About This Page
This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
The Bottom Line
- Asthma = variable airflow obstruction with airway hyperresponsiveness and airway inflammation
- Diagnosis is confirmed by spirometry showing obstruction with reversibility: FEV1 increase >12% and >200 mL after bronchodilator
- Controller therapy is inhaled corticosteroid based; NAEPP supports ICS-formoterol SMART therapy for moderate to severe persistent asthma
- Acute exacerbation: inhaled albuterol + ipratropium, systemic corticosteroids, oxygen, and magnesium sulfate if severe
- Impending respiratory failure: silent chest, altered mental status, rising PaCO2, exhaustion, or worsening acidosis
Overview
Asthma is a chronic inflammatory disorder of the airways characterized by variable expiratory airflow limitation, bronchial hyperresponsiveness, and episodic symptoms. Triggers include viral infection, allergens, exercise, cold air, tobacco smoke, occupational exposure, aspirin or NSAIDs in aspirin-exacerbated respiratory disease, and beta-blockers. The key clinical distinction from COPD is variability and reversibility, although fixed obstruction can develop in long-standing disease.
Epidemiology
Asthma affects more than 25 million people in the United States, including approximately 1 in 13 people. It is common in children and persists or recurs in adulthood. Risk is increased by atopy, allergic rhinitis, eczema, family history, obesity, prematurity, occupational sensitizers, and environmental tobacco smoke. Asthma morbidity and mortality are disproportionately higher among Black, Puerto Rican, and low-income populations in the United States.
Clinical Features
Symptoms
Episodic wheeze, cough, chest tightness, and dyspnea
Nocturnal cough or early-morning symptoms
Symptoms triggered by exercise, allergens, viral infection, cold air, smoke, or occupational exposure
Symptoms improve with albuterol or removal from trigger
Severe exacerbation with inability to speak full sentences
Altered mental status, cyanosis, exhaustion, or syncope during an attack
Signs
Diffuse expiratory wheeze with prolonged expiratory phase
Tachypnea, tachycardia, and accessory muscle use during exacerbation
Pulsus paradoxus in severe exacerbation
Silent chest despite respiratory distress
Normal examination between attacks
Rising PaCO2 or respiratory acidosis in acute asthma
Investigations
First-line
Spirometry with bronchodilator testingObstructive pattern with reduced FEV1/FVC and reversibility: FEV1 increase >12% and >200 mL after albuterol supports asthma
Peak expiratory flowUseful for monitoring severity and variability. Acute severe asthma often has PEF <50% predicted; life-threatening if <25% or unmeasurable
Pulse oximetryAssess acute severity. SpO2 <92% suggests severe exacerbation and need for arterial or venous blood gas assessment
Second-line
Methacholine challengeUsed if symptoms suggest asthma but baseline spirometry is normal. Negative test has high negative predictive value
FeNOElevated fractional exhaled nitric oxide supports type 2 airway inflammation and steroid responsiveness; not diagnostic alone
Chest X-rayNot routine for stable asthma; obtain if pneumonia, pneumothorax, foreign body, or alternative diagnosis is suspected
Specialist
Allergy testingSkin prick or serum specific IgE if allergic triggers affect control or immunotherapy is being considered
ABG or VBGIn severe exacerbation. Normal or rising PaCO2 is ominous because early asthma usually causes hypocapnia
1
Long-term control
- Avoid triggers, assess inhaler technique, provide a written asthma action plan, and treat comorbid allergic rhinitis, GERD, obesity, and OSA
- Intermittent asthma: as-needed short-acting beta agonist remains acceptable in NAEPP; many clinicians use ICS whenever SABA is used to reduce exacerbation risk
- Persistent asthma: inhaled corticosteroid is the foundation of controller therapy
- Moderate to severe persistent asthma: ICS-formoterol single maintenance and reliever therapy is preferred in NAEPP for appropriate patients
- Add-on options: LABA with ICS, LAMA such as tiotropium, leukotriene receptor antagonist, or biologic therapy for severe type 2 asthma
2
Acute exacerbation
- Oxygen to maintain SpO2 93-95%
- Inhaled albuterol by MDI with spacer or nebulizer; repeat frequently in the first hour
- Add inhaled ipratropium for moderate to severe exacerbation
- Systemic corticosteroid: prednisone PO or methylprednisolone IV; give early because onset is delayed
- IV magnesium sulfate for severe exacerbation or poor response to initial bronchodilators
- Intubation if respiratory arrest, severe exhaustion, altered mental status, refractory hypoxemia, or worsening hypercapnia
3
Step-up and step-down care
- Step up if symptoms, rescue inhaler use, nighttime awakenings, or exacerbations indicate poor control
- Before stepping up, check adherence, inhaler technique, exposure, and alternative diagnosis
- Step down after at least 3 months of good control using the lowest effective controller dose
- Annual influenza vaccination and routine age-appropriate immunization
Complications
- Status asthmaticus: Severe exacerbation refractory to initial bronchodilator therapy with risk of respiratory failure
- Respiratory failure: Rising PaCO2, acidosis, altered mental status, and exhaustion require ventilatory support
- Pneumothorax: Barotrauma or severe air trapping can rupture alveoli
- Airway remodeling: Long-standing inflammation can cause fixed airflow obstruction
- Medication effects: Oral corticosteroids increase risk of hyperglycemia, osteoporosis, adrenal suppression, and infection
USMLE Step 2 CK Exam Tips
- 1Reversible obstruction on spirometry = FEV1 improves >12% and >200 mL after albuterol
- 2Acute severe asthma with normal PaCO2 is bad: early asthma should cause low PaCO2 from hyperventilation
- 3Silent chest + altered mental status = impending respiratory failure, not clinical improvement
- 4Next best step in severe exacerbation after albuterol/ipratropium/steroids = IV magnesium sulfate
- 5Aspirin sensitivity + asthma + nasal polyps = aspirin-exacerbated respiratory disease; avoid COX-1 NSAIDs
- 6Long-acting beta agonist monotherapy is contraindicated in asthma; LABA must be paired with ICS
practicetest your knowledge on asthmaApply what you've learnt with USMLE Step 2 CK-style questions from the iatroX Q-Bank — respiratory and beyond.
open q-bank