The Point of Maximal Impulse (PMI) is the location on the chest wall where the rhythmic beat of the heart’s apex can be most easily felt or seen. Also known as the apical impulse, this pulsation is created when the heart’s left ventricle contracts during systole, causing the tip of the heart to momentarily push against the chest wall. Because its location and characteristics change under various conditions, the PMI serves as a fundamental diagnostic landmark for assessing cardiac size and function. Evaluating the impulse provides a non-invasive way for healthcare professionals to gain insights into the health of the heart muscle and surrounding structures.
The Precise Normal Location
The normal position of the Point of Maximal Impulse is specific in a healthy adult, corresponding directly to the anatomical location of the heart’s apex. It is typically found on the left side of the chest in the fifth intercostal space (ICS), the space between the fifth and sixth ribs. To accurately locate this space, an examiner first finds the sternal angle, a ridge on the sternum that marks the articulation of the second rib. Counting downward from the second space allows for the identification of the fifth space.
The normal PMI is also situated along the mid-clavicular line (MCL), or just slightly medial to it. The MCL is an imaginary vertical line drawn downward from the midpoint of the clavicle on the left side. This coordinate system places the normal impulse approximately seven to nine centimeters left of the midsternal line, reflecting the natural positioning of the left ventricle’s apex.
Assessing the Impulse
Palpating the impulse allows for the assessment of several characteristics that reflect the underlying function of the left ventricle. A normal PMI is typically small, discrete, and localized, covering an area no larger than two to three centimeters. The duration is also important, as a healthy beat is brief and non-sustained, lasting for less than two-thirds of the systolic phase.
The quality of the normal impulse is often described as a brisk or tapping sensation felt with the fingertips. If the heart is working harder or the ventricle is structurally altered, the impulse changes significantly. For instance, an abnormal impulse may feel sustained, pushing against the fingertips for a longer duration, or it may be forceful, indicating an increased amplitude of contraction. Analyzing these features helps estimate the volume and pressure within the ventricle.
Conditions That Cause Displacement
A shift in the PMI’s location away from the normal position is a significant clinical finding, often caused by a change in the heart’s size or external pressure from surrounding structures. Cardiac causes primarily involve changes to the left ventricle’s structure. For example, left ventricular hypertrophy (LVH), a thickening of the heart muscle often due to high blood pressure, causes the impulse to become sustained and may shift it laterally.
When the left ventricle dilates, such as in heart failure or generalized cardiomegaly, the heart expands, pushing the apex further down and to the left. This displaces the PMI both laterally, beyond the mid-clavicular line, and often inferiorly, potentially down to the sixth intercostal space. This lateral and inferior shift is a classic sign of an enlarged heart.
Extracardiac conditions can also physically push or pull the heart, displacing the PMI even if the heart is structurally normal. A large collection of fluid in the pleural space (pleural effusion) or an accumulation of air in the chest cavity (pneumothorax) can push the heart to the opposite side. For example, a tension pneumothorax on the right side pushes the mediastinum, including the heart, significantly to the left, resulting in an abnormally lateral PMI.
Conversely, conditions that cause lung tissue to contract, such as extensive left-sided pulmonary fibrosis, can pull the heart toward the affected side, causing the PMI to shift medially and superiorly. Conditions that elevate the diaphragm, such as late pregnancy or a large abdominal mass, can lift the heart, resulting in an upward displacement. In cases of pericardial effusion, where fluid surrounds the heart, the PMI often becomes diffuse or entirely absent because the fluid muffles the impulse.