An infant weighing 1600 g (3 1/2 lb) is born at 32 weeks' gestation. Initial evaluation was benign, and the infant was transferred to the level 2 nursery for prematurity. The nurse there calls at 1 hour of life and reports the infant is tachypneic. Vital signs include a heart rate of 140 beats per minute, a respiratory rate of 80 breaths per minute, a temperature of 35degC (95degF), and a peripheral oxygen saturation of 98%. The lungs are clear with bilateral breath sounds and there is no murmur; the infant is in no distress. The child's chest radiograph is shown. Which of the following is the most appropriate next step in evaluating the infant?