Cardiac dysrhythmias are easy. Unlike the lung (which has formidable neuroendocrine, metabolic, and respiratory responsibilities), the heart is simple. It is an innervated muscular pump. A resting Purkinje or ventricular muscle cell membrane maintains a charge of about 90 millivolts. The five phases of a cardiac action potential are similar to the action potential in skeletal muscle, however, the cardiac action potential lasts a hundred times longer. When sodium specific "fast" channels and calcium specific "slow" channels open, positive ions rush into the myocardial cell, thus causing rapid membrane depolarization. In order to produce an action potential, some stimulus must decrease the membrane potential from -90 millivolts to "threshold" or -60 millivolts. Purkinje fibers do not have a stable phase for diastolic potential. These fibers continuously depolarize during diastole. Hypoxemia or hypokalemia may exacerbate this diastolic depolarization, thus promoting "hyperexcitability" or "automatic" ectopy. When myocardium is damaged, characteristically with myocardial ischemia, rapid conduction of cardiac impulses may be slowed dramatically. Very slow impulses may course through muscle such that by the time the activation wave front returns to the initiating site, this origin has had a chance to repolarize. This is the basis for re-entrant dysrhythmias. All cardiac dysrhythmias are automatic, re-entrant or both.