CEN Respiratory Emergencies: The 17 Items, From Asthma and Tension Pneumothorax to Croup

The 14 topics in the CEN respiratory area

The July 2026 CEN outline gives Respiratory Emergencies 17 scored items, 11.3% of the 150 in Passy's arithmetic, tied with Neurological for the second-largest area. Its 14 topics group naturally:

Two neighbors live elsewhere on the outline. Epiglottitis is a Throat topic in the Head, Eye, Ear, Nose, Throat area, and carbon monoxide and cyanide are Toxicology topics. The clinical statements below come from Merck Manual Professional Edition chapters, cited with their review dates; they are not BCEN statements.

Severe asthma on the CEN: the quiet chest is the loud warning

Merck's asthma chapter lists the usual signs of an exacerbation: wheezing, tachypnea, tachycardia, use of accessory muscles, speech limited by dyspnea, and pulsus paradoxus, a fall in systolic pressure of more than 10 mm Hg during inspiration. Then come the findings that change the priority:

• A silent chest. With severe bronchoconstriction, airflow can be so limited that no wheeze is audible, which "may indicate disease progression or impending respiratory failure"
• Signs of impending respiratory failure: some combination of altered consciousness, cyanosis, pulsus paradoxus above 15 mm Hg, oxygen saturation below 90%, PaO2 below 60 mm Hg and PaCO2 above 45 mm Hg
• Rising carbon dioxide. Merck's exacerbation chapter says a significant increase in PaCO2 indicates progression to respiratory failure

The same chapter calls anxiolytics and morphine relatively contraindicated in an exacerbation, because of respiratory depression. Reassurance is its approach when anxiety is the trigger. Merck also notes that pulse oximeters may overestimate saturation in patients with dark skin who are hypoxemic.

COPD exacerbation: give oxygen, but control it

One idea worth unlearning is that oxygen should be withheld in COPD. Merck's COPD exacerbation chapter says oxygen "is recommended" even though it may worsen hypercapnia. For patients prone to retaining carbon dioxide, it should be given by nasal prongs or a Venturi mask so it can be closely regulated, with close monitoring. Noninvasive positive-pressure ventilation is the chapter's alternative to full mechanical ventilation in severe exacerbations.

Tension pneumothorax and other CEN chest trauma

Sources: Merck's tension pneumothorax, hemothorax and flail chest chapters.

Tension pneumothorax is the respiratory topic most likely to meet a look-alike. Cardiac tamponade also causes hypotension and distended neck veins; Merck separates the two by the one-sided absent breath sounds and hyperresonance of tension pneumothorax. The CEN cardiovascular guide covers tamponade. A non-traumatic pleural effusion gives similar one-sided findings to a hemothorax: Merck's pleural effusion chapter lists decreased breath sounds, dullness to percussion and absent tactile fremitus on the affected side.

To practice telling these pictures apart in exam-style stems, 192 Respiratory questions are planned for full CEN prep.

Pulmonary embolus on the CEN: nonspecific until it isn't

Merck's PE chapter calls the symptoms and signs nonspecific: dyspnea and pleuritic chest pain, with tachypnea and tachycardia, and it notes that significant unexplained tachycardia may be a clue. A massive (high-risk) PE is defined by right ventricular impairment causing hypotension, a systolic pressure below 90 mm Hg or a drop of at least 40 mm Hg for 15 minutes, and it can present with syncope or cardiac arrest. Merck's shock chapter lists PE among the causes of obstructive shock, alongside tension pneumothorax and tamponade.

Croup, newly named in the 2026 CEN outline

Croup appears by name for the first time in the July 2026 outline. Merck's croup chapter describes a viral illness of infants and young children, most often from parainfluenza virus, with a barking, seal-like cough, hoarseness and inspiratory stridor, often worse at night and after cold symptoms.

• Diagnosis is clinical. Laboratory tests and imaging are not routinely recommended; neck radiographs are for cases of diagnostic uncertainty
• Mild croup means no stridor at rest and no respiratory distress, and most children can be cared for at home
• Moderate to severe croup is treated with humidified oxygen, nebulized racemic epinephrine and glucocorticoids such as dexamethasone
• After racemic epinephrine, the effect is transient, and children should be monitored in a medical setting for at least 2 hours
• Not epiglottitis. Epiglottitis, retropharyngeal abscess and bacterial tracheitis look more toxic and are not associated with a barking cough

The CEN page shows an example: you can try a pediatric croup scenario from planned CEN practice with its explanation.

Inhalation injuries, obstruction, ARDS and aspiration

Smoke inhalation. Merck's smoke inhalation chapter says to suspect it with respiratory symptoms, confinement in a burning space, or carbonaceous sputum; perioral burns and singed nasal hair are clues. Swelling of the posterior pharynx can prompt early prophylactic intubation, and everyone at risk gets 100% oxygen by face mask at first. Carbon monoxide and cyanide toxicity should be considered.

Obstruction. Merck's choking procedure chapter reserves abdominal thrusts for severe obstruction: the person can't speak, cough or breathe adequately. If the person can speak, cough forcefully or breathe adequately, no intervention is required.

Respiratory distress syndrome. Merck's AHRF chapter defines acute hypoxemic respiratory failure as severe hypoxemia, such as PaO2 below 60 mm Hg or SpO2 below 88%, and ARDS as one type, caused by diffuse inflammatory lung injury.

Aspiration. The outline's aspiration topic spans two conditions that Merck's aspiration chapter separates: chemical pneumonitis, a noninfectious reaction to inhaled irritants such as gastric contents, and aspiration pneumonia, a bacterial infection after large-volume aspiration of colonized contents.

A Passy-written CEN respiratory practice item

This is an original practice item, not a BCEN question.

A patient with known COPD arrives short of breath, with an SpO2 of 84% on room air. A serum bicarbonate from a recent visit was elevated. Which oxygen plan is BEST?

• A. Withhold oxygen until a blood gas result is back
• B. Give controlled oxygen by Venturi mask and monitor
• C. Apply a high-flow nonrebreather and recheck in an hour
• D. Avoid oxygen to protect the hypoxic drive to breathe

Preferred response: B. Merck says oxygen is recommended in a COPD exacerbation even though it may worsen hypercapnia, and that for patients prone to hypercarbia, which an elevated bicarbonate may indicate, it should be given by nasal prongs or Venturi mask so it can be closely regulated, with close monitoring. Withholding oxygen (A, D) leaves the hypoxemia untreated. Unregulated high-flow oxygen without close monitoring (C) ignores the hypercapnia risk. The CEN priority questions guide explains why "withhold" options are rarely preferred.

How to review the 17 CEN respiratory items

A practical plan, not a BCEN rule:

1. Learn the "getting worse" signs for each topic. Silent chest and rising PaCO2 in asthma, tracheal deviation in pneumothorax, stridor at rest in croup.
2. Pair look-alikes across areas. Tension pneumothorax with tamponade, croup with epiglottitis, PE with ACS.
3. Use a reference from BCEN's list. Its 2026 resource list includes ENA's core curriculum and Sheehy's Manual of Emergency Care; the CEN study plan pairs Respiratory with HEENT in week 2.
4. Check yourself with mixed items. The free CEN practice questions include respiratory items across the 11 areas.

For short daily review of the thresholds above, 61 Respiratory flashcards are planned for full CEN prep; checkout isn't open yet.