Does PaCO2 Increase During Sleep?

Yes, PaCO2 physiologically increases during sleep in normal individuals, with a typical rise of 2 to 7 mm Hg from wakefulness to sleep. 1

Normal Physiological Changes

In healthy subjects, sleep causes mild alveolar hypoventilation that results in PaCO2 increases of 2 to 8 mm Hg and PaO2 reductions of 3 to 11 mm Hg 2
The American Academy of Sleep Medicine references the 1999 Chicago consensus paper stating that normal PaCO2 increase from wakefulness to sleep ranges from 2 to 7 mm Hg 1
Studies using transcutaneous CO2 monitoring in 33 healthy volunteers demonstrated an average maximal increase of 6 mm Hg (13% of awake PtcCO2) during sleep 3
These increases occur because breathing during sleep becomes primarily dependent on chemical control rather than behavioral control, making ventilation more vulnerable to instability 4

The ventilatory response to CO2 appears decreased during sleep when measured by minute ventilation, but central CO2 chemosensitivity actually remains intact 5
The apparent decrease in ventilatory response is due to mechanical factors and changes in brain blood flow rather than true chemoreceptor insensitivity 5
During REM sleep specifically, increased brain blood flow may cause cerebral venous PCO2 to not increase proportionally to arterial PCO2, affecting the ventilatory drive 5

Pathological hypoventilation during sleep is defined as either:
- PaCO2 >55 mm Hg for ≥10 minutes, OR
- ≥10 mm Hg increase in PaCO2 during sleep (compared to awake supine value) reaching >50 mm Hg for ≥10 minutes 1, 6
Patients with chronic obstructive pulmonary disease show similar percentage increases in PaCO2 during sleep as normal subjects, but starting from higher baseline values 3, 7
Patients with scoliosis demonstrate significantly greater increases (mean 11 mm Hg or 21% of awake PtcCO2), which is pathological 3
In patients with respiratory muscle weakness, daytime hypercapnia is unlikely unless respiratory muscle strength is reduced to <40% of predicted and vital capacity is <50% of predicted 1

Transcutaneous PCO2 monitoring shows increases of 0.1-0.3 kPa with each apneic event, with cumulative increases during repeated long apneas, especially in REM sleep 8
The American Academy of Sleep Medicine recommends transcutaneous or end-tidal PCO2 as surrogates for arterial PaCO2 during polysomnography 1, 6
Arterial blood gas obtained immediately upon awakening provides definitive evidence of sleep hypoventilation, particularly useful when daytime PaCO2 is borderline 6

Do not assume that normal daytime PaCO2 excludes significant nocturnal hypoventilation, particularly in patients with neuromuscular disease or obesity hypoventilation syndrome 1, 6
Elevated serum bicarbonate (≥27 mmol/L) in obese patients should prompt investigation for sleep-related hypoventilation even if daytime blood gases appear acceptable 6
Standard pressure support ventilation without a backup rate can paradoxically worsen central apneas through hyperventilation and hypocapnia 9, 4