Normal tidal volume is 6-8 cc/kg. Normal vital capacity is 60 cc/kg +/- 20% (i.e. 10-fold increase of TV), and is correlated with the ability to cough and take deep breaths. Inspiratory capacity (IC, the maximal volume inspired from FRC) is one of the only tests that can detect extrathoracic airway obstruction.
FRC profoundly influences the venous admixture of the lung (V/Q) and if low can lead to increased hypoxemia – when FRC is reduced, lung compliance necessarily falls as well, resulting in tachypnea and subsequent hypoxemia. Note that FRC falls by 10% simply by lying down. FRC cannot be measured (because residual volume is unknown).
Of all the spirometric exams which correlate with pulmonary complications (FVC, MVV, RV/TLC, FEV1%, ppoFEV1%), ppoFEV1 is the most predictive of pulmonary complications. [British Thoracic Society. Thorax 56: 98, 2001]
ppoFEV1% = preoperative FEV1% x (1 – %functional lung tissue removed/100)
Generally, a ppoFEV1% > 40% is low-risk for respiratory complications, and ppoFEV1% < 30% is high risk [Nakahara K et al. Ann Thor Surg 46: 549, 1988, 156 patients]. Note that because of compensatory hyperinflation of the residual lung, eventual FEV1 will exceed ppo FEV1 by 250 mL in lobectomies and 500 mL in pneumonectomies. [Zehier B et al. Chest 105: 753, 1995]
By spirometry, a vital capacity (maximal ins./exp.) of < 50% of predicted or < 2L is predictive of increased risk [Gass GD and Olsen GN. Chest 89: 127, 1986] – according to Barash, “abnormal” vital capacity yields a 10% risk of mortality. FEV1 may be a better indicator than vital capacity – if FEV1 is > 2L, mortality is ~ 10%, whereas for FEV1 < 1L, mortality may increase to > 20% [Lockwood P. Respiration 30: 529, 1973]. Maximum voluntary ventilation < 50%, and RV/TLC > 50% [Mittman C. Am Rev Respir Dis 84: 197, 1961] may also be predictive.
Keep in mind, however, that some of the above data are based on the pre-thorascopy and pre-epidural eras, and may no longer be as applicable. For instance, classically FEV1 < 800 cc used to be an absolute contraindication to thoracic surgery, but no longer is. Note also that in order to cough effectively, VC ≥ 3 x TV.
Graphical output of a spirometric measurement. Actual volumes are on the x-axis, flow rates are on the y-axis. Note that the x-axis is numbered in reverse (0 is to the right). Thus, patients with obstructive disease, whose lungs exist at larger volumes, will be shifted to the left.
The F-V loop begins at FRC (furthest to the right). Inspiration is represented by the fraction of the F-V loop below the x-axis, exhalation is represented by the fraction above it. The width of a flow-volume loop is equal to vital capacity (which is why patients with restrictive lung disease have very narrow F-V loops).
Expiration in the flow-volume loop is made of two components, an effort-dependent component (accelerating flow, dependent on larger airways [lung volumes are large]) and an effort-dependent component (decelerating flow, dependent on smaller airways as the lung volumes are smaller).