Spirogram Analysis- Understanding Respiratory Volumes and Lung Function

What Is Spirogram Analysis?

A spirogram is a graphical recording of lung volumes and capacities during breathing. It's the primary diagnostic tool for assessing ventilatory function — how well your lungs move air in and out.

Doctors order this test when you have shortness of breath, chronic cough, or suspected lung disease. The results tell them if your problem is obstructive (difficulty exhaling) or restrictive (difficulty inhaling).

That's it. That's the point. Everything else is details that support that decision.

Lung Volumes vs. Lung Capacities

You need to understand this distinction before reading any spirogram:

• Volumes are the individual compartments of air in your lungs — they don't combine with other measurements.
• Capacities are combinations of two or more volumes. They give you a broader picture of lung function.

The Four Primary Lung Volumes

Tidal Volume (TV) — The air you breathe in or out during normal breathing. Roughly 500 mL in healthy adults. This is your baseline respiratory effort.

Inspiratory Reserve Volume (IRV) — The air you can forcibly inhale after a normal breath. About 3000 mL. This is your backup capacity when you need more oxygen.

Expiratory Reserve Volume (ERV) — The air you can forcibly exhale after a normal breath. About 1200 mL. This empties your lungs below normal resting levels.

Residual Volume (RV) — The air left in your lungs after maximum expiration. Roughly 1200 mL. You can never fully empty your lungs — this volume stays trapped.

The Four Lung Capacities

Total Lung Capacity (TLC) = TV + IRV + ERV + RV. The total amount of air your lungs can hold. Normally around 6000 mL.

Vital Capacity (VC) = TV + IRV + ERV. The maximum air you can breathe out after maximum inhalation. This is the functional measurement doctors care about most.

Inspiratory Capacity (IC) = TV + IRV. The air you can inhale from normal resting expiration.

Functional Residual Capacity (FRC) = ERV + RV. The air remaining in your lungs after normal expiration.

Reading the Spirogram Graph

The spirogram plots volume against time during different breathing maneuvers. Here's what you'll see:

• The baseline represents resting tidal breathing
• Inspiratory efforts move the tracing upward (volume increases)
• Expiratory efforts move the tracing downward (volume decreases)

Key Spirometry Measurements

Forced Vital Capacity (FVC) — The total amount of air you can forcibly exhale in one breath after maximum inhalation. This is the headline number.

Forced Expiratory Volume in 1 Second (FEV1) — The air you can exhale in the first second of the FVC maneuver. This tells you how quickly your lungs empty.

FEV1/FVC Ratio — This ratio is the first thing pulmonologists look at. It's the key to distinguishing obstructive from restrictive disease.

Normal Values: What Should Your Numbers Be?

These values adjust for age, height, and ethnicity. Your doctor uses predicted values based on your demographics, then compares your actual results to that baseline.

Obstructive vs. Restrictive Patterns

This is the clinical decision tree. Every spirogram interpretation starts here.

Obstructive Pattern

Seen in asthma, COPD, and bronchiectasis.

• FEV1 is reduced more than FVC
• FEV1/FVC ratio is low (below 70%)
• Air gets trapped — RV increases
• TLC may be normal or elevated

The problem: airways narrow and collapse during expiration. Air gets trapped behind them.

Restrictive Pattern

Seen in pulmonary fibrosis, sarcoidosis, and chest wall deformities.

• FVC is reduced
• FEV1 is reduced proportionally (ratio stays normal)
• All volumes decrease — TLC, RV, ERV all drop

The problem: the lungs can't expand fully or the chest wall is constrained.

How to Tell Them Apart

What Reduced FEV1 Actually Means

Your FEV1 percentage of predicted correlates directly with severity:

• > 80% — Normal or mildly impaired
• 50-79% — Moderately impaired
• 30-49% — Severely impaired
• < 30% — Very severely impaired

These numbers matter for treatment decisions and prognosis. A COPD patient with FEV1 of 35% predicted has different management than one at 65%.

Common Conditions Diagnosed by Spirogram

Asthma — Reversible airflow obstruction. FEV1 improves > 12% after bronchodilator use. This reversibility is diagnostic.

COPD — Irreversible obstruction. FEV1 doesn't improve significantly with bronchodilators. FEV1/FVC remains low.

Pulmonary Fibrosis — Restrictive pattern. Both FVC and FEV1 drop, but the ratio stays normal. Reduced DLCO confirms impaired gas exchange.

Neuromuscular Disease — Reduced VC and FVC with preserved ratio. The problem is weak respiratory muscles, not lung tissue.

How to Get a Spirogram Test

You need a doctor's order. Spirometry is available at:

• Pulmonary function laboratories
• Hospital respiratory departments
• Some primary care offices with the equipment
• Specialist pulmonology clinics

The test takes 15-30 minutes. You'll breathe through a mouthpiece attached to a spirometer. Technician guidance is essential — poor technique produces invalid results.

Before the Test

• Avoid smoking for 1 hour before
• Don't exercise vigorously beforehand
• Wear loose clothing that doesn't restrict breathing
• Skip bronchodilators if instructed (for baseline measurements)

During the Test

You'll perform the maximal expiratory maneuver: inhale fully, then blast the air out as hard and fast as possible. Hold it until you're completely empty. Repeat at least 3 times for consistency.

The best two FVC and FEV1 values should be within 150 mL of each other. If they're not, you need more attempts.

Limitations of Spirogram Analysis

Spirogram measures ventilation, not gas exchange. You can have normal spirometry but impaired diffusion (measured by DLCO test).

It doesn't tell you why your lungs are abnormal. A restrictive pattern could be lung fibrosis, chest wall disease, or muscle weakness. You need additional testing to find the cause.

Effort dependence is real. Patients who don't give full effort will look worse than they are. Experienced technicians know how to coach proper technique.

Bottom Line

Spirogram analysis tells you if your lungs have an obstructive problem (difficulty exhaling), a restrictive problem (difficulty expanding), or are functioning normally. The FEV1/FVC ratio is the primary discriminator.

Everything else — specific volumes, capacities, and how much each is reduced — tells you the severity and helps monitor disease progression or treatment response.

If your doctor ordered this test, ask specifically: "What is my FEV1/FVC ratio and is it obstructive, restrictive, or normal?" That's the answer that matters.