Actually, I am not responding as a paramedic with ALS equipment/medication, but as a first line BLS responder. Cardiac arrest will occur post respiratory arrest. If the patient is not breathing adequately and the heart not circulating oxygenated blood sufficiently for adequate perfusion, cardiac arrest is inevitable. While rescue breathing may provide air exchange within the lungs, if the oxygenated blood is not circulated to heart, which requires oxygen for metabolic function, the heart muscle becomes irritable, leading to Vfib and eventual asystole.

Pete