More recently the interaction between NIV use and the change in chemosensitivity to CO2 has been reexplored, this time using a combination of invasive (nonvolitional) and noninvasive measures of respiratory function.10 A direct comparison with data from the study by Annane et al9 is not straightforward as measurements were made at different time points (baseline, after 5 days, and after 3 months of NIV therapy). However, the overall results are similar to those in the restrictive group (NMD, 12 patients; scoliosis, 8 patients), and the mean hypercapnic ventilatory response increased significantly from 2.8 L/min/kPa (SD, 2.3 L/min/kPa) to 3.6 L/min/kPa (SD, 2.4 L/min/kPa) at 5 days, and further to 4.3 L/min/kPa (SD, 3.3 L/min/kPa) after 3 months of nocturnal NIV therapy (p = 0.044). There was an interesting “dose response” in that those patients using NIV for > 4 h per night showed a significant improvement in hypercapnic ventilatory response and Paco2 at 3 months, whereas in those patients averaging < 4 h of use per night, chemosensitivity and arterial blood gas tensions returned to baseline levels at 3 months, after initial improvements at 5 days. Invasive measurements of respiratory muscle strength and lung/chest wall compliance obtained using esophageal and gastric manometers showed no significant improvement. Sleep stage distribution was not assessed in this study, so it is not clear whether it played a part, as sleep deprivation has previously been shown to depress chemosensitivity to CO2.