FEV1
FEV1 is the volume of air you can forcibly push out in the very first second after taking a full breath and blowing as hard as possible. If exhaling is like draining a sink, FEV1 measures how much water gushes out in that first second when you yank the plug.
Because narrowed or floppy airways slow the rush of air out, FEV1 falls in obstructive diseases such as asthma and COPD, where it is the headline number for severity and for tracking response to treatment. It can also fall in restrictive disease, but there it drops in step with the total volume, leaving the ratio preserved.
FEV1 is highly reproducible and so makes an excellent yardstick over time — for example, the accelerated yearly decline of FEV1 in smokers is a classic signature of COPD progression. Like all of spirometry, a meaningful FEV1 needs a maximal, blast-like start to the blow; a hesitant or sub-maximal effort falsely lowers it.
After two puffs of an inhaled bronchodilator, an asthmatic's FEV1 rises from 2.0 L to 2.4 L — a 20 percent and 400 mL gain that signals significant reversibility.
A rise in FEV1 after a bronchodilator points toward asthma.
FEV1 is usually reported both as an absolute volume in litres and as a percent of the predicted value for someone of the same age, sex and height; severity grading uses the percent.