This 57-year-old diabetic hypertensive woman, with end-stage renal failure on hemodialysis, is seen in the Emergency Department with fever (38.6 C), nausea, weakness and diarrhea. Her medication includes Atenolol 100 mg daily:

• Marked sinus bradycardia (30/min) with junctional escape complexes (escape-capture bigeminy). Ventricular rate 60/min.
• Left axis deviation
• Left ventricular hypertrophy with QRS widening

Comments: On admission the patient appeared dehydrated and acutely ill. Her serum potassium was 6.5 mmol/L. Her blood cultures grew a staphylococcus aureus. She was dialysed and vigorously treated with antibiotics. Atenolol was withheld. The ECG shows a good example of “escape-capture bigeminy”. The sinus rate is 30/min and there is a junctional escape complex preceding each P-QRS complex. There is a slight change in morphology of the escape QRS complex, compatible with incomplete left bundle branch block. Occasionally there may be a conduction abnormality after a longer pause (“phase 4 aberrancy”). The tall T waves may be due to hyperkalemia, however, the findings are not specific.