Final answer:
The first thing to do during CPR is begin cardiac massage to manually compress the chest and push blood within the heart into the pulmonary and systemic circuits.
Step-by-step explanation:
During cardiopulmonary resuscitation (CPR), the first thing to do is to start with chest compressions, which falls under the category of beginning cardiac massage. According to current guidelines, high-quality chest compressions are prioritized over artificial respiration. The responder must compress the chest at least 5 cm deep and at a rate of 100 compressions per minute. This manual compression of the heart helps to push blood through the pulmonary and systemic circuits, providing critical blood flow to the brain and other vital organs. Following the initiation of chest compressions, if the rescuer is trained and able to do so, ventilating the animal and other advanced procedures, such as intubation or medication administration, may follow based on the situation and available resources.