Final answer:
The nurse identifies the cardiac border by a change from a resonant to a dull note during percussion, a technique that aids in mapping the heart's size and position. Auscultation with a stethoscope complements this by detecting heart sounds and murmurs.
Step-by-step explanation:
Using percussion, the nurse identifies the cardiac border by a change from a resonant to a dull note. Percussion is a technique used to determine the underlying structure of the chest cavity and the borders of the organs within, such as the heart. When percussing over the lung, the sound is typically resonant due to air in the lungs. However, as the nurse moves towards the heart, the sound changes to dull because the heart is a solid organ that does not contain air and does not resonate as the lung does. This change in sound helps to map out the size and position of the heart within the thoracic cavity. The auscultation technique with a stethoscope is also used to listen to heart sounds, identifying anomalies like heart murmurs, which can be indicative of turbulent blood flow within the heart.