A previously healthy 64-year-old woman comes to the physician because of a dry cough and progressively worsening shortness of breath for the past 2 months. She has not had fever, chills, or night sweats. She has smoked one pack of cigarettes daily for the past 45 years. She appears thin. Examination of the lung shows a prolonged expiratory phase and end-expiratory wheezing. Spirometry shows decreased FEV1:FVC ratio (< 70% predicted), decreased FEV1, and a total lung capacity of 125% of predicted. The diffusion capacity of the lung (DLCO) is decreased. Which of the following is the most likely diagnosis?

Chronic obstructive pulmonary disease

A 62-year-old female presents with complaint of chronic productive cough for the last 4 months. She states that she has had 4-5 month periods of similar symptoms over the past several years. She has never smoked, but she reports significant exposure to second-hand smoke in her home. She denies any fevers, reporting only occasional shortness of breath and a persistent cough where she frequently expectorates thick, white sputum. Vital signs are as follows: T 37.1 C, HR 88, BP 136/88, RR 18, O2 sat 94% on room air. Physical exam is significant for bilateral end-expiratory wheezes, a blue tint to the patient's lips and mucous membranes of the mouth, and a barrel chest. Which of the following sets of results would be expected on pulmonary function testing in this patient?

Decreased FEV1, Decreased FEV1/FVC ratio, Increased TLC, Normal DLCO

Normal FEV1, Normal FEV1/FVC, Normal TLC, Normal DLCO

Decreased FEV1, Normal FEV1/FVC, Decreased TLC, Decreased DLCO

Decreased FEV1, Increased FEV1/FVC ratio, Decreased TLC, Normal DLCO

Decreased FEV1, Decreased FEV1/FVC ratio, Increased TLC, Decreased DLCO

A 63-year-old woman comes to the physician because of worsening shortness of breath, cough, and a 4-kg (8.8-lb) weight loss over the last year. She has no history of serious illness and takes no medications. She has smoked one pack of cigarettes daily for 35 years. Her temperature is 37°C (98.6°F), pulse is 92/min, respirations are 20/min, blood pressure is 124/78 mm Hg, and pulse oximetry on room air shows an oxygen saturation of 93%. Physical examination shows decreased breath sounds. A flow-volume loop obtained via pulmonary function testing is shown. Which of the following is the most likely cause of this patient's respiratory symptoms?

Chronic obstructive pulmonary disease

Unilateral mainstem obstruction

A 55-year-old man with a 60 pack-year smoking history is referred by his primary care physician for a pulmonary function test (PFT). A previously obtained chest x-ray is shown below. Which of the following will most likely appear in his PFT report?

Residual volume increased, total lung capacity increased

Residual volume increased, total lung capacity decreased

Residual volume normal, total lung capacity decreased

Residual volume normal, total lung capacity normal

Residual volume decreased, total lung capacity increased

A 68-year-old man comes to the emergency room with difficulty in breathing. He was diagnosed with severe obstructive lung disease a few years back. He uses his medication but often has to come to the emergency room for intravenous therapy to help him breathe. He was a smoker for 40 years smoking two packs of cigarettes every day. Which of the following best represents the expected changes in his ventilation, perfusion and V/Q ratio?

Low ventilation, normal perfusion and low V/Q ratio

Normal ventilation, low or nonexistent perfusion and infinite V/Q ratio

Medium ventilation and perfusion, V/Q that equals 0.8

Higher ventilation and perfusion with lower V/Q ratio

Lower ventilation and perfusion, but higher V/Q ratio

Spirometry interpretation for USMLE Step 3: High-Yield Physiology Lessons

Spirometry Basics - Lung Volume Vitals

Lung Volumes (single measurements)
- Tidal Volume (TV): Air in a normal breath (~500 mL).
- Inspiratory Reserve Volume (IRV): Max air inhaled post-normal inspiration.
- Expiratory Reserve Volume (ERV): Max air exhaled post-normal expiration.
- Residual Volume (RV): Air left after max expiration.
Lung Capacities (≥2 volumes)
- Inspiratory Capacity (IC): $IC = IRV + TV$
- Functional Residual Capacity (FRC): $FRC = ERV + RV$
- Vital Capacity (VC): $VC = IRV + TV + ERV$
- Total Lung Capacity (TLC): $TLC = VC + RV$

Spirogram: Lung Volumes and Capacities

⭐ Residual Volume (RV) and, consequently, FRC and TLC, cannot be measured by simple spirometry. They require techniques like helium dilution or body plethysmography.

The Main Algorithm - Obstructive vs. Restrictive

Step 1: Check FEV₁/FVC Ratio
- The primary differentiator. Is the proportion of air exhaled in the first second reduced?
Step 2: Interpret the Ratio
- If FEV₁/FVC < 70%: Obstructive defect. Airflow is limited. Proceed to check FEV₁ to grade severity.
- If FEV₁/FVC ≥ 70%: Not obstructive. Now, check lung volumes.
Step 3: Evaluate FVC for Restriction
- If FVC is < 80% of predicted, suspect a Restrictive pattern. Reduced volume is key.
- Confirmation requires Total Lung Capacity (TLC) < 80%.

⭐ A "normal" or preserved FEV₁/FVC ratio in the presence of a low FVC and FEV₁ is characteristic of a restrictive pattern. Both volumes are reduced proportionally.

Flow-Volume Loops - Shape Shifters

Obstructive: Classic "scooped-out" expiratory limb from ↓ airflow.
Restrictive: "Witch's hat" shape; a miniaturized, narrow loop shifted right due to low lung volumes.
Upper Airway Obstruction:
- Fixed: Flattened top (expiration) and bottom (inspiration).
- Variable Extrathoracic: Flattened inspiratory limb.
- Variable Intrathoracic: Flattened expiratory limb.
- 📌 Mnemonic: Extrathoracic affects Inspiration; Intrathoracic affects Expiration.

⭐ The hallmark of obstructive disease is the "coved" or "scooped-out" shape of the expiratory limb.

Clinical Patterns - The Usual Suspects

Start with the FEV1/FVC ratio to differentiate patterns. A low ratio points to obstruction; a normal ratio with low lung volumes suggests restriction.

Obstructive Pattern (e.g., COPD, Asthma)
- Hallmark: ↓ FEV1/FVC (< 0.7)
- ↓ FEV1 is profound; FVC may be normal or ↓.
- TLC is often normal or ↑ (air trapping).
Restrictive Pattern (e.g., Fibrosis, Scoliosis)
- Hallmark: ↓ TLC is definitive.
- FEV1/FVC is normal or ↑ (≥ 0.7).
- Both FEV1 and FVC are reduced proportionally.

⭐ A post-bronchodilator increase in FEV1 or FVC by >12% AND >200 mL indicates significant reversibility, classic for Asthma.

High‑Yield Points - ⚡ Biggest Takeaways

FEV1/FVC ratio is the key: < 70% suggests obstruction; ≥ 70% suggests restriction.

Obstructive patterns show a disproportionately low FEV1. Severity is graded by FEV1.

Restrictive patterns show a low FVC and TLC, with a normal or high FEV1/FVC ratio.

Reversibility with a bronchodilator (>12% & 200 mL ↑ in FEV1/FVC) points to asthma.

DLCO helps specify the cause: ↓ in emphysema/fibrosis, but normal in asthma or chronic bronchitis.

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