Scenario: A 78-year-old male is brought to the emergency department by his family, who states that he has been acting “very tired” for several days. The patient has a history of coronary artery disease, hypertension, and diabetes. The patient is lethargic and hypotensive (80/40), the radial pulse is thready and irregular, and he is jaundiced. The patient’s current medications include aspirin, metoprolol, lisinopril, and metformin. Notably, the family states that the patient has not been urinating for more than 1 week. Below is the initial ECG rhythm strip in lead II.Sinus arrest with atrial (beats 2,4,6) and junctional escape beats (1,3,5)The conduction system is designed so that if the sinoatrial (SA) node fails, an escape rhythm from a lower pacemaker site occurs. Escape rhythms typically originate from the atrioventricular (AV) junction at 40 to 60 beats/min or the Purkinje system at a rate of 20 to 40 beats/min. Here, the absence of P waves indicates SA node dysfunction, and the 3.4-second pause indicates lower pacemaker failure.Dysfunction in the sinus node may be intrinsic, meaning actual pathology of the tissue, or extrinsic, involving outside factors such as drugs. Typically, if the cause of sinus arrest is intrinsic, a diagnosis of “sick sinus syndrome” is appropriate. Beats 1, 3, and 5 are not preceded by a discernible P wave, hence they are not sinus beats; instead, a lower pacemaker site is responsible for initiating these beats. In deciding the location of escape beats (junctional or ventricle), the duration of the QRS complex should be assessed. Because the duration of QRS complexes here are within normal range (< 0.12 seconds), the site of the escape beat is supraventricular and likely junctional in origin. Although a P wave preceeds beats 2, 4, and 6, the waveform is predominantly negative, with an abnormal direction in lead II, indicating retrograde (opposite direction) conduction, a diagnostic feature of a junctional foci in this lead. However, the PR interval is normal, indicating that the origin of the beat is above the AV node. These beats therefore are not junctional but atrial, with retrograde conduction. This physiological phenomenon, impulse formation in atrial tissue above the AV node, has been described previously.The immediate treatment is restoration of cardiac output. Prepare to initiate Advanced Cardiac Life Support guidelines for symptomatic bradycardia, which include temporary pacing, and atropine. Possible causes must be identified and treated, if possible. This patient is jaundiced and has low urine output, indicating hepatic and renal dysfunction that may have led to drug toxicity. In addition, a standard 12-lead ECG should be obtained because sinus arrest can occur in those with ischemic heart disease; myocardial ischemia can be ruled out. It was determined this patient had metoprolol toxicity.