MEDREH · EP 02 · PULMONARY
Before You Listen
Episode Setup
- Topic in one line: the obstructive vs restrictive vs mixed pulmonary function test (PFT) framework, the Global Initiative for Chronic Obstructive Lung Disease (GOLD) staging of chronic obstructive pulmonary disease (COPD), the diffusing capacity of the lung for carbon monoxide (DLCO), flow-volume loop pattern recognition, breathing techniques (pursed-lip auto-positive end-expiratory pressure (PEEP), diaphragmatic), the strict objective oxygen prescription criteria, ventilator weaning by the rapid shallow breathing index (RSBI), and tracheostomy management with the Passy-Muir speaking valve and the cuff-deflation safety rule.
- Prerequisites: basic respiratory mechanics, the alveolar-capillary unit, the oxygen-hemoglobin dissociation curve, normal arterial blood gas (ABG) interpretation, and the autonomic and somatic innervation of the diaphragm and accessory respiratory muscles.
- Runtime: approximately 33 minutes for Part 1.
- Scope boundary: Part 1 builds the diagnostic vocabulary (PFT patterns, DLCO, flow-volume loops), grades COPD severity (GOLD, BODE, absolute FEV1 in liters), prescribes oxygen by strict objective criteria, walks ventilator weaning through the rapid shallow breathing index, and locks in the Passy-Muir cuff-deflation rule. Part 2 turns those diagnostics into a complete program: the four pillars of comprehensive pulmonary rehabilitation, airway clearance technique (chest physiotherapy, flutter, vest, active cycle of breathing with the huff cough), the COPD exacerbation triad, and the special populations (cystic fibrosis, interstitial lung disease, asthma exercise-induced bronchoconstriction, neuromuscular disease, diaphragm pacing in high cervical spinal cord injury).
Vignette. A 68-year-old man with a 50-pack-year smoking history and chronic exertional dyspnea has post-bronchodilator spirometry showing FEV1 1.05 L (38 percent predicted), FVC 2.40 L (62 percent predicted), and FEV1/FVC ratio 0.44. DLCO is 38 percent of predicted. Resting SaO2 is 92 percent on room air. Six-minute walk test stops at 240 m with desaturation to 86 percent. He is referred to outpatient pulmonary rehabilitation. He asks whether his FEV1 will improve, whether he qualifies for ambulatory oxygen, and what the program will accomplish.
What is his GOLD stage, what specific findings indicate emphysema-predominant rather than chronic-bronchitis-predominant disease, what oxygen prescription does he qualify for, and what realistic outcomes should he expect from the program?
(Answer at the end of this chapter)
Section 1: PFT Pattern Recognition — Obstructive, Restrictive, Mixed, and DLCO
Bottom line: pulmonary function tests classify into three patterns by the FEV1/FVC ratio and the total lung capacity (TLC); obstructive disease has a reduced ratio (post-bronchodilator FEV1/FVC less than 0.70) with reduced FEV1 and a normal or increased TLC from hyperinflation, restrictive disease has a normal or increased ratio with a reduced TLC, and mixed disease combines a reduced ratio with a reduced TLC; spirometry alone cannot diagnose restriction (TLC measurement is required); the diffusing capacity of the lung for carbon monoxide (DLCO) is decreased in emphysema, pulmonary fibrosis, and pulmonary embolism, normal in chronic bronchitis and asthma, and increased in pulmonary hemorrhage and polycythemia, so a normal DLCO in obstruction points to chronic bronchitis or asthma while a reduced DLCO points to emphysema; positive bronchodilator response is greater than 12 percent and greater than 200 mL improvement in FEV1.
The three pulmonary function patterns are obstructive, restrictive, and mixed.
Obstructive patterns reflect airflow limitation. The hallmark is a reduced FEV1/FVC ratio (post-bronchodilator below 0.70). FEV1 is reduced because air cannot exit quickly through narrowed, inflamed, or compliant airways. FVC may be normal or reduced (when reduced, the cause is air trapping rather than small lungs). TLC is normal or increased due to hyperinflation. Residual volume is increased because air is trapped behind collapsing airways. The classic obstructive diseases are COPD, asthma, and bronchiectasis. The pathology localizes differently in each: in asthma the airway is narrowed from the outside in by smooth muscle constriction and mucosal inflammation; in chronic bronchitis the lumen is narrowed from the inside out by excessive mucus and chronic inflammation; and in emphysema the parenchymal elastic recoil is lost so the small airways, missing their structural tethering, collapse prematurely under positive intrathoracic pressure during forced exhalation.
Restrictive patterns reflect reduced lung volumes. The hallmark is a reduced TLC (less than 80 percent of predicted). FEV1/FVC is normal or increased because both values fall proportionally, or FEV1 is relatively preserved compared to FVC. Restrictive diseases fall into three buckets. Intrinsic lung disease, where the parenchyma itself becomes scarred and stiff (idiopathic pulmonary fibrosis, other interstitial lung diseases). Chest wall disorders, where a rigid bony cage traps healthy lung (severe kyphoscoliosis, ankylosing spondylitis). And neuromuscular disease, where the lung parenchyma and chest wall are fine but the respiratory muscles cannot generate the negative pressure needed to inflate the system (amyotrophic lateral sclerosis (ALS), Guillain-Barre syndrome (GBS), high cervical spinal cord injury).
Mixed patterns combine reduced ratio (obstruction) and reduced TLC (restriction). This pattern occurs with two concurrent diseases (COPD plus pulmonary fibrosis) or one disease producing both.
A critical board-tested point: spirometry alone cannot diagnose restriction. Spirometry measures FEV1 and FVC but not TLC. A reduced FVC with a normal or high ratio suggests restriction but must be confirmed by full lung volumes via body plethysmography or nitrogen washout. The trap is that severe hyperinflation in advanced obstruction can artificially depress the FVC: the residual volume is so large that the patient cannot draw in much additional air on top of it, mimicking a small container on the spirometry printout. Without TLC measurement, the report can only say “consistent with restriction.”
Bronchodilator responsiveness is tested in the asthma-vs-COPD distinction. A positive response requires both an increase in FEV1 of greater than 12 percent and greater than 200 mL after a short-acting bronchodilator (typically albuterol). Both thresholds must be crossed simultaneously. This indicates reversible obstruction (asthma). COPD typically shows minimal or no response because permanent structural damage and lost elasticity cannot be reversed pharmacologically.
The diffusing capacity of the lung for carbon monoxide (DLCO) measures gas transfer across the alveolar-capillary membrane. It is a separate test from spirometry. Carbon monoxide is used as the tracer because its affinity for hemoglobin is roughly 200 times that of oxygen, so its uptake rate is limited almost entirely by membrane diffusion properties rather than by capillary blood flow. Decreased DLCO occurs in conditions that damage, thicken, or reduce the surface area of the membrane: emphysema (alveolar wall destruction collapses the cluster-of-grapes architecture into floppy inefficient sacs), pulmonary fibrosis (collagen deposition thickens the membrane like trying to hear through concrete instead of glass), pulmonary embolism (PE; healthy membrane but no blood flow past it to pick up the gas), and anemia (insufficient hemoglobin to bind the tracer). Normal DLCO in obstruction distinguishes chronic bronchitis and asthma (the airway, not the membrane, is the problem) from emphysema. Increased DLCO is less commonly tested but occurs in pulmonary hemorrhage (alveolar blood absorbs the carbon monoxide) and polycythemia (more hemoglobin).
Flow-volume loops display flow against volume. The normal loop has a rapid rise to peak expiratory flow followed by a near-linear decline back to zero, with a smooth semicircular inspiratory limb below the axis. The obstructive pattern shows a scooped-out (concave) expiratory limb that reflects dynamic airway collapse: as the patient bears down to exhale, positive intrathoracic pressure squeezes the floppy small airways closed before all the air can escape, dropping flow precipitously. The restrictive pattern is a miniature normal loop with preserved architecture but shrunken in both flow and volume axes. The upper-airway patterns are reasoned through transmural pressure direction. Fixed upper airway obstruction (tracheal stenosis from prolonged intubation, fixed tumor) flattens both limbs into a boxy plateau because a rigid hole limits flow equally in both directions. Variable extrathoracic obstruction (vocal cord paralysis, cervical tracheomalacia) flattens the inspiratory limb only: negative intraluminal pressure during inspiration sucks the floppy neck lesion closed, while positive pressure during expiration blows it open. Variable intrathoracic obstruction (lower tracheomalacia) flattens the expiratory limb only because highly positive pleural pressure during forced exhalation compresses the lesion, while negative pleural pressure during inspiration pulls outward and props it open.
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## High Yield — PFT pattern recognition
- Obstructive: FEV1/FVC < 0.70, reduced FEV1, normal or increased TLC (hyperinflation).
- Restrictive: TLC < 80 percent predicted; ratio normal or increased; spirometry alone cannot diagnose.
- Mixed: reduced ratio + reduced TLC.
- DLCO: decreased in emphysema/fibrosis/PE/anemia; normal in asthma/chronic bronchitis; increased in hemorrhage/polycythemia.
- Bronchodilator response: ≥ 12 percent AND ≥ 200 mL FEV1 improvement (asthma).
- Loops: scooped-out = obstructive; mini-normal = restrictive; flat both = fixed; flat inspiratory only = variable extrathoracic; flat expiratory only = variable intrathoracic. :::
Mnemonic — “RATIO restricts your diagnosis, TLC seals it”
The FEV1/FVC RATIO sets the obstructive vs restrictive direction (low ratio = obstructive). But you cannot prove restriction without TLC. Whenever a vignette gives you spirometry only and asks whether a patient is restricted, the answer is “consistent with restriction; needs lung volumes.” DLCO then sub-types: emphysema drops it, asthma and chronic bronchitis preserve it.
Spirometry alone only measures flow rates and exhaled volumes. It measures the FEV1 and the FVC. It absolutely cannot measure the total lung capacity or the residual volume. Because it can’t measure the air left in the lungs that the patient literally can’t blow out.
— MEDREH-02-a podcast, ~09:21