The Point of Maximal Impulse (PMI) is a physical finding used in standard cardiovascular examinations. It represents the brief, localized sensation felt on the chest wall as the heart contracts. Assessing this impulse through touch provides medical professionals with non-invasive information about the heart’s size, position, and function. Analyzing the PMI is a foundational technique used to screen for potential cardiac issues.
Defining the Point of Maximal Impulse
The Point of Maximal Impulse (also known as the apical impulse) is the lowest and most lateral point on the chest where the cardiac pulsation is felt most strongly. In a healthy adult, the PMI is typically found at a specific anatomical location. This spot is the fifth intercostal space (between the fifth and sixth ribs) along the midclavicular line. This imaginary line is drawn vertically downward from the midpoint of the collarbone, placing the normal PMI approximately 7 to 9 centimeters to the left of the midsternal line.
Since the left ventricle forms the apex, or tip, of the heart, the PMI is a physical manifestation of the left ventricle’s activity. This consistent location serves as the baseline for assessing the heart’s position within the thoracic cavity.
The Mechanics Behind the Impulse
The physical sensation of the PMI is directly caused by the heart’s muscular action during systole, the phase when the ventricles contract to eject blood. As the left ventricle shortens and stiffens to push blood into the aorta, the heart undergoes a slight rotation and moves forward. This mechanical action causes the apex of the heart to briefly tap or thrust against the inner chest wall.
The palpable impulse represents this momentary contact between the contracting left ventricle and the rib cage. It is a brief, outward movement, similar to a localized tap, lasting for less than two-thirds of the total systolic duration.
What a Normal PMI Tells Doctors
A normal PMI provides reassurance regarding the health and structure of the left side of the heart. Palpating the impulse at the expected location confirms the heart is of normal size and correctly positioned. The physical assessment focuses on the location, size, and character of the pulsation. A healthy impulse is discrete, localized, and typically measures only 1 to 2 centimeters in diameter. Its character is a quick, non-sustained tap, signaling efficient ventricular contraction.
These findings suggest the left ventricle is functioning effectively without abnormal pressure or volume strain, meaning it is neither enlarged nor overworked.
When the PMI is Displaced or Altered
Any deviation from the normal PMI indicates a potential underlying change in cardiac anatomy or function. Displacement is the most common alteration, usually presenting as a shift laterally or inferiorly from the fifth intercostal space. A PMI found outside the midclavicular line suggests the heart has enlarged (cardiomegaly). This lateral displacement is commonly seen in conditions causing left ventricular dilation, such as heart failure. The character of the impulse can also be altered, providing specific diagnostic clues.
A forceful and sustained impulse that lasts throughout systole suggests a chronic pressure overload on the left ventricle. This finding is characteristic of left ventricular hypertrophy, where the muscle has thickened to overcome high resistance, such as hypertension. Conversely, the PMI may be absent or difficult to locate in patients with conditions like pericardial effusion or significant obesity.