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Pathophysiology
Clinical meaning
the clinician must master pulmonary function test interpretation, advanced ventilator management, and evidence-based ARDS protocols.
Pulmonary Function Tests (PFTs):
• Spirometry: Measures airflow and lung volumes during forced breathing maneuvers.
- FVC (Forced Vital Capacity): Total volume exhaled during a forced expiration. Reduced in restrictive disease.
- FEV1 (Forced Expiratory Volume in 1 second): Volume exhaled in the first second. Reduced in both obstructive and restrictive disease.
- FEV1/FVC Ratio: KEY diagnostic ratio. Normal ≥70% (or ≥LLN). <70% = Obstructive pattern (air trapping prevents rapid exhalation). Normal or elevated = Restrictive pattern (lungs can't fill, but what fills empties quickly).
- Bronchodilator Response: ≥12% AND ≥200 mL improvement in FEV1 post-bronchodilator = significant reversibility (supports asthma diagnosis). Minimal/no reversibility supports COPD.
• Lung Volumes (measured by body plethysmography or gas dilution):
- TLC (Total Lung Capacity): Increased in obstructive disease (hyperinflation/air trapping). Decreased in restrictive disease.
- RV (Residual Volume): Air remaining after maximal expiration. Increased in obstructive disease (air trapping). Decreased in restrictive.
- FRC (Functional Residual Capacity): Volume at end-expiration during tidal breathing. Increased in emphysema.
• DLCO (Diffusion Capacity for Carbon Monoxide): Measures gas transfer across the alveolar-capillary membrane.
- Decreased: Emphysema (alveolar destruction reduces surface area), pulmonary fibrosis (thickened membrane impairs diffusion), pulmonary vascular disease, anemia.
- Increased: Polycythemia, pulmonary hemorrhage (hemoglobin in alveoli absorbs CO), asthma (during acute bronchospasm: air trapping increases contact time), left-to-right shunts (increased pulmonary blood flow).
- Normal: Asthma (between attacks), chronic bronchitis (airway disease, not parenchymal), chest wall disease.
Advanced Ventilator Management:
• Driving Pressure: Plateau pressure minus PEEP. Should be <15 cmH2O. Emerging evidence suggests driving pressure is the strongest predictor of mortality in ARDS (Amato et al., NEJM 2015). Optimizing driving pressure may be more important than targeting specific VT or plateau pressure.
• Transpulmonary Pressure Monitoring: Using esophageal balloon to estimate pleural pressure and calculate true transpulmonary pressure. Allows individualized PEEP titration based on actual lung mechanics rather than empiric tables.
• Recruitment Maneuvers: Brief application of sustained high pressure (e.g., 40 cmH2O for 40 seconds) to open collapsed alveoli. Controversial: ART trial showed increased mortality with aggressive recruitment maneuvers. Use with caution.
• Airway Pressure Release Ventilation (APRV): Time-cycled pressure mode that spends most time at high pressure (P-high) with brief releases to low pressure (P-low) for CO2 elimination. May improve oxygenation in ARDS. Allows spontaneous breathing throughout. Controversial and center-dependent.
• ECMO for Severe ARDS: VV-ECMO (veno-venous) provides gas exchange support when conventional ventilation fails. EOLIA trial: Trend toward mortality benefit but did not reach significance (35% vs 46%). Consider when PaO2/FiO2 <80 despite optimized ventilator management and prone positioning, or uncompensated respiratory acidosis with pH <7.20.
Exam Focus
Diagnosis & workup
Diagnostics & workup:
- Order and interpret ABGs with acid-base analysis
- Order pulmonary function tests (FEV1, FVC, FEV1/FVC ratio, DLCO)
- Order CT chest for parenchymal evaluation
- Order D-dimer and CTPA for PE evaluation
- Order sputum cultures with antibiotic sensitivities
- Order bronchoscopy for diagnostic evaluation when indicated
- Order sleep study (polysomnography) for suspected OSA
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