What is electromechanical systole?

What is Electromechanical Systole

Electromechanical systole (QS2) is the total duration from the onset of ventricular electrical activation (Q wave on ECG) to the closure of the aortic valve (A2 heart sound), representing the complete period of ventricular electrical and mechanical activity during systole. 1

Components and Measurement

Electromechanical systole consists of two main phases that together comprise the total duration of ventricular systolic activity:

• Pre-ejection period (PEP): The interval from the onset of the QRS complex to the beginning of left ventricular ejection, which includes both the electromechanical interval and isovolumetric contraction time 2

• Left ventricular ejection time (LVET): The period during which blood is actively ejected from the left ventricle into the aorta 1, 3

The relationship is expressed as: QS2 = PEP + LVET 1, 3

Normal Physiological Relationship

In healthy individuals, a critical relationship exists between electrical and mechanical systole:

• The QT interval (electrical systole) is normally shorter than QS2 (electromechanical systole) by approximately 26 ± 13 ms 4

• This creates a positive "electromechanical window" (EMW), calculated as QS2 minus QT interval, which is typically positive (+22 ± 19 ms) in healthy controls 5

Clinical Significance in Disease States

Heart Failure and Conduction Abnormalities

Impaired electromechanical coupling occurs when left ventricular systolic dysfunction progresses to clinical heart failure, manifesting as prolonged atrioventricular and interventricular conduction (most commonly left bundle branch block) 6:

• Prolonged interventricular and intraventricular conduction causes regional mechanical delay within the left ventricle 6
• This results in reduced ventricular systolic function, altered myocardial metabolism, functional mitral regurgitation, and adverse remodeling with ventricular dilatation 6
• Approximately one-third of patients with advanced heart failure develop QRS prolongation associated with ventricular electromechanical delay ("dyssynchrony") 6

Left Bundle Branch Block

In patients with left bundle branch block, electromechanical systole demonstrates characteristic abnormalities:

• Abnormal prolongation of QS2 occurs due to lengthening of both PEP and the Q-M1 intervals 2
• Isovolumetric contraction time (ICT) is prolonged in the majority of these patients 2

Coronary Artery Disease and Risk Stratification

When QT duration exceeds QS2 duration (QT > QS2), this represents a powerful mortality risk indicator in coronary artery disease 4:

• Patients with QT > QS2 after myocardial infarction have a 5-year survival rate of only 35% compared to 91% in those with normal QT ≤ QS2 relationship 4
• This abnormal relationship is a more potent prognostic indicator than QT corrected for heart rate (QTc) alone 4
• The presence of abnormal PEP/LVET ratio further stratifies risk when combined with QT > QS2 4

Long QT Syndrome

In genotyped long QT syndrome patients, the electromechanical window becomes profoundly negative:

• EMW is negative (-43 ± 46 ms) in LQTS patients compared to positive values in controls, and is even more negative in symptomatic patients (-67 ± 42 ms) versus event-free patients (-27 ± 41 ms) 5
• Negative EMW is a better discriminator of arrhythmic events than resting QTc (AUC 0.77 vs 0.71) and predicts events independently of QTc 5

Hypertension

In essential hypertension, electromechanical systole changes reflect cardiac adaptation:

• PEP shows positive correlation with left ventricular mass in hypertensive patients 1
• LVET shortening occurs only at the decompensated stage of hypertensive heart disease 1
• Pheochromocytoma produces a distinct pattern with marked shortening of both QS2 and LVET with normal PEP 1

Factors Influencing Electromechanical Systole

The duration of electromechanical systole is affected by multiple physiological variables 1:

• Heart rate: Inverse relationship with QS2 duration
• Preload: Increased preload prolongs LVET component
• Afterload: Increased afterload prolongs PEP and may shorten LVET
• Myocardial contractility: Enhanced contractility shortens PEP and may affect LVET

Common Pitfalls in Interpretation

• Do not assume that QTc alone adequately captures electromechanical risk in coronary artery disease; the relationship between QT and QS2 provides superior prognostic information 4

• Recognize that electromechanical systole abnormalities in heart failure reflect impaired coupling between electrical activation and mechanical contraction, not simply conduction delay alone 6

• Avoid interpreting systolic time intervals without considering the clinical context (hypertension, coronary disease, heart failure), as different disease states produce distinct patterns 1, 3