Dead space loading and exercise limitation in patients with interstitial lung disease

We tested the hypothesis that maximal exercise performance in subjects with interstitial lung disease (ILD) is limited by respiratory factors. Assuming this is so, ventilatory stimulation by added dead space (VD) should impair exercise capacity.

Six subjects with ILD each underwent three maximal incremental exercise studies on a bicycle ergometer; control 1, added VD, and control 2. During the VD study, external VD (500 ml) was added to the circuit, and results obtained were compared with the mean results from the control studies.

Exercise duration (TLIM) was significantly less in the VD study when compared to the control study (369 +/- 50 vs 439 +/- 55, p < 0.05), as was work rate (102 +/- 13 vs 125 +/- 14 W, p < 0.05) and peak oxygen uptake per minute (VO2) (1.08 +/- 0.09 vs 1.43 +/- 0.14 L/min, p < 0.05). At end-exercise, the Borg scale was higher in the VD study when compared to the control study (6 +/- 1 vs 5 +/- 1, p < 0.05), while no significant difference in minute ventilation (VI) or oxygen desaturation was noted. When compared to the control study at matched times during exercise, the addition of VD resulted in a significant increase in VI while no significant change was noted in VO2, carbon dioxide output (VcO2), or heart rate (HR).

The decrease observed in TLIM, work rate, and peak VO2 with added VD, associated with a lack of change in VI or oxygen desaturation at end-exercise, suggests that exercise limitation in ILD is primarily due to respiratory factors.