The cardiac notch is a depression found in the anterior border of the left lung. This indentation is a direct consequence of the heart’s position, which is situated mostly toward the left side of the body. The notch is an adaptation of the lung’s shape, allowing the heart and lung to coexist within the limited space of the thorax.
Defining the Cardiac Notch and Its Location
The cardiac notch, or incisura cardiaca pulmonis sinistri, is a concavity carved into the superior lobe of the left lung. This feature is absent on the right lung, which has three lobes and a uniform anterior border. The notch is a lateral deflection of the lung’s anterior edge, created to accommodate the space occupied by the heart and its surrounding pericardium.
The notch is defined by the lingula, a thin, tongue-like projection of the superior lobe that extends below it. The lingula is situated inferior to the notch and superior to the oblique fissure separating the lung’s two lobes. The notch is continuous with the large cardiac impression, the concave surface on the lung’s medial side where the heart rests.
This indentation is positioned roughly at the level of the fifth and sixth intercostal spaces along the anterior border of the left lung. The apex of the heart fits into the space created by this notch. This precise positioning ensures the lung is shaped around the heart, optimizing the use of the thoracic cavity volume.
Functional and Clinical Significance
The function of the cardiac notch is to provide physical clearance for the heart’s asymmetrical placement and movement. Since the majority of the heart projects toward the left, the notch demands a reduction in the volume of the left lung. This indentation allows the heart ample space to contract and move freely without restriction during the respiratory cycle.
The notch creates a small area where the pericardium, the sac surrounding the heart, comes into direct contact with the sternum and rib cage. This direct contact makes the area a significant landmark in clinical examinations. When a clinician performs chest percussion (tapping on the chest wall), this area produces a dull sound.
This dull sound is known as the area of superficial cardiac dullness, distinct from the resonant sound produced by the surrounding air-filled lung tissue. The absence of lung tissue over the heart also makes the cardiac notch an important reference point for auscultation (listening to heart and lung sounds). The heart’s close proximity to the chest wall here allows for clearer assessment of heart sounds during a physical examination.