Why the Normal Heartbeat is Palpable During Systole on the Chest Wall
The normal heartbeat is palpable during systole because this is when the left ventricle contracts forcefully, rotating and striking the anterior chest wall at the apex, creating the apical impulse (also called the point of maximal impulse or PMI).
Physiological Mechanism
During systole, several mechanical events make the heartbeat palpable:
- Ventricular contraction and rotation: The left ventricle contracts and rotates anteriorly, bringing the apex into contact with the chest wall
- Ejection of blood: Blood is forcefully ejected through the aortic outflow tract, creating momentum
- Ventricular wall thickening: The myocardium thickens during contraction, increasing the force transmitted to the chest wall
The apex beat represents the point where the contracting left ventricle strikes the chest wall during systole, not diastole when the heart is relaxed and filling.
Normal Characteristics of the Apical Impulse
In healthy adults examined in the full left lateral position 1:
- Location: 5th or 6th intercostal space, within 3 cm of the left anterior axillary line toward the mid-clavicular line
- Size: Mean diameter of 2.5 ± 0.5 cm and area of 5.0 ± 2.5 cm²
- Palpability: Present in approximately 87% of healthy adults
- Intensity: Medium degree in 51%, weak in 25%, strong in 24%
Clinical Relevance
The presence of a palpable cardiac impulse has diagnostic value:
- Predicts adequate echocardiographic windows: A palpable apex is the only independent predictor of adequate acoustic imaging quality (92% of patients with palpable apex had adequate images versus only 55% without) 2
- Detects left ventricular enlargement: An apex beat diameter ≥4.0 cm is both sensitive (96%) and specific (96%) for left ventricular dilatation 1
Why Not Diastole?
During diastole, the ventricle is relaxed and filling passively. There is no forceful contraction or anterior rotation to create a palpable impulse against the chest wall. The heart essentially "rests" during this phase, making it imperceptible to palpation under normal circumstances.
Important caveat: Abnormal diastolic impulses can occasionally be palpated in pathological conditions (such as severe left ventricular hypertrophy with reduced compliance), but this is distinctly abnormal and represents disease, not normal physiology.