Fifth left intercostal space

The fifth left intercostal space is the space between the left fifth and sixth ribs. Intercostal spaces are numbered by the rib above them, so this space lies just below rib five. It is one of the most clinically important surface landmarks in the body because, at the midclavicular line, it overlies the apex of the heart.

To locate it, clinicians start at the sternal angle (angle of Louis), which sits at the level of the second costal cartilage, then count down the intercostal spaces. The heart's apex — formed by the tip of the left ventricle — normally points inferiorly, anteriorly, and to the left, reaching the fifth left intercostal space about 8 to 9 cm from the midline.

This landmark is where the apical impulse, or point of maximal impulse (PMI), is palpated, and where the mitral valve is best auscultated, since sound radiates in the direction of blood flow toward the apex. A PMI displaced laterally or inferiorly from this position suggests left ventricular enlargement or cardiomegaly. The apex is also the standard site for counting an apical heart rate and placing certain ECG electrodes (V4 sits in the fifth intercostal space at the midclavicular line).

USMLE Step 1 tests this landmark within cardiovascular anatomy — expect questions tying the fifth left intercostal space at the midclavicular line to the apex beat, the left ventricle, mitral auscultation, and the significance of a displaced PMI.