When the blood potassium is gradually increased, the excitability of the myocardium should be increased first and then decreased. This is because when the potassium ion in the blood is mildly elevated, the concentration gradient of potassium ions inside and outside the myocardial cells decreases, the absolute value of the resting potential decreases and the excitability increases close to the valve potential, while when the potassium ion in the blood is significantly increased, the absolute value of the resting potential decreases excessively, resulting in the inactivation of sodium channels and the complete loss of myocardial excitability. Therefore, the excitability of the myocardium should first increase and then decrease when the potassium ion in the blood is gradually increased. The key to this decrease is the inactivation of sodium channels, which is the reason for the risk of cardiac arrest in hyperkalemia.