Time-Expired Spirogram

After a maximum inspiratory effort, a subject exhales as forcefully and rapidly as possible, and the volume of gas is called the forced vital capacity (FVC). The exhaled volume is recorded with respect to time. The rate of airflow during this rapid, forceful exhalation indirectly reflects the flow resistance properties of the airways. When airway obstruction occurs, the FVC tends to be less than the standard VC because airways reach flow limitation early and air trapping occurs. In healthy subjects, the two maneuvers usually result in almost equal measured volumes. Because the FVC maneuver is an artificial one, patients must be instructed carefully, and they often require practice attempts before performing the test adequately. Generally, three acceptable tracings are required for analysis. These FVC maneuvers must be characterized by an initial full inspiration to TLC, followed by an abrupt onset of exhalation and continued maximum effort throughout exhalation to RV. The exhalation should take at least 4 seconds and should not be interrupted by coughing, glottic closure, or any mechanical obstruction.^[2]

The FVC is reduced by the same conditions that reduce VC. To identify airway obstruction, flow rates are determined by calculation of the volume exhaled during certain time intervals. Most commonly measured is the volume exhaled in the first second, called the forced expiratory volume in 1 second (FEV₁ ). The FEV₁ can be expressed as the absolute volume in liters, and typical values for various patient groups are listed in Table 26-1 . The FEV₁ provides an even better perspective on the degree of airway obstruction when it is expressed as a percentage of the FVC (FEV₁ /FVC).

For the purposes of reporting and calculating values, the largest observed FVC and FEV₁ from any of the three acceptable spirograms are used, even if they are not obtained from the same curve.^[2] Normal, healthy subjects can exhale 75% to 80% of FVC in the first second; the remaining volume is exhaled in two or three additional seconds ( Fig. 26-1 ). Diseases such as asthma and bronchitis,

TABLE 26-1 -- Clinical ranges for 1-second forced expiratory volume (FEV₁ )

Clinical Range (L)	Patient Group
3.0–4.5	Normal adult
1.5–2.5	Mild-to-moderate obstruction
<1.0	Qualifies as handicapped
0.8	Disability
0.5	Severe emphysema

which obstruct the airway, reduce expiratory flow rates and therefore reduce FEV₁ and FEV₁ /FVC. An obstructive defect is essentially characterized by a disproportionate decrease in the airflow rate relative to the actual volume exhaled (i.e., FVC) and indicates airway narrowing and flow limitation during expiration. Values for FEV₁ /FVC that are lower than 70% reflect mild obstruction, those lower than 60% suggest moderate obstruction, and those lower than 50% indicate severe obstruction. Because the FEV₁ /FVC represents a ratio, it is important to realize that identical percentage values may not indicate equivalent degrees of lung dysfunction. For example, a patient with an FEV₁ of 1.5 L and an FVC of 3.0 L does not have the same degree of impairment as a similar-sized patient with an FEV₁ of 0.75 L and an FVC of 1.5 L, although both have an FEV₁ /FVC ratio of 50%.

Restrictive diseases are not usually associated with airway obstruction but do cause decreases in FVC. Although the absolute volume of FEV₁ may be reduced on a similar

Figure 26-1 Forced vital capacity (FVC) maneuver in a subject with a normal lung. Exhaled volume is plotted against time as the subject expires forcefully, rapidly, and completely to residual volume (RV) after a maximum deep inspiration to total lung capacity (TLC). FEV₁ , forced expired volume in 1 second; FEF_200–1200 , forced expiratory flow between 200 and 1200 mL of expired volume; FEF_25%–75% , forced expiratory flow over the midportion of vital capacity (i.e., from 25% to 75% of expired volume).

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The maximum flow rate during an FVC maneuver occurs in the initial 0.1 second and is called the peak flow rate (measured in liters per second or liters per minute). Peak flow can be estimated by drawing a tangent to the steepest part of the FVC spirogram, but this method is subject to large errors. More commonly, maximum flow is measured as the average flow during the liter of gas expired after the initial 200 mL during an FVC maneuver (see Fig. 26-1 ). This is usually designated as forced expiratory flow between 200 and 1200 mL of FVC (FEF_200–1200 ); however, the term maximum expiratory flow rate has also been used. This flow is slightly lower than the true peak flow, which can be measured conveniently with a handheld flow meter ( Fig. 26-2 ) or more accurately with a pneumotachygraph. Peak flow is markedly affected by obstruction of large airways and is particularly responsive to bronchodilator therapy. Because repeat measurements are convenient to obtain, the peak flow rate can be used to monitor therapeutic responses in patients with acute asthma. Normal values in healthy men younger than 40 years are typically 500 L/min or more. Values less than 200 L/min in surgical candidates suggest impaired cough efficiency and a strong likelihood of postoperative complications.^[3] The test is much less unpleasant and exhausting for patients than the FVC maneuver; it therefore provides a valuable tool to identify gross pulmonary disability at the bedside.

Although peak flow is largely a function of the caliber of the airways, it also greatly depends on expiratory muscle strength and on the patient's effort and coordination. As a result, the measurement can be variable. In contrast, high degrees of effort are not required to achieve maximum expiratory flow at intermediate and low lung volumes during forced expiration. Flow is often measured over the middle half of the FVC (i.e., between 25% and 75% of the expired volume). This parameter, formerly called the maximum midexpiratory flow, is now referred to as the forced midexpiratory flow (FEF_25%–75% ). Because the flow does not include the initial, highly effort-dependent portion of forced expiration, FEF_25%–75% is often referred to as effort independent. This designation is not entirely appropriate, because FEF_25%–75% can be decreased by marked reductions in expiratory effort and by a submaximal inspiration before the maneuver. The latter artificially

TABLE 26-2 -- Forced vital capacity (FVC) and 1-second forced expiratory volumes (FEV₁ ) in disease states

Disease States	FVC	FEV1	FEV1 /FVC
Airway obstruction (asthma, bronchitis)	Normal	Decreased	Decreased
Stiff lungs (pneumonia, pulmonary edema, pulmonary fibrosis)	Decreased	Decreased	Normal
Respiratory muscle weakness (myasthenia gravis, myopathies)	Decreased	Decreased	Normal

Figure 26-2 Three hand-held peak flow meters. The classic Wright meter (Air Med Limited, Harlow, England) and the Assess peak flow meter (Health Scan Products, Cedar Grove, NJ) quantitate the peak flow rate. The smaller peak flow monitor (Biotrine Corporation, Woburn, MA) allows the patient to tape over the holes before blowing. If the flow rate indicated by the first uncovered hole is reached, a hornlike sound is produced by a metal, reedlike device within the tube.

The FEF_25%–75% is a highly variable spirometric index, largely because of its dependence on the absolute volume of FVC and on changes in expiratory time with various degrees of airway obstruction. Values for FEF_25%–75% in healthy young men average 4.5 to 5.0 L/sec, but because of wide variations even in normal subjects, the predicted limits of normal may be as low as 2 L/sec. The measurement has often been proposed as a sensitive indicator of early obstruction in the small distal airways. However, patients undergoing spirometry for suspected airway obstruction almost always had normal values for FEF_25%–75% when FEV₁ /FEV was 75% or greater.^[6] One exception may occur in patients with restrictive ventilatory defects, in whom the FEF_25%–75% may be markedly reduced and the FEV₁ /FVC normal. However, lung volumes, as reflected by the TLC, the FVC, and the absolute volume of FEV₁ , are all reduced, and the FEF_25%–75% does not appear to be any more sensitive than FEV₁ in detecting mild abnormalities of lung dysfunction.