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Respiratory Therapy · Patient Data Evaluation

Pulmonary Function Testing and Diagnostic Procedures

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On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Study tools

In 30 seconds

Pulmonary function tests (PFTs) quantify how the lungs move air and transfer gas. Spirometry reports , , the ratio, and ; an obstructive pattern lowers the FEV1/FVC ratio, while a restrictive pattern lowers lung volumes with a preserved or elevated ratio. Lung volumes that spirometry cannot measure directly (, RV, ) are obtained with body plethysmography, nitrogen washout, or helium dilution. assesses alveolar-capillary gas transfer, and plus gauge respiratory muscle strength.

Why this matters

PFT data guide diagnosis and treatment, but only when the numbers are trustworthy. Poor coaching, submaximal effort, or a leaking mouth seal can produce a false "obstructive" or "restrictive" pattern and lead to the wrong conclusion. The respiratory therapist's professional responsibility is to obtain acceptable, repeatable maneuvers, recognize when results are technically invalid, and never over-interpret a single value. Any pattern suggesting severe disease — especially markedly reduced lung volumes, a very low DLCO, or evidence of a large-airway obstruction — should be flagged for prompt clinician review. All reference ranges, interpretation thresholds, and testing protocols must be verified against the current NBRC detailed content outline, AARC clinical practice guidelines, ATS/ERS standards, facility policy, and manufacturer instructions for use.

The college version

1. Spirometry and Airflow Patterns

Spirometry measures volumes and flows during a forced maneuver. Key values:

  • FVC (forced vital capacity): the total volume exhaled forcefully after a maximal inhalation.
  • FEV1 (forced expiratory volume in 1 second): the volume exhaled in the first second of that maneuver.
  • FEV1/FVC ratio: the fraction of the total breath expelled in the first second; normally around 0.70–0.80 or above (age-dependent).
  • PEFR (peak expiratory flow rate): the fastest flow achieved during the forced exhale.

The two classic patterns:

  • Obstructive: reduced FEV1, normal or mildly reduced FVC, and a reduced FEV1/FVC ratio (airflow limitation). PEFR is typically reduced.
  • Restrictive: FVC and FEV1 are both reduced proportionally, so the FEV1/FVC ratio stays normal or is elevated. PEFR is usually well preserved.

2. Lung Volumes and Gas Transfer (DLCO)

Spirometry cannot measure the air left behind after a full exhale. Total lung capacity (TLC), residual volume (RV), and functional residual capacity (FRC) require special techniques:

  • Body plethysmography ("body box"): the person sits in a sealed chamber and pants against a closed shutter; Boyle's law relates pressure and volume changes to compute thoracic gas volume, including trapped gas.
  • Nitrogen washout: the person breathes 100% oxygen, and the nitrogen washed out of the lungs is measured to compute the volume of communicating gas.
  • Helium dilution: a known amount of helium mixes with lung gas; the diluted concentration reveals the volume of gas that communicates with the airway.

Vital capacity (VC) is the largest volume that can be exhaled after a maximal inspiration (or inhaled after a maximal exhalation) and can be obtained from spirometry.

DLCO (diffusing capacity of the lung for carbon monoxide) measures how well gas crosses the alveolar-capillary membrane. A low DLCO points to a gas-transfer problem:

  • Emphysema: low DLCO from destruction of alveolar walls and loss of capillary surface area.
  • Pulmonary fibrosis: low DLCO from a thickened, scarred alveolar-capillary membrane.
  • By contrast, asthma (obstructive but without tissue destruction) shows a normal or high DLCO, and extrapulmonary restriction (obesity, neuromuscular or chest-wall disease) shows a normal DLCO. This is why DLCO helps separate emphysema from asthma and interstitial fibrosis from chest-wall restriction.

3. Flow-Volume Loops and Muscle Strength

A flow-volume loop plots flow on the vertical axis against volume on the horizontal axis. The shape localizes obstruction:

  • Obstructive: a scooped, concave expiratory limb.
  • Restrictive: a narrow but tall loop with reduced volume.
  • Fixed upper-airway obstruction (for example, tracheal stenosis): flattened both inspiratory and expiratory limbs.
  • Variable extrathoracic obstruction: flattened inspiratory limb.
  • Variable intrathoracic obstruction: flattened expiratory limb.

MIP/NIF (maximal inspiratory pressure / negative inspiratory force) measures inspiratory muscle strength — how negative a pressure the person can generate against an occluded airway. Values closer to zero (less negative) indicate weakness; a NIF less negative than about −20 cmH2O suggests significant inspiratory muscle weakness. MEP (maximal expiratory pressure) measures expiratory muscle strength, which is needed for an effective cough. These are used to evaluate neuromuscular disease and to help assess a person's readiness to breathe independently, always as part of a broader clinical judgment.

How it works

  1. The person performs a maximal inhalation, then exhales as hard and fast as possible into the spirometer.
  2. The device records flow over time; software integrates flow to compute volumes (FVC, FEV1) and derives the ratio and PEFR.
  3. For lung volumes, gas is either diluted (helium), washed out (nitrogen), or its pressure-volume relationship is measured (body box).
  4. For DLCO, a small, known concentration of carbon monoxide is inhaled; the amount taken up reflects the membrane's gas-transfer capacity.
  5. For MIP/MEP, the person breathes against an occluded mouthpiece while a pressure manometer records the maximum negative (inspiratory) or positive (expiratory) pressure generated.
  6. Results are compared against predicted normal values based on the person's age, height, sex, and other factors, then reported as a pattern.

Common confusions

Do not confuseWithDifference
Obstructive pattern (low FEV1/FVC)Restrictive pattern (low FVC, normal ratio)Obstruction slows airflow; restriction shrinks lung volume
DLCO (gas transfer)Spirometry (airflow)DLCO measures membrane gas exchange; spirometry measures volumes and flows
Nitrogen washout / helium dilutionBody plethysmographyWashout and dilution measure only communicating gas; the body box also includes trapped gas
MIP/NIF (inspiratory strength)MEP (expiratory strength)MIP reflects the ability to breathe in; MEP reflects cough/expiratory force
Bronchodilator reversibility (≥12% and 200 mL)Bronchoprovocation (≥20% FEV1 fall)Reversibility shows improvement after a bronchodilator; challenge shows hyperresponsiveness to an inhaled irritant

Memory aids

"OBstructed = Out Blocked" — in obstruction the FEV1/FVC ratio drops because the out flow is blocked. And "REstrict = REduced Everything" — in restriction FVC and FEV1 fall together, keeping the ratio normal.

Quick review

Topic Recap

PFTs separate airflow (speed) from volume (size). Spirometry yields FVC, FEV1, FEV1/FVC, and PEFR; a low ratio means obstruction and a low FVC with a normal ratio means restriction. Lung volumes (TLC, RV, FRC) require the body box, nitrogen washout, or helium dilution; DLCO measures gas transfer and drops in emphysema (surface-area loss) and fibrosis (membrane thickening). Flow-volume loops localize obstruction, and MIP/NIF plus MEP assess muscle strength. Reversibility and bronchoprovocation testing identify reactive airway disease. Always confirm test quality, recognize the pattern, and escalate to the clinician — never diagnose from numbers alone.

Knowledge Check

  1. Which pattern shows a reduced FEV1/FVC ratio?
  2. Which lung volume is the amount of air remaining after a maximal exhalation?
  3. A person with an obstructive pattern and a low DLCO most likely has which condition?
  4. What defines clinically significant bronchodilator reversibility?
  5. Which flow-volume loop shape suggests a fixed upper-airway obstruction?

Answers and Rationales

  1. Obstructive. A reduced FEV1/FVC ratio means airflow out is disproportionately slow, the hallmark of obstruction.
  2. Residual volume (RV). RV is the air left after a complete exhale and cannot be measured by simple spirometry.
  3. Emphysema. Destruction of alveolar walls reduces capillary surface area, lowering DLCO; asthma typically shows normal or high DLCO.
  4. An increase in FEV1 (or FVC) of at least 12% and at least 200 mL after a bronchodilator — a pattern of reversible airway disease.
  5. Flattened inspiratory and expiratory limbs. A fixed large-airway obstruction limits flow in both directions.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Imagine the lungs as a balloon attached to a straw. Blowing all the way out tells you how big the balloon is and how fast air can leave. Spirometry asks the person to take the biggest breath possible and blow it out as hard and fast as they can. Two things can go wrong: the straw can be narrowed (hard to push air out fast — an obstructive problem, like asthma), or the balloon can be small and stiff (not much air fits — a restrictive problem, like scarring in the lung tissue).

To tell them apart, compare how fast air comes out versus how much air comes out. In obstruction the speed is the problem; in restriction the size is the problem. For a real-world comparison: a narrow straw versus a small balloon both reduce how much you can blow out in the first second, but only the narrow straw also reduces the fraction of your total breath you can get out quickly.

Where this stops being exact: PFTs report numbers, not diagnoses. They show a pattern. Deciding what disease is causing that pattern — and what to do about it — is the physician's job. The respiratory therapist gathers and quality-checks the data, recognizes the pattern, and escalates findings to the ordering clinician.

Simple Example

A person blows into the machine. Their total breath out (FVC) is near normal, but the amount they can blow in the first second (FEV1) is only 55% of that total. The ratio FEV1/FVC is low, so the speed of airflow is the problem — an obstructive pattern, the kind seen with asthma or COPD.

Worked example

Interpreting a set of PFT results follows a "why" behind each step:

  1. Confirm test quality first. Poor effort or a bad seal makes every number misleading. The technologist verifies acceptability and repeatability before anything is interpreted.
  2. Look at the FEV1/FVC ratio. A reduced ratio means obstruction; a normal or high ratio with reduced volumes suggests restriction. This single ratio is the first branch point.
  3. Check lung volumes when restriction is suspected. A low TLC confirms restriction; a normal TLC points elsewhere. RV and FRC help show whether gas trapping is present.
  4. Add DLCO to separate causes within a pattern. Low DLCO in an obstructive pattern favors emphysema; normal DLCO favors asthma. Low DLCO in a restrictive pattern favors interstitial fibrosis; normal DLCO favors extrapulmonary causes.
  5. Use the flow-volume loop to localize any obstruction. The shape of the inspiratory and expiratory limbs can point to a large-airway or upper-airway cause rather than small-airway disease.
  6. Assess reversibility or hyperresponsiveness when airway disease is in question. Post-bronchodilator improvement (below) or a positive challenge test helps distinguish variable, reactive airway disease from fixed obstruction.
  7. Report, never diagnose. The technologist documents the numbers and pattern and escalates anything abnormal to the ordering clinician for interpretation and management decisions.

Key takeaways

  • High yield: Obstructive = low FEV1/FVC ratio; restrictive = low FVC with normal or high FEV1/FVC ratio.
  • High yield: Spirometry cannot measure RV, FRC, or TLC directly — those require plethysmography, nitrogen washout, or helium dilution.
  • High yield: Significant bronchodilator reversibility is classically defined as an improvement in FEV1 (or FVC) of at least 12% and at least 200 mL — a pattern suggesting reversible airway disease.
  • High yield: Both emphysema and fibrosis lower DLCO, but for different reasons (surface-area loss versus membrane thickening); asthma and extrapulmonary restriction keep DLCO normal or high.
  • A methacholine (bronchoprovocation) challenge tests airway hyperresponsiveness; a ≥20% fall in FEV1 at a low-to-moderate challenge concentration indicates a reactive airway.
  • Fixed upper-airway obstruction flattens both limbs of the flow-volume loop; a variable extrathoracic lesion flattens only inspiration.
  • A NIF less negative than about −20 cmH2O suggests clinically important inspiratory muscle weakness.
  • Every abnormal value or pattern requires escalation to the ordering clinician; test numbers alone are not diagnoses.

Keep learning

Ready to build on this? Continue to the next lesson.

Practice Respiratory Therapy

This lesson has no separate scored set. Practice draws from the subject’s question bank.

Study tools & related lessonsYou’ll learn to · Key vocabulary · Related

You’ll learn to

  • Define spirometry and use FVC, FEV1, FEV1/FVC, and PEFR to distinguish obstructive from restrictive patterns.
  • Explain the criteria for bronchodilator reversibility and the purpose of bronchoprovocation (methacholine) testing.
  • Describe how lung volumes (TLC, RV, FRC, VC) are measured and what diffusing capacity (DLCO) reveals about gas transfer.
  • Interpret flow-volume loops and explain the role of MIP/NIF and MEP in assessing respiratory muscle strength.

Key vocabulary

FVC
Total air blown out forcefully after a full breath in
FEV1
Air blown out in the first second
FEV1/FVC
Fraction of the breath out in one second
PEFR
Fastest flow during a forced exhale
TLC
Total air in the lungs after a full breath in
RV
Air left after a complete exhale
FRC
Air left after a normal, relaxed exhale
VC
Largest volume that can be exhaled after maximal inspiration
DLCO
Gas transfer across the alveolar-capillary membrane
MIP/NIF
Most negative pressure generated on a strong inhale
MEP
Strongest pressure generated on a forceful exhale

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