Why does my chest sometimes feel a 'sharp, stabbing pain' when I take a deep breath?

This usually isn't a heart condition, but rather nerves in your chest wall getting 'pinched and irritated'! This phenomenon is medically known as 'precordial catch syndrome (PCS)' or 'anterior chest wall catch syndrome'. Between our ribs are numerous tiny 'intercostal nerves', and the outside of our lungs is covered by a smooth membrane called the pleura. When you're sitting still, have poor posture (like hunching), or suddenly take a deep breath, the parietal pleura inside the chest cavity or the intercostal nerves might get momentarily 'pinched' or stretched by surrounding tissues due to slight friction within the space! Because the nerves in this area are quite sensitive, the brain immediately receives a signal of extremely sharp, stabbing pain. When you slow your breathing or change your posture, the pinched or stretched nerve slips back into place, and the stabbing pain disappears instantly. It's completely harmless to your heart and lungs.