A critically ill 80-year-old man had blood pressure waves with a period of about 30 minutes. These blood pressure oscillations, lasting longer than 18 hours, were accompanied by oscillations in heart rate, central venous pressure, pulmonary arterial pressure, and oxygen saturation of mixed venous blood. No discernible variations in external conditions caused the oscillations. Oscillations with such long periods heretofore have not been reported.
Carroll GC. Fourth-Order Blood Pressure Waves. JAMA. 1990;263(6):856-857. doi:10.1001/jama.1990.03440060102040