A 71-year-old man returns from the operating room (OR) after undergoing a triple coronary bypass. His initial cardiac index is 2.8 L/ (min*m2 ). Hea rate is then noted to be 55 beats per minute, BP is 110/80 mm Hg, wedge pressure is 15, and his cardiac index has dropped to 1.6 L/(min*m2 ). He has a normal left ventricle. Which of the following maneuvers will increase his cardiac output?