Lung Volumes and Diffusing Capacity
Volumes by plethysmography, transfer by single breath
Plethysmography measures thoracic gas volume (VTG) while the patient pants against a closed shutter at roughly 1 Hz; mouth pressure is assumed to reflect alveolar pressure. FRC comes from Boyle's law, and TLC = FRC + inspiratory capacity. Correct for apparatus dead space and for thermal and pressure drift inside the box; uncontrolled panting frequency or a leaking shutter produces characteristic errors.
Gas dilution techniques (helium dilution, nitrogen washout) measure only communicating gas and therefore underestimate FRC and TLC in severe obstruction - a favorite trap. Plethysmography is preferred when air trapping is suspected.
Single-breath DLCO: the patient exhales to residual volume, inhales rapidly to near TLC a mixture containing about 0.3% CO with an inert tracer (helium or methane), holds the breath about 10 seconds (acceptable roughly 8–12), then exhales; the washout volume is discarded and an alveolar sample analyzed. Report DLCO and DLCO/VA, and correct for hemoglobin, carboxyhemoglobin, and inspired oxygen pressure (altitude).
Physiology that raises DLCO: supine position, exercise, polycythemia, left-to-right shunt, alveolar hemorrhage, asthma. That lowers it: anemia, emphysema, interstitial disease, pulmonary vascular disease, smoking (elevated COHb), and a Valsalva maneuver during breath-hold.
Trap: a low DLCO is not specific for fibrosis; check hemoglobin and COHb before interpreting.
Rule of thumb (hemoglobin correction): DLCOcorrected = DLCOmeasured × (10.22 + Hb) ÷ (1.7 × Hb). Worked example: measured DLCO 15 mL/min/mmHg with Hb 10 g/dL gives 15 × (20.22 ÷ 17.0) = 17.8, so anemia explained part of the low value.