Therapeutic application of herbal essential oil and its bioactive compounds as complementary and alternative medicine in cardiovascular-associated diseases

Depression is debilitating health mental disorder, a common and an independent risk factor associated with cardiovascular disease (CVD) and increased mortality. A deprived health condition in conjunction to the heart failure (e.g involving heart’s valve, pericardial, muscle) and malfunction of the blood vessels (e.g coronary artery, vascular) can ultimately lead to serious events such as heart Abstract


Introduction
Depression is debilitating health mental disorder, a common and an independent risk factor associated with cardiovascular disease (CVD) and increased mortality. A deprived health condition in conjunction to the heart failure (e.g involving heart's valve, pericardial, muscle) and malfunction of the blood vessels (e.g coronary artery, vascular) can ultimately lead to serious events such as heart and more than 1.5 million of people suffered from myocardial infarction [1,3,4]. In 2016, data from the Department of Statistics of Malaysia showed that ischaemic heart disease (e.g coronary heart disease, myocardial infarction) was the highest (15.3%) case of deaths among Malaysian as compared to other type of incidents or diseases [5]. The manifestation of CVD is often coupled with several risk factors such as family history, metabolic syndrome [e.g visceral obesity, glucose intolerance, insulin resistance, high triglyceride (TG), low high density lipoprotein (HDL)], hypertension and dietary composition in relation to atherogenesis and thrombosis [3,6]. Particularly, hypertension has become a prevalent risk factor for CVD morbidity whereby individuals with this risk factor was estimated to increase to about 60% with a total of about 1.6 billion people in 2025 [1][2][3]. Other risk factor such as coronary atherosclerosis is often pathologized by the accumulation of fats [e.g lipids, cholesterol and TG], within the arterial blood vessels, endothelial dysfunction as well as coagulation (e.g platelet-mediated) [7][8][9]. Important consequences of coronary atherosclerosis include coronary artery disease (CAD), angina (ischaemic chest pain) and myocardial infarction (MI). Many studies also showed that hyperlipidemia is the root cause of atherosclerosis, stroke and ischaemic heart disease whereby their correlation with single nucleotide polymorphisms of genes for lipid metabolism [e.g lipoprotein lipase (LPL) and apolipoprotein A5 (APOA5)] have been demonstrated [4,7,8]. Modern applications of pharmacological drugs (e.g Aspirin, statins, opioids) have their own limitations related to patient compliance, dose, effectiveness and side-effect mostly due to the differences in individual genetic makeup, foods, diets (e.g lipids ratio/type composition) and lifestyle [2,10]. Several drawbacks from great reliance on modern pharmaceutical drugs has caused long-term problem on inancial circumstances and health side-effect. Some CVD drugs have been implicated as causes of depression. Likewise, some antidepressant drugs (e.g Tricyclic antidepressants) are not suitable for CAD patient. Some other side-effects from the interaction between antidepressant and CAD drugs are also a concern in patient suffering depression with CAD. Noteworthy, patient with CVD is at an increased risk of developing depression. As high as 45% of patients with CAD (includes stable CAD), unstable angina or MI had experienced from clinically signi icant depressive symptoms. Hence, a new strategy to mitigate this disease using compounds of natural resources have been studied and proposed [8,[10][11][12][13][14].
Research disclosed that marine ish, cod liver and plant oils with a high amount of polyunsaturated fatty acids (PUFA, e.g omega-3] reduced TG and LDL levels (e.g triglyceridemia), halt thrombosis (e.g platelet aggregation/reactivity, plasma viscosity) as well as alleviate atherosclerotic plaque formation and rupture thus preventing cardiovascular disease (e.g acute coronary syndrome). Hence, oils and lipids from such sources have been widely studied and reviewed [2,4,6,15]. Little attentions have been given to evaluate and review the bene icial depressive-cardiovascular function of oil of other resources. This is due to the fact that depressivecardiovascular relevance of plant oils does not only rely on the presence of PUFA or their other functional lipids components (e.g phospholipids) [1,3]. Several analysis indicated that plant oils particularly herbal essential oils contain considerable amount of lipophilic bioactive compounds which are intervening CAD [13][14][15][16][17]. Several known lavonoids and other polyphenolic antioxidants [e.g ubiquinone, (vitamin E; tocopherols: tocotrienol), γ-oryzanol, ferulic acids triterpenyl esters] which were abundantly present in essential oils signi icantly inhibited oxidation of LDL-cholesterol and reduced thrombotic development [10][11][12]. These lipophilic antioxidants were capable of reducing thiobarbituric acid reactive substances (TBARS), lipid peroxidation (LPO), glutathione peroxidase (GPx) and superoxide dismutase (SOD) activity in vitro [18][19][20].
In these present years, several essential oils (EOs) have been demonstrated as alternative intervention therapies for both depression and cardiovascular disease. Recent advance in this new approach shows potential alternative to conventional dietary or pharmacological approach. EOs are unique as compared to vegetable/marine oils which the former contain many other distinctive bioactive compounds that potentially led to new discovery (e.g bioactive drugs or techniques) in the intervention of cardiovascular disease. In this present article, EOs and their bioactive compounds as potent complementary therapy in the intervention of depression associated with CAD are systematically reviewed using reliable sources and databases. Their pharmacological effects on lipid pro ile, biochemical and physiological (e.g heamodynamic) parameters are illustrated. This present article also elucidates mechanism of action and pharmacological targets of EOs, as well as its effectiveness and safety as phytotherapeutic compounds.

Search strategy and screening
Searches were performed using NCBI PubMed, PubMed Health, SCOPUS, Wiley Online, tandfonline, ScienceDirect and Espacenet databases. The records provide coverage of high quality and peer-reviewed articles in the ields of health and medicine. Searches were performed from January 2018 to February 2020 and were selected for review without any limitations. Terms and keywords relating to "essential oil" or "depression" and "cardiovascular" were used in the searches. Titles and abstracts were independently screened in accordance with our predetermined inclusion criteria. The reference lists of included studies were manually searched for potentially relevant studies. Studies that did not meet the inclusion criteria were excluded. Research topics were devised to essential oil and depression-CAD intervention. Review articles were excluded as well as studies examining the use of oil from non-plant origin. English and other language studies were included.

Data extraction
Data was collected based on author, date, intervention treatment (essential oils and bioactive compounds) and intervention results on depression and CAD. The outcomes were categorized and compared between studies with different essentail oils and their bioactive compounds.

Results
Searches in NCBI Pubmed and PubMed Health have retrieved works assessing the effect of essential oils and their bioactive compounds on CAD condition. In this systematic review, total of 1366 articles have been identi ied and 1269 of them have been excluded after screening of duplication and the titles/abstracts. The full-length research articles were retrieved in details and reviewed. A inal total of 23 articles were selected for inclusion in this review. A low-chart of the process of article selection is illustrated in igure 1.

Several in vitro studies
Based on the search criteria, 90 selected articles were evaluated (Tables 1,2). In vitro studies indicated that EO of O. basilicum L. and R. A. Tatarinowii, had cytoprotective effect in cultured cardiomyocytes (e.g depress pulse frequency and increase the viability of cardiomyocytes) [21,22]. Particularly, R. Acori Tatarinowii EO ameliorated cell viability of neonate rat cardiac myocytes and reduced its pulse frequency [22]. In comparison, T. capitata EO had cytoprotective effect against LPO product (4-hydroxy-2nonenal, pathophysiologic concentration, less than 10 μM)induced neonatal rat cardiomyocytes death [23]. This EO at 20- 40 parts per million (ppm) and pre-incubation at 12 h also reduced reactive oxygen species (ROS) generation and loss of mitochondrion membrane integrity but resembled cytotoxicity at concentration greater than 40 ppm [23]. The composition of the T. capitata EO revealed that it contained a considerable amount of monoterpenes [23]. In contrast, S. pinnatifolia EOs showed notable protective effect (high cell viability) on H 2 O 2 induced death in rat myocyte culture in vitro [24]. Some other EOs such as F. A. zerumbet EO protected human endothelial cells from injury caused by oxidized lowdensity lipoprotein molecules. Its protective effect was mainly due to an increase of glutathione (GSH) and SOD activities [17,25].
On the other hand, S. Chinensis Fructus EO and its bioactive compounds (e.g lignans, volatile oils and polysaccharides) have an ability to intervene several parameters in cardiovascular diseases mostly via antioxidant, apoptosis inhibition mechanism and anti-in lammation activity [26]. Study also showed that magnolol (C 18 H 18 O 2 ), bioactive component from M. of icinalis, up-regulated lipoprotein lipase (LPL) activity in a concentration-dependent, probably via alleviating LPL mRNA transcription, in mouse 3T3-L1 pre-adipocytes [27].

Study using mesenteric artery preparations
On the other hand, Cymbopogon citratus, C. winterianus, H. fruticosa EOs and β-citronellol antagonize the effects of contractions induced by both or either phenylephrine or potassium in mesenteric artery rings [51][52][53][54][55][56]. Several bioactive compounds such as α-pinene (C 10 H 16 ) and caryophyllene (C 15 H 24 ), and 1,8 cineole (C 10 H 18 O) in H. fruticosa EO were suggested to promote its hypotensive effect [54]. Comparatively, vasorelaxant activity of C. citratus, C. winterianus and H. fruticosa EOs was not affected in denuded endothelium [5254]. It was also suggested that hypotensive effect of C. citratus was not linked to K + channels where no effect was observed in the presence of tetraethylammonium or potassium ions [52]. Meanwhile, β-citronellol (C 10 H 20 O) also suppressed spontaneous or electrical-evoked contractions of isolated left or right atrium of an adult rat [55]. A. speciosa EO reduced rat left atrial force of contraction with an IC₅₀ of 292 μg/ml [57]. Compounds screening and identi ication via GC-MS analysis revealed that terpinen-4-ol (C 10 H 18 O, ~38%) and eucalyptol (C 10 H 18 O, ~18%) were high among 18 identi ied bioactive compounds in A. speciosa EO [57]. In another study, some bioactive compounds in EO such as carvone epoxide had higher relaxation effect on phenylephrine-induced contraction in mesenteric artery rings as compared to several other EO bioactive components such as limonene (C 10 H 16 ), rotundifolone ( C 10 H 14 O 2 ), pulegone epoxide (C 10 H 16 O 2 ), limonene epoxide (C 10 H 16 O) and pulegone (C 10 H 16 O) [29]. Apparently, molecular structures of previously mentioned bioactive compounds are functionally important in artery relaxation [29]. In vitro study will assist further analysis in vivo models to attain a deeper understanding and evaluation of EOs in cardiovascular disease. L. angustifolia EO (5-20 mg/Kg) with antioxidative property had cardioprotective effects in male Wistar rats with isoproterenol-induced myocardium infarction [81]. The previously mentioned EO ameliorated electrocardiogram (ECG) pattern by preventing ST-segment elevation and amplifying R-wave amplitude [81]. This is supported by the fact that L. angustifolia EO (1020 mg/Kg) notably reduced heart-body weight ratio and the increase of Malondialdehyde (MDA)-LPO and Myeloperoxidase (MPO)-neutrophils in myocardium and considerably lowered left ventricular enddiastolic pressure [81]. In experimental acute myocardium infarction (anterior interventricular branch of left coronary artery ligated with a 4/0 silk thread), administration of S. pinnatifolia EOs diminished deviation of ST-segment [24]. In biochemical parameters and serum marker enzymes analysis, it reduced level of related myocardial enzymes such as LDH, CK and Troponin-T with an increased in SOD activity as compared to myocardium infarction Male Wistar rats control [24]. Histopathological analysis showed that S. pinnatifolia EO had protective effect on myocardium infarction with lessen degree of necrosis and in iltration of in lammatory cells in rats [24]. Under hypoxia condition, S. pinnatifolia EOs (8-32 mg/kg) can prolong survival time of Kunming mice, suggesting its activity against hypoxia in experimental myocardium infarction [24]. Similarly, Nardostachyos Radix and S. pinnatifolia EOs exerted protective effect, thus preventing cell death-in chemical (e.g tert-Butyl hydroperoxide, H 2 O 2 )-induced injury in cardiomyocyte cultures (e.g H9c2, neonatal rat cardiac ventricular myocyte) [24,82]. The cell survival was higher with higher EO concentration due to signi icant reduction of ROS. EOs reduced the degree of myocardial infarction and the release of LDH and creatine kinase (CK), ameliorated the hemorheology index, increased SOD and glutathione peroxide activity in the myocardium and decreased MDA level [82]. Hesperetin had anti-apoptotic action on cardiomyoblasts via mitochondrion JNK/Bax pathway. Nobiletin activated the PI3K-Akt pathway, reduced cell apoptosis, and reduced myocardium infarct size, hence lowered the risk for myocardium ischemia and reperfusion injury [83,84]. These studies showed the effect of EOs as potent antimyocardial ischemia/infarction, and antimyocardial injury.

Hypolipidemic effect in rats and rabbits:
In preclinical trial, Vallianou, et al. [30], and Abass, et al. [56], showed that EOs from Chios mastic gum had hypolipidemic effect in young and hyperlipidemic rats. Further evaluation indicated that camphene in this particular EO plays an important role in reducing the constitutive biosynthesis of serum cholesterol and TG [30]. Administration of its bioactive compound, camphene at a concentration of 30μg/g into hyperlipidemic rat resulted in diminished of total cholesterol (TC), LDL-cholesterol and TG to about 33-55% [30]. Similarly, administration of C. jwarancusa EO in experimental high-fat-carbohydrate diet rats reduced hyperlipidemic effect (e.g reduction in body weight, fats and blood sugar levels), thus potentially alleviate the risk of cardiovascular disease [59]. In a study, the administration of magnolol, reduced the serum lipid TG level (up to 50%) in hyperglyceridemic heterozygous transgenic mice (knock-in mice carrying APOA5 c.553G>T variant) [27]. On the other hand, O. sanctum EO reduced serum lipid pro ile (e.g TG, cholesterol) in normal and hypercholesterolemic Male Wistar rats [19]. Other EOs such as P. asiatica EO also exerted hypocholesterolaemic effect in C57BL/6 mice with signi icant reduction of plasma total cholesterol and TG (29-46%) as compared to untreated control [31].  [32-35, 60, 61]. However, hypotensive effect of A. zerumbet EOs was much lower than that of its pure bioactive terpinen-4ol at same doses (1-10 mg/kg) [34,35]. In either experimented deoxycorticosterone-acetate (DOCA)-salt hypertensive or normotensive conscious rat, the effect of A. zerumbet EO, C. zehntneri EO, C. argyrophylloides EO, C. nepetaefolius EO (1 to 50 mg/kg), O. gratissimum EO, N. sativa oil, terpinen-4-ol and 1,8-cineole on hemodynamic parameters can be seen with reduction of mean aortic pressure, heart rate and arterial blood pressure [34,36,40,41,[61][62][63] . These hypotension evidences on hemodynamic parameters of such EOs (e.g A. canelilla, A. zerumbet, C. argyrophylloides and terpinen-4ol) were mainly caused by active vascular relaxation in lieu to the withdrawal of nervous system sympathetic activity [34,36]. Similarly, the cardiovascular-hypotensive effect of O. gratissimum EO was most probably mediated independent of operational autonomic nervous system whereby its vasodilatory activity may have direct interaction with vascular smooth muscle [62]. In conscious rabbit, administration of S. areira EO reduced its systolic blood pressure, diastolic blood pressure, and mean arterial pressure in a pattern comparatively similar to nifedipine [64]. In hypertensive rats, pre-treatment with hexamethonium (30 mg/kg) decreased the bradycardia elicited by C. nepetaefolius EO (50 mg/kg) exclusively affecting the increment of C. nepetaefolius EOstimulated hypotension [40,41]. This increment was linked to an increase in C. nepetaefolius EO-stimulated vascular smooth muscle relaxation with little evidence linked to the enhancement of sympathetic nervous system action in this hypertensive model (e.g its vasodilatory effects directly act upon vascular smooth muscle) [41]. Meanwhile, 1,8-cineole substantially reduced heart rate when only administrated at the highest dose (10 mg/kg) [35]. On the other hand, N. sativa and its thymoquinone (C 10 H 12 O 2 ) had cardiovascular depressant effects, mediated primarily via indirect and direct mechanisms involving both 5-hydroxytryptaminergic and muscarinic mechanisms [63]. Likewise, intravenous administration of EOs bioactive compounds (e.g pinenes, citronellol, bisabolol and linalool) also produced hypotensive effect in conscious normotensive rats. Particularly, very high hypotension effect was noted by induction of β-pinene, citronellol and bisabolol at concentration of 20 mg/kg as calculated from its haemodynamics parameters (e.g mean arterial pressure and heart rate) [65]. Intravenous pre-treatment of conscious rats with hexamethonium (30 mg/ kg) considerably reduced the resulted bradycardia produced from EOs administration (e.g A. canelilla bark, O. gratissimum) without affecting their hypotensive effect [49,62]. Unlike O. gratissimum EO, the hypotension and bradycardia created by A. canelilla bark EO were substantially decreased by pretreatment with methylatropine (1 mg/kg) [62]. Similarly, cardiovascular depressant effect of N. sativa oil (4-32 μL/kg) or thymoquinone (0.2-1.6 mg/kg) on rats was substantially antagonized by certain concentration of cyproheptadine, atropine and hexamethonium [63]. Likewise, hypotensive and bradycardic responses evoked by Mentha EO in rats were blocked by pre-treatment with atropine (2 mg/kg) [42]. On the other hand, pretreatment with methylatropine (1 mg/kg) reduced bradycardic response without affecting hypotensive response [42]. In conscious rats, pre-treatment with hexamethonium (30 mg/kg), methylatropine (1 mg/ kg) or atenolol (1.5 mg/kg) had no considerable effects on the 1,8cineole-stimulated hypotension, whereby bradycardic response to 1,8-cineole (10 mg/kg) was notably deceased by methylatropine [35]. These EOs possess the prospective of being an effective antihypertensive agent. Comparatively, P. elsholtzioides EOs contained high amount of bioactive compounds of sesquiterpenes and curzerene, benzophenone, α-cadinol and germacrone as analyzed by GC-FID and GC-MS [37]. These major compounds in this EO were suggested to play an important role in vasorelaxant and cardiovascular effects in Wistar rats whereby physiological and hemodynamic parameters indicated that this EO improved systolic and diastolic blood pressure, mean arterial pressure and heart beats after carotid artery cannulation [37].

Clinical studies
In clinical trial, it has been demonstrated that olfactory stimulation of the C. indicum Linné EO reduced systolic blood pressure and heart rate of the patients [66]. GCMS evaluation showed that 1,8-cineole and camphor as main biocompounds in this EO [66]. In a singleblinded randomized controlled trial, inhalation of EOs (e.g lavender and grapefruit) via olfactory stimulation (2% EOs for 10-20min) showed some repression on the in lated change values of diastolic blood pressure response in patients with stroke (with anxiety) and patients following coronary artery bypass and open-heart surgery [66][67][68][69][70]. While EOs alleviating stress and improved sleep quality in stroke patients, they had no noteworthy effects on mental stress and respiratory rate and other vital signs in patients underwent coronary artery bypass and open-heart surgery [69,70]. On the other hand, administration of A. calamus had signi icantly reduced chest pain, dyspnea, body weight index as well as improving ECG and lipid pro ile (serum cholesterol, LDL, HDL) in patients with ischemic heart disease [13]. Based on several studies, EO could be delivered via four different routes ( Figure 2).

Mechanism of action
Calcium (Ca 2+ ) channel blocker: Cardiac Ca 2+ channels (e.g T-type, L-type) in cardiac myocytes play functional role in heart, such as the resource of Ca 2+ -induced excitationcontraction and facilitate pacemaker depolarization of sinoatrial node in heart [8]. In particular, blockade of these channels has been used in the treatment of cardiovascular disease (e.g reduce contraction). Studies demonstrated that cardiodepressive effect of several EOs (e.g F. asafoetida, Citrus aurantium) and their bioactive components (e.g eucalyptol) rely onto their potential as Ca 2+ channel blocker [47,48,71]. Particularly, eucalyptol depressed rate of force development by steady-state contractions, postrest potentiation, force development isometric force as well as positive inotropic effect created by Ca 2+ [72]. F. asafoetida EO, A. speciosa EO, C. winterianus EO and Mentha x villosa EO had potent cardiodepressive and vasodilatory effect that mediated via endothelium-dependent (e.g EDRFs, NO and prostacyclin) and/or endothelium-independent mechanisms (e.g Ca 2+ channel blockade) [57,71]. The previously mentioned EOs decreased the in lux of Ca 2+ into the cell via plasma membrane Ca 2+ channels hence reduced the Ca 2+ -stimulated contractions [57]. Among EOs, A. speciosa EO probably have speci ic inhibition effect on L-type Ca²+ channels in rat heart [57]. The vasodilatory effect of F. asafoetida EO was decreased, but not fully inhibited, by either L-NAME or indomethacin [71]. H. fruticosa EO was also capable of antagonizing the doseresponse curves to Ca 2+ (3 μM-30 mM) in a concentrationrelated manner [54]. Concomitantly, the C. winterianus EO also antagonized the effect of Ca 2+ induced contractions in depolarizing potassium chloride solution which resulted in hypotension and vasorelaxation [53]. On the other hand, A. speciosa EO at 25 μg/ml and 250 μg/ml also inhibited left atrial force of contraction by up to ~33% and 89% respectively which was lower as compared to Nifedipine (a L-type Ca 2+ blocker) with an IC₅₀ of 12 μg/mL [71].
Anti-atherogenic effect: The effects of antiatherogenicity of several EOs have also been demonstrated. The O. sanctum EO reduced atherogenic index as well as several enzyme/protein released by dying myocardial cells such as serum lactate dehydrogenase (LDH) and creatinekinase (CK) activity with no adverse effect on high serum levels of aspartate/alanine aminotransferase and alkaline phosphatase in hypercholesterolemic rats [8,19]. EO also reduced elevated levels of TBARS, GPx and SOD with no adverse effect on catalase activity in the myocytes [19]. Histopathological evaluation indicated that EO was capable in preserving myocytes in experimental condition [19]. The compounds screening, identi ication and matching using GCMS revealed that eugenol and methyl eugenol as the major bioactive compounds in O. sanctum EO, thus suggested as being responsible for such pharmacological antihyperlipidemic effects [19]. In atherosclerotic rabbit model, treatment of A. senegal seeds (500 mg/kg/day) by 'Per os' for 45 d noticeably reduced serum level of TC, LDL-C, TG, and VLDL-C and atherogenic index (e.g with decreased atherosclerotic plaques in aorta, enlarged lumen volume) as compared to that of control [7]. The improvement on lipid pro ile and atherogenic index was noted almost comparatively similar to atorvastatin. Histopathological abnormality in aorta wall and other organs (e.g heart, kidney, liver) were reverted to normal condition with A. senegal seed supplementation, proven its antiatherosclerotic and cardioprotective effect.
Anti-coagulative, Anti-thrombotic and ibrinolysis effect: Heparin and coumarin derivatives are few of known anticoagulant compounds which help to reduce erythrocyte aggregation, platelet hyperactivity, arterial thrombosis and atherosclerosis, which were potentially useful to minimise the incidence of cardiovascular diseases (e.g coronary heart diseases myocardial infarction and cerebral arterial thrombosis) in human [85][86][87][88][89][90][91][92]. Particularly, Artemisia dracunculus leaves EO and F. Aurantii had a signi icant anticoagulative effect [87,88]. The former had considerable amount of coumarin derivatives while the latter had signi icant amount of lavonoids which could regulate several coagulation parameters such as lengthening prothrombin time as well as reducing fractional shortening and left ventricular out low, decreasing blood-clotting time in mice (e.g hematocrit and ibrinogen), and ameliorating the pathological alteration of myocytes in blood stasis model [87]. Some coumarins suppressed vitamin K-dependent γ-carboxylase action involved in the activation of coagulation factors [87]. Some citrus EO and its lavonoids (e.g hesperidin and hesperetin) inhibited the aggregation of erythrocytes and platelets. Hesperidin also inhibited ADP and thrombin-induced rat platelet aggregation [87]. Coumarins inhibited platelet functions via multiple mechanisms including scavenging of ROS and suppressing cyclic nucleotide phosphodiesterase and prostaglandin syntheses [87]. In addition, S. pinnatifolia EO at concentration of 1 to 5μg/mL also reduced adenosine 5'diphosphate (ADP) -induced rat platelet aggregation in vitro to about 33-47% [88]. In the anaesthetized Guinea-pigs, the C. bergamia Risso (bergamot) EO demonstrated a protective action against both pitressin-induced coronary arrhythmias and ouabain-induced ventricular arrhythmias [89]. In isolated heart of an adult rat, it also exerted a coronary-dilator action, reduced the hyperkinetic ventricular arrhythmias due to postischaemic reperfusion. The bergamot EO possesses signi icant cardiovascular effect comparable to that of an antidysrhythmic drug verapamil [89]. However, it is unknown whether it had similar action as verapamil by blocking voltage-sensitive Ca 2+ L-type channels. It is well-understood that Ca2+ reduces afterdepolarization and suppresses premature ectopic beats.

Safety and limitation
Safe applications and doses of several EOs have been demonstrated [90][91][92][93][94][95]. For instance, clinical status (morbidity or mortality), morphology and lung:body weight ratio were unaffected by the administration of the EO S. areira [64]. On the other hand, LD 50 of lemongrass EO based on a 24 h acute oral toxicity study in male Swiss mice was about ~3500 mg/ kg with no signi icant changes in organs [51]. On the other hand, tachycardia was signi icantly observed in mice/rat administrated with pinenes at 20 μg/mL, citronellol at 5 μg/ mL, and linalool at 20 μg/mL at increasing concentration while bisabolol at similar dose causes bradycardia (signi icant even at 5 μg/mL) [29,[93][94][95]. On the other hand, high dose of C. winterianus EO stimulated transient bradycardia as well as arrhythmias due to a cardiac muscarinic activation secondary to a vagal discharge [53]. Meanwhile, administration of A. canelilla bark EO elicited concentration-dependent bradycardia in rats. The bradycardia mostly depended in the presence of an operation-functional parasympathetic drive to the heart [38]. In conscious hypertensive rats, intravenous administration of C. zehntneri EO (1-20 mg/ kg) stimulated rapid (2-4s) and concentrationdependent bradycardia [32]. The effect of bradycardia of C. zehntneri EO was reversed into tachycardiac effects by methylatropine (1 mg/kg) pre-treatment [32]. In contrast, administration of C. argyrophylloides EOs via intravenous administration in conscious rats created dose-dependent tachycardia [36]. In non-anesthetized normotensive rats, Hyptis fruticosa EO (5-40 mg/kg) also induced tachycardia [54]. In rats, pretreatment of atropine, but not with atenolol or L-NAME, decreased tachycardiac responses by C. argyrophylloides EO [36]. However, hexamethonium pre-treatment converted the effect of the C. argyrophylloides EO-stimulated tachycardia into prevailing bradycardia [36]. Moreover, C. argyrophylloides EO-stimulated tachycardia in conscious rats may mediated via inhibition of vagal drive to the heart [36].

Conclusion
This systematic review demonstrated the therapeutic application of several EOs as complementary and alternative medicine in cardiovascular diseases. Terpenes are the most active components in EOs gave direct vasorelaxant effect as supported by the results of the present study. Other EOs lowered blood pressure, reduced risk for myocardial infarction, stroke and heart failure. This study provides a reference for the clinical research and utilization of EOs, as well as the application basis for co-treatment of cardiovascular diseases. This will also pave further evaluation of EOs for potential new application for cardiovascular diseases.