       |
| Exacerbation        | High pH (Resp Alk)    | minute ventilation blows off CO2 (PaCO2 25-34 mmHg).          |
+---------------------+-----------------------+---------------------------------------------------------------+
| Impending Failure   | Low PaO2, NORMAL      | CO2 Retention Imminent: Patient is fatiguing. Despite         |
| (OMINOUS TRANSITION)| PaCO2 (38-42 mmHg),   | persistent tachypnea and marked retractions, the PaCO2 has   |
|                     | Normal pH (~7.40)     | 'normalized'. Signals diaphragmatic exhaustion and imminent   |
|                     |                       | ventilatory failure.                                          |
+---------------------+-----------------------+---------------------------------------------------------------+
| Decompensated       | Marked Hypoxemia,     | Terminal Failure: Diaphragmatic arrest, severe hypercapnia    |
| Respiratory Failure | HIGH PaCO2 (>45-50),  | with mixed respiratory and lactic acidosis. Requires immediate|
|                     | Severe Acidosis (<7.30)| noninvasive or invasive ventilatory intervention.              |
+---------------------+-----------------------+---------------------------------------------------------------+

Emergency Pharmacological Escalation Ladder

1. Inhaled Short-Acting $\beta_2$-Agonists (SABA):
   • Albuterol: Dosed via metered-dose inhaler with valved holding chamber (4 to 8 puffs every 20 minutes) or nebulized solution ($2.5\text{ to }5.0\text{ mg}$ every 20 minutes for 3 doses). For severe distress, initiate continuous albuterol nebulization at $10\text{ to }20\text{ mg/hr}$.
2. Inhaled Anticholinergics:
   • Ipratropium Bromide: Administered as $0.25\text{ mg}$ (child $<20\text{ kg}$) or $0.50\text{ mg}$ (child $>20\text{ kg}$) combined with albuterol every 20 minutes for the first 3 doses. Blocks muscarinic cholinergic receptors, reducing bronchomotor tone and drying mucus secretions.
3. Systemic Corticosteroids:
   • Oral Dexamethasone $0.6\text{ mg/kg}$ (maximum $16\text{ mg}$) daily for 2 days, or oral Prednisolone $1\text{ to }2\text{ mg/kg/day}$, or IV Methylprednisolone $1\text{ to }2\text{ mg/kg}$ every 6 hours. Blunts the inflammatory cascade, up-regulates $\beta_2$-receptor expression, and prevents late-phase relapse.
4. Intravenous Magnesium Sulfate:
   • Dosing: $25\text{ to }50\text{ mg/kg}$ IV (maximum $2.0\text{ g}$) infused over $20\text{ to }30\text{ minutes}$.
   • Mechanism: Inhibits calcium influx into bronchial smooth muscle cells, blocks neuromuscular acetylcholine transmission, and promotes direct bronchodilation. Monitor for transient hypotension and facial flushing.
5. Intravenous Terbutaline:
   • Continuous IV $\beta_2$-agonist infusion: Bolus $2\text{ to }10\text{ mcg/kg}$ over 10 minutes, followed by continuous infusion at $0.1\text{ to }1.0\text{ mcg/kg/min}$ (titrated up to $2-4\text{ mcg/kg/min}$). Indicated when dense air trapping and minimal tidal excursion prevent inhaled aerosols from depositing in distal airways.
6. Heliox Therapy (Helium-Oxygen Mixture):
   • Mixtures: 80:20 (factor $1.8$) or 70:30 (factor $1.6$). Helium has a density one-third that of nitrogen, dramatically lowering Reynolds number and transforming turbulent airflow into smooth laminar flow across narrowed airways, reducing work of breathing and enhancing particle deposition.
7. Noninvasive Positive Pressure Ventilation (BiPAP):
   • Dosed with IPAP $10\text{ to }14\text{ cmH}_2\text{O}$ and EPAP $4\text{ to }6\text{ cmH}_2\text{O}$. Offloads fatigued inspiratory muscles, stents open dynamic airways, and recruits microatelectatic zones, frequently avoiding invasive intubation.

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Foreign Body Aspiration (FBA)

Foreign body aspiration is a major cause of accidental pediatric mortality, predominantly occurring in children between 1 and 3 years of age due to oral exploratory behavior, lack of molar teeth for grinding, and uncoordinated chewing/swallowing.

Pathophysiology & Anatomical Predisposition

• Common Objects: Organic foodstuffs (peanuts, seeds, raw carrots, hot dog rounds) and small toy parts or coins. Peanuts and oily nuts release organic free fatty acids, triggering intense local chemical bronchitis and granulation tissue.
• Anatomical Site: The right mainstem bronchus is the most frequent site of lodgment due to its slightly larger caliber, more direct vertical alignment, and greater airflow distribution.
• Airway Obstruction Dynamics:
  • Bypass Valve: Partial obstruction allowing air to enter and exit; causes local wheezing.
  • Ball-Valve Obstruction: The airway expands during inspiration, allowing air to bypass the foreign body, but constricts during expiration, trapping air distally. Produces dynamic obstructive emphysema (hyperinflation).
  • Stop-Valve Obstruction: Complete occlusion preventing any airflow; alveolar air distal to the obstruction is resorbed, causing complete absorption atelectasis.

Clinical Presentation & Imaging

• Clinical Triad: The classic triad consists of: 1) Sudden, witnessed choking/coughing episode, 2) Unilateral monophonic wheeze, and 3) Asymmetric, decreased breath sounds. (Note: The triad is present in $<60%$ of cases; a clear history of sudden choking is sufficient to mandate investigation).
• Radiographic Imaging:
  • Standard chest X-rays miss most organic foreign bodies because food items are radiolucent.
  • Inspiratory and Expiratory CXR (or Bilateral Lateral Decubitus Views in Infants): During expiration (or when the child is placed in the lateral decubitus position with the affected lung down), the normal lung deflates and becomes smaller. The obstructed lung, however, exhibits air trapping: failure to collapse, persistent hyperlucency, and mediastinal shift away from the affected side.

Emergency & Definitive Management

1. Acute Choking Emergency (BLS Protocol):
   • Infant ($<1\text{ year}$): Deliver 5 firm back blows (slaps) between the scapulae, followed by 5 downward chest thrusts. Repeat until object is dislodged or infant becomes unconscious.
   • Child ($>1\text{ year}$): Perform series of subdiaphragmatic abdominal thrusts (Heimlich maneuver).
   • ABSOLUTE RULE: NEVER perform a blind finger sweep. Blind sweeping easily pushes a movable supraglottic foreign body deeper into the laryngeal aperture, causing complete obstruction.
2. Definitive Extraction:
   • Bronchoscopic removal is the definitive approach for most aspirated airway foreign bodies. Rigid bronchoscopy commonly provides airway control and a large working channel; flexible bronchoscopy can be diagnostic and, in selected centers and objects, therapeutic. Choose technique and location with the pediatric airway and bronchoscopy team.

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Worked Calculation: Heliox Flowmeter Conversion

Heliox gas mixtures have a lower physical density than 100% oxygen. Because standard hospital flowmeters are calibrated specifically for $100%\text{ }\text{O}_2$, they deliver a higher volume of heliox gas than the scale indicates. To calculate actual flow delivered:

$\text{Actual Flow Delivered} = \text{Observed Flowmeter Reading} \times \text{Heliox Factor}$

• For an 80:20 Heliox mixture, the conversion factor is $1.8$.
• For a 70:30 Heliox mixture, the conversion factor is $1.6$.

Worked Example

A respiratory therapist is delivering an 80:20 heliox mixture via a non-rebreather mask to a 6-year-old child in severe status asthmaticus. The oxygen flowmeter is currently set at an indicated reading of $10\text{ L/min}$.

Calculate the actual flow rate delivered to the patient:

$\text{Actual Flow} = 10\text{ L/min} \times 1.8 = 18\text{ L/min}$

If the clinician desires to deliver an actual flow of $14.4\text{ L/min}$ using a 70:30 heliox mixture, what should the flowmeter be set to?

$\text{Indicated Flow} = \frac{\text{Desired Actual Flow}}{\text{Heliox Factor}} = \frac{14.4\text{ L/min}}{1.6} = 9\text{ L/min}$

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NPS Exam Traps

Exam Trap 1: The 'Normal' PaCO2 in Severe Pediatric Asthma

In an asthmatic child exhibiting marked tachypnea, severe suprasternal retractions, and agitation, a 'normal' $\text{PaCO}_2$ of $38\text{ to }42\text{ mmHg}$ on arterial blood gas is NOT reassuring. A tachypneic child should be blowing off carbon dioxide ($ ext{PaCO}_2 < 32\text{ to }35\text{ mmHg}$). A 'normal' $\text{PaCO}_2$ indicates the child's diaphragm is exhausting; ventilatory failure and respiratory arrest are imminent. The correct action is immediate escalation (continuous albuterol, IV magnesium sulfate, BiPAP, or PICU transfer), not de-escalation.

Exam Trap 2: The Tongue Depressor in Epiglottitis

High fever, drooling, toxic appearance, tripod position, and stridor suggest a precarious supraglottic airway. Keep the child calm and upright, avoid forceful pharyngeal examination or nonessential procedures, and mobilize experienced pediatric anesthesia/airway and ENT clinicians in a setting with a complete difficult-airway and surgical-rescue plan. Do not let imaging or cultures delay control of a deteriorating airway.

Exam Trap 3: Bronchodilators and Steroids in Viral Bronchiolitis

Exam vignettes often present an infant with tachypnea, wheezing, and RSV bronchiolitis. Distractors will include 'nebulized albuterol every 20 minutes' or 'oral dexamethasone'. Based on AAP guidelines, bronchodilators and corticosteroids are not indicated for routine viral bronchiolitis. Select supportive care: superficial suctioning, hydration, and heated high-flow nasal cannula (HFNC).