Effect of hypercapnia on total pulmonary resistance during wakefulness and during NREM sleep

We investigated the effect of different levels of hypercapnia on total pulmonary resistance (RL) in 13 subjects ranging from nonsnorers with low RL to snorers with high RL and dynamic narrowing of the upper airway during inspiration. Added CO2 was adjusted to achieve a steady-state increase in PETCO2 of +2, +4, or +6 mm Hg. RL was measured at peak inspiratory flow (RLpf), at maximal resistance within breath (RLmax), and at 10 equally spaced points within inspiration in several trials. During wakefulness, hypercapnia was associated with decreased RLmax. During steady state +6 mm Hg hypercapnia, RLmax decreased by 30% (p less than 0.01). During NREM sleep, low levels of hypercapnia did not affect RL. However, +6 mm Hg hypercapnia was associated with decreased RLmax in six of eight subjects (p = 0.07), especially in subjects with high RLmax during room air breathing. The effects of hypercapnia on RLpf paralleled its effect on RLmax. We concluded that (1) the decrease in RL during awake hypercapnia suggests an increase in upper airway dimensions and stiffness, (2) the absence of increased RL during low level NREM hypercapnia (despite the increase in inspiratory flows and collapsing pressures) also suggests an increase in upper airway dimensions and stiffness, and (3) upper airway dilating muscles appear to be recruited in a coordinated fashion with inspiratory muscles in normal humans during NREM sleep. The implications of these findings in patients with obstructive sleep apnea are not clear at this point.