Spirometry

The forced breath, second by second

Spirometry captures a single deliberate manoeuvre. The patient seals their lips around a mouthpiece with a nose clip on, breathes normally for a few cycles, then inhales to total lung capacity and blows out as fast and as completely as possible, sustaining the effort until the lungs are essentially empty. A pneumotachograph or turbine measures airflow, and the machine integrates flow over time to recover volume. From that one breath it constructs two complementary pictures: the volume-time curve, which rises steeply then plateaus, and the flow-volume loop, which spikes to a sharp peak flow and then falls almost linearly as the lungs empty.

The first second is decisive. In a healthy lung the airways are wide and the chest recoils briskly, so the bulk of the air leaves early — typically 75 to 85 percent of the entire vital capacity is gone within that first second. That fraction is the FEV1/FVC ratio, and it is the most informative single output of the whole test, because it normalises FEV1 against the size of the breath. A small person and a large person have very different absolute volumes but a similar ratio when both are healthy.

The numbers and what they mean

Raw litres mean little without context, so spirometry results are reported as a percent of predicted — the value expected for a person of the same age, sex, height, and ethnicity, derived from large reference populations (the GLI-2012 equations are the current global standard). A few key measurements:

• FVC — forced vital capacity. The total volume blown out, around 4.8 L in an average-sized adult man and 3.3 L in an average woman. It shrinks in restriction and, in severe obstruction, when air trapping prevents full emptying.
• FEV1 — forced expiratory volume in one second. Roughly 3.7 L in that same man. It is the workhorse for tracking obstruction over time; COPD severity (GOLD stages 1–4) is graded largely by FEV1 percent predicted: ≥80%, 50–79%, 30–49%, and <30%.
• FEV1/FVC ratio. Normally 0.75–0.85 in young adults, declining with age. Below the lower limit of normal it defines an obstructive ventilatory defect.
• FEF25-75. The average flow over the middle half of the exhalation. It is sensitive to small-airway narrowing and can dip before FEV1 falls, but it is noisy and not used alone.
• PEF — peak expiratory flow. The maximum flow rate, reached within the first 100 milliseconds; it is effort-dependent and is what cheap handheld peak-flow meters track at home in asthma.

A clinically meaningful change in FEV1 over time is usually taken as about 100 mL or roughly 10–12 percent; healthy adults lose around 25–30 mL of FEV1 per year, while smokers with susceptible lungs can lose 60 mL or more annually — the accelerated decline that defines emphysematous COPD.

Obstructive versus restrictive

Almost every spirometry interpretation begins by sorting the result into one of two patterns. In obstructive disease the airways are narrowed — by bronchospasm and inflammation in asthma, by mucus and small-airway collapse in COPD — so air leaves slowly. FEV1 falls more steeply than FVC, dragging the ratio down, and the expiratory limb of the flow-volume loop develops a characteristic concave "scooped-out" shape as flow collapses at low lung volumes. In restrictive disease the lungs or chest wall cannot expand fully — fibrosis stiffens the parenchyma, kyphoscoliosis cages the thorax, weak muscles cannot pull in a full breath — so every volume is small but airflow per litre is preserved or even brisk. FVC and FEV1 fall together, the ratio stays normal or rises, and the loop looks like a tall, narrow miniature of the normal curve.

Feature	Obstructive (asthma, COPD)	Restrictive (fibrosis, weakness)
FEV1	Reduced (often markedly)	Reduced
FVC	Normal or reduced	Reduced
FEV1/FVC ratio	Low (< 0.70 / < LLN)	Normal or increased
Flow-volume loop	Concave, scooped expiratory limb	Tall, narrow, normal shape
Total lung capacity	Normal or increased (air trapping)	Reduced (confirms restriction)
Confirmed by spirometry alone?	Yes — low ratio is diagnostic	No — needs lung volumes

This is the crucial caveat: spirometry can diagnose obstruction outright, because a reduced FEV1/FVC ratio is itself definitional. But it can only suggest restriction. A low FVC with a normal ratio raises the suspicion, yet confirmation requires demonstrating a reduced total lung capacity on body plethysmography or gas-dilution testing, since a poorly performed manoeuvre or coexisting obstruction can mimic a restrictive pattern. A "mixed" defect — both ratio and volumes low — needs full lung-volume testing to disentangle.

Reversibility and the asthma–COPD distinction

Once obstruction is found, the next question is whether it reverses. The patient inhales a short-acting bronchodilator such as salbutamol (typically 400 micrograms via spacer), waits 10–15 minutes, and repeats the test. A rise in FEV1 (or FVC) of at least 12 percent and 200 mL is a significant bronchodilator response and supports asthma, in which airflow obstruction is largely reversible. COPD, by contrast, is defined by airflow limitation that is not fully reversible — though many COPD patients still show a partial response, which is why reversibility is supportive rather than absolute. The 2022 ERS/ATS standards recast reversibility in terms of change as a percent of the predicted value, reducing the bias that the older percent-of-baseline definition introduced for people with small lungs.

The flow-volume loop also reveals where an obstruction sits. A flattened inspiratory limb suggests a variable extrathoracic obstruction (vocal cord dysfunction, an extrathoracic tumour); a flattened expiratory limb suggests an intrathoracic lesion; and a box-shaped loop with both limbs truncated points to a fixed obstruction such as tracheal stenosis. These shapes are often the first clue to a large-airway problem that the bare numbers would miss.

Where spirometry changes management

• Diagnosing and staging COPD. A post-bronchodilator FEV1/FVC below 0.70 confirms COPD; the FEV1 percent predicted then sets the GOLD stage and guides inhaler choice and prognosis.
• Confirming asthma. Reversible obstruction, or variability on serial peak-flow monitoring, supports the diagnosis and distinguishes asthma from fixed obstruction.
• Monitoring interstitial lung disease. Serial FVC is the central trial endpoint and clinical tracker in idiopathic pulmonary fibrosis; a 10 percent annual FVC decline marks progression and worse survival.
• Preoperative risk assessment. Before lung resection, FEV1 helps predict postoperative pulmonary reserve and whether a patient can tolerate losing lung tissue.
• Occupational and disability evaluation. Spirometry quantifies impairment from asbestosis, silicosis, and other dust diseases, and is used in surveillance of exposed workers.
• Neuromuscular disease. A falling FVC, especially the drop from sitting to lying flat, flags diaphragm weakness in ALS or Guillain–Barré and helps time ventilatory support.

What spirometry cannot see

Spirometry is effort-dependent and patient-cooperative, which is both its strength and its weakness. A submaximal blow, an early cough, a leak around the mouthpiece, or stopping before the lungs empty all produce spuriously low values; ATS/ERS quality criteria — a sharp start, no glottic closure, at least six seconds of exhalation, and two readings agreeing within 150 mL — exist precisely to catch these errors. Even a perfect test has blind spots. It measures mechanics, not gas exchange: a patient with early pulmonary fibrosis, pulmonary embolism, or pulmonary vascular disease can have entirely normal spirometry yet a markedly reduced diffusing capacity (DLCO) and oxygen desaturation on exertion. That is why spirometry is the entry point to pulmonary function testing rather than the whole of it, typically paired with lung volumes, DLCO, and, when needed, a six-minute walk test.

This article is educational and is not medical advice. Spirometry interpretation depends on validated reference equations and clinical context; diagnosis and treatment decisions should be made by a qualified clinician.