Pulmonary Pharmacology  ·  Module 7 of 7

Pharmacology of Respiratory Failure and Mechanical Ventilation

ARDS pathophysiology and dexamethasone · ICU sedation and analgesia · Neuromuscular blockade · Inhaled vasodilators · Ventilator weaning pharmacology

AKI = acute kidney injury  ·  ARDS = acute respiratory distress syndrome  ·  cAMP = cyclic adenosine monophosphate  ·  cGMP = cyclic guanosine monophosphate  ·  CK = creatine kinase  ·  ECMO = extracorporeal membrane oxygenation  ·  GABA-A = gamma-aminobutyric acid type A receptor  ·  Hb = hemoglobin  ·  iNO = inhaled nitric oxide  ·  IP = prostacyclin receptor  ·  MetHb = methemoglobin  ·  NIV = non-invasive ventilation  ·  NMB = neuromuscular blocking agent  ·  NO = nitric oxide  ·  PEEP = positive end-expiratory pressure  ·  sGC = soluble guanylate cyclase  ·  V/Q = ventilation-perfusion

ARDS: Berlin Severity, Lung-Protective Ventilation, and Dexamethasone
Berlin Definition — Severity
PaO2/FiO2 Thresholds
  • All measured with PEEP ≥5 cmH2O
  • Mild: PaO2/FiO2 201–300 mmHg
  • Moderate: 101–200 mmHg
  • Severe: ≤100 mmHg — threshold for NMB and prone positioning consideration
  • Bilateral infiltrates; onset within 1 week; not explained by cardiac failure or fluid overload
ARDSNet — Lung-Protective Ventilation
6 mL/kg Predicted Body Weight
  • Heterogeneous ARDS lung: flooded/collapsed + recruitable + still-aerated units
  • Traditional 12 mL/kg: overdistends aerated units → volutrauma; cyclic opening/closing → atelectrauma
  • Tidal volume 6 mL/kg predicted body weight + plateau pressure <30 cmH2O
  • ARDSNet trial: 22% mortality reduction versus conventional tidal volumes
  • Standard of care — USMLE Step 1 tidal volume target
DEXA-ARDS Trial
Dexamethasone in Moderate-Severe ARDS
  • Mechanism: suppresses sustained cytokine-mediated diffuse alveolar damage in the inflammatory phase
  • Regimen: 20 mg IV daily ×5 days, then 10 mg daily ×5 days
  • DEXA-ARDS: reduced ventilator-free days and 60-day mortality in moderate-to-severe ARDS
  • Early use only — must initiate <14 days from onset; late use (fibroproliferative phase) not beneficial
  • Preferred: no mineralocorticoid activity; favorable potency and duration
ICU Sedation and Analgesia — Analgesia-First Approach
Class Drug Mechanism Advantage Key Safety / Concern
Sedative Propofol GABA-A potentiator (propofol binding site) Very short context-sensitive half-time → ideal for daily awakening trials and extubation readiness Propofol infusion syndrome: >5 mg/kg/h + >48 h → metabolic acidosis, rhabdomyolysis, AKI, arrhythmias; lipid emulsion → hypertriglyceridemia
Sedative Dexmedetomidine Selective alpha-2 agonist (locus coeruleus) → natural NREM sleep pathway Cooperative sedation — arousable, follows commands; no respiratory depression; useful for NIV and extubation readiness (MIDEX/PRODEX: shorter time to extubation) Bradycardia + hypotension from reduced sympathetic outflow; more expensive
Sedative Midazolam Benzodiazepine GABA-A potentiator (BZD binding site) Provides amnesia; useful for short procedures Accumulates with prolonged infusion → extended sedation duration, ICU delirium; deprioritized for routine ICU sedation in contemporary practice
Analgesic Fentanyl Mu-opioid receptor agonist No histamine release (unlike morphine); rapid onset; highly titratable by continuous infusion Respiratory depression — managed in context (patient on mechanical ventilatory support); accumulation with prolonged infusion
Neuromuscular Blockade and Inhaled Pulmonary Vasodilators
NMB in ARDS — Cisatracurium
Hofmann Elimination
  • Hofmann elimination: spontaneous non-enzymatic degradation at physiological temperature and pH — independent of hepatic and renal function
  • Ideal for multi-organ dysfunction (avoids accumulation when liver/kidneys fail)
  • Indication: severe ventilator dyssynchrony or life-threatening hypoxemia refractory to optimized sedation
  • ICU-acquired weakness: most important adverse consequence — generalized flaccid weakness weeks-months after ICU; denervation-type muscle changes, myosin filament loss
  • Train-of-four monitoring: target 1–2/4 twitches — minimize residual block while preventing overdose
  • ACURASYS (benefit) vs ROSE (no benefit) — current practice: selective use, not routine
Rocuronium + Sugammadex
Rapid Reversal at Any Depth
  • Rocuronium: rapid-onset nondepolarizing NMB; rapid sequence intubation + shorter-term ICU paralysis
  • Sugammadex: modified γ-cyclodextrin that encapsulates rocuronium (and vecuronium) in a 1:1 complex → rapidly removed from neuromuscular junction
  • Reversal within minutes at ANY depth of block — does not require spontaneous recovery
  • No cholinergic adverse effects (unlike neostigmine + glycopyrrolate)
  • Supersedes neostigmine/glycopyrrolate for rocuronium/vecuronium reversal
Rescue Oxygenation — iNO
Inhaled Nitric Oxide
  • 5–80 ppm via ventilator circuit → diffuses into pulmonary arterial smooth muscle adjacent to ventilated alveoli → sGC → cGMP → vasodilation
  • Selective: rapidly inactivated by Hb in capillary blood → no systemic escape → no systemic hypotension
  • Redirects perfusion from flooded/collapsed to ventilated units → V/Q matching → improved oxygenation
  • Rebound pulmonary hypertension on abrupt discontinuation — wean gradually
  • Methemoglobinemia (NO oxidizes Hb Fe²⁺ to Fe³⁺ → MetHb cannot carry O2)
  • Nitrogen dioxide (NO₂) in circuit when NO contacts O2 — toxic byproduct
  • No mortality benefit in ARDS — rescue therapy only; bridge to prone/ECMO
Rescue Oxygenation — Inhaled Epoprostenol
Inhaled Prostacyclin
  • IP receptor → cAMP → pulmonary vasodilation in ventilated lung units
  • Same mechanism of selective V/Q improvement as iNO (drug only reaches ventilated units)
  • Substantially less expensive than iNO; no specialized delivery equipment required
  • Similar degree of oxygenation improvement to iNO in ARDS
  • No mortality benefit in ARDS — rescue therapy; bridge to prone/ECMO
  • Practical alternative to iNO in many ICU settings
Ventilator Weaning Pharmacology
Post-Extubation Stridor
Corticosteroid Prophylaxis
  • Laryngeal edema from prolonged intubation → post-extubation stridor → possible reintubation
  • Cuff leak test: deflate cuff — no air bypasses tube = high risk for post-extubation obstruction
  • Methylprednisolone or dexamethasone IV hours before extubation in cuff-leak-test failures → reduces laryngeal mucosal edema → lowers stridor and reintubation rate
Neonatal Apnea of Prematurity
Caffeine — Preferred Methylxanthine
  • Adenosine receptor antagonism → stimulates central respiratory drive → reduces apneic episodes
  • CAP trial: reduces apnea, accelerates extubation, reduces bronchopulmonary dysplasia and patent ductus arteriosus
  • Preferred over theophylline: wider therapeutic window
  • Same class as theophylline — methylxanthine
Sedation Minimization
Paired SAT + SBT Protocol
  • Spontaneous Awakening Trial (SAT): daily sedation interruption → assess neurological status and readiness for extubation
  • Spontaneous Breathing Trial (SBT): brief period of minimal ventilator support to assess readiness for extubation
  • Paired SAT + SBT on same day: reduces total sedative exposure, ventilator days, and ICU length of stay
Propofol Infusion Syndrome

Rare but potentially fatal. Triggered by high-dose infusion (>5 mg/kg/hour) for more than 48 hours. Features: metabolic acidosis, rhabdomyolysis, acute kidney injury, cardiac arrhythmias (QRS widening, bundle branch block, ventricular arrhythmias), lipemic plasma. Mechanism: impairment of mitochondrial electron transport chain. Monitor triglycerides, lactate, and creatine kinase in patients receiving prolonged high-dose infusions. Propofol's lipid emulsion formulation also contributes caloric load and hypertriglyceridemia independently of the syndrome. If suspected: stop propofol immediately and switch to an alternative sedative agent.

Pulmonary Pharmacology — Chapter Complete

The seven modules of Pulmonary Pharmacology map the full pharmacological landscape of obstructive, inflammatory, vascular, and critical airway disease. The unifying thread is airway smooth muscle tone and its two governing axes — the Gq/IP3/calcium bronchoconstriction pathway targeted by anticholinergics and blocked by leukotriene modifiers, and the Gs/cAMP/PKA bronchodilation pathway activated by beta-2 agonists and prolonged by PDE inhibitors. Inhaled corticosteroids suppress the T2-high eosinophilic inflammation that drives asthma; biologic agents extend this suppression to specific cytokine nodes when ICS/LABA combination therapy is insufficient. Pulmonary arterial hypertension therapy restores three endothelial vasoactive pathways in parallel — prostacyclin, nitric oxide/cGMP, and endothelin — and the same NO/cGMP biology that governs inhaled nitric oxide in ARDS governs riociguat and PDE-5 inhibitors in PAH. CFTR modulator therapy closes the chapter by targeting protein biology rather than downstream pathophysiology — correcting the molecular defect itself in the approximately 90% of CF patients with a tractable mutation.

Four pharmacological principles recur across this chapter and are worth internalizing as cross-cutting themes: first, selectivity achieved through pharmacokinetic design rather than receptor selectivity — inhaled drugs concentrate locally, minimizing systemic toxicity (ICS, bronchodilators, inhaled vasodilators); second, the distinction between symptom suppression and disease modification — bronchodilators relieve airflow obstruction but do not prevent exacerbations without concurrent anti-inflammatory therapy, and CFTR modulators modify disease biology while non-modulator therapies manage residual structural consequences; third, the fatal interaction principle — LABA monotherapy in asthma, PDE-5 inhibitors plus nitrates, abrupt epoprostenol discontinuation, and abrupt iNO discontinuation each represent pharmacological combinations or transitions where the consequence of error is death, not merely adverse effect; fourth, biomarker-guided therapy — eosinophil count in COPD and severe asthma, IgE and FeNO in biologic selection, sweat chloride in CFTR monitoring, PaO2/FiO2 in ARDS escalation — represents the unifying methodological advance that converts population-level drug categories into patient-specific treatment decisions.

Suggested References
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