Respiratory

Acute respiratory failure: hypoxemic vs hypercapnic

Clinical Overview and When to Suspect Acute Respiratory Failure

— Hypoxemic (Type 1): PaO₂ < 60 mm Hg on room air with normal/low PaCO₂; problem is gas exchange across the alveolar–capillary membrane

— Hypercapnic (Type 2): PaCO₂ > 45 mm Hg with pH < 7.35; problem is alveolar ventilation (the "pump")

— Mixed: both abnormalities (e.g., COPD exacerbation with superimposed pneumonia)

— RR > 24 or < 8, accessory muscle use, paradoxical abdominal movement, tripoding

— SpO₂ < 90% on room air, or rapid desaturation with minimal exertion

— Acute mental status change in a dyspneic patient (CO₂ narcosis vs hypoxic encephalopathy)

— "Quiet chest" in asthma, single-word dyspnea, inability to lie flat

— Acute hypercapnia: PaCO₂ rises ~10 → pH falls ~0.08

— Chronic hypercapnia: PaCO₂ rises ~10 → pH falls only ~0.03 (renal HCO₃⁻ compensation)

— A COPD patient with PaCO₂ 70 and pH 7.36 is compensated; same PaCO₂ with pH 7.22 is an acute decompensation requiring ventilatory support

— V/Q mismatch (most common — pneumonia, asthma, PE)

— Shunt (ARDS, atelectasis, pulmonary edema) — does not correct with 100% O₂

— Hypoventilation (opioids, neuromuscular) — A-a gradient normal

— Diffusion impairment (ILD, emphysema)

— Low FiO₂ / low barometric pressure (altitude)

Board pearl: The single fastest bedside discriminator between Type 1 and Type 2 failure is whether the patient is somnolent or agitated — CO₂ retention causes flapping tremor, headache, and progressive obtundation; pure hypoxemia causes restlessness and tachypnea first. Always get an ABG (not just SpO₂) when failure is suspected — pulse oximetry tells you nothing about PaCO₂ or pH.

Definition by ABG, not by symptoms alone:

When to suspect in the ED:

Acute vs chronic distinction is pH-driven:

Pathophysiologic categories of hypoxemia (memorize the five):

Presentation Patterns and Key History

— Pneumonia: fever, productive cough, focal crackles, lobar infiltrate

— Cardiogenic pulmonary edema: orthopnea, PND, pink frothy sputum, S3, JVD, bilateral B-lines

— ARDS: sepsis/aspiration/pancreatitis 12–72 h prior, bilateral infiltrates, refractory hypoxemia

— PE: sudden pleuritic chest pain, unilateral leg swelling, clear lungs, tachycardia out of proportion

— Pneumothorax: sudden unilateral pain, absent breath sounds, tall thin smoker or trauma

— COPD exacerbation: smoker, increased sputum purulence/volume, prolonged expiration, wheeze

— Asthma status: young patient, silent chest, fatigue after prolonged tachypnea (ominous — rising CO₂ in asthma = pre-arrest)

— Opioid/benzo overdose: pinpoint pupils, bradypnea, found-down

— Neuromuscular: Guillain-Barré (ascending weakness, ↓FVC), myasthenic crisis, ALS

— Obesity hypoventilation / OSA decompensation: BMI > 35, daytime somnolence, morning headaches

— Chest wall: kyphoscoliosis, flail chest, large pleural effusion

— Speed of onset (minutes = PTX/PE/anaphylaxis; hours = pneumonia/CHF; days = ILD/effusion)

— Position dependence (orthopnea → CHF; platypnea → hepatopulmonary, intracardiac shunt)

— Recent surgery/immobilization (PE)

— Aspiration risk (stroke, dementia, intoxication)

— Medication review — sedatives, beta-blockers, ACEi (angioedema)

— Occupational exposures, smoking pack-years, home O₂ requirement

Key distinction: A COPD patient who arrives somnolent with a low respiratory rate has decompensated to hypercapnic failure — do not be reassured by the "calmer" appearance. Conversely, an asthmatic who stops wheezing and looks tired is approaching respiratory arrest, not improving. Step 3 management: Both scenarios trigger immediate ventilatory support (NIV or intubation) regardless of SpO₂ that may still look acceptable.

Hypoxemic failure — classic stems:

Hypercapnic failure — classic stems:

History red flags to elicit:

Physical Exam Findings and Hemodynamic Assessment

— Accessory muscle use: sternocleidomastoid, scalene, intercostal retractions

— Nasal flaring, pursed-lip breathing (auto-PEEP in COPD)

— Tripod position, inability to speak full sentences

— Paradoxical abdominal motion (abdomen moves inward on inspiration) = diaphragmatic fatigue → imminent failure

— Bilateral crackles + S3 → cardiogenic edema

— Focal crackles + bronchial breath sounds → pneumonia/consolidation

— Diffuse wheezing → asthma/COPD; silent chest = severe obstruction

— Unilateral absent breath sounds + hyperresonance → pneumothorax; + dullness → effusion/hemothorax

— Stridor → upper airway obstruction (angioedema, foreign body, epiglottitis)

— Asterixis (flapping tremor), bounding pulse, conjunctival injection, papilledema, somnolence progressing to coma

— JVD + clear lungs + hypotension → massive PE, tension PTX, RV infarct, tamponade (obstructive shock pattern)

— JVD + crackles + S3 → LV failure

— Hypotension + warm extremities + fever → septic ARDS

— Pulsus paradoxus > 12 mm Hg → severe asthma, tamponade

— Unreliable in poor perfusion, nail polish, dark skin pigmentation (overestimates), CO poisoning (reads falsely normal — get co-oximetry)

— Methemoglobinemia: SpO₂ stuck around 85% regardless of FiO₂

Board pearl: A tachypneic patient whose RR suddenly drops from 35 to 12 with rising PaCO₂ is not improving — they are decompensating. This is a CCS pearl: intubate immediately rather than wait for further ABGs. Document neuro status before sedation to enable later prognostication.

Work-of-breathing signs (most sensitive early markers):

Auscultation patterns:

Signs of CO₂ retention:

Cyanosis: central (tongue/lips) requires ≥5 g/dL deoxyhemoglobin — insensitive in anemia, falsely present in polycythemia

Hemodynamic clues:

Pulse oximetry caveats:

Diagnostic Workup — Initial Labs, Imaging, ECG, Biomarkers

— Calculate A-a gradient: PAO₂ − PaO₂, where PAO₂ = (FiO₂ × 713) − (PaCO₂/0.8)

— Normal A-a gradient = (age/4) + 4

— Elevated A-a gradient → V/Q mismatch, shunt, diffusion defect, or low mixed venous O₂

— Normal A-a gradient with hypoxemia → hypoventilation or low FiO₂

— CBC (leukocytosis, anemia worsening O₂ delivery, polycythemia from chronic hypoxia)

— BMP (HCO₃⁻ elevation suggests chronic CO₂ retention)

— Lactate (tissue hypoxia, sepsis)

— BNP/NT-proBNP (CHF vs non-cardiac dyspnea; BNP < 100 makes CHF unlikely)

— Troponin (demand ischemia, PE-related RV strain, myocarditis)

— D-dimer (only if PE pretest probability is low/moderate by Wells)

— Procalcitonin (bacterial vs viral etiology, antibiotic stewardship)

— Bilateral infiltrates + cardiomegaly + Kerley B lines → cardiogenic edema

— Bilateral infiltrates without cardiomegaly → ARDS

— Lobar consolidation → pneumonia

— Hyperinflation, flattened diaphragms → COPD

— Wedge-shaped opacity (Hampton hump), Westermark sign → PE

— Pneumothorax (deep sulcus sign supine), pleural effusion (blunted costophrenic angle)

— Sinus tachycardia is most common in PE; S1Q3T3 is classic but only ~20% sensitive

— RV strain pattern (T-wave inversions V1–V4) — high-risk PE

— Atrial fibrillation with RVR can precipitate failure in compromised lungs

— B-lines (edema/ARDS), lung sliding absent (PTX), pleural effusion, RV dilation (PE), IVC plethora (right heart strain)

Step 3 management: Order ABG, CXR, ECG, CBC, BMP, troponin, BNP, lactate simultaneously while initiating oxygen. CCS pearl: Don't forget to advance the clock — reassess saturation and mental status every 15–30 minutes during initial stabilization.

ABG is the cornerstone — must precede definitive classification:

Initial labs:

CXR — first imaging in nearly every case:

ECG:

Bedside POCUS (rapid, high-yield):

Diagnostic Workup — Advanced or Confirmatory Studies

— Gold standard for PE when D-dimer positive or Wells score > 4

— Also identifies pneumonia, ARDS pattern, masses, dissection

— Use V/Q scan if pregnancy, contrast allergy, severe renal impairment

— Suspected ILD, atypical infection, organizing pneumonia

— Ground-glass opacities, honeycombing, traction bronchiectasis

— Differentiates cardiogenic (low EF, regional wall motion) from non-cardiogenic edema

— RV dilation/D-shaped septum → acute cor pulmonale (massive PE, severe ARDS)

— Tamponade physiology, valvular catastrophes (acute MR, aortic dissection with AI)

— Foreign body removal, massive hemoptysis localization, BAL for opportunistic infection in immunocompromised (PCP, CMV)

— Not first-line in undifferentiated failure

— Spirometry only when patient stabilized; negative inspiratory force (NIF) < −20 cm H₂O and FVC < 15 mL/kg in neuromuscular disease → intubation criteria

— Suspected OSA/OHS contributing to chronic hypercapnia — outpatient after stabilization

— 30–60 minutes after initiating NIV or changing FiO₂ — assess CO₂ trend and pH correction

— Failure of pH to rise above 7.25 or CO₂ to fall after 1–2 h of NIV → intubate

— Blood cultures × 2, sputum Gram stain and culture, urinary Strep pneumoniae and Legionella antigens, respiratory viral PCR (influenza, RSV, SARS-CoV-2), HIV test if risk factors

— Carboxyhemoglobin (smoke inhalation, faulty heater), methemoglobin (cyanosis with normal PaO₂), TSH (myxedema coma → hypoventilation)

Board pearl: Refractory hypoxemia despite 100% FiO₂ that does not correct → think right-to-left shunt (ARDS, intracardiac shunt, AVM, hepatopulmonary syndrome). Key distinction: V/Q mismatch corrects with supplemental O₂; true shunt does not.

CT pulmonary angiogram (CTPA):

High-resolution CT chest:

Echocardiogram:

Bronchoscopy:

Pulmonary function in subacute presentations:

Sleep study / nocturnal oximetry:

Repeat ABG cadence:

Microbiologic workup when infection suspected:

Specialty labs:

Risk Stratification and First-Line Management Logic

— Assess patency, protective reflexes (gag, cough), GCS

— GCS ≤ 8 or inability to protect airway → intubate regardless of ABG

— Most patients: SpO₂ 92–96%

— COPD / chronic CO₂ retainers: 88–92% (avoid hyperoxia → worsens V/Q matching and CO₂ retention via Haldane effect)

— Never withhold O₂ from a hypoxemic patient out of fear of CO₂ retention — titrate carefully

— Nasal cannula (1–6 L) → simple mask (6–10 L) → non-rebreather (10–15 L, FiO₂ ~80–90%)

— High-flow nasal cannula (HFNC): up to 60 L/min, FiO₂ to 100%, provides modest PEEP; preferred in hypoxemic failure not responding to standard O₂ (ROX index guides success)

— NIV (BiPAP): first-line in COPD exacerbation with pH 7.25–7.35 and cardiogenic pulmonary edema

— CPAP: cardiogenic edema, OSA decompensation

— Intubation/mechanical ventilation: failure of above, altered mental status, hemodynamic instability, exhaustion

— Cardiac/respiratory arrest, inability to protect airway, facial trauma, vomiting, severe agitation, hemodynamic instability, recent upper GI surgery

— (SpO₂/FiO₂) / RR; > 4.88 predicts success, < 3.85 predicts failure → intubate

Step 3 management: A COPD patient with pH 7.28, PaCO₂ 68, alert → BiPAP with IPAP 12 / EPAP 5, titrate up. Reassess ABG at 1 hour. If pH < 7.25 or worsening mental status → intubate. CCS pearl: Order continuous pulse oximetry, telemetry, ABG in 1 hour, and place patient in ICU or step-down location before starting NIV.

ABCDE first — airway always:

Oxygen titration targets:

Escalation ladder:

NIV contraindications:

ROX index (HFNC efficacy at 2, 6, 12 h):

HACOR score predicts NIV failure in hypoxemic failure (Heart rate, Acidosis, Consciousness, Oxygenation, Respiratory rate)

Pharmacotherapy — First-Line Drug Regimens by Etiology

— Albuterol 2.5 mg + ipratropium 0.5 mg nebulized q20min × 3, then q4–6h

— Systemic corticosteroids: prednisone 40 mg PO daily × 5 days (or methylprednisolone 40 mg IV if NPO)

— Antibiotics if ≥2 of: increased dyspnea, sputum volume, sputum purulence (Anthonisen criteria) — azithromycin or doxycycline or amoxicillin-clavulanate × 5 days

— Albuterol continuous neb, ipratropium added in severe

— Methylprednisolone 60–125 mg IV or prednisone 40–60 mg PO

— Magnesium sulfate 2 g IV in severe (PEF < 40% predicted)

— Consider IV epinephrine, heliox, ketamine in near-fatal asthma

— IV furosemide (1–2.5× home dose, or 40 mg if naive)

— Nitroglycerin sublingual then IV drip for preload reduction (caution in RV infarct, severe AS)

— CPAP/BiPAP reduces preload and work of breathing

— Avoid morphine (associated with worse outcomes)

— CAP requiring ICU: ceftriaxone + azithromycin (or respiratory fluoroquinolone) ± vancomycin/linezolid if MRSA risk, antipseudomonal coverage if HCAP risk

— Influenza season: empiric oseltamivir

— Heparin IV bolus + drip; systemic tPA (alteplase 100 mg over 2 h) if hemodynamically unstable

Board pearl: Magnesium sulfate is the most commonly missed adjunct in severe asthma. Step 3 management: Steroids in COPD/asthma reduce relapse and hospital LOS — give within the first hour. Do not delay antibiotics in septic pneumonia beyond 1 hour from suspicion (Surviving Sepsis bundle).

COPD exacerbation:

Asthma exacerbation:

Cardiogenic pulmonary edema:

Pneumonia:

PE (high-risk/massive):

Opioid overdose: naloxone 0.04–0.4 mg IV, titrate

Anaphylaxis: IM epinephrine 0.3–0.5 mg, repeat q5–15min; H1/H2 blockers, steroids adjunctive

Mechanical Ventilation and Invasive Management

— Induction: etomidate 0.3 mg/kg (hemodynamically neutral) or ketamine 1–2 mg/kg (bronchodilation, preferred in asthma/COPD); avoid propofol bolus in hypotension

— Paralytic: rocuronium 1.2 mg/kg or succinylcholine 1.5 mg/kg (avoid succ in hyperkalemia, crush injury, denervation > 72 h, severe burns)

— Preoxygenate with NRB or HFNC; consider delayed sequence intubation in agitated hypoxemic patients

— Mode: volume-control or PRVC

— Tidal volume 6 mL/kg ideal body weight (lung-protective; mandatory in ARDS)

— RR 14–18, FiO₂ 100% initially then titrate down to maintain SpO₂ 88–95%

— PEEP 5 cm H₂O baseline; higher in ARDS per PEEP/FiO₂ table

— Plateau pressure target < 30 cm H₂O

— Low TV ventilation (6 mL/kg IBW)

— Permissive hypercapnia (pH > 7.20 acceptable)

— Prone positioning ≥ 16 h/day if P/F < 150

— Neuromuscular blockade (cisatracurium) considered in severe early ARDS

— Conservative fluid strategy

— ECMO referral if P/F < 80 despite optimization

— Low RR (8–10), prolonged expiratory time (I:E 1:4 or 1:5), permissive hypercapnia

— Watch for auto-PEEP / breath-stacking — disconnect from vent if sudden hypotension

— Daily spontaneous awakening trial (SAT) + spontaneous breathing trial (SBT)

— Cuff leak test before extubation if risk of laryngeal edema

— RSBI (RR/TV in L) < 105 predicts successful extubation

CCS pearl: After intubation, order post-intubation CXR to confirm ETT position (2–4 cm above carina), sedation analgesia (fentanyl + propofol), DVT prophylaxis, stress ulcer prophylaxis (PPI in high-risk), HOB ≥ 30°, oral chlorhexidine, daily SAT/SBT.

Rapid sequence intubation (RSI):

Initial ventilator settings:

ARDS-specific (Berlin criteria, PaO₂/FiO₂ ratio):

Obstructive disease (asthma/COPD) ventilation:

Liberation/weaning:

Special Populations — Elderly and Renal/Hepatic Impairment

— Diminished respiratory reserve: ↓ chest wall compliance, ↓ diaphragmatic strength, ↓ FEV1

— Baseline PaO₂ declines ~3 mm Hg per decade after 30; expected PaO₂ ≈ 100 − (age/3)

— Atypical presentations: confusion or falls may be the only manifestation of pneumonia/PE

— Higher risk of delirium with sedation — use lowest effective doses, avoid benzodiazepines, prefer dexmedetomidine for ICU sedation

— Higher aspiration risk → swallow evaluation before resuming PO

— Goals-of-care discussions early — many elderly patients have advance directives limiting intubation

— Contrast risk: weigh CTPA against V/Q scan; hold metformin

— Furosemide dosing: doubled in CKD (e.g., 80–160 mg IV for cardiogenic edema)

— Avoid nephrotoxins: NSAIDs, aminoglycosides where possible

— Aminoglycoside, vancomycin, and many antibiotics require renal dose adjustment

— Magnesium accumulates in renal failure — reduce dose, monitor reflexes

— Heparin preferred over LMWH for VTE prophylaxis/treatment when CrCl < 30 (or use reduced enoxaparin 30 mg daily)

— Hepatopulmonary syndrome: platypnea-orthodeoxia, intrapulmonary vascular dilations, A-a gradient ≥ 15 with cirrhosis → liver transplant referral

— Hepatic hydrothorax can cause respiratory failure — diuretics + thoracentesis; avoid chest tube (high mortality)

— Sedation: avoid benzodiazepines (precipitate encephalopathy); fentanyl preferred over morphine

— Adjust theophylline, clindamycin, macrolides

— Coagulopathy increases bleeding risk with central lines and tPA

Board pearl: In an elderly COPD patient with hypercapnic failure, NIV is preferred over intubation when feasible — reduces mortality, VAP, ICU LOS. Step 3 management: Always document a goals-of-care conversation and code status within the first 24 hours of admission for any patient ≥ 65 with respiratory failure.

Elderly considerations:

Renal impairment:

Hepatic impairment:

Special Populations — Pregnancy, Pediatrics, Other Subgroups

— Physiologic changes: ↑ minute ventilation, PaCO₂ baseline 28–32, mild respiratory alkalosis; ↓ FRC; ERV ↓ 20%

— A "normal" PaCO₂ of 40 in a pregnant patient = impending failure

— Target SpO₂ ≥ 95% (fetus requires higher maternal O₂)

— PE is leading cause of maternal mortality — V/Q scan or low-dose CTPA acceptable; D-dimer often elevated baseline

— Amniotic fluid embolism: sudden hypoxemia + DIC + cardiovascular collapse during labor

— Asthma: continue ICS/LABA; oral steroids safe; avoid leukotriene inhibitors initiation

— Position left lateral decubitus to relieve IVC compression

— Intubation: difficult airway (mucosal edema) — use smaller ETT (6.0–6.5)

— Bronchiolitis (RSV): supportive, HFNC, no routine bronchodilators or steroids

— Croup: nebulized epinephrine + dexamethasone

— Epiglottitis: do not agitate; OR for intubation with anesthesia/ENT

— Asthma: same drugs, weight-based; consider IV magnesium, terbutaline

— Foreign body: unilateral wheeze, hyperinflation on expiratory film

— Pediatric tidal volumes 6–8 mL/kg IBW; use cuffed tubes age-appropriate

— Broaden differential: PCP (LDH elevated, ground-glass), CMV, aspergillus, TB

— Empiric TMP-SMX + steroids if PaO₂ < 70 in suspected PCP

— Preoxygenation with HFNC and reverse Trendelenburg

— Ramped position for intubation (ear-to-sternal-notch)

— OHS: chronic hypercapnia, polycythemia, cor pulmonale — outpatient PAP therapy

Key distinction: A pregnant woman with PaCO₂ of 38 is hypercapnic relative to her baseline and may be in early failure. CCS pearl: Always check the trend, not just absolute values, and consult OB early for fetal monitoring when GA > 24 weeks.

Pregnancy:

Pediatrics:

Immunocompromised (HIV, transplant, chemo):

Obesity (BMI > 40):

Complications and Adverse Outcomes

— VAP (ventilator-associated pneumonia): new infiltrate + fever + leukocytosis + purulent secretions > 48 h after intubation; prevent with HOB elevation, oral chlorhexidine, subglottic suctioning, daily SAT/SBT

— Barotrauma: pneumothorax, pneumomediastinum from high plateau pressures

— Volutrauma/biotrauma: large tidal volumes → cytokine release

— Auto-PEEP: dynamic hyperinflation → hypotension, PEA arrest

— Oxygen toxicity with prolonged FiO₂ > 60%

— Positive pressure reduces venous return → hypotension, especially post-intubation in volume-depleted patients

— Right heart strain → acute cor pulmonale in ARDS/PE

— ICU-acquired weakness (critical illness myopathy/neuropathy) — minimize sedation, early mobilization

— Tracheal stenosis, vocal cord paralysis, tracheomalacia

— Tracheostomy considered after 10–14 days of intubation

— VTE (prophylax with heparin/LMWH or SCDs)

— Stress ulcer/GI bleed (PPI in high-risk: mechanical ventilation > 48 h, coagulopathy)

— Pressure injuries, delirium, malnutrition (enteral feeds within 24–48 h)

— Acute kidney injury — common, multifactorial

— Cognitive impairment, PTSD, depression, physical deconditioning

— Up to 50% of ARDS survivors at 1 year

— Within 48–72 h of extubation portends worse outcome

— Risk factors: positive cuff leak, weak cough, > 7 days intubation, age > 65

— ARDS: ~40% (severe ~45–50%)

— COPD exacerbation requiring intubation: ~25%

— Massive PE without tPA: > 50%

Board pearl: Sudden hypotension on the ventilator → think DOPES (Displaced tube, Obstruction, Pneumothorax, Equipment failure, Stacked breaths). Step 3 management: Disconnect from vent, bag manually, get stat CXR.

Ventilator-associated complications:

Hemodynamic:

Sequelae of prolonged ventilation:

Hospital complications:

Post-ICU syndrome (PICS):

Reintubation:

Mortality benchmarks:

When to Escalate Care — ICU, Consult, Triage

— Need for invasive mechanical ventilation

— NIV with close monitoring (BiPAP for hypercapnic failure with pH < 7.30)

— FiO₂ requirement > 50% to maintain SpO₂ ≥ 90%

— Hemodynamic instability requiring vasopressors

— Altered mental status, GCS ≤ 13

— Severe acid-base derangement (pH < 7.25 or > 7.55)

— Multi-organ involvement (AKI, shock, DIC)

— Stable on NIV with improving ABG

— Weaning from HFNC

— Telemetry needed but not vasoactive support

— Pulmonary/Critical care: all intubated patients, refractory hypoxemia, ARDS, massive hemoptysis

— Cardiology: acute MI complicated by edema, cardiogenic shock, tamponade

— Interventional radiology / Vascular surgery: catheter-directed thrombolysis for submassive PE

— Cardiothoracic surgery: ECMO candidate, surgical embolectomy

— Anesthesia/ENT: difficult airway, awake fiberoptic intubation, surgical airway

— Palliative care: advanced COPD/ILD, terminal cancer with respiratory failure

— Infectious disease: immunocompromised with atypical infection, drug-resistant organisms

— Need for ECMO, lung transplantation evaluation, specialized burn/trauma care, or pediatric/obstetric ICU not available locally

— Initial ED → ICU when: intubated, on NIV with concern for failure, requiring vasopressors, severe ABG abnormality

— ICU → floor when: ≥ 24 h off vent, stable O₂ requirement ≤ 4 L NC, no vasopressors, mental status at baseline

Step 3 management: A patient improving on BiPAP can transition to floor only after demonstrating tolerance of intermittent removal and stable ABG off NIV. CCS pearl: "Change patient location to ICU" early — do not let a deteriorating patient remain on the floor with an inadequate monitoring environment.

ICU admission criteria for respiratory failure:

Step-down/intermediate care:

Consultant triggers:

Transfer criteria:

CCS-style triage triggers to "move location":

Key Differentials — Same-Category (Pulmonary) Causes

— Pneumonia (CAP/HAP/VAP): focal consolidation, fever, productive cough; cover typicals + atypicals

— ARDS: bilateral infiltrates, P/F ≤ 300, non-cardiogenic, within 7 days of insult (Berlin criteria)

— Cardiogenic pulmonary edema: elevated BNP, S3, JVD, cardiomegaly — technically cardiac but presents as hypoxemic respiratory failure

— PE: clear lungs, sudden onset, elevated D-dimer, RV strain

— Pulmonary contusion: trauma history, opacity within 24 h

— Diffuse alveolar hemorrhage: dropping hemoglobin, hemoptysis, bloody BAL — vasculitis (GPA, anti-GBM, SLE)

— Pulmonary embolism (fat, amniotic, air): post-trauma long bone fracture, peripartum, central line manipulation

— COPD exacerbation: chronic smoker, hyperinflation, prolonged expiration

— Asthma status: wheezing → silent chest, normal CXR

— Bronchiectasis exacerbation: chronic purulent sputum, CT tram-tracking

— OSA/OHS decompensation: obese, witnessed apnea, daytime CO₂ retention

— Obesity hypoventilation: BMI > 30, PaCO₂ > 45 awake, no other cause

— Severe interstitial lung disease end-stage: restrictive pattern, honeycombing

— COPD with superimposed pneumonia, CHF with COPD, ARDS with auto-PEEP physiology

— Angioedema (ACEi-induced, hereditary), epiglottitis, retropharyngeal abscess, vocal cord dysfunction, post-extubation stridor

— Massive pleural effusion (parapneumonic, malignant, cardiac, hepatic hydrothorax)

— Pneumothorax: spontaneous (tall thin males), secondary (COPD bullae), tension (trauma, mechanical ventilation)

Key distinction: ARDS vs cardiogenic edema — both show bilateral infiltrates, but ARDS has normal LV function on echo, no JVD, often P/F ≤ 200, and an identifiable trigger (sepsis, pancreatitis, aspiration, transfusion, trauma). Board pearl: TRALI presents within 6 h of transfusion with ARDS pattern; TACO is fluid overload with elevated BNP.

Within hypoxemic failure:

Within hypercapnic failure:

Mixed pattern:

Upper airway:

Pleural disease:

Key Differentials — Other-Category (Extrapulmonary) Causes

— Guillain-Barré: ascending weakness, areflexia, albuminocytologic dissociation in CSF; monitor NIF and FVC q4h; FVC < 20 mL/kg or NIF less negative than −30 → intubate

— Myasthenic crisis: worsening bulbar weakness, often triggered by infection or medications (aminoglycosides, beta-blockers, magnesium); treat with IVIG or plasmapheresis

— ALS: progressive bulbar/limb weakness; consider NIV early

— Botulism: descending paralysis, dilated pupils, dry mouth

— Tick paralysis, organophosphate toxicity, hypermagnesemia

— Brainstem stroke, hemorrhage, infection (encephalitis, meningitis)

— Drug overdose (opioids, benzos, barbiturates) — bradypnea

— Central hypoventilation syndromes (Ondine's curse)

— Severe metabolic acidosis (DKA, lactic acidosis, salicylate toxicity, methanol/ethylene glycol) → compensatory tachypnea, Kussmaul breathing; can fatigue into failure

— Myxedema coma → hypoventilation, hypothermia, bradycardia

— Severe hypophosphatemia → diaphragmatic weakness

— Acute MI with pulmonary edema, cardiogenic shock

— Tamponade — Beck's triad

— Massive PE — obstructive shock

— Severe AS, acute MR (papillary muscle rupture)

— Flail chest, kyphoscoliosis, ankylosing spondylitis, large effusion, hemothorax

— Carbon monoxide (cherry-red, normal PaO₂, elevated COHb)

— Cyanide (smoke inhalation, almond breath, elevated anion gap with normal lactate clearance)

— Methemoglobinemia (chocolate-brown blood, dapsone/topical anesthetics)

Board pearl: Always check TSH in unexplained hypercapnic failure with hypothermia and bradycardia — myxedema coma kills if missed. Step 3 management: Stat levothyroxine + hydrocortisone IV (cover possible adrenal insufficiency).

Neuromuscular causes of hypercapnic failure:

CNS causes:

Metabolic causes mimicking respiratory failure:

Cardiac causes:

Chest wall/pleural:

Toxic exposures:

Secondary Prevention, Discharge Meds, Long-Term Plan

— Long-acting bronchodilators: LAMA (tiotropium) ± LABA ± ICS based on GOLD group

— Smoking cessation counseling + varenicline or NRT (mandatory at every visit)

— Influenza vaccine annually; pneumococcal (PCV20 or PCV15+PPSV23); COVID-19 boosters; RSV ≥ 60; Tdap

— Pulmonary rehabilitation referral (post-exacerbation within 4 weeks reduces readmission)

— Home O₂ if resting PaO₂ ≤ 55 mm Hg or SpO₂ ≤ 88% (or PaO₂ ≤ 59 with cor pulmonale/polycythemia)

— Action plan with rescue antibiotics/steroids for exacerbations

— Step-up ICS-LABA, biologics (omalizumab, mepolizumab, dupilumab) for eosinophilic/allergic phenotypes

— Trigger avoidance, asthma action plan, peak flow monitoring

— Vaccinations as above

— GDMT: ACEi/ARB/ARNI, beta-blocker, MRA, SGLT2 inhibitor

— Diuretic optimization, daily weights, low-sodium diet (< 2 g/d)

— ICD/CRT evaluation if EF ≤ 35% after 3 months GDMT

— DOAC (apixaban, rivaroxaban) ≥ 3 months; longer if unprovoked or persistent risk factor

— Hypercoagulability workup only if family history or unusual site

— Pulmonary rehab, mental health screening (PTSD, depression at 50%), cognitive assessment

— Spirometry at 3 and 6 months

— CPAP/BiPAP at home with adherence monitoring (> 4 h/night, ≥ 70% of nights)

— Weight loss program, bariatric surgery referral if BMI > 35 with comorbidities

Step 3 management: A COPD patient hospitalized for exacerbation should leave with inhalers, prednisone taper or completion, antibiotic completion, smoking cessation counseling, pulm rehab referral, vaccines updated, and follow-up in 1–2 weeks. Board pearl: Pulmonary rehab within 4 weeks of discharge reduces 1-year mortality and readmission.

COPD discharge bundle:

Asthma:

Cardiogenic edema:

PE:

ARDS survivors:

OSA/OHS:

Follow-Up, Monitoring Parameters, Counseling

— Hospital discharge → PCP visit within 7–14 days (CMS readmission penalty window is 30 days)

— Pulmonology within 4 weeks for new diagnosis (ILD, severe COPD, ARDS survivor)

— Repeat ABG/PFTs at 6–8 weeks when stable to redefine baseline

— COPD: symptom diary, exacerbation frequency, spirometry annually, SpO₂ on home O₂, BMI/nutrition

— Asthma: ACT score, peak flow at home, FeNO if available, eosinophil count for biologic candidates

— CHF: daily weights (alert if +2 lb/day or +5 lb/week), BP, K+/Cr 1–2 weeks after diuretic/ACEi changes, BNP trend

— PE on anticoagulation: bleeding signs, medication adherence, no CBC monitoring needed for DOACs

— OSA: CPAP adherence reports, Epworth score, repeat sleep study after weight changes

— 6–12 weeks, 2–3 sessions weekly, exercise + education + psychosocial support

— Improves quality of life, dyspnea, exercise tolerance — evidence A

— Smoking cessation at every encounter (5 A's: Ask, Advise, Assess, Assist, Arrange)

— Inhaler technique demonstration — most patients use incorrectly

— Recognize exacerbation triggers, when to call/return

— Advance directives, especially in advanced lung disease

— Sexual activity, air travel guidance (air travel hypoxia simulation if SpO₂ < 92% at rest)

— 30-day readmission rate for COPD/CHF/pneumonia (CMS-tracked)

— Time to first follow-up, medication reconciliation accuracy

CCS pearl: Always include medication reconciliation at discharge — biggest source of post-discharge adverse events. Step 3 management: Schedule the follow-up appointment before the patient leaves the hospital; do not rely on the patient calling.

Outpatient follow-up cadence:

Monitoring parameters by diagnosis:

Pulmonary rehab structure:

Counseling pillars:

Quality metrics:

Ethical, Legal, and Patient Safety Considerations

— Document on admission for any patient with chronic lung/heart disease, frailty, or age > 65

— Distinguish between DNR (no chest compressions/defibrillation) and DNI (no intubation) — they are separable

— A DNI patient may still receive NIV, vasopressors, ICU-level care

— Reassess goals after any major event (failed extubation, prolonged ventilation, new diagnosis)

— Emergent intubation: implied consent applies in life-threatening respiratory failure

— Pre-procedural consent for elective intubation (e.g., bronchoscopy) requires capacity assessment; surrogate (durable POA, then next of kin hierarchy) if absent

— Capacity is decision-specific — a hypoxic patient may regain capacity after oxygenation

— Ethically equivalent to withholding; requires consensus among surrogates and team

— Palliative extubation protocol: opioids + benzodiazepines titrated to comfort, not to hasten death (double-effect doctrine)

— Document discussions, witnesses, and surrogate identity

— Suspected tuberculosis → public health department

— Occupational lung disease (asbestos, silicosis) — workers' comp implications

— Pediatric or elder neglect contributing to recurrent aspiration

— Hand hygiene, oral care, HOB elevation reduce VAP

— Daily SAT/SBT shorten ventilator days

— High-alert medications: heparin, insulin, opioids, paralytics — independent double-check

— Transition-of-care risk: medication errors at discharge — reconcile inhalers, anticoagulants, diuretics; use teach-back method

— Handoff communication (SBAR, I-PASS) at every transfer; respiratory failure patients are highest-risk handoffs

— Pulse oximetry inaccuracy in dark skin pigmentation — recent FDA guidance; corroborate with ABG when clinical picture mismatches SpO₂

Step 3 management: A family disagreement about withdrawal should trigger ethics committee consultation, palliative care involvement, and clear documentation. Board pearl: The physician's duty is to provide medically appropriate care; futile interventions are not obligatory even if family requests them.

Goals of care and code status:

Informed consent edge cases:

Withdrawal of life-sustaining therapy:

Mandatory reporting:

Patient safety priorities:

High-Yield Associations and Rapid-Fire Facts

— Normal A-a + hypoxemia → hypoventilation or high altitude

— Elevated A-a + corrects with O₂ → V/Q mismatch

— Elevated A-a + does not correct → shunt

— pH 7.20 / PaCO₂ 65 / PaO₂ 55 → acute hypercapnic failure

— pH 7.36 / PaCO₂ 65 / HCO₃ 36 → chronic compensated (COPD baseline)

— pH 7.50 / PaCO₂ 25 / PaO₂ 50 → hypoxemic failure with respiratory alkalosis (PE, early ARDS, pneumonia)

— Kerley B lines, cephalization → cardiogenic edema

— S1Q3T3 → PE (specific, not sensitive)

— Westermark sign (oligemia), Hampton hump (peripheral wedge) → PE

— Cherry-red skin + normal SpO₂ → CO poisoning

— Chocolate-brown blood + cyanosis with normal PaO₂ → methemoglobinemia (treat with methylene blue)

— Albuminocytologic dissociation in CSF → Guillain-Barré

— Anti-AChR antibodies → myasthenia gravis

— Tachycardia + clear lungs + hypoxemia + recent surgery → PE until proven otherwise

— Ipratropium peaks in 30–60 min; albuterol in 5 min

— Magnesium 2 g IV for severe asthma

— Steroids onset 4–6 h — give early

— tPA for massive PE: alteplase 100 mg over 2 h

— Naloxone half-life shorter than most opioids — expect renarcotization

— TV 6 mL/kg IBW in ARDS; plateau < 30

— High PEEP in ARDS; low PEEP with prolonged expiratory time in obstructive disease

— SpO₂ goal 88–92% in COPD, 92–96% otherwise

— BiPAP indication: COPD with pH < 7.35; cardiogenic edema

— ARDS Berlin: P/F ≤ 300 mild, ≤ 200 moderate, ≤ 100 severe

Board pearl: The most commonly missed Step 3 answer is NIV before intubation for COPD exacerbation with respiratory acidosis — reduces mortality, VAP, and LOS. Key distinction: Do not use NIV in altered mental status, vomiting, or hemodynamic instability.

A-a gradient:

Classic ABG snapshots:

Pearls by etiology:

Treatment trivia:

Vent settings:

Numbers to memorize:

Board Question Stem Patterns

— 68-yo smoker, 60 pack-years, increased sputum, somnolent, ABG pH 7.28 / PaCO₂ 68 / PaO₂ 52 on 2 L NC

— Answer: BiPAP, nebulized albuterol/ipratropium, IV methylprednisolone, antibiotics; reassess ABG in 1 h

— 45-yo with severe pancreatitis, develops bilateral infiltrates, P/F 150 on ventilator, normal echo

— Answer: low TV ventilation 6 mL/kg IBW, PEEP per table, prone positioning, conservative fluids

— Post-op day 4 hip replacement, sudden hypoxemia, hypotension, RV strain on echo

— Answer: systemic tPA + heparin; surgical/catheter embolectomy if contraindication to thrombolysis

— 70-yo with HFrEF, orthopnea, JVD, bilateral crackles, SpO₂ 84%, BP 170/95

— Answer: CPAP/BiPAP, IV furosemide, IV nitroglycerin

— Young patient, silent chest, drowsy, normalizing PaCO₂ from previous low value

— Answer: intubate (ketamine induction), continuous albuterol, IV steroids, IV magnesium

— Ascending weakness post-Campylobacter, FVC declining to 18 mL/kg

— Answer: intubate before further decline; IVIG or plasmapheresis

— Found-down, RR 6, pinpoint pupils

— Answer: naloxone, monitor for renarcotization, no intubation if responds

— House fire, headache, confusion, SpO₂ 99%, normal PaO₂

— Answer: 100% O₂ NRB; hyperbaric if LOC, neuro signs, pregnancy, COHb > 25%

— COPD patient given high-flow O₂, becomes obtunded; PaCO₂ 90

— Answer: titrate O₂ to SpO₂ 88–92%; BiPAP; intubate if not improving

— Cyanotic infant after topical anesthetic; SpO₂ 85% unresponsive to O₂

— Answer: methylene blue 1–2 mg/kg IV

CCS pearl: Most stems hinge on choosing the next best step, not the diagnosis. Always identify Type 1 vs Type 2, then match the ladder: oxygen → HFNC/NIV → intubation. Board pearl: When a stem describes a patient becoming "calmer" while ill, suspect deterioration, not improvement.

Stem 1 — COPD exacerbation with hypercapnia:

Stem 2 — ARDS:

Stem 3 — Massive PE:

Stem 4 — Cardiogenic pulmonary edema:

Stem 5 — Asthma near-fatal:

Stem 6 — Guillain-Barré:

Stem 7 — Opioid overdose:

Stem 8 — CO poisoning:

Stem 9 — Hospital-acquired CO₂ retention:

Stem 10 — Methemoglobinemia:

One-Line Recap

Acute respiratory failure is defined by ABG — hypoxemic (PaO₂ < 60) reflects a gas-exchange problem treated with oxygen/PEEP/HFNC, while hypercapnic (PaCO₂ > 45 with pH < 7.35) reflects a ventilatory pump problem first treated with BiPAP, and both escalate to intubation when mental status, hemodynamics, or trajectory fail.

— Check ABG → classify Type 1 vs Type 2 → calculate A-a gradient

— Type 1 hypoxemic: O₂ → HFNC → intubation with lung-protective ventilation

— Type 2 hypercapnic: NIV (BiPAP) first-line if alert and protecting airway; intubate if failing

— Always treat the underlying cause: bronchodilators + steroids + antibiotics (COPD), diuresis + nitrates (CHF), anticoagulation/tPA (PE), antibiotics (pneumonia)

— Target SpO₂ 88–92% in COPD; 92–96% otherwise

— "Calming" or normalizing CO₂ in a tiring asthmatic = pre-arrest

— Refractory hypoxemia on 100% FiO₂ = shunt physiology

— Daily SAT/SBT, HOB ≥ 30°, low TV ventilation save lives

— Discharge bundle: inhalers, vaccines, smoking cessation, pulm rehab, 7–14 day follow-up

— Medication reconciliation and teach-back reduce readmission

— Goals-of-care documented for every patient

— BiPAP before intubation in COPD with pH 7.25–7.35 — mortality benefit

— Lung-protective ventilation (6 mL/kg IBW) is mandatory in ARDS

— Prone positioning ≥ 16 h/day in severe ARDS (P/F < 150)

— Magnesium in severe asthma, systemic tPA in massive PE, naloxone in opioid OD

Board pearl: The single most tested concept is matching ventilatory support to physiology — oxygen alone fixes V/Q mismatch but not hypoventilation; positive pressure fixes both shunt (PEEP) and pump failure (pressure support). CCS pearl: Reassess every 30–60 minutes after initiating support; escalate decisively rather than wait for arrest.

Decision algorithm:

Top safety pearls:

Transition-of-care essentials:

Step 3 must-knows: