The investigators intend to prospectively enroll consecutive patients, without known ischemic heart disease, referred by their physician for routine exercise MPI at a tertiary care center.

Prior to the MPI (nuclear stress test), eligible and consenting patients will undergo a one-hour period of Holter ECG digital recording. Application of ECG electrodes will be performed by medical technicians following standard recommendations, utilizing approved Holter device & stickers. The Holter ECG data will be used for the offline heart rate variability (HRV) analysis by the HeartTrends device.

Immediately following data acquisition, all patients will undergo an exercise MPI according to accepted clinical practice guidelines.

Following the MPI, there will be no interference with patient management, which will be conducted by the patients' treating physicians according to current guidelines, unaware of HRV results.

Analysis of recorded ECG data for HRV will be performed blinded to the EST and MPI results, by HeartTrends device.

The results of the HRV tests will not be available to treating physicians and will not be used to guide patient management. Patients will be followed for 6 months for the occurrence of coronary interventions and MACE, defined as: death, myocardial infarction, unstable angina, or any coronary revascularization.

Inclusion Criteria:

• Age ≥ 21
• Referral for exercise MPI by treating physicians
• Willing and able to provide written informed consent

Exclusion Criteria:

• Established ischemic heart disease
• Atrial fibrillation or flutter
• Acute Coronary Syndrome
• Cardiac Pacemaker
• Clinical diagnosis of heart failure
• Moderate or severe COPD
• Active myocarditis, constrictive pericarditis, any cardiomyopathy, cardiac or systemic amyloidosis
• Known drug or alcohol dependence or any other factors which will interfere with the study conduct or interpretation of the results or in the opinion of the investigator are not suitable to participate;
• Any illness that might reduce life expectancy to less than 1 year from screening
• Left bundle branch block (LBBB), significant intra-ventricular conduction delay (IVCD) or significant (>1mm) ST deviations on baseline ECG
• Inability to perform an exercise stress test (i.e. orthopedical or neurological limitations)
• Any significant established myocardial or valvular disease

The study will prospectively enroll consecutive patients, without known ischemic heart disease, referred by their physician for an exercise MPI.