Treatment of obstructive sleep apnea with CPAP improves chronic inflammation measured by neutrophil-to-lymphocyte ratio

Abstract

Study Objectives:

Obstructive sleep apnea (OSA) is associated with chronic inflammation likely triggered by nocturnal, intermittent hypoxemia and increased adrenergic tone. The neutrophil-to-lymphocyte ratio (NLR) was recently described as a measure of subclinical systemic inflammation. Studies on the effect of continuous positive airway pressure (CPAP) therapy in OSA on subclinical inflammation measured by NLR are lacking. We hypothesize that NLR levels would improve as chronic inflammation diminishes in patients with OSA treated with CPAP.

Methods:

We retrospectively reviewed patients in whom OSA was diagnosed and who were treated with CPAP therapy. Complete blood count (CBC) were obtained pretreatment and posttreatment for calculation of NLR, which was calculated by dividing the number of neutrophils by the number of lymphocytes. Patients with conditions known to affect NLR such as chronic infections, inflammatory diseases, active cardiovascular disease, and malignancies were excluded from the study. CPAP adherence downloads were obtained for all patients.

Results:

Out of 184 patients in whom OSA was diagnosed and who were treated with CPAP, 109 met our study criteria, including baseline polysomnogram, baseline and posttreatment CBC, and available adherence download. We compared the NLR before and after treatment with CPAP. There was a significant difference in NLR before and after treatment with CPAP (P < .0001). There was also a significant difference in apnea-hypopnea index before and after treatment (P < .0001). We also assessed the relationship between CPAP adherence (percentage of days used for > 4 hours) and the change in NLR. NLR decreased significantly in both the adherent (CPAP use ≥ 70% of days; P = .014) and nonadherent groups (CPAP use < 70% of days; P = .0003). Finally, we noticed a significant direct correlation between CPAP adherence beyond 70% and the change in NLR (ΔNLR) (P = .046) in patients who had ≥ 70% adherence with CPAP, which was not observed in patients with < 70% adherence.

Conclusions:

The NLR may be a useful marker for monitoring improvement, as CPAP had a desirable effect on the chronic inflammation induced by OSA when measured by NLR in this study. Our results specifically suggest that the NLR values decrease significantly in patients using CPAP regardless of adherence, but with a more direct relationship in those who use it beyond 70% of days, at least 4 hours a day.

Citation:

Al-Halawani M, Kyung C, Liang F, et al. Treatment of obstructive sleep apnea with CPAP improves chronic inflammation measured by neutrophil-to-lymphocyte ratio. J Clin Sleep Med. 2020;16(2):251–257.

BRIEF SUMMARY

Current Knowledge/Study Rationale: Obstructive sleep apnea (OSA) is associated with subclinical systemic inflammation that is likely triggered by intermittent hypoxia and activation of the sympathetic nervous system. The neutrophil-to-lymphocyte ratio (NLR) is a reliable measure of subclinical systemic inflammation. Our study aims to evaluate the effect of OSA treatment with continuous positive airway pressure (CPAP) on inflammation measured by NLR.

Study Impact: Our data show that treatment of OSA with CPAP has a desirable effect on systemic subclinical inflammation measured by NLR. The NLR may be a useful marker for monitoring treatment response in patients with OSA treated with CPAP.

INTRODUCTION

Obstructive sleep apnea (OSA) is a common disorder characterized by repetitive obstruction or narrowing of the upper airway leading to intermittent hypoxia and sleep fragmentation. It has been associated with cardiovascular diseases including hypertension, stroke, coronary artery disease, heart failure, and atrial fibrillation as well diabetes mellitus.^1–6 OSA also contributes to a poor quality of life due to excessive daytime sleepiness and has a reported prevalence of 9% in women and 24% in men.⁷ However, recent studies show a relative increase in prevalence between 14% and 55% in the past two decades depending on the studied subgroup, likely due to increasing rates of obesity.⁸ The gold standard for diagnosis of OSA is an in-laboratory polysomnography, and the primary outcome measure is the apnea-hypopnea index (AHI), which is defined as the number of apnea and hypopnea events per hour of sleep.⁹ The American Academy of Sleep Medicine (AASM) clinical practice guidelines recommend that clinicians use continuous positive airway pressure (CPAP) to manage OSA in adults with excessive sleepiness, impaired sleep-related quality of life, and in patients with comorbid hypertension.¹⁰

The association between OSA and its comorbidities have been linked to systemic inflammation and oxidative stress by free radicals caused by repetitive episodes of hypoxia, also called intermittent hypoxia and activation of the sympathetic nervous system.^11,12 The neutrophil-to-lymphocyte ratio (NLR) is a marker of chronic subclinical inflammation that has recently been used as a prognostic marker in cardiovascular, malignant, and inflammatory diseases.^13–15 A recently published meta-analysis, including 38 studies (n = 76,002), assessed the risk of NLR on cardiovascular diseases and showed that high NLR was significantly associated with all cardiovascular disease outcomes including coronary artery disease, acute coronary syndrome, stroke, and composite cardiovascular events with pooled odds ratio ranging from 1.62 to 3.86. NLR was also significantly higher in coronary artery disease, acute coronary syndrome, and patients who have had a stroke compared to control groups.¹⁶ The role of NLR has also been explored in several malignancies. In a study on patients with inoperable pancreatic cancer, NLR levels were significantly higher in patients with pancreatic cancer compared with healthy individuals and were also predictive of response to chemotherapy.¹⁴ Furthermore, NLR was found to be a significant prognostic factors in patients with lung cancer and malignant pleural effusion, with higher values being predictive of shorter overall survival.¹⁷

A correlation between the severity of OSA and the NLR was also noted. A study by Altintas et al demonstrated significantly higher NLR in patients with severe OSA compared to healthy individuals and patients with mild to moderate disease.¹² Another study by Günbatar et al showed a positive correlation of higher NLR and OSA severity.¹⁸ However, there are few data evaluating the effects of treatment of OSA on NLR. We hypothesize that treatment of OSA with CPAP will improve chronic inflammation measured by NLR and that increased adherence with CPAP will show larger and more significant improvement in the NLR ratio as a marker of improving inflammation.

METHODS

This is a retrospective observational study at the Veterans Affairs Hospital in Brooklyn, New York, and was approved by the Institutional review board at the same institution. The study involved patients with an OSA diagnosis who were treated with CPAP between October 1, 2015 and September 30, 2016.

Demographics including age, sex, body mass index, and medical history were obtained from the patients’ charts, and adherence data from the CPAP devices. Inclusion criteria include age older than 18 years of age, a diagnosis of OSA, and treatment with CPAP. White blood cell (WBC) count obtained from a 12-month window prior to the index polysomnogram were reviewed. The complete blood count (CBC) closest in time to the polysomnography was labeled as pretreatment NLR. Posttreatment WBC count at least 3 months after index polysomnogram and initiation of CPAP was required. Exclusion criteria include age younger than 18 years, any active inflammatory medical conditions including acute cardiac disease or recent myocardial infarction, infections, malignancies or rheumatological diseases, use of immunomodulators, or charts with incomplete data.

All patients underwent diagnostic in-laboratory polysomnography under the supervision of a sleep technologist, with measurement of several parameters including electroencephalography, electrooculography, electromyography, electrocardiography, airflow (nasal pressure transducer and thermistor), respiratory effort (by evaluation of chest movement), and oxygen saturation. The data were acquired using a SomnoStar Sleep System (Somnostar 10.2 by CareFusion, consolidated into Vyaire Medical, Mettawa, IL). Studies were scored by a sleep technologist using the American Academy of Sleep Medicine scoring criteria in use at the time of the study. Apneas were scored when there was more than or equal to a 90% decrease in the thermistor flow for at least 10 seconds, and hypopneas were scored when there was a 30% or more reduction in amplitude of nasal pressure for at least 10 seconds associated with a 3% oxygen desaturation or arousal.

All scored studies were reviewed and interpreted by a board-certified sleep physician. An apnea-hypopnea index (AHI) of > 5 events/h was considered diagnostic of OSA. Patients were then prescribed CPAP after a CPAP-titration study or autotitrating positive airway pressure where indicated. All patients had close follow-up with a sleep specialist and a respiratory therapist. Adherence data as well as posttherapy average AHI were downloaded during each visit and analyzed using ResScan (ResMed, San Diego, CA).

CBC was obtained via retrospective chart review in a window of 12 months prior to diagnosis of OSA and at least 3 months after treatment with CPAP as described earlier. CBC was obtained by venipuncture in an outpatient setting, and the blood was placed into ethylenediaminetetraacetic acid, or EDTA, containing tubes as per standard clinical protocol at our center. If blood work was only available during acute hospitalizations, the patient was excluded from the study. The NLR was calculated by dividing the absolute neutrophils count by the absolute lymphocyte count obtained from the CBC.

GraphPad Prism 7 software (GraphPad Software, La Jolla, CA) was used for processing of the statistics. The relationships between the variables were assessed using Pearson or Spearman correlation coefficients for parametric or nonparametric analyses, respectively. A one-way analysis of variance and t test were used to assess the differences in parameters between groups. A value of P < .05 was considered statistically significant.

RESULTS

A total of 356 patients were reviewed and the charts of 184 had data on CPAP usage. Of those, 109 met our inclusion criteria (Figure 1) These patients were predominantly middle-aged (mean [standard deviation] 58.1 [14.01] years), obese (body mass index 34.12 [6.43] kg/m²), male (97%) military veterans with OSA on CPAP with a high prevalence of hypertension and hyperlipidemia. Baseline characteristics including comorbidities, as well as parameters that changed with therapy are summarized in Table 1.

Figure 1. Flowchart of the study population.

CPAP = continuous positive airway pressure, OSA = obstructive sleep apnea.

Table 1.

Characteristics of study group.

The pretreatment NLR for the study population was 2.08, which is elevated relative to 1.49 and 1.81 reported for control patients without OSA in the literature.^19,20 We compared pretreatment NLR values in patients with mild OSA (AHI < 15 events/h, n = 18), moderate OSA (AHI 15–29 events/h, n = 36), and severe OSA (AHI ≥ 30 events/h, n = 55), NLR was elevated in all three groups, Mean ± SD in mild, moderate, and severe OSA were 2.14 ± 0.99; 1.88 ± 0.74; and 2.20 ± 1.20, respectively, but the differences were not statistically significant (Figure 2). There was a trend toward increasing NLR with increasing AHI, although this correlation did not reach statistical significance (r = .15, P = .11). We found a significant inverse correlation between the nadir oxygen saturation and the NLR (r = −.21, P = .04), but not with time spent below oxygen saturation of 90% (P = .4). Among other baseline variables, there was a correlation between NLR and age (r = .27, P < .005), hypertension (r = .22, P = .019), and hyperlipidemia (r = .25, P = .007). There was no correlation between baseline NLR and any of the other variables.

Figure 2. Difference in NLR between patients with mild, moderate, and severe OSA.

There was no significant difference in NLR values between the three groups. Bars represent mean, minimum, and maximum values. NLR = neutrophil-to-lymphocyte ratio, OSA = obstructive sleep apnea.

In this population, NLR decreased after treatment of OSA with CPAP with a mean NLR of 2.08 before treatment and 1.74 after treatment (P < .0001) (Figure 3). There was also a significant improvement in AHI with CPAP treatment from an average of 40.12 events/h before treatment to 1.88 events/h after treatment (P < .0001).

Figure 3. Change in NLR before and after treatment with CPAP.

There was a significant decrease in NLR values, mean 2.08 to 1.74 (P < .0001) after treatment. Bars represent mean, minimum, and maximum values. CPAP = continuous positive airway pressure, NLR = neutrophil-to-lymphocyte ratio.

We assessed the relationship between the change in AHI before and after treatment (ΔAHI; pretreatment AHI minus posttreatment AHI) and the change in NLR (ΔNLR; pretreatment NLR minus posttreatment NLR) regardless of the degree of adherence with CPAP. There was a significant direct correlation between the two variables (r = .24, P = .01) indicating a dose- dependent relationship between the change in AHI and the change in NLR (Figure 4). Interestingly, we also found a positive correlation between the baseline NLR and ΔNLR (r = .69, P < .0001) suggesting that the larger the degree of inflammation at baseline, the more robust the response to treatment will be.

Figure 4. Correlation between the change in AHI and the change in NLR.

There was a significant correlation between the two variables (r = .24, P = .01) indicating a dose-dependent relationship between the change in AHI and the change in NLR. AHI = apnea-hypopnea index, NLR = neutrophil-to-lymphocyte ratio.

We also evaluated the change in NLR in patients who were adherent with CPAP use (n = 25) and those who were nonadherent (n = 84). CPAP use for ≥ 70% of days, at least 4 hours a day was used as the cutoff for adherence; patients with CPAP use for at least 4 hours and < 70% of days were considered nonadherent. NLR decreased significantly in both the CPAP-adherent (1.96 to 1.68, P = .01) and CPAP-nonadherent groups (2.11 to 1.75, P < .001). We noticed that some patients who were rendered nonadherent were using CPAP up to 98% of days (average % days used was 53% in nonadherent group, regardless of hours of use) but did not reach the 4-hour threshold. Last, we explored the relationship between CPAP adherence (% of days used > 4 hours) and ΔNLR. Adherence ranged from 0 to 100% (mean 43.21 SD 28.79). We found no correlation between CPAP adherence and the magnitude of change in NLR. However, when looking at a subgroup of patients who had ≥ 70% adherence with CPAP (n = 25), we found a direct correlation between CPAP adherence beyond 70% and the change in NLR (r = .34, P = .046) (Figure 5), a finding that was not observed in patients with < 70% adherence.

Figure 5. Correlation between the change in NLR and the percent of days with CPAP use for 4 hours or more, ≥ 70% of days.

There is a direct correlation between CPAP adherence and the change (decrease) in NLR (r = .34, P = .046). CPAP = continuous positive airway pressure, NLR = neutrophil-to-lymphocyte ratio.

DISCUSSION

This study showed that treatment of patients with OSA with CPAP improved chronic subclinical inflammation measured by NLR from 2.08 pretreatment to 1.74 posttreatment. Values of NLR were also higher in patients who had a more severe degree of hypoxemia with a lower SpO₂ nadir rather than prolonged hypoxemia. This suggests that OSA-associated hypoxemia may be the major pathway for increasing NLR and inflammation associated with it. We also noted that patients who had higher NLR values at baseline had a larger magnitude of NLR decrease with treatment of OSA. This finding suggests that the response was more pronounced in patients who had a more severe inflammation as measured by NLR.

Another observation in this study was the dose-dependent relationship between treatment for OSA and NLR. A significant correlation between the change in AHI and the change in NLR was demonstrated. However, the correlation was weak likely because of differences in adherence, which may have played a significant role. Although some patients had a significant change in AHI before and after treatment, their suboptimal adherence may have led to a small change in NLR. This would contribute to a weaker correlation as the relationship between the two deltas was evaluated regardless of the degree of adherence with CPAP.

Perhaps the most clinically relevant finding in the cohort is the relationship between CPAP adherence and improvement in NLR, in which we found that CPAP use to any degree and duration was associated with a significant decrease in NLR in both patient groups, those deemed adherent and nonadherent by CMS criteria; our data suggest that there is some benefit in using CPAP even if used for periods shorter than the recommended adherence threshold. This benefit was reflected as a decrease in chronic, subclinical inflammation measured by NLR. We also noticed that CPAP adherence beyond a critical cutoff ≥ 70% of days of use for more than 4 hours had a significant dose-dependent relationship with the change in NLR values (ΔNLR). This relationship was not observed in those who were less adherent. These findings suggest that better adherence to a certain threshold (use > 70% of days) carries a qualitative improvement with a better, more significant effect on inflammation associated with OSA.

The pathophysiology behind the association between OSA and its comorbidities have been linked to systemic inflammation and oxidative stress.²¹ By causing sleep fragmentation and intermittent hypoxia, OSA increases sympathetic nervous system outflow and generation of reactive oxygen species. This leads to endothelial damage,²² and activation of the hypoxia inducible factor-1 and nuclear factor-κB pathways, both of which promote systemic inflammation.¹¹ OSA can also mimic cardiac ischemia/reoxygenation injury, causing adenosine triphosphate depletion and xanthine oxidase activation, which also increases oxygen-derived free radicals.¹² Multiple studies have also shown an overexpression of proinflammatory cytokines in OSA such as interleukin (IL)-8, IL-6, C-reactive protein (CRP), and tumor necrosis factor alpha (TNF-α).²³

The pathophysiology between OSA and its effects on the cardiovascular and neurological systems can be attributed, in part, to intermittent hypoxia during sleep. This decreases the amount of nitric oxide and increases the number of circulating neutrophils, monocytes, lymphocytes, dendritic cells, and platelets leading to an increase in reactive oxygen species and proinflammatory cytokines such as TNF-α, IL-6, and IL-8. This potentiates the interaction between endothelial cells and blood cells, which causes endothelial injury and atherosclerosis through the formation of foam cells and dendritic cells.^21–23

An emerging topic in sleep medicine is investigating the extent of decrease in systemic inflammation as a result of CPAP therapy. A meta-analysis performed in 2013 by Xie et al demonstrated that with longer duration of therapy and adequate adherence, CPAP therapy can decrease the levels of CRP, IL-6, IL-8 and TNF-α.²⁴ Further meta-analyses have shown significant decreases in CRP levels with the use of CPAP, highlighting the decrease in inflammation in OSA resulting from treatment.^25,26

The NLR has been recognized as a useful marker of systemic inflammation with prognostic value in various diseases.²⁷ The ratio can be easily calculated from the differential WBC count and uses two different immune pathways to obtain a value to assess inflammation and physiological stress. There is an increase in neutrophils due to nonspecific inflammation and a decrease in lymphocytes as they represent the regulatory portion of the immune system. Lymphocytes undergo apoptosis in the setting of inflammation and release of endogenous glucocorticoids, which occurs during sleep apnea and also contributes to a rise in NLR.^28,29

A study by De Jager et al showed a positive correlation between NLR values and CURB-65 scores in patients admitted with community-acquired pneumonia. Increased NLR values were seen in patients with bacteremia, prolonged hospitalization and intensive care unit admission. Overall, the NLR was a better predictor of disease severity and mortality compared to neutrophil count, lymphocyte count, WBC count, and C-reactive protein levels.³⁰ In patients with ST elevation acute myocardial infarction, a higher NLR was associated with a significantly higher incidence of in-hospital cardiovascular mortality.^31,32

Several studies have shown that NLR is increased in OSA and may correlate with disease severity.³³ In a study by Altintas et al, NLR was examined in 481 patients with OSA compared with 80 healthy individuals. They showed that patients with severe OSA had significantly higher NLR ratios compared to healthy individuals and patients with mild and moderate disease.¹² A similar study by Günbatar et al examined 111 individuals including a healthy control population and showed a positive correlation of higher ratios as the severity of OSA increased.¹⁸ Yenigun et al studied 178 patients and found a positive correlation between the severity of AHI and NLR values. In this study the individuals were separated into five groups including: control, mild, moderate, severe OSA and treatment with CPAP arm. The severe OSA group had a significantly higher NLR than the control, mild and CPAP treated group. As the severity of OSA increased, the NLR also increased.³³ These studies have consistently shown that in patients with OSA, the NLR can be used as a marker for severity.

Although there is a handful of studies that assessed the relationship between OSA and NLR, there is limited data evaluating the response of NLR to treatment. One such study by Oyama et al (n = 29) showed that there was a significant decrease (P < .05) in the NLR in patients with moderate and severe OSA using CPAP for 3 months.³⁴

Another study by Al-Halawani et al showed a significant decrease in NLR in patients who were successfully treated for OSA with mandibular advancement device, which further supports the findings of our current study.³⁵

To the best of our knowledge, this study is the first to demonstrate a dose-dependent improvement of chronic subclinical inflammation measured by NLR in patients with OSA treated with CPAP.

However, it has some limitations. First, this was a single-center retrospective study with a limited number of patients specifically in a predominantly male, veteran population; therefore, our findings may not be generalizable. Second, the laboratory values were obtained by chart review for both the CBC values prior to and after treatment with CPAP. Additionally, these values were obtained from routine laboratory draws during outpatient visits and not specifically for this study. Finally, minor upper respiratory tract infections, seasonal allergies, and other causes of minor inflammation cannot be controlled for in this study given its retrospective design. The effect of such random events would be expected to dilute the results of this study. Nevertheless, a robust relationship was found between CPAP use and decrease in NLR.

CONCLUSIONS

An increased NLR, a marker of subclinical systemic inflammation, is associated with OSA. Significant improvement in the NLR can be seen with CPAP treatment in patients with OSA. This relationship also shows a dose-dependent improvement with the most benefit in patients with CPAP use of more than 4 hours per night, > 70% of the nights recorded. We hope that these findings will lead to larger, prospective studies to show that the NLR could potentially be used as a surveillance marker for the treatment of OSA.

DISCLOSURE STATEMENT

All authors have seen and approved the manuscript. The authors report no conflicts of interest.

ABBREVIATIONS

AASM

American Academy of Sleep Medicine

AHI

apnea-hypopnea index

CBC

complete blood count

CPAP

continuous positive airway pressure

IL

interleukin

NLR

neutrophil-to-lymphocyte ratio

OSA

obstructive sleep apnea

WBC

white blood cell