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ASTHMA IN THE EMERGENCY DEPARTMENT

Postgraduate Emergency Medicine - Exam-Oriented Notes

Based on GINA 2026 | Rosen's EM | Goldman-Cecil | GINA 2026 Strategy Report

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1. DEFINITIONS (High-Yield for MCQ)

Exam trap: In asthma, a "NORMAL PaCO2" during an attack = early DANGER sign (patient is tiring, losing ability to hyperventilate). Rising PaCO2 = impending respiratory failure.

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2. PATHOPHYSIOLOGY (Key Points)

• Triad: Airway inflammation + Bronchospasm + Mucus hypersecretion
• Air trapping causes dynamic hyperinflation and auto-PEEP (intrinsic PEEP)
• Increased work of breathing leads to respiratory muscle fatigue
• Pulsus paradoxus (>10 mmHg inspiratory BP drop) indicates severe obstruction
• Hyperlactatemia in acute asthma - from albuterol/increased work of breathing - does NOT indicate deterioration (exam favorite!)
• Leukocytosis with asthma: caused by corticosteroids + catecholamines demarginating PMNs - does NOT indicate infection

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3. SEVERITY CLASSIFICATION (GINA 2026)

Adults, Adolescents, Children 6-11 years

NEW in GINA 2026: Oxygen supplementation NOT recommended unless SpO2 <92%. Target SpO2 = 92-95% in adults/adolescents/children 6-11. Target ≥92% in children ≤5 years.

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4. INITIAL ASSESSMENT IN ED (GINA 2026)

What to do IMMEDIATELY:

1. Measure SpO2 by pulse oximetry (routine)
2. PEF or FEV1 - document BEFORE starting treatment (strongly recommended)
3. Monitor lung function at 1 hour and at intervals thereafter
4. ABG and chest X-ray: NOT routinely required
   • ABG: only if SpO2 <92% or unable to assess clinically
   • CXR: only if suspecting pneumothorax, pneumonia, pneumomediastinum, heart failure
5. VBG screening trick: PaCO2 on VBG <40 mmHg likely excludes hypercapnia

POCUS (Point-of-Care Ultrasound):

• Comet-tail sign (B-lines) = high accuracy in distinguishing acute heart failure vs. asthma/COPD

Pulsus paradoxus:

• 10 mmHg = significant obstruction

• 25 mmHg = severe

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5. TREATMENT - STEP-BY-STEP (GINA 2026 + Rosen's EM)

STEP 1 - OXYGEN

• Give O2 only if SpO2 <92%
• Target: 92-95% (avoid hyperoxia)
• Deliver via nasal cannula or face mask

STEP 2 - BRONCHODILATORS (SABA)

• pMDI + spacer: 4-8 puffs (100 mcg/puff) every 20 minutes x 3 doses
• Nebulizer: 2.5-5 mg every 20-30 minutes (can give continuously in severe cases)
• GINA 2026 NEW: Shake the inhaler before EVERY actuation (suspension pMDIs including salbutamol, budesonide-formoterol, fluticasone-salmeterol) - prevents ultra-high accidental dosing
• GINA 2026 NEW: ICS-formoterol (budesonide-formoterol) is now an ALTERNATE option to SABA for mild exacerbations in primary care/ED in adults, adolescents, and children 6-11 years

STEP 3 - IPRATROPIUM BROMIDE (Add for Moderate-Severe)

• Dose: 0.5 mg nebulized (or 8 puffs MDI, 18 mcg/puff) every 20 min x 3 doses
• Add with EACH salbutamol treatment for first 3 doses
• Mechanism: blocks parasympathetic bronchoconstriction
• Benefit: reduces hospitalizations + greater PEF improvement vs. SABA alone
• Onset: 30-120 min; duration 6 hours

STEP 4 - SYSTEMIC CORTICOSTEROIDS

• Give to ALL patients with acute asthma exacerbation (as early as possible)
• Oral = IV in efficacy (oral preferred unless vomiting/very ill)
• Prednisone: 40-50 mg/day orally (standard)
• IV Methylprednisolone: 40-60 mg (Goldman-Cecil) or 125 mg/day in 1-2 divided doses (Rosen's) if oral not feasible
• Dexamethasone alternative: 16 mg/day x 2 days (oral) or 10 mg IM single dose at discharge
• Effects begin within hours, peak at ~24 hours
• NO dose tapering needed for courses <2 weeks
• ICS + systemic steroids: No established benefit to routinely adding ICS on top of systemic steroids in ED

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6. SECOND-LINE / ADJUNCT THERAPIES (Exam Favorites)

IV Magnesium Sulfate

• Mechanism: Ca2+ channel blockade → smooth muscle relaxation; inhibits cholinergic transmission; stabilizes mast cells; stimulates NO/prostacyclin
• Indications (GINA/Rosen's):
  • Adults: severe asthma (PEF <25%) or persistent hypoxia after initial treatment
  • Children: PEF <60% after 1 hour of care
• Dose: 2 g IV over 20 min (adult); 25-75 mg/kg (max 2 g) over 20 min (pediatric)
• Evidence: Meta-analysis 2025 (PMID 40562459) confirms benefit of IV MgSO4 added to SABA + steroids in children with acute severe asthma
• Side effects: warmth, flushing, nausea, muscle weakness, hypotension, loss of DTRs, respiratory depression (dose-related)
• Exam trap: Studies excluding patients with severe asthma showed NO benefit - use only in SEVERE cases

Epinephrine

• Indication: Asthma WITH anaphylaxis/angioedema
• GINA 2026 NEW KEY POINT: If patient has BOTH anaphylaxis and asthma features, give epinephrine FIRST, then bronchodilators (previously unclear)
• Dose: 0.3-0.5 mg IM (EpiPen)
• Also adjunct for status asthmaticus (SC/IM 0.5 mg)

Heliox (Helium-Oxygen mixture)

• Low-density gas reduces turbulent airflow, improves laminar flow
• Consider for severe refractory asthma not responding to 1 hour of conventional therapy
• Can be used to drive nebulized albuterol
• Limited evidence; not first-line

Ketamine

• Preferred induction agent for RSI in asthmatics
• Dose: 1-2 mg/kg IV
• Properties: bronchodilatory + sympathomimetic → ideal in asthmatic
• Infusion: 1 mg/kg/hr post-intubation (per Harriet Lane)
• Alternative induction: Propofol 1.5-2 mg/kg (also bronchodilatory; caution - hypotension in volume-depleted patients)

Montelukast (Leukotriene Receptor Antagonist)

• Add as early as possible if patient not already on it
• Dose: 10 mg (adult); zafirlukast 20 mg alternative
• Adjunct role in ED

Theophylline / Methylxanthines

• NOT recommended for acute asthma in ED (last-line historical option only)
• High toxicity, narrow therapeutic window, no added benefit over current regimens

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7. NONINVASIVE VENTILATION (NIV/BiPAP)

• Indications: Severe resistant asthma, impending respiratory failure, to buy time before intubation
• Benefits: Reduces respiratory muscle fatigue, improves FRC, may decrease need for intubation and hospitalization
• Requirements: Alert mental status + intact airway reflexes + ICU admission mandatory
• BiPAP well tolerated in children with status asthmaticus
• Monitor: Serial ABGs during NIV - watch for rising PaCO2 → sign of failure → proceed to intubation
• Important: NIV is NOT a substitute for intubation when clearly indicated

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8. ENDOTRACHEAL INTUBATION + MECHANICAL VENTILATION

Indications for Intubation:

• Coma or altered consciousness
• Cardiac/respiratory arrest
• Paradoxical breathing pattern
• Refractory hypoxemia
• Failure of NIV
• Rising PaCO2 with clinical deterioration

RSI Protocol (Rosen's EM):

• Use large ETT (≥8.0 mm adult) - facilitates suctioning, mucus plug removal, bronchoscopy
• Induction: Ketamine 1-2 mg/kg (preferred) or Propofol 1.5-2 mg/kg
• Paralytic: Succinylcholine 1.5 mg/kg OR Rocuronium 1 mg/kg
• Post-intubation sedation: Propofol, Lorazepam, or Fentanyl (avoid histamine-releasing opioids)

Ventilator Strategy - "Permissive Hypercapnia" Approach:

Key concept: Priority = DECREASE HYPERINFLATION, not correct hypercarbia. Auto-PEEP/breath-stacking is the main killer in intubated asthmatics. Prolonged I:E ratio (1:3 to 1:5) is critical.

Auto-PEEP Detection:

• Look for breath-stacking on flow-time waveform
• "Disconnect test" - disconnect and allow passive deflation; if O2 improves, confirms auto-PEEP

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9. DISCHARGE CRITERIA (GINA 2026)

• PEF or FEV1 >70% predicted (or personal best) after treatment
• Symptoms minimal, no nocturnal awakening expected
• SpO2 ≥92% on room air
• Able to use inhaler correctly
• Discharge medications arranged

1. ICS-containing maintenance therapy (regular; SABA-only discharge is NOT recommended - GINA 2026)
2. Reliever medication (as-needed, not scheduled routine)
3. Short course OCS: Prednisone 40-50 mg/day x 5-7 days
4. Review inhaler technique before discharge

• Adults: within 2-7 days
• Children: within 2-5 days

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10. ADMISSION INDICATIONS

• Failure to improve with treatment (PEF <40% at 2 hours)
• Severe/life-threatening features persist
• SpO2 <92% despite O2
• Need for resuscitation on arrival
• RR >22/min at reassessment
• PEF <50% predicted on exit
• Previous history of intubation or ICU admission for asthma
• Lack of response to initial bronchodilator therapy
• Poor social circumstances / lack of reliable follow-up

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11. STATUS ASTHMATICUS - SPECIFIC MANAGEMENT

• Rising PaCO2 without airflow improvement
• Major complication: pneumothorax, pneumomediastinum, acute MI, cardiac arrhythmia (other than sinus tach)

• Hospital admission (mandatory)
• Frequent inhaled beta-agonists (continuous nebulization option)
• IV corticosteroids
• IV Magnesium 2g over 20 min
• Consider BiPAP trial (if alert + intact airway reflexes)
• If failing NIV or obtunded → RSI with ketamine
• Permissive hypercapnia on ventilator
• Consider isoflurane inhalation therapy (refractory pediatric cases - Harriet Lane)

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12. SPECIAL SITUATIONS

Pregnancy (High-Yield Exam Topic)

• Normal PaCO2 in pregnancy = 28-32 mmHg (due to chronic hyperventilation)
• Therefore: A PaCO2 of 40 mmHg in a pregnant asthmatic = HYPERCARBIA = danger sign
• Treat the same; beta-agonists, steroids are safe
• Oxygen more critical - fetal hypoxia threshold lower

Pediatric (Children ≤5 years - GINA 2026 New)

• Salbutamol 4-6 puffs pMDI/spacer OR 2.5 mg nebulized, every 20 min
• Ipratropium 4 puffs with each salbutamol, up to 3 times
• Target SpO2 ≥92%
• Treat immediately with O2 + SABA before completing full assessment if in acute distress
• Systemic steroids (can be given)
• IV MgSO4 25-75 mg/kg (max 2g) over 20 min if severe

Anaphylaxis + Asthma (GINA 2026 NEW)

• Give epinephrine FIRST, then bronchodilators (exam-ready new guideline point)

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13. NEW BIOLOGICS IN SEVERE ASTHMA (GINA 2026)

GINA 2026 NEW: Depemokimab (long-acting anti-IL-5) and generic anti-IgE added to biologic options for severe eosinophilic asthma.

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14. HIGH-YIELD EXAM MNEMONICS

"ATOM" for Acute Asthma Treatment:

• A - Albuterol (SABA, first line)
• T - Theophylline (NOT recommended acutely)
• O - Oxygen (only if SpO2 <92%)
• M - Magnesium (severe, refractory)

Signs of Life-Threatening Asthma - "PACES":

• P - PaCO2 rising (or normal)
• A - Altered consciousness (drowsy/confused)
• C - Cyanosis
• E - Exhaustion/unable to speak
• S - Silent chest

Ventilator in Asthma - "LOW LOW HIGH":

• Low tidal volume (6-8 mL/kg)
• Low respiratory rate (<10/min)
• High inspiratory flow rate (>60 L/min)

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15. KEY GINA 2026 UPDATES - EXAM ALERT TABLE

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16. EXAM-STYLE QUICK FACTS

• Drug of choice for RSI in asthmatic: Ketamine
• Best surrogate for ventilatory failure in ED (quick): VBG PaCO2 <40 mmHg excludes hypercapnia
• Lactate in acute asthma: Elevated (from albuterol/work of breathing) - does NOT reflect deterioration
• Leukocytosis in asthma: From steroids/catecholamines - does NOT indicate infection
• Pulsus paradoxus >25 mmHg: Severe obstruction
• Normal PaCO2 in acute asthma attack: DANGER SIGN (patient fatiguing)
• Theophylline in acute asthma: NOT recommended (last-line historical)
• Oral vs IV steroids: Equivalent efficacy - prefer oral unless vomiting/critically ill
• No benefit: ICS added to systemic steroids in ED
• CXR indication in asthma: Suspected pneumothorax, pneumonia, or no response to therapy
• Tiotropium in ED: No clear role currently
• Auto-PEEP target: Minimize by low RR + high inspiratory flow + low TV

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17. SOURCES & REFERENCES

• Rosen's Emergency Medicine: Concepts and Clinical Practice - Chapters 59 & 32 (Asthma in ED, Asthma Medications)
• Goldman-Cecil Medicine, International Edition - Asthma in the Emergency Department
• GINA 2026 Strategy Report (published May 5, 2026) - ginasthma.org GINA 2026 PDF
• GINA 2026 Key Highlights via Guideline Central summary
• IV MgSO4 in pediatric acute severe asthma: PMID 40562459 - Systematic review, Arch Dis Child, 2025
• Second-line treatment in pediatric severe asthma: PMID 40562457 - Meta-analysis, Arch Dis Child, 2025
• Chinese Thoracic Society Guidelines for Bronchial Asthma 2024 Edition: PMID 40050074
• AHA 2025 CPR Guidelines - Special Circumstances (Asthma): PMID 41122889

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Exam Tip: GINA 2026 is the most current guideline (published May 2026). Focus areas for PG EM exams: severity classification, stepwise treatment, O2 targets, magnesium dosing, ketamine for RSI, permissive hypercapnia ventilation strategy, discharge criteria, and the new epinephrine-first rule for anaphylaxis+asthma. Biologics are increasingly tested in written and viva exams.

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APPROACH TO ACUTE DYSPNEA IN THE EMERGENCY ROOM

Postgraduate Emergency Medicine - Exam-Oriented Notes

Sources: Tintinalli's EM | Harrison's Principles 22E (2025) | SFMU/FICS Guidelines 2026 | AHA/ACC 2026 PE Guidelines | Recent PubMed Evidence

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1. DEFINITION (Harrison's 22E, 2025)

"Dyspnea is a subjective experience of breathing discomfort that consists of qualitatively distinct sensations that vary in intensity. The experience derives from interactions among multiple physiological, psychological, social, and environmental factors and may induce secondary physiological and behavioral responses." -- American Thoracic Society Consensus Statement, as cited in Harrison's 22E

• Dyspnea = a SYMPTOM - can ONLY be self-reported by the patient
• Signs of increased work of breathing (tachypnea, accessory muscle use, intercostal retraction) = measurable by clinicians and are NOT synonymous with dyspnea

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2. EPIDEMIOLOGY (Harrison's 22E)

• Accounts for 3-4 million ED visits per year
• Present in up to 50% of inpatients and 25% of ambulatory patients
• Prevalence: 9-13% in general community; rises to 37% in adults aged ≥70 years
• Cardiac + pulmonary causes account for up to 85% of all dyspnea
• Up to one-third of patients have multifactorial dyspnea
• Post-COVID syndrome: increasing cause of chronic dyspnea (Harrison's 22E, Ch. 205)
• Degree of dyspnea predicts outcomes in COPD better than FEV1 (GOLD guidelines)

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3. MECHANISMS OF DYSPNEA (Harrison's 22E - Exam Favorite)

A. Afferent Signals to CNS

1. Peripheral chemoreceptors (carotid body, aortic arch) + central chemoreceptors (medulla)
   • Activated by: hypoxemia, hypercapnia, acidemia
   • Produce: sensation of "air hunger"
2. Mechanoreceptors (upper airways, lungs, chest wall)
   • Stretch receptors, irritant receptors, J receptors (lung)
   • Muscle spindles, tendon organs (chest wall)
   • Increased resistance → chest tightness (asthma/COPD)

B. Efferent Signals (CNS to respiratory muscles)

• Increased motor output to overcome obstruction/stiffness

C. Efferent-Reafferent Mismatch (Key Concept)

• When CNS motor commands do not match the mechanical response (e.g., neuromuscular disease, severe obstruction) → most intense dyspnea
• Also called neuromechanical dissociation

Exam tip: "Efferent-reafferent mismatch" = the single most important mechanism for the most severe dyspnea. Appears in Harrison's 22E viva questions.

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4. QUALITATIVE DESCRIPTORS - DIAGNOSTIC CLUES (Harrison's 22E)

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5. DIFFERENTIAL DIAGNOSIS OF ACUTE DYSPNEA IN ED

"CARDS" Framework for Life-Threatening Causes (Must Not Miss):

Full Differential by System (Harrison's 22E, Table 34-3):

• Airways disease: Asthma, COPD exacerbation, upper airway obstruction (foreign body, epiglottitis, anaphylaxis)
• Parenchymal: Pneumonia, ARDS, ILD exacerbation
• Pleural: Pneumothorax, massive pleural effusion, hemothorax
• Vascular: Pulmonary embolism, pulmonary hypertension
• Neuromuscular: Guillain-Barré, myasthenic crisis, diaphragm paralysis

• Acute decompensated heart failure (HFrEF, HFpEF)
• ACS / acute MI
• Cardiac tamponade
• Arrhythmias (new-onset AF, SVT, VT)
• Valvular emergencies (acute MR, AR)

• Severe anemia (high-output dyspnea)
• Metabolic acidosis (Kussmaul breathing)
• DKA, salicylate toxicity, CO poisoning
• Anaphylaxis
• Anxiety/panic disorder (diagnosis of exclusion)
• Obesity hypoventilation, obstructive sleep apnea

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6. INITIAL ASSESSMENT IN THE ED - ABC FIRST

SFMU/FICS 2026 Guidelines (France - Joint Expert Consensus, Ann Intensive Care 2026):

• Patients with respiratory distress must be immediately triaged and assessed for severity
• RR should be measured for a minimum of 30 seconds on arrival (not just "eyeballed")
• Continuous monitoring of: SpO2, HR, SBP, RR, temperature, level of consciousness throughout ED stay

• Continuous vital sign monitoring for moderate-severe dyspnea reduces morbidity/mortality
• Use a numerical scale to assess dyspnea components (strong agreement, expert opinion)

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7. HISTORY - KEY ELEMENTS (Harrison's 22E + Tintinalli's EM)

Onset and Timeline:

• Sudden onset (<1 hour): Tension pneumothorax, PE, arrhythmia, ACS, foreign body, anaphylaxis
• Hours: Asthma exacerbation, ADHF, pneumonia
• Days: COPD exacerbation, pleural effusion, worsening ILD, slow progression PE

Positional Symptoms:

• Orthopnea → CHF, obesity, diaphragmatic weakness
• PND (2-3 hours after sleep) → CHF
• Platypnea → hepatopulmonary syndrome, ASD, left atrial myxoma (RARE - exam classic)
• Trepopnea (better in lateral decubitus) → pleural effusion, unilateral lung disease

Associated Symptoms:

• Chest pain + dyspnea → ACS, PE, pneumothorax, aortic dissection, pericarditis
• Fever + dyspnea → pneumonia, empyema, pericarditis, myocarditis, sepsis
• Leg swelling + dyspnea → DVT-PE, CHF
• Palpitations + dyspnea → arrhythmia, SVT, VT
• Stridor + dyspnea → upper airway obstruction (epiglottitis, foreign body, anaphylaxis, angioedema)
• Wheezing + dyspnea → asthma, COPD, cardiac asthma (CHF), anaphylaxis

Risk Factors (Tintinalli's EM):

• Immobility, recent surgery, DVT/PE history, malignancy → PE
• Smoking, occupational exposure → COPD, malignancy
• Drug injection → pneumonia, septic emboli, pulmonary edema, TB
• Known CHF, LVEF impairment → ADHF
• Prior intubation → asthma severity marker

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8. PHYSICAL EXAMINATION - TARGETED APPROACH

Vital Signs - Diagnostic Clues:

Systematic Examination:

• Stridor → upper airway obstruction (inspiratory = above glottis; expiratory = below)
• Position (tripod, sniffing) → epiglottitis, severe obstruction

• Symmetric chest expansion? No → pneumothorax, effusion, endobronchial obstruction
• Accessory muscle use → severe obstruction or reduced compliance
• Silent chest → life-threatening asthma (no air movement)
• Tracheal deviation → tension pneumothorax (AWAY from affected side)
• Percussion: Hyperresonance → PTX; Dullness → effusion/consolidation

• JVP raised: ADHF, tamponade, tension PTX, massive PE
• S3 gallop: ADHF (high sensitivity for elevated LVEDP)
• Muffled heart sounds: tamponade (Beck's triad: hypotension + raised JVP + muffled sounds)
• Peripheral edema: CHF, DVT, cor pulmonale

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9. INVESTIGATIONS - STEPWISE APPROACH

Immediate (All Acute Dyspnea):

1. SpO2 (continuous pulse oximetry) - mandatory
2. 12-lead ECG - ACS, arrhythmia, RV strain (PE), LBBB (CHF)
3. CXR (portable) - effusion, cardiomegaly, pneumothorax, consolidation, pulmonary edema
4. ABG / VBG - oxygenation, ventilation, acid-base status
   • VBG: PaCO2 <40 mmHg reliably excludes hypercapnia (Rosen's EM)
   • ABG: required when SpO2 <92% or clinical concern for hypercapnia

Biomarkers:

Exam Alert - SFMU/FICS 2026: There is insufficient evidence to recommend CRP or procalcitonin to guide antibiotic initiation in acute dyspnea with suspected lower respiratory tract infection.

Point-of-Care Ultrasound (POCUS) - NOW STANDARD OF CARE:

• Taheri et al., Eur J Emerg Med, 2025 (PMID 39630617) - Systematic review + meta-analysis: POCUS in prehospital non-trauma acute dyspnea improves diagnostic accuracy
• Russell et al., Am J Emerg Med, 2024 (PMID 38522242) - Prehospital lung ultrasound for ADHF: high diagnostic accuracy
• Jeffers et al., J Emerg Med, 2025 (PMID 40845678) - POCUS utility for pulmonary edema: systematic review

• B-lines (≥3 per zone in bilateral zones) = interstitial edema
• LV dilation with decreased function = cardiogenic shock
• Comet-tail sign: high diagnostic accuracy differentiating acute heart failure from COPD/asthma

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10. CLINICAL DECISION TOOLS

A. Pulmonary Embolism (PE)

1. Age <50
2. HR <100
3. SpO2 ≥95%
4. No unilateral leg swelling
5. No hemoptysis
6. No recent surgery/trauma
7. No prior PE/DVT
8. No exogenous estrogen use

If ALL 8 absent in LOW pre-test probability patient → PE excluded. No D-dimer needed.

• Score >4: PE likely → CTPA
• Score ≤4 + D-dimer negative: PE excluded
• Age-adjusted D-dimer (if age >50): threshold = age × 10 µg/L (increases specificity)

• Risk stratification now drives care setting decisions
• Low-risk PE: can be discharged from ED with outpatient management
• Intermediate-high risk: hospitalization required
• High-risk (hemodynamically unstable): systemic thrombolysis or catheter-directed therapy

B. Acute Heart Failure (Framingham Criteria):

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11. SYSTEMATIC APPROACH - DIAGNOSTIC ALGORITHM FOR ACUTE DYSPNEA IN ED

ACUTE DYSPNEA ARRIVES IN ED
         ↓
IMMEDIATE: ABC → SpO2, IV access, monitor
         ↓
IS PATIENT IN IMMEDIATE DANGER?
- SpO2 <90%, altered consciousness, impending arrest
         ↓ YES                    ↓ NO
  Airway management         Focused assessment
  RSI if needed             History + Exam
  (Ketamine preferred       CXR, ECG, SpO2
  if bronchospasm)          Blood tests
         ↓                         ↓
    POCUS (BLUE Protocol)
         ↓
  A-lines?    B-lines?    Effusion?    No lung sliding?
  Asthma/COPD  CHF/edema   PTX/effusion  Pneumothorax
         ↓
  Biomarkers: BNP/NT-proBNP, troponin, D-dimer, lactate
         ↓
DIAGNOSE AND TREAT

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12. SPECIFIC DIAGNOSES - EMERGENCY MANAGEMENT

A. Acute Decompensated Heart Failure (ADHF)

• First-line: IV furosemide (0.5-1 mg/kg), nitrates (sublingual/IV) for preload/afterload reduction
• CPAP/BiPAP: Reduces need for intubation; improves outcomes in cardiogenic pulmonary edema
• BNP-guided therapy improves outcomes (Fuster & Hurst's, 15th Edition)
• Avoid aggressive fluid in HFpEF

B. COPD Exacerbation

• SABA + SAMA nebulization, systemic steroids, controlled O2 (target SpO2 88-92%)
• NIV (BiPAP): First-line for moderate-severe COPD exacerbation (reduces intubation rates, improves mortality)
• Antibiotics if purulent sputum or CXR infiltrate

C. Acute Asthma

• SABA (salbutamol), ipratropium, systemic steroids
• IV MgSO4 2g for severe refractory asthma
• GINA 2026: O2 only if SpO2 <92%; target 92-95%
• RSI with ketamine if intubation needed

D. Pulmonary Embolism

• Anticoagulation: LMWH or UFH (UFH preferred if high-risk/lytic candidate)
• High-risk PE (hemodynamic instability): systemic thrombolysis (alteplase 100 mg IV over 2 hours)
• AHA/ACC 2026: PE Response Team (PERT) activation for complex cases
• Low-risk PE: direct discharge on DOAC (rivaroxaban/apixaban)

E. Tension Pneumothorax

• DO NOT WAIT for CXR
• Immediate needle decompression: 2nd ICS, midclavicular line (or 4th/5th ICS, anterior axillary line)
• Followed by chest tube insertion

F. Anaphylaxis

• Epinephrine IM 0.3-0.5 mg = first-line (vastus lateralis)
• Airway management (early intubation if angioedema progressing)
• Antihistamines + steroids = adjuncts

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13. OXYGENATION STRATEGIES IN ACUTE DYSPNEA (2026 Evidence)

SFMU/FICS 2026 (prehospital): NIV in prehospital setting is supported by evidence for COPD + CHF; HFNC emerging in prehospital context.

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14. SPECIAL POPULATIONS

Pregnancy

• Normal PaCO2 = 28-32 mmHg (physiologic hyperventilation)
• PaCO2 of 40 mmHg in pregnant patient = HYPERCARBIA = danger sign
• Common causes: PE (increased risk), peripartum cardiomyopathy, amniotic fluid embolism (AFE)
• AFE: acute hypoxia + cardiovascular collapse + DIC in peripartum → catastrophic

Injection Drug Users (Tintinalli's EM, Chapter on substance-related presentations):

• Infectious: Pneumonia, TB, septic pulmonary emboli (right-sided endocarditis), opportunistic infections (HIV)
• Non-infectious: Noncardiogenic pulmonary edema (heroin/cocaine), talc lung, reactive airway disease (cocaine), hypersensitivity reactions
• "Pocket shooting" → PTX/hemothorax
• Place all febrile IVDU with dyspnea in respiratory isolation until TB is excluded

Elderly:

• Dyspnea prevalence rises to 37% (age >70) - Harrison's 22E
• Atypical presentations of ACS (no chest pain, only dyspnea)
• Higher rates of HFpEF, pulmonary hypertension

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15. DISPOSITION CRITERIA

Discharge criteria (low-risk):

• SpO2 ≥95% on room air after treatment
• Symptoms resolved, underlying cause identified and treated
• Reliable follow-up arranged
• No high-risk features (abnormal vitals, high-risk ECG, elevated troponin, large effusion)

Admit criteria:

• SpO2 <92% despite treatment
• Hemodynamic instability
• Persistently abnormal vital signs
• High-risk diagnoses (PE, ADHF, ACS, pneumonia requiring IV antibiotics)
• Failure to improve with ED therapy
• Uncertain diagnosis with high-risk features

ICU/HDU criteria:

• Respiratory failure requiring NIV or intubation
• Hemodynamic compromise (septic shock, massive PE, tamponade)
• Rising PaCO2 with altered consciousness
• ARDS

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16. ASSESSING DYSPNEA SEVERITY - SCALES

Modified Medical Research Council (mMRC) Dyspnea Scale (Harrison's 22E, Table 39-1)

Borg Scale (Acute Setting)

• 0 = nothing at all; 10 = maximal
• Use numerical rating scale in ED (SFMU/FICS 2026 recommendation)

AVPU / GCS for Mental Status

• Altered consciousness + dyspnea = emergency; do not delay airway management

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17. HIGH-YIELD EXAM QUICK FACTS

• Most common cause of acute dyspnea in ED: CHF and COPD exacerbation
• Cardiac + pulmonary causes combined: account for 85% of all dyspnea (Harrison's 22E)
• BNP <100 pg/mL: NPV 90% for ruling out CHF as cause of dyspnea
• PERC negative in low-probability patient: no further PE workup needed
• Platypnea (dyspnea worse sitting up): hepatopulmonary syndrome, left atrial myxoma, ASD - EXAM CLASSIC
• Silent chest in asthma: no wheeze = air entry too poor to generate wheeze = worse than wheeze
• Normal PaCO2 in an asthmatic attack = danger sign (fatiguing patient)
• Tracheal deviation AWAY from affected side: tension pneumothorax (not always present)
• Beck's Triad: hypotension + raised JVP + muffled heart sounds = cardiac tamponade
• POCUS B-lines: bilateral = pulmonary edema; absent lung sliding = pneumothorax
• Lung point on POCUS: pathognomonic for pneumothorax
• CRP/PCT: insufficient evidence to guide antibiotics in acute dyspnea with suspected LRTI (SFMU/FICS 2026)
• Morphine for dyspnea (palliative): start at 0.05 mg/kg IV (HALF the pain dose) - Tintinalli's EM
• Efferent-reafferent mismatch: the key mechanism of severe dyspnea (Harrison's 22E)
• Post-COVID dyspnea: now a recognized entity; often multifactorial (Harrison's 22E, Ch. 205)
• Age-adjusted D-dimer (age × 10 µg/L for age >50): increases specificity without losing sensitivity
• HFNC: generates ~2 cmH2O PEEP per 10 L/min flow; can reach FiO2 100%

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18. MNEMONICS

Causes of Acute Dyspnea - "ASTHMA + CPE":

• A - Asthma/Airway obstruction
• S - Sepsis/SIRS
• T - Tension pneumothorax/Tamponade
• H - Heart failure (acute decompensated)
• M - Massive PE/MI
• A - Anemia/Acidosis
• +
• C - COPD exacerbation
• P - Pneumonia/Pneumothorax
• E - Embolism (PE)

Life-Threatening Dyspnea - "TOPPLE":

• T - Tension pneumothorax
• O - Obstructed airway (foreign body, epiglottitis, anaphylaxis)
• P - PE (massive)
• P - Pulmonary edema (flash)
• L - Left ventricular failure
• E - Embolism/Electrical (cardiac tamponade, arrhythmia)

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19. CARDIOPULMONARY EXERCISE TESTING (CPET) - Harrison's 22E

• Achieves predicted maximal ventilation
• Increased dead space
• Desaturation on exercise
• Bronchospasm development

• HR >85% predicted max
• Early anaerobic threshold
• Falling O2 pulse (stroke volume proxy)
• Exercise-induced ECG ischemic changes
• BP fall with exercise

CPET is the definitive tool for unexplained dyspnea when basic workup is non-diagnostic.

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20. KEY REFERENCES

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Exam Strategy: In PG EM exams, acute dyspnea questions test: (1) identifying the life-threatening cause first (tension PTX, massive PE, flash pulmonary edema, tamponade), (2) correct use of POCUS BLUE protocol and biomarkers, (3) knowing cutoffs for BNP/D-dimer/PERC, (4) mechanism questions from Harrison's (efferent-reafferent mismatch), and (5) ventilation strategy selection (HFNC vs BiPAP vs ETI). Descriptors like "platypnea" and "air hunger" are classic differentiating MCQ targets from Harrison's 22E.

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Presentation Overview — 13 Slides

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APPROACH TO ACUTE HYPOXIA IN THE EMERGENCY ROOM

Postgraduate Emergency Medicine - Exam-Oriented Notes

Sources: Tintinalli's EM | Harrison's Principles 22E (2025) | SFMU/FICS Guidelines 2026 | UK-ROX Trial (JAMA 2025) | Recent PubMed Evidence

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1. DEFINITIONS - MUST KNOW (Tintinalli's EM + Harrison's 22E)

Hypoxia vs Hypoxemia - KEY DISTINCTION (Exam Trap)

"Hypoxia and hypoxemia are NOT interchangeable; one can occur without the other."

• Tintinalli's EM, Chapter 62

• Polycythemia: low PaO2 (hypoxemia) but normal tissue O2 delivery → NO tissue hypoxia
• Severe anemia: normal PaO2 (no hypoxemia) but low O2 carrying capacity → TISSUE HYPOXIA
• CO poisoning: normal PaO2 (pulse oximetry falsely normal!) → SEVERE tissue hypoxia

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2. OXYGEN PHYSIOLOGY - KEY FORMULAE (Tintinalli's EM, Chapter 62)

Arterial Oxygen Content (CaO2):

• Dissolved O2 (0.0031 × PaO2) = tiny contribution (~1.5% at room air)
• Hb-bound O2 (1.38 × Hb × SaO2) = dominant factor (~98.5%)

Oxygen Delivery (DO2):

Clinical implication: Tissue hypoxia can occur even with normal SpO2 if:

• Cardiac output is low (cardiogenic shock)
• Hemoglobin is low (severe anemia)
• Hemoglobin is non-functional (CO, MetHb)

Alveolar Gas Equation (A-a Gradient Calculation):

• Young healthy adults: <10 mmHg
• Age-adjusted: 2.5 + (0.21 × age in years) ± 11 mmHg (Tintinalli's EM)
• Simplified formula: A-a gradient = 145 - PaO2 (breathing room air at sea level)

Exam tip: Supine position (common ED patient position) raises A-a gradient even in healthy patients - factor this into interpretation (Tintinalli's EM).

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3. FIVE MECHANISMS OF HYPOXEMIA (Tintinalli's EM + Harrison's 22E)

Critical Exam Points:

• Hypoventilation = only mechanism with raised PaCO2 + NORMAL A-a gradient
• Shunt = only mechanism where 100% O2 FAILS to correct hypoxemia (hallmark)
• V/Q mismatch = most common mechanism of hypoxemia in clinical practice
• V/Q mismatch and shunt are on a spectrum - shunt = extreme V/Q mismatch (V=0, Q persists)
• PaO2/FiO2 ratio: used to quantify severity of gas exchange impairment

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4. TYPES OF HYPOXIA BY CAUSE (Harrison's 22E, Chapter 42)

A. Respiratory/Hypoxemic Hypoxia

• Ventilation-perfusion mismatch (most common)
• Hypoventilation (associated with raised PaCO2)
• Right-to-left shunt (intrapulmonary or intracardiac)
• Diffusion impairment

B. Anemic Hypoxia

• PaO2 NORMAL; SaO2 NORMAL (unless concurrent lung disease)
• Absolute O2 carrying capacity reduced
• Venous PO2 drops more than usual (greater extraction per unit blood)
• Causes: hemorrhage, hemolysis, iron deficiency, thalassemia

C. Circulatory/Stagnant Hypoxia

• PaO2 NORMAL; SaO2 NORMAL
• Reduced tissue perfusion → increased O2 extraction → low venous PO2
• Arteriovenous O2 difference (a-v O2 gap) INCREASES
• Causes: cardiogenic shock, distributive shock, hypovolemic shock, local ischemia
• Can precipitate CHF in pre-existing heart failure (increased demand on impaired heart)

D. Carbon Monoxide (CO) Hypoxia - SPECIAL CASE

• Hemoglobin binds CO (forming carboxyhemoglobin, COHb) → unavailable for O2 transport
• COHb shifts Hb-O2 dissociation curve to the LEFT → O2 unloads only at lower tissue tensions
• PaO2 normal; pulse oximetry FALSELY NORMAL (reads COHb as OxyHb)
• Causes: smoke inhalation, gas heaters, car exhaust, house fires
• Clinical sign: Cherry-red skin/flush (Harrison's 22E) - NOT cyanosis

E. Histotoxic Hypoxia

• Cells cannot utilize O2 despite adequate delivery
• Cause: Cyanide toxicity (blocks cytochrome c oxidase in mitochondria)
• PaO2 normal; mixed venous O2 HIGH (cells cannot extract O2)
• Paradox: high SvO2 + lactic acidosis = cyanide poisoning

F. High Altitude Hypoxia

• Reduced FiO2 due to ↓ barometric pressure
• Denver (5400 ft): barometric pressure 620 mmHg; PiO2 = 130 mmHg (vs 160 at sea level) - Tintinalli's EM
• At 3000 m (~10,000 ft): alveolar PO2 drops to ~60 mmHg
• High altitude illness spectrum: Acute Mountain Sickness → HACE → HAPE

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5. RESPONSES TO HYPOXIA (Harrison's 22E, Chapter 42)

Cellular Response:

• Aerobic → Anaerobic metabolism switch (Pasteur effect)
• ATP production falls dramatically
• Severe hypoxia: membrane depolarization → Ca2+ influx → phospholipases/proteases activation → cell swelling → apoptosis → cell death

Molecular Response:

• HIF-1 (Hypoxia-Inducible Factor-1): master transcription factor for hypoxia response
• Upregulates: glycolytic enzymes (PGK, PFK), glucose transporters (GLUT-1, GLUT-2), VEGF, erythropoietin

Vascular Response:

• Systemic arterioles: DILATE (KATP channels open → reduced ATP) → ↑ tissue perfusion
• Pulmonary vasculature: CONSTRICT (K+ channel inhibition → depolarization → Ca2+ influx → smooth muscle contraction)
  • Hypoxic pulmonary vasoconstriction (HPV) = redirects blood from poorly-ventilated to better-ventilated lung segments
  • But increases pulmonary vascular resistance and RV afterload
  • Can cause acute cor pulmonale in diffuse lung disease

Cardiovascular Response:

• Acute: ↑ myocardial contractility → ↑ cardiac output
• Prolonged: depressed myocardial contractility
• Systemic vasodilation → ↑ cardiac output (dangerous in pre-existing heart disease)
• Cerebrovascular resistance ↓ → ↑ cerebral blood flow
• BUT if hypoxia + hyperventilation → PaCO2 ↓ → cerebrovascular resistance ↑ → cerebral blood flow ↓ → worsens brain hypoxia

CNS Effects:

• Mild: impaired judgment, motor incoordination (resembles alcohol intoxication)
• Moderate: headache, dizziness, somnolence, insomnia
• Severe: seizures, coma
• Critical (PaO2 <20 mmHg): brainstem dysfunction → respiratory failure → death (Tintinalli's EM)

Compensatory Mechanisms (Tintinalli's EM):

1. ↑ Minute ventilation (immediate)
2. Pulmonary arterial vasoconstriction (regional - improves V/Q, worsens if diffuse)
3. ↑ Sympathetic tone → ↑ heart rate → ↑ cardiac output
4. Chronic: ↑ red cell mass (erythropoietin-driven polycythemia)
5. Chronic: ↓ tissue O2 demands

• Key threshold: Compensatory mechanisms ALWAYS activated when PaO2 reaches 60 mmHg
• Failure threshold: PaO2 <20 mmHg - compensatory mechanisms fail

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6. CLINICAL FEATURES OF HYPOXIA

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7. CYANOSIS (Harrison's 22E, Chapter 42)

Definition:

Critical Exam Distinctions:

Central Cyanosis:

• SaO2 is reduced OR abnormal Hb derivative present
• Skin AND mucous membranes both affected
• Causes: lung disease, R→L shunt, high altitude, MetHb, SulfHb

Peripheral Cyanosis:

• SaO2 is NORMAL; peripheral vasoconstriction → increased O2 extraction locally
• Mucous membranes usually SPARED (check sublingual/conjunctivae)
• Causes: cold exposure, shock, heart failure, peripheral vascular disease

Why Cyanosis is an Unreliable Indicator (Harrison's 22E):

• Central cyanosis detectable at SaO2 85% in fair-skinned persons
• In dark-skinned patients: not detectable until SaO2 drops to 75% → SIGNIFICANT delay
• Examine mucous membranes (oral cavity, conjunctivae) in dark-skinned patients
• Anemia + hypoxemia: may have NO visible cyanosis (absolute reduced Hb too low despite ↓ SaO2)
• Polycythemia: cyanosis at higher SaO2 levels (more reduced Hb in absolute terms)
• CO poisoning: cherry-red flush (NOT cyanosis) - Harrison's 22E

Methemoglobinemia:

• MetHb = Fe3+ form of Hb, cannot carry O2
• Causes: dapsone, nitrites, local anaesthetics (benzocaine, prilocaine), sulfonamides, nitrates
• Cyanosis + chocolate-brown blood (cyanosis refractory to O2)
• Pulse oximetry reads ~85% regardless of true saturation
• Treatment: Methylene blue 1-2 mg/kg IV
• SpO2 falsely reads ~85% regardless of true SaO2

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8. PULSE OXIMETRY - PRINCIPLES AND LIMITATIONS

Principles:

• Beer-Lambert law: uses light absorption at 660 nm (red) and 940 nm (infrared)
• Measures SpO2 (peripheral O2 saturation) as proxy for SaO2

SFMU/FICS 2026 Guidelines (KEY):

• R3.2.2: SpO2 alone is PROBABLY SUFFICIENT to assess oxygenation in most patients
• SpO2 >96% on room air makes PaO2 <60 mmHg UNLIKELY
• In COPD patients: SpO2 >92% makes significant hypoxemia unlikely
• R3.2.1: Venous blood gas (VBG) is NOT recommended for assessing hypoxemia (GRADE 1, high evidence)

Limitations of Pulse Oximetry - EXAM FAVORITES:

2024 Landmark Evidence: Martin et al., Br J Anaesth 2024 (PMID 38368234) - Systematic review + meta-analysis: Darker skin tone associated with overestimation of arterial O2 saturation by pulse oximetry. This has implications for occult hypoxemia in dark-skinned patients (health equity issue).

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9. ACUTE RESPIRATORY FAILURE - CLASSIFICATION

Definition (SFMU/FICS 2026):

• PaO2 <60 mmHg (SpO2 <90%) on room air
• OR PaO2/FiO2 ratio <300 mmHg on supplemental oxygen

Type I vs Type II Respiratory Failure:

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10. BERLIN DEFINITION OF ARDS (Tintinalli's EM)

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11. ASSESSMENT IN THE ED - SYSTEMATIC APPROACH

Step 1: Stabilize First (ABC)

• Airway patency
• SpO2 monitoring (continuous) - MANDATORY
• IV access + cardiac monitoring

Step 2: Quantify Hypoxia

ABG Interpretation Framework for Hypoxia:

Step 1: Is PaO2 <60 mmHg? → YES = Hypoxemia confirmed
Step 2: Is PaCO2 elevated (>45 mmHg)?
        → YES = Type II (Hypoventilation component)
        → NO  = Type I (Hypoxemic failure)
Step 3: Calculate A-a gradient
        NORMAL  → Hypoventilation OR Low FiO2
        ELEVATED → V/Q mismatch / Shunt / Diffusion impairment
Step 4: Response to 100% O2 trial:
        Corrects → V/Q mismatch / Diffusion / Hypoventilation
        Does NOT correct → R→L Shunt (cardiac or pulmonary)

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12. DIFFERENTIAL DIAGNOSIS OF ACUTE HYPOXIA IN ED

By Onset:

By SpO2 Response to Supplemental O2:

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13. OXYGEN THERAPY - TARGETS AND DELIVERY (2025-2026 Evidence)

O2 Saturation Targets (Updated 2025-2026):

LANDMARK: UK-ROX Trial (JAMA, August 2025 - PMID 40501321): Conservative oxygen therapy (SpO2 target 90-94%) in mechanically ventilated critically ill adults - large multicenter RCT. Results showed conservative targets were safe and potentially beneficial compared to liberal O2 targets.

OXY-BREATHES Meta-analysis (Crit Care Med, May 2026 - PMID 41661051): Conservative vs liberal oxygen targets in mechanically ventilated patients - systematic review confirms conservative O2 targets are safe and reduce exposure to hyperoxia.

Dangers of Hyperoxia - "Oxygen Toxicity":

• Reactive oxygen species (ROS) → pulmonary toxicity, tracheobronchitis
• Absorption atelectasis: pure O2 washes out N2 that normally "splints" alveoli open
• COPD: suppresses hypoxic drive (Haldane effect); worsens hypercapnia
• AMI/Stroke: coronary/cerebral vasoconstriction at high PaO2
• Reperfusion injury post-ROSC
• Retinopathy of prematurity in neonates
• Avoid SpO2 98-100% in most patients without specific indication

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14. OXYGEN DELIVERY DEVICES AND FiO2

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15. HIGH-FLOW NASAL CANNULA (HFNC) - KEY FACTS

Mechanism:

• Delivers up to 60 L/min of heated, humidified O2
• Generates 2-3 cmH2O of PEEP (positive pressure) - minimal but helps recruit alveoli
• Washes out nasopharyngeal dead space → reduces effective dead space → improves CO2 clearance
• Reduces work of breathing
• FiO2 up to 1.0 (100%)

ROX Index (HFNC Failure Predictor):

• ROX ≥4.88 at 12 hours: HFNC likely to succeed - avoid intubation
• ROX <3.85: High risk of HFNC failure → consider intubation
• [Yau et al., Crit Care 2023 (PMID 37605238)]: ROX index validated for COVID-19 HFNC prediction

Indications for HFNC:

• Acute hypoxemic respiratory failure (Type I)
• Post-extubation support
• Apneic oxygenation during RSI
• High-risk intubation pre-oxygenation

HFNC for Apneic Oxygenation During RSI (2026):

• [Geary et al., J Emerg Med 2026 (PMID 42407213)]: Systematic review - HFNC during RSI in ED decreases desaturation during intubation attempt

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16. NONINVASIVE VENTILATION (NIV) IN HYPOXIA

CPAP:

• Single pressure applied throughout breathing cycle
• Increases FRC; prevents alveolar collapse; reduces work of breathing
• Best for: cardiogenic pulmonary edema, OSA, mild hypoxemia
• Does NOT assist ventilation (no inspiratory pressure support)

BiPAP:

• IPAP (inspiratory) higher than EPAP (expiratory)
• Provides ventilatory assistance + PEEP
• Best for: COPD exacerbation (Type II failure), moderate hypercapnia, ADHF, post-extubation
• Reduces intubation rates in COPD and CHF

NIV Contraindications (relative):

• Inability to protect airway (reduced GCS, uncooperative)
• Vomiting (aspiration risk)
• Facial trauma/burns
• Untreated pneumothorax
• Cardiovascular instability (relative)

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17. INVASIVE MECHANICAL VENTILATION - LUNG-PROTECTIVE STRATEGY

ARDS Net Protocol (Tintinalli's EM + Harrison's 22E):

Auto-PEEP: Check that expiratory flow returns to zero before next breath. If present: disconnect from ventilator 15-20 seconds + decrease RR (Tintinalli's EM).

Prone Positioning in ARDS:

• PROSEVA trial: 16 hours prone per day in moderate-severe ARDS (PF ratio <150) → mortality benefit
• Mechanism: improved V/Q matching, drainage of secretions, better lung homogeneity

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18. SPECIAL CONDITIONS IN ED HYPOXIA

A. CO Poisoning

• Pulse oximetry FALSELY NORMAL
• Diagnosis: co-oximetry (ABG machine) shows elevated COHb
• Treatment: 100% O2 via NRM (half-life COHb 5 hrs → 90 mins with 100% O2)
• Severe (COHb >25%, neuro symptoms, pregnancy): Hyperbaric O2 (HBO)
• Cherry-red skin = COHb > ~30-40% (NOT cyanosis)

B. Methemoglobinemia

• Pulse oximetry reads ~85% regardless of true SaO2
• "Chocolate-brown" blood on ABG (visual inspection of blood sample)
• SpO2 does NOT improve with 100% O2 (key diagnostic clue)
• PaO2 may be normal on ABG (dissolved O2 still diffuses normally)
• Treatment: Methylene blue 1-2 mg/kg IV (reduces MetHb → Hb via NADPH pathway)
• Note: Methylene blue FAILS in G6PD deficiency → use ascorbic acid or exchange transfusion

C. PE with Hypoxia

• V/Q mismatch (most common mechanism)
• Dead space ventilation (unperfused lung)
• R→L shunt via reopened PFO (raised RV pressure)
• Treatment: anticoagulation; systemic thrombolysis if hemodynamically unstable

D. Tension Pneumothorax

• Immediate life-threatening hypoxia
• Clinical diagnosis: tracheal deviation (away), absent breath sounds, hypotension
• Do NOT wait for CXR
• Needle decompression: 2nd ICS, MCL; followed by chest drain

E. Flash Pulmonary Edema (Hypertensive Emergency)

• Sudden ADHF → severe bilateral infiltrates → shunt + V/Q mismatch
• Treatment: IV nitrates (first-line), CPAP, IV furosemide

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19. LACTATE AND OXYGEN DEBT

DO2/VO2 Relationship (Tintinalli's EM, Chapter 32):

• DO2-independent VO2: Normal state - O2 consumption maintained despite ↓ delivery by increasing extraction ratio (OER)
• DO2-dependent VO2: When DO2 falls below critical level, VO2 also falls → anaerobic metabolism → lactate rises
• Critical DO2 (~10 mL/kg/min): threshold below which VO2 becomes dependent on delivery

Central Venous O2 (ScvO2):

• Normal: 65-75%
• ScvO2 <65%: Increased O2 extraction → ↓ DO2 or ↑ VO2 (tissue hypoxia)
• ScvO2 >75-80%: Reduced O2 extraction → sepsis (maldistribution), histotoxic hypoxia (cyanide), or high CO
• Tintinalli's EM, Chapter 32: troubleshoot abnormal ScvO2 by assessing DO2 and VO2 components

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20. HIGH-YIELD EXAM QUICK FACTS

• Hypoxia ≠ Hypoxemia: One can occur without the other (Tintinalli's EM + Harrison's 22E)
• CO poisoning: Normal SpO2, normal PaO2 on ABG - tissue hypoxia; cherry-red skin - NOT cyanosis
• Shunt: the ONLY mechanism where 100% O2 fails to correct hypoxemia
• Hypoventilation: the ONLY mechanism with NORMAL A-a gradient + elevated PaCO2
• HIF-1 (Hypoxia-Inducible Factor-1): master regulator of hypoxia adaptation (Harrison's 22E)
• Cyanosis in anemia: May be ABSENT even with severe desaturation (not enough absolute reduced Hb)
• MetHb: SpO2 reads ~85% regardless of actual SaO2; chocolate-brown blood; treat with methylene blue
• SFMU/FICS 2026: VBG NOT recommended to assess degree of hypoxemia (GRADE 1, high evidence)
• SFMU/FICS 2026: SpO2 >96% on room air makes PaO2 <60 mmHg unlikely
• SpO2 and dark skin: Overestimates saturation in dark-skinned patients (Martin et al., 2024)
• UK-ROX Trial (JAMA 2025): Conservative O2 (SpO2 90-94%) safe in ICU ventilated patients
• OXY-BREATHES (Crit Care Med 2026): Meta-analysis confirms conservative O2 targets reduce hyperoxia exposure
• ROX Index ≥4.88 at 12 hrs: HFNC success; <3.85 = high failure risk → consider intubation
• ARDS target: SpO2 88-95% OR PaO2 55-80 mmHg (NOT 100%)
• Histotoxic hypoxia (cyanide): HIGH mixed venous O2 + lactic acidosis (cells cannot use O2)
• Hypoxic pulmonary vasoconstriction: beneficial regionally (V/Q); harmful diffusely (cor pulmonale)
• Critical PaO2: 60 mmHg (activates all compensatory mechanisms); 20 mmHg (compensatory failure)
• Peripheral cyanosis vs central: Check sublingual mucosa and conjunctivae (especially in dark-skinned patients) - Harrison's 22E
• Hypertonic HFNC PEEP: ~2 cmH2O per 10 L/min of flow; 60 L/min = ~6 cmH2O PEEP

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21. MNEMONICS

Types of Hypoxia - "ARCH":

• A - Anemic (low Hb)
• R - Respiratory (V/Q mismatch, shunt, diffusion)
• C - Circulatory (low cardiac output/perfusion)
• H - Histotoxic (cyanide; cells can't use O2)
• (+ High altitude + CO - special types)

Five Mechanisms of Hypoxemia - "HV-DRS":

• H - Hypoventilation (normal A-a, high PaCO2)
• V - V/Q Mismatch (raised A-a, improves with O2)
• D - Diffusion Impairment (raised A-a, improves with O2)
• R - Right-to-Left Shunt (raised A-a, DOES NOT improve)
• S - Subnormal FiO2 (normal A-a, improves with O2)

Pulse Oximetry False Readings - "MCAP":

• M - Methemoglobinemia (~85% regardless)
• C - Carboxyhemoglobin (falsely HIGH)
• A - Anaemia/low perfusion (unreliable)
• P - Pigmented skin (overestimates)

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22. KEY REFERENCES

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Top Exam Tips for PG EM Boards:

1. Distinguish hypoxia vs hypoxemia (Harrison's - always examinable)
2. Know the 5 mechanisms by A-a gradient + O2 response (Tintinalli's)
3. Memorize pulse oximetry pitfalls: CO (falsely normal), MetHb (~85%), dark skin (overestimates)
4. SFMU/FICS 2026: VBG cannot assess hypoxemia; SpO2 >96% excludes PaO2 <60 mmHg
5. Conservative O2 targets now supported by UK-ROX (JAMA 2025) - avoid routine high-flow O2
6. ARDS: target SpO2 88-95%, NOT 98-100%
7. ROX Index for HFNC failure prediction (≥4.88 = success at 12 hrs)
8. Cyanosis absent in anemia despite severe hypoxemia (key exam trap from Harrison's 22E)
9. HIF-1 = master transcription factor governing hypoxia response (Harrison's viva)
10. Critical threshold: all compensatory mechanisms activated at PaO2 = 60 mmHg; fail at PaO2 <20 mmHg