Describe pulmonary function tests and use of spirometry in clinical medicine

Reading File

Finding Sources

Reading File

Finding Sources

Loading Image

Finding Sources

Reading File

Loading Image

Reading File

Loading Image

I now have everything I need for a comprehensive, well-illustrated answer. Let me compose it.

Pulmonary Function Tests and Spirometry in Clinical Medicine

Overview

Pulmonary function tests (PFTs) permit accurate, reproducible assessment of the functional state of the respiratory system. Importantly, PFTs do not diagnose specific diseases on their own - different diseases cause different patterns of abnormality across a battery of tests, allowing clinicians to quantify severity, detect disease early, characterize natural history, and monitor response to treatment. Accuracy of interpretation depends on a thorough knowledge of the physiologic basis of each test, properly validated equipment, and standardized protocols.

Murray & Nadel's Textbook of Respiratory Medicine, p. 704

The Components of a Full PFT Battery

Lung Volume Partitions

Understanding PFTs begins with lung volumes:

Lung volume partitions: IRV = inspiratory reserve volume; TV = tidal volume; ERV = expiratory reserve volume; RV = residual volume; IC = inspiratory capacity; FRC = functional residual capacity; VC = vital capacity; TLC = total lung capacity. - Goldman-Cecil Medicine, p. 867

Term	Definition
TLC	Total lung capacity - volume at end of maximal inspiration
FRC	Functional residual capacity - resting lung volume where elastic recoil forces are balanced
RV	Residual volume - gas remaining after maximal exhalation
ERV	Expiratory reserve volume = FRC - RV
IC	Inspiratory capacity - volume inspired from FRC to TLC
VC	Vital capacity = IRV + TV + ERV

Three volumes (TV, IRV, ERV) can be measured with a simple spirometer. Measurement of RV, FRC, or TLC requires more sophisticated methods: body plethysmography, inert gas dilution, or the nitrogen washout technique.

Goldman-Cecil Medicine, p. 866

Spirometry

What It Measures

Spirometry is the measurement of the volume of air inhaled or exhaled. It is commonly performed with a pneumotachometer that measures airflow and integrates it to obtain volume during a series of ventilatory maneuvers. The simplest and most commonly performed PFT, spirometry measures:

FVC (Forced Vital Capacity): the total volume of air forcefully expelled from TLC down to RV
FEV₁ (Forced Expiratory Volume in 1 second): volume expelled in the first second
FEV₁/FVC ratio: the fraction of the total exhaled volume expelled in 1 second
FEF₂₅₋₇₅ (Forced Expiratory Flow at 25-75% of FVC): a mid-expiratory flow measure, more sensitive for small airway disease but with poor reproducibility
MVV (Maximal Voluntary Ventilation): maximal ventilation over a timed period, estimates ventilatory reserve
Goldman-Cecil Medicine, p. 865

The Physiology of Forced Expiration

The first 25-30% of a maximal expiratory maneuver is effort-dependent. Beyond this, flow becomes effort-independent due to dynamic airway compression at the equal pressure point (EPP) - the point along the airway where intraluminal pressure equals pleural pressure. Increasing expiratory effort beyond this point merely compresses airways further without increasing flow. This is why the FEV₁ and the descending limb of the flow-volume curve are reproducible and reliable:

Model of expiratory flow limitation - equal pressure point theory

Equal pressure point model: Increased expiratory effort (bottom panel) only compresses airways more; maximum flow (6.5 L/s) remains the same as in the middle panel. - Murray & Nadel's, p. 704

The Two Graphical Displays

Results can be displayed as:

Volume-time curve (spirogram): shows exhaled volume on the y-axis vs. time on x-axis. A prolonged expiratory time indicates obstruction.
Flow-volume loop: plots expiratory and inspiratory flow at each lung volume. The shape of this curve is highly informative.

Flow-volume loop and volume-time curve in COPD, showing concave expiratory limb and prolonged expiration

COPD flow-volume loop (A): concave expiratory limb with flow falling below predicted (dots) at all lung volumes. Volume-time curve (B): prolonged expiratory time, with predicted FEV₁ shown as a dot. - Murray & Nadel's, p. 1471

Interpreting Spirometry: Obstructive vs. Restrictive Patterns

Obstructive Pattern

FEV₁/FVC ratio reduced (< 0.70 by GOLD criteria, or below the fifth percentile lower limit of normal by ATS/ERS)
FEV₁ reduced out of proportion to FVC
Flow-volume curve shows a concave (scooped-out) expiratory limb
Diseases: COPD, asthma, bronchiectasis, cystic fibrosis

A fixed low ratio (post-bronchodilator) is consistent with COPD. In asthma, the ratio may vary between exacerbations and return to normal. The GOLD criteria (fixed ratio < 0.70) has the advantage of simplicity but can overdiagnose obstruction in older subjects because FEV₁/FVC naturally declines with age. ATS/ERS prefers the lower limit of normal approach for this reason.

Restrictive Pattern

FEV₁/FVC ratio normal or elevated
Both FEV₁ and FVC reduced
TLC reduced (confirmed by body plethysmography - required to formally diagnose restriction)
Diseases: interstitial lung disease, chest wall deformity, neuromuscular weakness, pleural disease, obesity

Note: Low FVC with normal FEV₁/FVC on spirometry alone does not confirm restriction - a normal TLC in this setting gives the "nonspecific pattern."

Mixed Pattern

Both a reduced FEV₁/FVC ratio AND reduced TLC
Goldman-Cecil Medicine, p. 865-868; Murray & Nadel's, p. 1500-1501

Bronchodilator Responsiveness

Spirometry is routinely performed before and after inhaled bronchodilator (typically albuterol/salbutamol 400 mcg, ipratropium, or both). A significant bronchodilator response is defined as:

Improvement in FEV₁ or FVC of ≥ 12% AND ≥ 200 mL above baseline

A positive response suggests reversible airflow obstruction and is consistent with (but not diagnostic of) asthma. The response is generally greater in asthma and less in COPD, but varies considerably between individuals. Notably, lack of acute bronchodilator response does not predict lack of clinical benefit from long-term bronchodilator therapy.

Before testing, patients should withhold LABAs for ≥12 hours and short-acting bronchodilators for ≥6 hours.

Murray & Nadel's (Asthma chapter), p. 229; Goldman-Cecil Medicine, p. 866

Severity Grading of Obstruction (GOLD System)

Stage	FEV₁ % Predicted	Severity
GOLD 1	≥ 80%	Mild
GOLD 2	50-79%	Moderate
GOLD 3	30-49%	Severe
GOLD 4	< 30%	Very severe

Modern COPD staging (updated GOLD guidelines) also incorporates symptom burden (mMRC, CAT score) and exacerbation history alongside spirometric severity.

Diffusing Capacity (DLCO)

The single-breath diffusing capacity for carbon monoxide (DLCO), also called the transfer factor or TLCO, measures the capacity of the lungs to exchange gas. It collectively reflects all factors affecting diffusion across the alveolar-capillary membrane: the surface area available, membrane thickness, capillary blood volume, and ventilation-perfusion homogeneity.

Clinical interpretation of DLCO:

DLCO Result	Common Causes
Reduced	Emphysema, interstitial lung disease, pulmonary vascular disease (e.g., pulmonary hypertension), combined pulmonary fibrosis-emphysema
Normal/elevated	Asthma (elevated due to increased upper zone perfusion and large lung volumes), polycythemia, pulmonary hemorrhage

In obstructive disease: low DLCO suggests emphysema or parenchymal/vascular disease; normal DLCO more likely suggests asthma or chronic bronchitis
In restrictive disease: low DLCO suggests parenchymal (interstitial) cause; normal DLCO suggests chest wall or neuromuscular limitation
An isolated low DLCO with normal spirometry and TLC should prompt consideration of pulmonary vascular disease
Goldman-Cecil Medicine, p. 868

Body Plethysmography and Lung Volume Measurement

Body plethysmography is the preferred (most accurate) method for measuring absolute lung volumes (especially FRC and TLC). Based on Boyle's law (P₁ × V₁ = P₂ × V₂), the patient sits in a sealed box and pants against a closed shutter. Changes in box pressure and mouth pressure allow calculation of thoracic gas volume. It is particularly valuable in patients with airway obstruction, where helium dilution tends to underestimate TLC by failing to equilibrate into poorly communicating air spaces (gas trapping).

Inert gas dilution (helium, neon) and nitrogen washout are alternative methods. These measure only communicating air spaces and will underestimate TLC when air trapping is significant.

Goldman-Cecil Medicine, p. 867

The Flow-Volume Curve: Pattern Recognition

The shape of the flow-volume loop provides pattern recognition for many specific disorders:

Pattern	Appearance	Examples
Normal	Smooth convex expiratory limb	-
Obstructive	Concave (scooped) expiratory limb	COPD, asthma
Restrictive	Normal shape but smaller overall TLC	ILD, obesity, NMD
Variable extrathoracic obstruction	Inspiratory limb flattened	Vocal cord dysfunction, tracheal stenosis (extrathoracic)
Variable intrathoracic obstruction	Expiratory limb flattened	Tracheomalacia, relapsing polychondritis
Fixed central obstruction	Both limbs flattened (box-shaped)	Fixed tracheal stenosis
Neuromuscular weakness	Reduced peak flow, normal shape	Myopathy, MND
Sawtooth pattern	Oscillatory irregular flow	Obstructive sleep apnea

Goldman-Cecil Medicine, p. 867 (Figure 73.3 caption)

The PFT Interpretation Algorithm

Algorithm for interpreting pulmonary function tests

Systematic algorithm for PFT interpretation. Begin by examining the flow-volume curve and FEV₁/FVC ratio, then proceed through TLC, DLCO, bronchodilator response, and airway resistance as needed. - Goldman-Cecil Medicine, p. 868

The algorithm proceeds as follows:

Inspect the flow-volume curve for quality and shape
FEV₁/FVC ratio: if low → obstructive pathway; if normal/high → restrictive pathway
Obstructive pathway: assess FVC → if low, measure TLC (distinguish simple obstruction, obstruction with air trapping/hyperinflation, or mixed pattern) → assess bronchodilator response → grade severity → use DLCO to differentiate asthma vs. emphysema
Restrictive pathway: measure TLC → if low, restriction confirmed → assess DLCO (low = parenchymal; normal = chest wall/NMD) → measure MIP/MEP if neuromuscular disease suspected
Nonspecific pattern: normal FEV₁/FVC + low FVC + normal TLC; possible causes include small airways disease, early restriction, or obesity

Other PFTs

Maximal Voluntary Ventilation (MVV)

Maximal ventilation achievable, expressed in L/min. Reduced in central airway obstruction, muscle weakness, and poor effort. Because effort-dependent, may be a better predictor of postoperative respiratory complications than FEV₁ alone.

Bronchial Provocation Testing (Methacholine Challenge)

Used when spirometry is normal but asthma is suspected. Nebulized methacholine is inhaled in doubling concentrations until FEV₁ falls by ≥20% (PC₂₀). Thresholds:

PC₂₀ < 1 mg/mL: severe airway hyperresponsiveness
PC₂₀ < 4 mg/mL: moderate AHR
PC₂₀ < 16 mg/mL: mild AHR

Impulse Oscillometry (IOS)

A non-effort-dependent technique superimposing sound waves on tidal breathing. Measures resistance at 5 Hz (R5, peripheral airways) and 20 Hz (R20, central airways) and reactance (X5). Useful in children, elderly, or those unable to perform forced maneuvers. Can detect small airway disease before spirometric abnormalities appear.

Murray & Nadel's (Asthma chapter), p. 241-244

Arterial Blood Gas (ABG)

Complements PFTs by directly measuring:

PaO₂: partial pressure of oxygen
PaCO₂: partial pressure of CO₂ (reflects ventilatory adequacy)
pH: acid-base status

Normal arterial pH range is 7.35-7.45.

Clinical Indications for Spirometry

According to Goldman-Cecil Medicine, spirometry is recommended for:

Diagnosis of respiratory symptoms (cough, dyspnea, wheeze)
Monitoring response to therapeutic interventions or potentially harmful drug/occupational exposures
Quantifying physiologic impairment objectively
Disability evaluations
Preemployment and lung health monitoring in at-risk occupations
Preoperative assessment (in conjunction with MVV)

Spirometry as a screening test in asymptomatic smokers is not strongly recommended - evidence shows that an abnormal result does not improve smoking cessation rates, and a normal result may be falsely reassuring.

Goldman-Cecil Medicine, p. 865

Quality Assurance

The FVC, FEV₁, FEV₁/FVC ratio, and flow-volume curve shape are highly reproducible with a properly trained technician and modern spirometry equipment. However:

Poor-quality tests are biased toward lower values, giving false impressions of disease
Poor performance often shows a delayed peak flow or cough within the first second on the flow-volume curve
ATS criteria require at least 3 acceptable maneuvers with the two best FVC values within 150 mL of each other

The PFT laboratory should have a medical director overseeing quality. The ATS Pulmonary Function Laboratory Manual provides detailed protocols and standards.

Reference Equations and Predicted Values

Predicted values for lung function vary with height, age, sex, and socioeconomic/environmental factors affecting lung development. The Global Lung Function Initiative (GLI) has published race-neutral reference equations (replacing earlier race-specific adjustments) to avoid systematic underestimation in certain populations. Results are expressed as percentage predicted or - more rigorously - as the z-score relative to the lower limit of normal (LLN), defined as the 5th percentile of the reference population.

Summary Table

Test	Measures	Clinical Use
FVC	Total forced exhaled volume	Detects restriction (if low with normal ratio)
FEV₁	Volume in first second	Grades obstruction severity
FEV₁/FVC	Ratio	Distinguishes obstruction vs. restriction
FEF₂₅₋₇₅	Mid-expiratory flow	Small airway disease marker (poor reproducibility)
MVV	Max ventilatory capacity	Preoperative risk, NMD
TLC / FRC / RV	Absolute volumes	Confirm restriction, quantify hyperinflation
DLCO	Gas transfer capacity	Emphysema, ILD, pulmonary vascular disease
Methacholine PC₂₀	Airway hyperresponsiveness	Asthma diagnosis when spirometry normal
IOS (R5, X5)	Airway resistance/reactance	Small airway disease, non-effort-dependent
ABG	Gas exchange, ventilation	Hypoxemia, hypercapnia, acid-base disorders

Sources:

Murray & Nadel's Textbook of Respiratory Medicine, 7th ed., Chapters 31-32 (PFTs: Physiologic and Technical Principles; Clinical Interpretation)
Goldman-Cecil Medicine, International Edition, Chapter 73 (Pulmonary Function Testing)
Murray & Nadel's Textbook of Respiratory Medicine - Asthma chapter (Chapter 64), including COPD spirometric criteria and GOLD staging

This is a shared conversation. Sign in to Orris to start your own chat.