Determinants of daytime hypercapnia in obstructive sleep apnea: is obesity the only one to blame?

Serum uric acid and lactate levels among patients with obstructive sleep apnea syndrome: which is a better marker of hypoxemia?

BACKGROUND AND OBJECTIVES

Tissue hypoxia due to repeated sleep apneas leads to increased serum levels of uric acid (UA) and lactate in patients with obstructive sleep apnea syndrome (OSAS). Studies on assessment of serum level of UA in patients of OSAS are available. However, research on simultaneous evaluation of levels of serum lactate and UA is lacking.

DESIGN AND SETTING

Prospective, case-control study in a tertiary care institution.

PATIENTS AND METHODS

Forty patients suffering from OSAS, diagnosed by night polysomnography (PSG), were included in this study. Forty age- and sex-matched subjects in whom the presence of OSAS was ruled out by night PSG were included as healthy controls. Participants underwent a procedure for the measurement of serum UA and lactate before and after sleep.

RESULTS

Both before and after sleep UA levels of patients with OSAS were found to be significantly higher (P=.001 and .002, respectively) as compared to UA levels of controls. A statistically significant (P=.02) overnight (after sleep) rise was observed in the serum lactate level of OSAS patients. The correlation between serum UA values and %TSTs (percentage of total sleep time spent) below 95% SaO2 (arterial oxygen saturation) was statistically significant (P=.02). The correlation was positive with %TSTs below 90% SaO2, whereas it was found negative with normal basal oxygen. No significant correlation was observed between serum UA and the AHI (apnea-hypopnea index). Polysomnographic variables failed to show significant correlation with serum UA on respective multiple regression models controlling for age, body mass index, and waist-hip ratio. However, plasma lactate levels after sleep correlated with %TSTs below 95% of SaO2 and AHI with P values of .02 and .01, respectively.

CONCLUSIONS

Both serum UA and lactate were positively correlated with the degree of hypoxia in OSAS. The plasma UA levels in patients with OSAS did not show an overnight rise. However, the plasma lactate levels were higher in the morning. The measurement of serum lactate level was a better marker of oxidative stress among patients with OSAS.

Postoperative hypoxemia: common, undetected, and unsuspected after bariatric surgery

BACKGROUND

Patients undergoing gastric bypass are at greater than ordinary risk for postoperative respiratory insufficiency, presumably related to obstructive sleep apnea (OSA) and patient-controlled analgesia (PCA). This study was proposed to quantify the magnitude of the problem.

METHODS

Fifteen patients undergoing gastric bypass had oxygen saturation (SpO(2)) recorded continuously, but not displayed, for 24h postoperatively; eight also had arterial blood analysis every 4h. All received narcotic PCA. SpO(2)<90% lasting more than 10 s was reviewed. Results are mean+/-SEM.

RESULTS

Mean age was 44+/-4 y, and mean BMI was 48+/-2kg/m(2); 77% had OSA. Every patient had more than one episode with SpO(2)<90% for longer than 30s undetected by routine monitoring; most had multiple episodes. Nadir SpO(2) averaged 75% +/- 8%. Mean longest duration of desaturation below 90% averaged 21+/-15min. Mean PaCO(2) was 37+/-3mm Hg; maximum PaCO(2) was 47mm Hg.

CONCLUSIONS

Severe and prolonged episodes of hypoxemia were a consistent finding, despite aggressive preoperative diagnosis and treatment of OSA, including use of CPAP postoperatively. Although some postoperative hypoventilation was expected, the degree and frequency of desaturation were surprising. No patient exhibited arterial PaCO(2) evidence of hypoventilation. No patient experienced cardiopulmonary arrest/instability, in spite of severe, repeated episodes of hypoxemia. In no instance was a significant hypoxemic episode suspected or detected. Continuous pulse oximetry monitoring, with an audible alarm set for a saturation less than 90% for 10 s, would have alerted providers to 100% of significant hypoxemic episodes. Our recommendation is routinely monitoring (with alarm capability enabled) every bariatric surgical patient, to prevent such occurrence.