The critical care nurse is caring for a client who has been experiencing bradycardia after cardiovascular surgery. The nurse knows that the heart rate is determined by myocardial cells with the fastest inherent firing rate. Under normal circumstances, where are these cells located?

The sinoatrial (SA) node, located in the right atrium near the superior vena cava, is the primary pacemaker of the heart under normal circumstances. It fires at an inherent rate of 60–100 beats per minute, which is faster than any other pacemaker tissue in the heart. Because the SA node fires fastest, it depolarizes the rest of the myocardium before other sites can reach their own firing threshold, effectively overriding them and establishing the heart's normal sinus rhythm. This principle is called overdrive suppression. The AV node has an inherent rate of 40–60 beats per minute and serves as a secondary pacemaker if the SA node fails. The Bundle of His and Purkinje cells have inherent rates of only 20–40 beats per minute, making them the slowest potential pacemakers and only relevant when higher centers fail, which can produce dangerous idioventricular rhythms. After cardiovascular surgery, bradycardia may result from damage to or suppression of the SA node, causing lower pacemaker sites to take over. Understanding which site normally fires fastest helps the nurse anticipate why the heart rate dropped and why temporary pacing or pharmacological intervention may be required in the postoperative period.