A substantial proportion of patients presenting to the emergency department (ED) with acute chest discomfort have normal electrocardiograms (ECGs) and initially reassuring conventional cardiac troponin concentrations, but still subsequently develop myocardial infarction
]. So the goal and greatest challenges are the integration of multiple techniques and tests into clinical practice in a logical and cost-effective strategy to answer the following questions, Does this patient have ACS or can he or she be discharged immediately? Should this patient undergo early catheterization? What is this patient’s risk?
The prognostic significance of HRV has been extensively studied in different populations especially in patient who survived an AMI. SDNN is a universally recognized parameter with the highest specificity and sensitivity as compared with other lethal outcomes predictors [8
Several studies demonstrated that reduced heart variability can be clinically relevant in the diagnostic evaluation of the coronary artery disease [11
This study aimed to create a non-invasive, economical and risk-free method in clinical evaluation and diagnosis of significant CAD among patients with unstable angina.
In the present study, we demonstrated:
The relationship between heart rate variability indices and stress test results in patient diagnosed clinically as low risk acute coronary syndrome.
The relationship between heart rate variability indices and diagnosis of obstructive CAD.
As a main finding, HRV indices were significantly reduced in patient with obstructive CAD.
Reduced HRV indices have a significant negative correlation with obstructive CAD.
A lot of previous studies showed the relation between HRV and presence of CAD, but the results was controversial according to the population who had been investigated.
Previous studies support our findings like Trigulova and Kurbanov, 2011 found that ECG Holter monitoring started in patients with unstable angina pectoris within first 24 hours after the last pain attack demonstrated all HRV time parameters confidently lower than those in healthy subjects of the same age [13
] But analysis of ECG Holter monitoring parameters detected no informative HRV markers to predict either adverse or favorable IHD course.
Also Huang et al, 1995 all measures of HRV were reduced in patients with acute coronary syndrome compared to normal controls (p<0.001) while there were no significant difference in measures of HRV between unstable angina and myocardial infarction patients [14
]. In patients with unstable angina who stabilized after admission, HRV had increased by the second 24 hours of monitoring.
In another study done by Goldkorn et al, 2015 who used HRV as a screening tool for detection of myocardial ischemia in patient without known CAD found that reduced HRV indices were highly correlated with the presence of significant CAD (p=<0.001) in patients who underwent coronary angiography during follow-up [15
Celik et al, 2011 used HRV to discriminate true CAD in patients with ST segment depression without angina during exercise stress testing [16
]. They showed that HRV and HRT parameters are blunted in patients with CAD and that is even more pronounced in those with obstructive CAD. SDNN, LF, pNN50, RMSSD, total power, and HF was significantly lower in patients with obstructive CAD (p<0.001).They recommend that SDNN ≤ 69.63 msec had high diagnostic accuracy for predicting obstructive CAD while SDNN ≤ 75.84 msec and LF ≤ 943 msec had significant diagnostic accuracy for predicting non-obstructive CAD.
Pivatell et al 2012 showed that patients with stable angina had significantly reduced HRV indices, especially SDNN (p=0.0209), RMSSD (p=0.03), NN50 (p=0.04) and HF (p=0.007), in patient with obstructive CAD [17
Takei et al. 2007 found that among ACS patients; all indicators of HRV in multi-vessels diseased group were lower significantly than single vessel diseased; same finding by Hayano et al 1990 [12
Tamoši et al 2005 showed that the value of HRV during deep breathing test was found to be lower in patients with coronary artery stenosis ≥ 50% comparing to patients with stenosis ≥ 30% (p=0.02) which showed diminished vagal cardiac activity in patients with CAD [19
]. They also found that liability of variability during deep breathing was expressed most in the myocardial infarction patients who is expected to have the highest lesions.
Inspecting these studies revealed that they support our finding that patients of obstructive CAD had reduced heart rate variability indices.
In disagreement with our finding, Wennerblom et al 2000 showed that patient with uncomplicated CAD and no previous myocardial infarction had reduced HRV mainly affecting the high and low frequencies [20
]. They also suggest that the small number of patient in their study is the probable reason for the fact that the SDNN and SDANN reduction in the angina patient was non-significant.
Lanza et al collected data from 1997-2001 using Holter recordings that were started within 24 hours of hospital admission in 543 UA patients [21
]. Primary endpoints were in-hospital and 6-month deaths, and a secondary endpoint was nonfatal acute MI. the SDNN index and LF power were significantly associated with in-hospital mortality in multivariate analysis, while other parameters shows no statistically significance.
Interestingly, alteration of RMSSD related to parasympathetic nerve
in HRV is not identical in different studies which means that there is no clear correlation between RMSSD and severity of CAD [18
The mechanism of low heart rate variability in patient with CAD is not exactly known but previous studies found that HRV can be used as a predictor for progression of atherosclerosis [23
At last, HRV measured close to the ACS onset may assist in risk stratification. HRV cut-points may provide additional, incremental prognostic information to established assessment guidelines, and may be worthy of additional study [24
Practice standards recommend 24 hours of cardiac monitoring for ACS patients after ED presentation. Our Holter findings suggest that use of HRV measurements to assist in identifying patients at highest risk for adverse events might be a practical addition to continuous ECG monitoring.
HRV represent a non-invasive bedside test that could be incorporated into heart rate monitoring devices already present in ambulances and emergency departments.
Our study had some limitations. First, long term follow up of the adverse cardio-vascular outcome for these patients needed to be studied. However, this would need a larger scale study which a larger sample of patients and longer duration of follow up which is beyond the scope of our study. Second, recent ESC guidelines recommended the assessment of serum copoptien level if the first two troponin measurements are not conclusive and the clinical condition is still suggestive of ACS [25
]. However, these recommendations were published after finishing our work, so we were unable to evaluate this test. Finally, the effect of drug therapy was not included in that work. Similar to long term adverse cardio-vascular outcome, this needs large scale studies to produce conclusive results.