Peer-review started: August 23, 2019
First decision: November 12, 2019
Revised: December 16, 2019
Accepted: December 23, 2019
Article in press: November 12, 2019
Published online: January 8, 2020
Processing time: 126 Days and 7.7 Hours
In the obese patient population, some patients have severe obstructive sleep apnea (OSA) with daytime hypoventilation. Such patients are generally identified on the basis of the presence or absence of daytime hypercapnia, and the condition is called obesity hypoventilation syndrome (OHS). However, mechanisms for such daytime hypoventilation remain unclear.
Because patients with OHS consume greater levels of healthcare resources, and are associated with increased morbidity and mortality compared with eucapnic controls, identifying patients with OHS is highly valuable.
To investigate metabolic syndrome and daytime hypercapnia association based on hypercapnia prevalence in obese OSA patients in a nested case-control study.
Consecutive obese patients (body mass index ≥ 30 kg/m2) who underwent polysomnography due to suspected OSA were included. Among them, patients with severe OSA (apnea hypopnea index ≥ 30/h) were divided into two groups according to the presence or absence of hypercapnia during wakefulness (arterial partial pressure of carbon dioxide ≥ or < 45 Torr, respectively).
Among 97 eligible patients, 25 patients (25.8%) had daytime hypercapnia. There were no significant differences in age, gender, body mass index, apnea-hypopnea index, and Epworth Sleepiness Scale scores between the two groups. However, patients with hypercapnia had a significantly lower arterial partial pressure of oxygen level (75.8 ± 8.2 torr vs 79.9 ± 8.7 torr, P = 0.042) and higher arterial partial pressure of carbon dioxide level (46.6 ± 2.5 torr vs 41.0 ± 2.9 torr, P < 0.001). Additionally, patients with hypercapnia were more likely to have metabolic syndrome (MetS) (72.0% vs 48.6%, P = 0.043) and a higher metabolic score (the number of satisfied criteria of metabolic syndrome). In multivariate logistic regression analysis, the presence of MetS was associated with the presence of hypercapnia (OR = 2.85, 95%CI: 1.04-7.84, P = 0.042).
This study is the first to show that the presence of MetS and increased metabolic scores were associated with a greater risk of daytime hypercapnia in these patients. The importance of the present study is that physicians will be able to identify patients at risk for OHS, on noting MetS or high metabolic scores in obese patients with severe OSA, without determining arterial blood gas. Physicians should consider the possibility of daytime hypercapnia in cases of obese patients with severe OSA presenting with coexisting MetS. In addition, we have suggested that hyperleptinemia and leptin resistance may play a key role in the relationship between MetS and daytime hypercapnia in obese patients with severe OSA independent of abdominal obesity.
Although a causal relationship between MetS and daytime hypercapnia remains to be elucidated, and although circulating levels of leptin were not measured in the present study, this study may contribute to generating hypotheses that hyperleptinemia in association with MetS contributes to the daytime hypoventilation in obese patients with severe OSA. Thus, further large-scale prospective studies including measurement of circulating levels of leptin are needed.