A 43-year-old woman develops acute renal failure following an emergency resection of a leaking abdominal aoic aneurysm. One week after surgery, the following laboratory values are obtained: Serum electrolytes (mEq/L): Na + 127, K+ 5.9, Cl - 92, HCO3 - 15 Blood urea nitrogen: 82 mg/dL Serum creatinine: 6.7 mg/dL The patient has gained 4 kg since surgery and is mildly dyspneic at rest. Eight hours after these values are repoed, the following electrocardiogram is obtained. Which of the following is the most appropriate initial treatment in the management of this patient?