The temperature and blood pressure of the patient drops.
The bonds that hold H2O2 together break apart and the atoms are rearranged to form water (H2O) and oxygen (O2) molecules. This happens slowly in store-‐bought hydrogen peroxide, so even in dark bottles, it will 'expire' in a year, or less if it's opened. ... And higher concentrations of H2O2 produce more oxygen.
