Episodic nocturnal hypercapnia (eNH) can identify patients with advanced but stable COPD, who are at risk of  COPD exacerbations, more accurately than sleep-associated hypoventilation, suggests a recent reported in the International Journal of Chronic Obstructive Pulmonary Disease.¹

To find out a possible association between exacerbations of COPD and episodic nocturnal hypercapnia, Kitajima et al recruited 24 patients with stable, severe or very severe COPD. These patients had a daytime arterial carbon dioxide pressure (PaCO2) less than 55 mmHg and they underwent overnight transcutaneous carbon dioxide pressure (PtcCO2) monitoring from April 2013 to January 2017. A retrospective analysis of the prevalence of eNH and sleep-associated hypoventilation was carried out.

For the purpose of the study, eNH was defined as an episodic increase of ≥5 mmHg from baseline PtcCO2 for 5 min or more at least once during nighttime. The criteria for sleep-associated hypoventilation for adults, as per the American Academy of Sleep Medicine (AASM), include increase in PaCO2 or PTcCO2 to >55 mmHg for ≥10 min, ≥10 mmHg increase in PaCO2 or PtcCO2 during sleep to >50 mmHg for ≥10 min.

The mean daytime PaCO2 among the study participants was 43.3 ± 6.8 mmHg; the nocturnal PtcCO2 was 42.9 ± 9.6 mmHg. Six (25%) out of the 24 study subjects fit the criteria for sleep-associated hypoventilation, while 11 (~46%) satisfied the criteria for eNH. The odds ratio of SH for minimum one annual exacerbation was 1.0, while the odds ratio was 11.1 for eNH. For a minimum of two exacerbations every year, the odds ratio for sleep-associated hypoventilation was 0.3 and for eNH it was 6.6.

PaCO2 greater than 50 mm Hg is a feature of type 2 or hypercapnic respiratory failure, which is common in COPD patients with advanced disease. It becomes evident earlier during sleep at night  before the daytime hypercapnia appears. Episodic nocturnal hypercapnia is common in patients with advanced COPD and mainly occurs during rapid-eye-movement (REM) sleep.

The results of this study establish episodic nocturnal hypercapnia (eNH) as a more sensitive indicator of exacerbations vis a vis sleep-associated hypoventilation, in patients with advanced COPD and a daytime PaCO2 less than 55 mmHg, though it had similar specificity as sleep-associated hypoventilation. This the authors note is because “eNH is exclusively focused on nocturnal episodic PtcCO2 increases, which corresponds to REM-related hypoventilation… whereas SH does not always correspond to episodic PtcCO2 increases, but also sustained hypercapnia during the night”.

Reference

1. Takamasa Kitajima, et al. Relationship between episodic nocturnal hypercapnia and history of exacerbations in patients with advanced chronic obstructive pulmonary disease. Int J Chron Obstruct Pulmon Dis. 2022 Jul 7;17:1553-1563. doi: 10.2147/COPD.S361914.