 Forced expiratory volume in one second

Forced expiratory volume in one second, abbreviated FEV1, is the gas volume expelled during the first second of a forced expiration timed according to the specified start-of-test convention. It is a volume, normally reported in litres, rather than a volumetric flow rate. FEV1 measures early emptying, while FVC describes the total volume expelled in the maneuver.^[ref]

For an ideal start with no timing offset,An original exponential model $E(t)=V_{\infty }(1-e^{-t/\tau })$ therefore gives ${\mathrm{FEV}}_1=V_{\infty }(1-e^{-1\mathrm{s}/\tau })$. At stipulated $V_{\infty }=4$ L and $\tau =2$ s, the first-second volume is about 1.57 L. The integral's units are litres even though the integrand is expressed in litres per second.

Flow-history ambiguity of a timed expiratory volume explains why FEV1 is not a complete flow trace. Over the first second, constant flow $Q_1=2\mathrm{L}{\mathrm{s}}^{-1}$ and the decreasing flow $Q_2=(3-2t)\mathrm{L}{\mathrm{s}}^{-1}$, with numerical $t$ in seconds, both integrate to 2 L. Their initial flows differ, and their instantaneous flows agree only at 0.5 s. These constructed histories show that equal FEV1 need not imply equal peak flow or equal waveform.

Real timing does not simply begin whenever a recorder is switched on. The 2019 statement uses back-extrapolation to define Time 0 and includes the volume expired before that time in both FEV1 and FVC.^[ref] State the convention rather than silently replacing it with an arbitrary one-second window.

The FEV1 to FVC ratio divides early emptying by total expelled volume. It is dimensionless and depends on both measurements. A reproducible FEV1 alone does not establish complete expiration, absolute lung size or the pressure-flow relation; these require additional observations or a different measurement model.