Chest
Volume 147, Issue 2, February 2015, Pages 362-368
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Original Research: Sleep Disorders
Is a Raised Bicarbonate, Without Hypercapnia, Part of the Physiologic Spectrum of Obesity-Related Hypoventilation?

https://doi.org/10.1378/chest.14-1279Get rights and content

BACKGROUND

Obesity hypoventilation syndrome (OHS) conventionally includes awake hypercapnia, but an isolated raised bicarbonate, even in the absence of awake hypercapnia, may represent evidence of “early” OHS. We investigated whether such individuals exhibit certain features characteristic of established OHS.

METHODS

Obese subjects (BMI > 30 kg/m2) were identified from a variety of sources and divided into those with (1) normal blood gas measurements and normal acid-base balance, (2) an isolated raised base excess (BE) (≥ 2 mmol/L), and (3) awake hypercapnia (> 6 kPa; ie, established OHS). Two-point ventilatory responses to hypoxia and hypercapnia were performed. Polygraphic sleep studies were done to identify intermittent and prolonged hypoxia.

RESULTS

Seventy-one subjects (BMI, 47.2; SD, 9.8; age, 52.1 years; SD, 8.8 years) were recruited into three groups (33, 22, and 16 respectively). The Paco2 and BE values were 5.15, 5.42, 6.62 kPa, and +0.12, +3.01, +4.78 mmol/L, respectively. For nearly all the ventilatory response and sleep study measures, group 2 (with only an isolated raised BE) represented an intermediate group, and for some of the measures they were more similar to the third group with established OHS.

CONCLUSIONS

These data suggest that obese individuals with a raised BE, despite normocapnia while awake, should probably be regarded as having early obesity-related hypoventilation. This has important implications for clinical management as well as randomized controlled treatment trials, as they may represent a group with a more reversible disease process.

TRIAL REGISTRY

ClinicalTrials.gov; No.: NCT01380418; URL: http://www.clinicaltrials.gov

Section snippets

Study Design and Setting

This was an open cross-sectional study of obese subjects with and without conventionally defined OHS. The work was carried out in the Oxford Sleep Unit, Oxford Centre for Respiratory Medicine, Churchill Hospital, England, which is a National Health Service secondary and tertiary referral center. Subjects were recruited and studied between June 2011 and September 2013. The study was registered prospectively with a global trials registry site (NCT01380418). It has been reported in accordance with

Results

Seventy-one obese subjects (37, 52%, men) were studied during the 27-month recruitment period. Demographic and anthropometric data are reported for each group in Table 1. There were no differences in age, weight, or BMI between the groups, although the P value for BMI approached significance (P = .056) between groups.

The ventilatory drive measurements at baseline and after hypoxic and hypercapnic challenge testing are shown in TABLE 2, TABLE 3, TABLE 4, respectively. Table 5 shows the sleep

Discussion

In this prospective observational cohort study, we have demonstrated that obese subjects with a raised BE, but with a normal daytime Paco2, have a ventilatory response between those of normal obese subjects (without evidence of awake hypoventilation) and those with hypercapnia and, thus, conventionally defined OHS. This evidence suggests that these subjects are in the middle of a spectrum, and indeed they could be considered as patients with “early OHS,” albeit we do not have longitudinal data

References (16)

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FUNDING/SUPPORT: This work was supported by the Oxford Health Services Research Committee, the Oxford Biomedical Research Centre, and the Oxford Radcliffe Hospital Charitable Funds. The NICO 2 device was kindly donated by Koninklijke Philips N.V.

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