For a person who is primarily a laboratory scientist, an individual who has four decades of research experience on “Vagal sensory mechanisms in health and disease”, developing interest in the field “sleep apnea” was purely accidental. It happened because of a curious but peculiar finding that we observed in the rabbit which showed that mild fluid accumulation in the pulmonary extravascular space resulted in an increase in urine flow and it was abolished by sectioning of the vagi as well as by the renal sympathetic nerves suggesting that the afferent for this reflex resided in the vagi and the efferent in the renal sympathetics. Further experiments demonstrated that this reflex was due to an increase in renal production of nitric oxide (NO) through the stimulation of renal sympathetic nerves. Why these results were peculiar was because the majority of the findings reported that an increase in renal sympathetic nerve activity decreased urine volume and sodium excretion, and an increase in renal NO facilitated water reabsorption in the renal tubules.
To translate our findings from bench to the bed side, I was looking for clinical conditions where there was enhanced sympathetic nerve activity as well as diuresis. On surveying through the literature, I realized that patients with sleep apnea had hypertension, natriuresis and nocturnal polyuria. The urge to urinate was so high among these patients that it resulted in frequent arousals from sleep. I started wondering whether the nocturnal polyuria and natriuresis were due to renal nitric oxide as a result of an increased sympathetic drive! Being a respiratory physiologist, I became curious and thus, “Sleep apnea” became another area of interest to me.
It was at this time that a tall, slim man wearing a turban joined my department to do his MD in Physiology. As is customary, I asked him whether he wanted to work in any particular area for his dissertation. He told me that he had a good working knowledge of yoga and he would like to investigate its beneficial effects on patients with respiratory ailments.
During the course of his conversation, he made a mention that he had some exposure to the sleep laboratory also while he was in Chandigarh. It was the year 2006 and by then, a Sleep Laboratory had started functioning in our Institute. I gave him the wild idea that the nocturnal polyuria in patients with obstructive sleep apnea (OSA) could be due to excessive sympathetic drive leading to an increase in renal NO and the NO was prevented from exerting its anti-diuretic action due to oxidative stress which converted NO to peroxynitrite as a result of the rapid reaction between superoxide and NO. In that case, the natriuresis and polyuria would get corrected by an anti-oxidant intake. This hypothesis was against the findings in the rabbit which demonstrated that increased NO caused diuresis.
The student I was referring to is none other than the editor of this News Letter Dr. Tripat Deep Singh. For this study, we managed to include Dr. V.K. Vijayan, a renowned pulmonologist and our Director too. The working hypothesis was that oxidative stress was an underlying mechanism for the increased sympathetic drive, natriuresis and nocturnal polyuria reported in patients with OSAS. The basis for the hypothesis was simple. In sleep apnea, due to recurrent hypoxia-re-oxygenation cycles, there would be oxidative stress as evidenced during ischemia reperfusion. In fact, there had been several studies which reported that the lipid peroxidation levels were higher and the anti-oxidant status was lower in patients with OSAS. After continuous positive airway pressure (CPAP) treatment, a reduction in the oxidative stress was reported also. Till we got into this field, there was no study which examined whether treatment with anti-oxidants alone had any beneficial effects upon the polysomnography (PSG) measurements and/or improved the sleep quality in patients with OSAS. The details of the selection of the subjects, the study plan and the data collection are mentioned in our publication (Singh et al., 2009, IJCD, 51: 217-224). After confirmation, PSG was performed on 20 male patients with OSAS and CPAP was given for two consecutive nights to every patient. Subsequently, each one of them took vitamin C (100 mg) and vitamin E (400 IU) twice daily orally for 45 days following which a repeat PSG was done. Ten healthy normal subjects served as controls and they underwent all the procedures except the CPAP therapy. It was observed that in OSAS patients alone, there was oxidative stress which was reduced significantly by both CPAP as well as anti-oxidant treatment. After anti-oxidant intake, there was a significant decrease in Epworth sleepiness score and the number of apneic episodes These patients spent more time in stages 3 and 4 of sleep. Also, there was a significant decrease in the optimum pressure of CPAP device. In summary, these findings demonstrated that anti-oxidant intake improved the quality of sleep in these patients.
To determine whether NO played any role in the diuresis and natriuresis reported in OSAS patients, 12-h urine volume in the day and 12-h urine volume in the night were measured and the concentrations of urinary sodium and nitrate were determined in the above patients. The frequency of urination was noted also. Unlike the control subjects, there was no diurnal variation in OSAS patients – the night urine volume and urinary sodium concentration were similar to those during the day. With CPAP therapy, the diurnal variation was restored – the night urine volume as well as the urinary sodium decreased significantly compared to those during the day. Such a reversal was not evident with anti-oxidant treatment. Indeed, the night urine volume which was higher to begin with, increased further. In the control subjects, while no significant difference was seen between the day and night samples with respect to urinary nitrate, there was a significant increase in urinary nitrate in the night sample compared to that in the day sample in OSAS patients. With CPAP therapy, the urinary nitrate level in the night sample became similar to that in the day sample.
However, instead of the reversal, the urinary nitrate in the night sample remained higher after anti-oxidant treatment. The urination frequency which was more became similar to the controls after CPAP as well as anti-oxidant treatment (Singh et al., 2011, IJCD, 53: 11-20). The major conclusions that emerge from this study are: 1) Even though renal NO produced by sympathetic stimulation gets converted to peroxynitrite by the oxidative stress, whatever NO is bioavailable is capable of causing diuresis and natriuresis, 2) A further increase in NO by anti-oxidant intake promotes further diuresis without natriuresis suggesting that they may be independent phenomena, 3) In man also, conditions which favour an increase in renal NO can result in diuresis – supporting our results in the rabbit and 4) the increase in the frequency of urination in these patients is because their sleep is disturbed and not due to an increase in the bladder pressure due to filling of urine.
After Dr. Tripat completed his MD, we continued our studies on OSAS patients using the anti-oxidant N-acetylcysteine (NAC). We confirmed our previous finding that there was oxidative stress in them (n=10). With oral intake of NAC (600 mg thrice daily for 30 days), along with a reduction in oxidative stress, and optimum pressure in CPAP, there was an overall improvement in the quality of sleep with significant decreases in apnea-hypopnea index, apnea related arousals, longest apneic episode duration, number of oxygen desaturation events per hour and Epworth sleepiness score. Additionally, the relative snore time, duration of longest snore episode and even number of snore episodes decreased significantly. Such effects were not seen in OSAS patients of the placebo group (n=10). NAC was found to be effective in reducing the parameters of the metabolic syndrome also. These results reiterate the usefulness of the anti-oxidant treatment in patients with OSAS (Sadasivam et al., 2011, IJCD, 53: 153-162).
In another study performed by us, Dr. Puneet Kumar demonstrated that in patients with OSAS (n=20), oral intake of grape seed extract (300 mg for 5 weeks) not only improved the quality of sleep but also decreased the level of the inflammatory mediator TNF-α (MD dissertation, 2012 submitted to Delhi University, unpublished observation).
To conclude, this fundamental scientist is of the opinion that anti-oxidants are extremely beneficial in alleviating most of the symptoms of sleep apnea. Even though the sample size is small to reach definite conclusions, the results are consistent, statistically significant and reproducible. Through this letter, I make an appeal to all the pulmonary physicians and sleep specialists to take note of these results, improve upon the findings, make a multicentric clinical trial on the therapeutic potential of various anti-oxidants in treating patients with sleep apnea and come out with recommendations.