The present study was done in Kalinga Institute of Medical Sciences with prior ethical clearance from institutional ethical committee from a period of August 2015 to December 2016. Written informed consent was taken from all the study subjects after explaining the aim and benefits of the study.

Study group included 75 known hypertensive cases of age group 25-60 years who were under antihypertensive treatment for at least more than three months. Forty age and sex matched normotensive subjects were selected.

Subjects with resistant hypertension (taking at least 3 medicines and one amongst them is a diuretic and their blood pressure not controlled), with recent intramuscular injection, known muscular disorder, heart disease, renal impairment, statin intake and persons who recently donated blood or received any kind of vaccination within three months were excluded from the study.

All the subjects were asked for abstinence from muscular exercise for at least three days prior to the test procedure. Normal daily activities were allowed.

Height and weight were recorded and BMI was calculated. Blood pressure was measured in a quiet and relaxed surrounding in the sitting position in the dominant arm at heart level. Mean of three consecutive blood pressure readings were taken into account to avoid the variability. Subjects were classified as treated controlled hypertension for those taking antihypertensive drugs and with SBP <140 mm of Hg and DBP <90 mm of Hg, treated uncontrolled hypertensives were those who were taking antihypertensive drugs with SBP ≥140 mm of Hg and DBP ≥90 mm of Hg, persons with SBP <140 mm of Hg and DBP <90 mm of Hg at three different sittings with no history intake of antihypertensive drugs were considered as normotensives.

CK activity was measured after three days of rest by Creatine Kinase N-Acetyl Cystine (CK-NAC) activated enzymatic UV method within six hours of sample collection in automated analyzer (Roche Diagnostics) by modified International Federation of Clinical Chemistry (IFCC) method. CK catalyzes reversible transfer of phosphate group from phosphocreatine to ADP. The ATP formed along with glucose is catalysed by hexokinase to form glucose 6 phosphate, which reduces Nicotinamide Adenine Dinucleotide Phosphate (NADP) to NADPH (reduced NADP) in presence of glucose 6 phosphate dehydrogenase. The rate of reduction of NADP to NADPH is measured as increase in absorbance at 340 nm which is proportional to CK activity in the serum [15].

Data were presented as mean±SD. Wilcoxon Mann-Whitney U test was done for better comparison of Creatine Kinase activity in normotensive control and hypertensive subjects. Kruskal Wallis test was done to compare the parameters in all the three study groups. Pairwise comparison of status and Bonferroni adjustment was done to know which specific mean differed. Spearman’s correlation between Creatine kinase and SBP and DBP was done in all the study groups. Area Under Curve (AUC) and the cut-off value for serum Creatine Kinase were calculated by Receiver Operating Curve (ROC). A p-value <0.05 was considered as significant. All statistical analysis was done with SPSS software version 20.0.

[Table/Fig-1] shows that there is significant difference in BMI, SBP, DBP, CK in the three study groups with statistically significant increase in hypertensive patients. BMI in hypertensive patients without control was found to be (31.27±3.35 kg/m²) which was significantly higher compared to hypertensive patients with control (29.70±4.30 Kg/m²). [Table/Fig-2] shows the comparison of BMI and CK between normotensives and hypertensives showing statistically significant increase in parameters in hypertensive group. The distribution of creatine kinase in the study groups has been depicted in a box plot showing the mean CK in hypertensive patients with treatment control was (99.55±31.34) U/L which was significantly higher compared to the normotensives (72.66±25.42) U/L. CK in hypertensive patients without treatment control (313.92±133.12) U/L showed significantly higher values than hypertensives with treatment control group (p<0.001) [Table/Fig-3]. There is statistically significant correlation between CK with SBP and DBP in hypertensive patients without treatment control as shown in [Table/Fig-4a,b]. [Table/Fig-5,6] shows the ROC curve with AUC of 0.956 for creatine kinase in hypertensive subjects. [Table/Fig-7] shows a cut-off value of serum CK of 168.5 U/L in hypertensive patients with 33 subjects with hypertension without treatment control and one patient with hypertension and treatment control above this value.

[Table/Fig-3]:

Box plot showing values of CK (U/L) in all the three study groups.

[Table/Fig-4a,b]:

Correlation between CK and SBP and DBP in hypertensive without treatment control.

SBP: Systolic blood pressure; DBP: Diastolic blood pressure

[Table/Fig-5]:

ROC curve for Creatine Kinase in hypertensive subjects.

Creatine kinase an enzyme involved with cellular energy system increases the capacity of the cells to meet high-energy demands of contractile processes and active trans-membranous transport by readily providing ATP [9].

In the present study we observed significantly higher CK level in hypertensive patients (199.59±142.31) compared to normotensive controls (72.66±25.42) U/L suggesting CK may be an associating factor contributing to increase in blood pressure. In the study, done by Johnsen SH et al., per log CK increases adjusted BP by 7.98 (3.27-12.68) mm of Hg of SBP and 4.69 (1.88-7.50) mm of Hg of DBP [10]. Our findings are also in consistence with the studies done by Brewster L et al., who found out that circulating CK is the main predictor of blood pressure in random sample of multiethnic origin [8]. In another study, done by them 48% of subjects with idiopathic increase in serum creatine kinase was hypertensive as compared to 19% of random population controls [16]. Studies have shown higher CK activity in all tissues in black population of Sub-Saharan African decent and hypothesised that it increases hypertension risk in this subgroup due to increase in pressure response [14]. According to Karamat FA et al., a strong association was found between human resistance artery CK mRNA and systemic SBP and DBP [17]. In an in vitro study done by Taherzadeh Z et al., isolated resistance arteries showed reduced vascular contractibility by 24.3% on addition of CK inhibitors like 2,4 dinitro-1-fluorobenzene [18]. Low serum CK levels are also associated with increase in fainting episodes [19]. Creatine Kinase regulates, buffers and transports energy to the sites of energy consumption like myofibrils and membrane ion pumps. At these cellular locations it is involved in contraction and active trans membranous transport processes by providing ATP readily. Creatine kinase also prevents acidosis due to ischaemia as it utilises the H⁺ and ADP for ATP formation. These factors result in increases trophic response resulting in increase of cardiovascular contractile reserve facilitating the development of arterial hypertension and hence, greater hypertension risk [10]. Common cause of increase in CK activity is exercise hence to avoid false results we suggested the study group a resting period of three days. In the absence of any tissue damage or any form of dysfunction increase in serum CK level suggests CK from the tissue activity [20].

In our study there was increase in serum CK in hypertensive treatment failure (313.92±133.12) U/L, which was significantly higher p<0.001 compared to hypertensive-controlled patients (99.55±31.34) U/L. A significant correlation (p<0.001) was also found out between serum CK and SBP and DBP in hypertensive treatment failure patients. These findings are in accordance to the findings of Oudman I et al., [13]. They found 72.9% treatment failure within highest CK tertile whereas 46.7% within lowest tertile. CK was found as the main predictor of treatment failure independent of age, sex, BMI, ethnicity or education level. They even found out a dose dependent association of serum CK with treatment failure. Studies have suggested that there is no direct evidence of increase in serum CK in hypertension and even hypertension does not effect the clearance of CK [8]. Even there is no such evidence that high blood pressure causes cardiovascular muscle damage causing increase in creatine kinase level. Hence resting subjects without muscle damage, serum CK is the direct measure of tissue CK. High serum CK which may be causal or induced, results in increase in contractile responses and salt retention. This increase in mean BP which is one of the important cause behind difficult to treat hypertension and hence results in hypertensive treatment failure.

Present study showed a positive association between CK and BMI in hypertensive patients. Studies have suggested that mean levels of SBP and DBP increases as BMI increases [21]. Strong and independent association between CK and BMI have also been found out [22]. In skeletal muscle higher activity of this enzyme is typically seen in type II muscle fibers, which are specifically characterised by low vascularisation and decreased capacity of uptake and oxidation of fatty acids and glucose. This promotes storage of fatty acid and glucose in adipose tissue leading to obesity [23]. Hence high CK phenotype may be responsible for obesity and hypertension. But no significant association was found between BMI and CK in hypertensive failure patients. This suggests that though BMI increases the hypertension risk, hypertensive treatment failure is independently associated with the serum CK value. A cut-off value for serum CK was found to be 168.5 from the co-ordinates of ROC curve for hypertensive patients with a sensitivity of 0.943. There were 40 hypertensive subjects above this level with 39 subjects having hypertension without treatment control. Hence a cut-off value for hypertensive subjects can be used as an independent predictor of development of hypertensive treatment failure. This can help us to segregate patients who may present with hypertensive treatment failure requiring more aggressive management and repeated follow up.

These findings suggest that creatine kinase estimation can help us to differentiate true hypertension from pseudo-hypertension and can also be used as a prognostic marker for suspected difficult to treat hypertension.

In congruence with previous findings our study also suggested the association of CK with hypertension. CK is also found to be higher in hypertensive treatment failure cases which is even independent of BMI. As hypertension is a multifactorial condition being affected by environmental and physiological factors, treatment failure cases must be studied cautiously in patients having multiple risk factors and co-morbidities.

Our study had certain limitations; no detailed history of antihypertensive medication was taken into account. Presence of other comorbidities, patient’s adherence to treatment, diet, alcohol consumption that may affect the creatine kinase was not taken into account. Size of the study sample was not large enough to confer significant differences.

Hence prospective studies with history of different hypertensive medications should be taken into account for detailed association of CK with hypertension and treatment failure hypertension cases. Further studies are necessary to justify the causal association so that CK can better be used as the prognostic marker and treatment strategies can be adopted in cases of difficult to treat hypertensive cases.