Flow/volume loops now supplement or have replaced volume/time spirograms in the analysis of forced exhalation. With the use of computers to analyze spirometric maneuvers, volume and flow can be translated many times per second into numbers, and the data can be graphed either as a volume/time spirogram or as a flow/volume curve. To illustrate the relationship of the volume/time spirogram to the flow/volume curve, I present a three-dimensional coordinate system, with flow on the vertical axis and volume (on the horizontal axis) and time forming a base grid. The height of the flow curve corresponds to the steepness of the volume/time spirogram at any point; therefore, the FEF25, -50, and -75 correspond to the slope of the spirogram at those volumes. The FEV₁ is the volume value of the point where the flow curve crosses the 1-second line on the time axis; this is clearly shown by the flow curve's 'shadow,' the spirogram on the base grid. The peak flow is the highest point reached by the flow curve and is directly above the steepest part of the spirogram. As diagnostic tools, the curves have limitations: When viewed from an angle with true perspective, a curve becomes distorted and does not align well with unit lines; when viewed from an angle without true perspective, the three-dimensional aspect is difficult to picture. The three-dimensional coordinate system is an example of how computerized spirometry can provide new ways of looking at available information. (Stewart J. A Three-Dimensional View of the Flow/Volume Curve. Respir Care 1984;29:380-383.
