Shivering following cardiac surgery: Hemodynamic changes and reversal

The effects of shivering on hemodynamics and systemic oxygenation, as well as the effectiveness of therapeutic interventions in decreasing shivering and increasing mixed venous oxygen saturation, were studied. Thirty adult patients undergoing cardiopulmonary bypass with systemic hypothermia were observed for 1 1/2 to 5 hours postoperatively for signs of shivering associated with a simultaneous decrease in oxygen transport. Systemic and pulmonary hemodynamic measurements were made, blood temperature and mixed venous oxygen saturation were monitored via the pulmonary arterial catheter, and oxygen consumption and delivery were calculated. Shivering was graded by a single investigator on a scale of 0 to 4, with 0 = no shivering and 4 = continuous violent muscle activity. Therapy was instituted when shivering reached grade 4 or when SvO₂ decreased to less than two thirds of its value on arrival in the intensive care unit (ICU). Patients were randomly assigned to receive either morphine sulfate, 5 to 10 mg, or meperidine, 25 to 50 mg intravenously (IV), followed by the other narcotic if the initial drug failed to improve SvO₂ or decrease shivering within ten minutes. The end-point for successful treatment was a return of SvO₂ to within 5% to 10% of its value upon arrival in the ICU or a cessation of shivering that did not recur within 45 minutes. Twenty of the thirty patients shivered sufficiently to decrease SvO₂ by more than one third of its initial value, thus requiring pharmacologic therapy. As shivering increased from a score of 0.8 ± 1.1 to 3.4 ± 0.9, SvO₂ decreased from 74 ± 6% to 57 ± 12%. This corresponded to an increase in oxygen consumption from 158 ± 58 to 324 ± 144 mL/min, an increase of 105%; with a coincident increase in oxygen delivery of only 20%, from 613 ± 130 to 738 ± 230 mL/min. Initial treatment with morphine was not effective either in increasing SvO₂ or decreasing shivering in any case. However, meperidine proved to be effective in 11 of 20 patients, resulting in a return of SvO₂ to within 5% to 10% of values on arrival in the ICU and a significant decrease in shivering. The remaining nine patients required the administration of pancuronium to achieve clinically acceptable values. At the termination of therapy, SvO₂ was 71 ± 1%, shivering score returned to 0.2 ± 0.4, and oxygen consumption decreased to 195 ± 63 mL/min. It is concluded that SvO₂ is an early indicator of increased oxygen consumption and inadequate oxygen delivery, and that meperidine appears to be effective in the treatment of shivering and may be the therapy of choice in the awakening patient.