Ambulance resuscitation. Paramedics carrying out cardiopulmonary resuscitation (CPR) on a patient in an ambulance. The heart massage (cardiac compression) is done following cardiac arrest. Each downward compression squeezes blood out of the heart and around the body. The other paramedic is using an Ambu bag (blue, not clearly seen) to push air into the patient's lungs. The bag is used when a patient is breathing weakly or not at all.