Shivering following cardiac surgery: predictive factors, consequences, and characteristics

BACKGROUND: Shivering is common after cardiac surgery and may evoke harmful hemodynamic changes. Neither those changes nor factors increasing probability of shivering are well defined. OBJECTIVES: (1) To identify factors linked with risk of shivering by comparing age, weight, body surface area, gender, intraoperative details, anesthetics, postoperative temperatures, hemodynamics, and therapeutics in shivering vs nonshivering patients. (2) To describe temperatures, hemodynamics, therapeutics, myocardial oxygen consumption correlates (rate-pressure product, heart rate, systemic vascular resistance) in shivering and nonshivering groups, and shivering and nonshivering periods. (3) To characterize the electromyogram to determine whether the tremor is cold-induced. METHODS: A descriptive design with a time series component was used to study a convenience sample of 10 shivering and 10 nonshivering adults for 4 hours during early recovery from cardiac surgery. Pulmonary artery and skin (facial, calf, trunk) temperature were measured every 60 seconds; heart rate and arterial pressure, every 15 minutes; cardiac output, 3 times. Electromyogram was recorded intermittently. Medications and treatments were noted. RESULTS: Lower skin temperature was significantly related to shivering risk. Heart rate was significantly higher initially in shiverers and remained higher by 13.6 beats per minute. Significantly more nitroprusside was used to control arterial pressure before than after shivering. No significant differences were noted between groups in core temperature, age, weight, body surface area, anesthesia type, intraoperative temperature; or surgery, circulatory bypass, or cardiac cross-clamp duration. The electromyogram pattern during shivering was typical of that produced by cold. CONCLUSIONS: These results suggest that true shivering occurs after cardiac surgery. Skin, but not core, temperature and elevated heart rate predict shivering. Shivering may be more likely in hemodynamically unstable patients.