Spirometry and Flow-Volume Loop Simulator

What is a flow-volume loop?

A flow-volume loop is the graph a spirometer draws of one forced vital capacity manoeuvre: the flow of air at the mouth plotted against the volume breathed out, while the person blows out as hard and as fast as they can from full lungs and then breathes straight back in. Breathing out is drawn above the axis and breathing in below it, so the two limbs close into a loop. The number that matters most is the ratio of the volume out in the first second, FEV1, to the whole volume out, the forced vital capacity or FVC, and the ATS/ERS definition of airflow obstruction is FEV1/FVC < LLN, a ratio below its lower limit of normal.

The same blow drawn against time is the volume-time curve. The two graphs carry the same information and show different parts of it well. The loop shows shape: the sharp early peak, a straight or scooped fall, and a plateau where a narrow central airway caps the flow. The volume-time curve shows timing: where one second falls, when the curve levels off, and how long the blow lasts.

Using the simulator

Choose a pattern and press Play: the forced breath animates along the loop and the volume-time curve, and the sketch of the airway beside the loop, on a wide screen, shows where a lesion sits and how far each phase squeezes it. Severity moves the chosen pattern from its mildest form to its most severe. The healthy vital capacity sets every predicted value, and the two lower limits decide the reading, because real ones come from a reference equation for the person’s age, height and sex. With the loop focused, the arrow keys change the pattern and the severity.

The readouts give FEV1, FVC and their ratio, peak expiratory flow, FEV1 and FVC as percentages of predicted, and the forced expiratory time. The reading and its severity grade come from those numbers and the shape of the loop, by the ATS/ERS rules and Miller and Hyatt’s, never from the pattern you chose: the mildest asthma reads as normal once the lower limit of FEV1/FVC is set to 0.68. Under the plot, FEF50/FIF50, FEV1/PEF and the length of any flat limb are the signs of an upper airway lesion. The healthy loop for the same lungs is dashed on both graphs.

Worked example: a healthy 4.6 L lung

The simulator opens on healthy lungs with a vital capacity of 4.6 L, Guyton and Hall’s textbook figure for a young adult man. They empty like a single compartment with a time constant of 0.5 s, so flow is the volume left divided by 0.5 s.

• With all of it in the lungs, flow could be VC/τ = 4.6 / 0.5 = 9.2 L/s.
• The blow accelerates until 0.05 × 4.6 = 0.23 L is out and meets that line at PEF = 9.2 × 0.95 = 8.74 L/s, or 524.4 L/min.
• The rise takes 2 × 0.23 / 8.74 = 0.0526 s, so time zero, halfway through it, is 0.0263 s into the blow, with 0.23 / 4 = 0.0575 L already out: 1.25 percent of the FVC, inside the ATS/ERS limit.
• After the peak the 4.37 L left empties exponentially. One second after time zero 4.37 × exp(−(1 − 0.0263) / 0.5) = 0.6234 L remains, so FEV1 = 4.6 − 0.6234 = 3.977 L.
• Each second takes the remainder down by a factor of e² = 7.389, so the last second brings out 25 mL with 0.025 / (e² − 1) = 0.0039 L left. The blow ends there, after an FET of 3.535 s, and FVC = 4.6 − 0.0039 = 4.596 L.
• The ratio is FEV1/FVC = 3.9766 / 4.5961 = 0.8652, well above 0.70.
• The breath back in peaks halfway at PIF = 1.1 × 4.596 = 5.056 L/s and lasts π / 2.2 = 1.428 s. Halfway out, with 2.302 L left, flow was 2.302 / 0.5 = 4.604 L/s, so FEF50/FIF50 = 4.604 / 5.056 = 0.9106.
• FEV1 in mL over PEF in L/min is 3976.6 / 524.4 = 7.583.

Every one of those is a readout at the opening settings, and the reading is normal spirometry: the ratio is above its limit, FVC is 100 percent of predicted and neither limb is flat.

Reading the volume-time curve

A real blow does not start crisply, so the ATS/ERS 2019 standard sets time zero by back extrapolation: a tangent drawn at peak flow, with a slope equal to PEF, crosses zero volume at time zero, and every timed volume is measured from there. The volume out before it, the back-extrapolated volume, must stay under 5 percent of the FVC or 0.100 L, whichever is greater, or the blow had a hesitant start. FEV1 is the volume out by one second after time zero, that early volume included.

The blow ends at a plateau, the first moment at which less than 0.025 L has come out over the last second, or once the forced expiratory time reaches 15 s. A healthy blow is over in a few seconds. An obstructed one creeps on: asthma at 50 percent severity runs for 12.20 s, and COPD at full severity reaches the 15 s limit with air still in the chest, which is one reason its FVC falls.

Obstructive vs restrictive spirometry

Obstruction means air comes out slowly, so FEV1 falls further than FVC and the ratio drops below its lower limit. The lower limit is the 5th percentile of healthy people of the same age, height and sex, a z-score of −1.645, from a reference equation such as GLI 2012. The GOLD strategy for COPD uses a fixed ratio of 0.70 instead, which is simpler but misclassifies people at both ends of adult life: the lower limit falls with age, and in Swanney and colleagues’ 2008 study it crossed 0.70 at a median age of 42 in men and 48 in women, so a fixed 0.70 calls some healthy older people obstructed. The mildest asthma here has a ratio of 0.6878: obstructive against 0.70, normal against a lower limit of 0.68.

Restriction means the lungs hold less, so FVC is low while the ratio is normal or high, because what air there is comes out quickly. Restriction at 50 percent severity gives an FVC of 61.55 percent of predicted with a ratio of 0.9215, a narrow, tall loop. Spirometry can only suggest it, though: a restrictive pattern is confirmed by a low total lung capacity, which needs a body box or gas dilution. Without that measurement, a low FVC with a normal ratio is reported as a possible restrictive pattern, and when FEV1 is low as well, as it is here, the 2022 ERS/ATS standard calls it preserved ratio impaired spirometry, PRISm. A low ratio with a low FVC can be obstruction with air trapping or a mixed defect, and only total lung capacity tells them apart.

ATS/ERS 2005 graded any abnormality by FEV1 percent predicted: mild at 70 percent or more, moderate at 60 to 69, moderately severe at 50 to 59, severe at 35 to 49 and very severe below 35. The 2022 standard grades by z-score instead: mild from −1.65 to −2.5, moderate from −2.5 to −4 and severe below −4. The simulator uses the 2005 bands, since a z-score needs a reference equation.

Asthma and COPD on the loop

Both scoop the expiratory limb into a concave curve: flow falls away quickly after the peak and then trickles. Asthma at 50 percent severity gives FEV1 2.397 L, 60.27 percent of predicted, and a ratio of 0.5738, which is moderate obstruction. COPD at 50 percent has a sharper early peak, a deeper scoop and a ratio of 0.5534, with FEV1 52.56 percent of predicted, moderately severe. At full severity COPD is still blowing at 15 s and its FVC is down to 70.80 percent of predicted, which the simulator reads as obstruction with a low FVC: the air trapping that a measured total lung capacity would show.

The loop cannot tell asthma from COPD. What separates them is the history and the response to a bronchodilator, which this model does not give. ATS/ERS 2005 counted a rise in FEV1 or FVC of at least 12 percent and 200 mL as a significant response; the 2022 standard counts a rise of more than 10 percent of the predicted value. Asthma often reverses well and COPD, by definition, does not fully. The two also differ in what happens to oxygen once it reaches the blood, which the oxygen dissociation curve simulator follows, and in gas exchange, which the A-a gradient calculator measures and spirometry cannot.

Upper airway obstruction: fixed, extrathoracic and intrathoracic

A narrowing of the larynx or trachea caps the flow through it, and the limb it caps runs flat. Where the lesion sits decides which limb. Outside the chest, above the thoracic inlet, the airway is surrounded by atmospheric pressure: breathing in hard drops the pressure inside below it and sucks a floppy lesion shut, while breathing out pushes it open, so a variable extrathoracic lesion, such as a paralysed vocal cord, flattens only the inspiratory limb. Inside the chest the airway is surrounded by pleural pressure, which a forced blow raises above the pressure inside, so a variable intrathoracic lesion, such as tracheomalacia, flattens only the expiratory limb. A rigid narrowing, such as a stricture after intubation, caps both at a similar flow.

Miller and Hyatt (1973) gave the classic ratios of mid-volume flow out to flow in, FEF50/FIF50: about 0.85 for fixed lesions, 2.2 for variable extrathoracic ones and 0.32 for variable intrathoracic ones. Empey (1972) found FEV1 in mL over PEF in L/min usually below 10 in healthy people, with a mean of 7.3, and above 10 in every one of his 18 patients with upper airway obstruction; ATS/ERS 2005 suggested a lower cut-off of 8. The simulator shows how both arise. A fixed lesion at 50 percent severity caps both limbs at 2.654 L/s, so FEF50/FIF50 is 1. Its plateau outlasts the first second, and on a plateau that does, FEV1 is the cap times one second, 2.654 L, which puts FEV1/PEF at 1000 / 60 = 16.67 and the ratio, 0.5775, below 0.70. A central lesion can mimic obstruction on the ratio alone, which is why the reading looks at the shape first. A variable extrathoracic lesion at 70 percent gives FEF50/FIF50 2.194, and a variable intrathoracic one at full severity 0.3457.

How the model works

The lungs empty along an envelope: with a volume V_R still in them above residual volume, the most flow they give is F = (VC/τ) (VR/VC)n. With n = 1 that is a single compartment emptying through a resistance, and the volume left falls exponentially; above 1 flow falls faster than the volume does, which scoops the curve; below 1 it bows outward, as stiff lungs do. The equation separates for any n, so every instant of the blow is a closed form. Flow first rises in proportion to time until 5 percent of the vital capacity is out, a lesion caps it at a fixed flow, the breath back in follows a half ellipse, and the end of the blow is found by halving an interval until it is settled to the limit of double precision. Each pattern is a set of these settings chosen to show the textbook shape, not a fit to any patient, and the predicted values are the same model with healthy lungs.

Reading a loop is pattern recognition, as reading a rhythm strip is, and the ECG rhythm simulator does for heart block what this does for airflow. The breath itself is driven by the diaphragm and the intercostal muscles, with the abdominal muscles joining in on a forced blow out, and the chest wall muscles explorer shows the diaphragm and all three intercostal layers in 3D.

What this model leaves out

• Reference equations. Predicted values are this model’s healthy lungs, whose FEV1/FVC is 0.8652 at 4.6 L and stays between 0.86 and 0.87 at every size, and the lower limits are inputs. GLI 2012 predicts each from age, height, sex and ethnicity.
• Effort and technique. Every blow here is maximal and perfectly started. A slow start, a cough, a leak or a blow stopped early changes the loop, and repeatability across at least three blows is part of a real test.
• Lung volumes and gas transfer. Spirometry cannot measure residual volume or total lung capacity, and neither can this; the sketch’s lung size is illustrative.
• Bronchodilators and variability. There is no reversibility test, and asthma that is normal between attacks is not modelled.
• Combined disease. Each pattern is one condition. Real patients can have COPD and a tracheal stenosis, or obstruction with restriction.
• Weak muscles and the chest wall. The restrictive pattern is stiff lungs. Neuromuscular weakness and chest wall disease restrict differently and change the loop in other ways.

Common mistakes

• Diagnosing obstruction from FEV1 alone. A low FEV1 with a normal ratio is not obstruction; restriction lowers FEV1 too.
• Using 0.70 at every age. It overcalls obstruction in older adults and misses it in some younger ones, whose lower limit is above 0.70.
• Calling restriction from spirometry. A low FVC with a normal ratio suggests it; only a low total lung capacity confirms it.
• Ignoring the inspiratory limb. A variable extrathoracic lesion can leave FEV1, FVC and PEF almost untouched and show only in the breath back in.
• Reading a flat top as obstruction. A plateau with a low ratio is a central lesion until shown otherwise.
• Stopping the blow early. An early stop cuts FVC and so raises FEV1/FVC, which can hide obstruction.