Upper left: Inferior margin of 2nd left costal cartilage
Apex of heart: Left 5th intercostal space at midclavicular line
Right border: From 3rd right costal cartilage to 6th right costal cartilage
Left border: From 5th left intercostal space at midclavicular line to 2nd left costal cartilage
Inferior border: From 6th right costal cartilage to 5th left intercostal space at midclavicular line
Superior border: From inferior margin of 2nd left costal cartilage to superior margin of 3rd right costal cartilage
Surface projection landmarks of the heart are crucial to remember for ‘step 1’ exam.
The right atrium will be pierced by the penetrating injury at the right sternal border at the 4th intercostal space.
The right ventricle will be pierced by the penetrating injury on the left sternal border at the 4th intercostal space.
The left ventricle will be pierced by the penetrating injury medial to the left midclavicular line at the 4th intercostal space.
There is very less likelihood of injury to left atrium due to its posterior location unless a penetrating trauma is deep enough to pass through the upper left side of the heart (bullet injury). In rare instances, dilation of left atrium can compress esophagus posteriorly and cause dysphagia.
Adapted from: Fundamentals of Cardiology: Concise Review for USMLE and General Medical Boards Image source: Frontal view of the chest cavity on chest x-ray, University of Auckland