Does Vitamin D Deficiency Effect Heart Rate Variability in Low Cardiovascular Risk Population?

AIM: This study aimed to evaluate the cardiac autonomic dysfunction and the cardiac arrhythmia risk using heart rate variability parameters in subjects with vitamin D deficiency and low cardiovascular risk. MATERIAL AND METHODS: One hundred five consecutive individuals, 54 patients with low vitamin D status and 51 healthy controls were enrolled in this study. The overall cardiac autonomic tone was quantified by using various heart rate variability parameters included mean RR interval, mean Heart Rate, mean of standard deviations of intervals for 24 hours (SDNN), standard deviation of averages of intervals (SDANN), mean of standard deviation of intervals for 5 minutes (SDNNI), root mean square of difference of successive intervals (rMSSD) and the proportion of intervals differing more than 50 ms (pNN50) values. The 12-lead ECG was recorded from each participant, and QT intervals were measured. RESULTS: Baseline demographic profiles were similar between two groups. The heart rate variability parameters such as mean RR interval, mean HR, SDNN, SDANN, SDNNI, rMSSD and pNN50 (%) values were not significantly different in patients with low vitamin D status compared to control group. The electrocardiography analysis revealed only slight but significant prolongation of corrected QT (QTc) intervals in the control group. CONCLUSION: HRV variables were not significantly altered in patients with vitamin D deficiency in low cardiovascular risk profile group. Further studies evaluating these findings in other cohorts with high cardiovascular risk are required.


Introduction
Vitamin D has a major role in a wide range of systems and organ functions including the cardiovascular system, beyond its effects on bone [1,2]. The prevalence of vitamin D deficiency is high in up to 50% of healthy adults [3]. Vitamin D deficiency has been implicated in a variety of cardiovascular disorders, including hypertension, coronary artery disease, heart failure, and peripheral vascular disease [4,5]. The experimental studies have shown the potential link between low vitamin D levels and the activation of the renin-angiotensin system, inflammation, peripheral insulin resistance, endothelial dysfunction, and adverse ventricular remodelling [6][7][8][9][10]. Recent studies revealed that vitamin D deficiency results in structural and ionic channel remodelling and autonomic dysfunction that may predispose the individuals to lethal cardiac arrhythmias and sudden cardiac death (SCD) [11][12][13][14][15].
Heart rate variability (HRV), the variation in the cardiac interbeat interval over time, provides a noninvasive assessment of cardiovascular autonomic functions [16]. It has been used as a predictor of sudden cardiac death or a marker of the progression of cardiovascular disease in several high-risk populations [17,18].
There has been only one study evaluating the effects of the vitamin D deficiency on HRV with confusing results. In this study, we aimed to evaluate the cardiac autonomic dysfunction using HRV parameters and in-patient with low cardiovascular risk and vitamin D deficiency [19].

Material and Methods
A total 105 subjects, 54 patients with vitamin D deficiency and 51 healthy controls were enrolled in this study. All subjects were free of cardiovascular risk factors and haed been living in a city of western http://www.mjms.mk/ http://www.id-press.eu/mjms/ Anatolia all through their life. Exclusion criteria were as follows: hypertension, diabetes mellitus, thyroid disorders, coronary artery disease, valvular heart disease; left ventricular dysfunction, use medications that interfere with vitamin D metabolism, primary hyperparathyroidism, hypercalciuria, malignancy, sarcoidosis, Paget's disease, malabsorption syndromes, estimated glomerular filtration rate ≤ 60 ml/min, and history of nephrolithiasis.
All subjects were recruited from Sakarya University School of Medicine (SAU) between October 2011 and September 2012. All the procedures were hby the ethical standards of the responsible committee on human experimentation (institutional or regional) and with the Helsinki Declaration of 1975, which was revised in 1983. The Ethics Committee of Sakarya University Faculty of Medicine approved the study protocol.

Biochemical Examination
For blood tests, subjects were asked to fast for at least 12 h, and the blood was centrifuged at 3,000 rpm for 15 min to obtain the serum. Serum 25(OH) D was analysed using 25 OH Vitamin D reagent based on the enzyme immunoassay (IDS Ltd. Boldon UK).

Electrocardiography and Holter analysis
The 12-lead electrocardiograms (ECG) were recorded from each participant with sinus rhythm (25 mm/s rate and 1 cm/mV amplitude). Electrocardiograms were transferred to a personal computer via a scanner, magnified by Adobe Photoshop software, and then the duration of QT and RR intervals were measured. QT interval, which is the duration from the beginning of QRS complex to the end of T wave, was measured in a derivation in which T wave was clearly seen. Measured QT intervals were corrected by Bazett's Formula (QT/ √RR), and defined as corrected QT interval (QTc).
All subjects underwent 1-hour Holter monitoring while they were in a sitting position after resting for 30 minutes for HRV analyses. The measurements were taken in the morning times by single investigator blinded to the diagnosis of the participants. Holter ECGs were analysed using the Del Mar Reynolds Pathfinder Holter system. The timedomain parameters were calculated: mean of all normal RR intervals (mean RR); standard deviations of all normal-to-normal (NN) intervals, (SDNN); mean of the standard deviations of all NN intervals for all 5minute segments of the entire recording (SDNNI); standard deviation of the averages of NN intervals in all 5-minute segments of the entire recording (SDANN); the square root of the mean of the sum of the squares of differences between adjacent NN intervals (rMSSD); the number of pairs of adjacent NN RR intervals differing by more than 50 ms divided by the total number of all NN intervals (pNN50 total). EKG exclusion criteria were as follows: atrial fibrillation, recordings with non-sinus beats that were more than 1% of a total number of beats, and artefacts.

Statistical Analysis
Statistical analysis was performed using the Statistical Package for Social Sciences (SPSS, Chicago, IL, USA), version 15.0 software for Windows. Descriptive statistics (frequency, mean, standard deviation, ranges) were calculated. The quantitative values between two groups were compared using a ttest, and the qualitative values were compared using the 2 test. Correlation of variables was demonstrated using Pearson's and Spearman's correlation coefficients in normal distributed parametric and nonparametric variables, respectively. For assessment of the association of variables, linear regression and logistic regression were used for parametric and binary variables, respectively. P value of <0.05 was considered as statistically significant in all cases.

Results
Fifty-four (7 male, 47 female) individuals with vitamin D deficiency and 51 healthy subjects (14 male, 37 female) were included this study. Age, sex, systolic blood pressure, diastolic blood pressure, mean heart rate, body mass index were not significantly different between two groups. Serum values of 25(OH) D, calcium, parathormone, creatinine, TSH, FT4 and glucose were not different between the two groups ( Table 1). Electrocardiogram and heart rate variability analysis revealed that mean RR, mean HR, SDNN, SDANN, SDNNI, RMSDD, and PNN50 in time domain parameters did not differ significantly between groups. When the QT intervals were compared, only a slight but significant proloangation of QTc intervals were detected in the control group. However, there was no significant difference between two groups in the QT intervals. Table 2 summarises the HRV and QT parameters in subjects with vitamin D deficiency and the control group. intervals for all 5-minute segments of the entire recording; SDANN, standard deviation of the averages of NN intervals in all 5-minute segments of the entire recording; RMSSD, the square root of the mean of the sum of the squares of differences between adjacent NN intervals; pNN50, the number of pairs of adjacent NN intervals differing by more than 50 ms divided by the total number of all NN interval.
Correlation analysis for HRV parameters in vitamin D deficiency group is shown in Table 3.  Table 2.

Discussion
This study showed that low vitamin D status is not associated with diminished HRV parameters in a low cardiovascular risk group. A probable explanation for the lacking association is that our study focuses on younger individuals who were free of any cardiovascular disease and risk factors. Furthermore, low vitamin D status alone has not any potential to initiate lethal arrhythmias, especially ventricular fibrillation as assessed by HRV and QT.
Our results are contrary the study of YJ TAK et al., in which they have studied HRV in a population with more advanced age and high cardiovascular risk. In their study, they have included a very high-risk group into the study as compared to our study group. We did not find any significant correlation in the HRV and QT data between the vitamin D deficiency and control groups [19]. HRV shows the effects of sympathetic and parasympathetic on the sinus node. HRV parameters evaluate the different types of autonomic nerve system effects on the heart with sinus rhythm. RMSSD and HF show variations in parasympathetic effects and LF is an indicator of sympathetic activity regulation on the sinus node. SDNN and LF show the influenced of both adrenergic and cholinergic activities on the sinus node. Any changes in all HRV parameters affect SDNN and SDNN is decreased in left ventricle systolic dysfunction [20]. SDANN, SDNNI, rMSDD and pNN50 are the other HRV parameters that show cardiac autonomic functions and evaluate the sympathetic and parasympathetic tones on cardiac functions [21]. Vitamin D has direct effects on myocytes. Vitamin D also affects cardiac contractility indirectly by calcium metabolism and directly via vitamin D receptors [12]. Our hypothesis mainly depends on the seasonal changes in arrhythmia related deaths in winter times and vitamin is a sunlight defendant [22,23]. So, we have chosen the HRV analyses for evaluation of the subclinical disturbances that poses the vitamin deficient people to malign cardiac arrhythmias.
Structural and ionic channel remodelling due to low vitamin D serum levels may prolong the repolarization interval [12]. These changes have been shown to cause altered myocardial calcium, prolongation of the repolarization interval and cardiac action potential over time [11][12][13][14]. Furthermore, low vitamin D levels also induce oxidative stress, vasoconstriction and cardiac hypertrophy. All of these mechanisms may also increase myocardial susceptibility to fatal cardiac arrhythmias and eventually lead to SCD [13,22,25]. Previous studies showed that seasonal variations in SCD rates are high during winter and are lowest in the summer months [23,24]. The synthesis of vitamin D is also dependent on the intensity of daylight exposure with peak values in the summer and lowest values in winter [26,27]. iAlso, prior studies have demonstrated seasonal variation in autonomic tone that may be induced by the intensity of daylight exposure and D vitamin status [28].
A recent study evaluated the association between SCD and the combination of low vitamin D status and parathyroid hormone (PTH) excess in participants who were free of clinical cardiovascular disease at baseline. However, the combination of vitamin D deficiency and high PTH concentrations did not increase the risk of SCD to 2.5 times, while individual risk factors, low vitamin D status nor high PTH levels were related with SCD events [13]. These associations have not been found in populations free of cardiovascular disease. Another strength of our study was a measurement of vitamin D levels only at a single point in time. However, we found no association between vitamin D deficiency and cardiac autonomic dysfunction using HRV which suggests that the effects of vitamin D deficiency on the cardiovascular system may not be immediate.
Limitations: Most important limitation of our study is the small sample size. Another issue is the absence of 24 or 48-hour Holter ECG monitoring and absence of follow-up for developing adverse events.
In conclusion, we found that there was no relationship between HRV variables and the vitamin D deficiency in subjects without cardiovascular risk factors. Vitamin D deficiency does not alter cardiac autonomic functions as assessed by HRV. Further studies are needed for evaluation of these findings in other cohorts with cardiovascular risk factors.