Circadian Variation in Ischemic Threshold in Patients with Stable Angina: Relation to Plasma Endothelin-1

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Abstract
To investigate circadian variation in ischemic threshold in chronic coronary heart disease (CHD) and its relation to plasma endothelin-1 (ET-1), 21 patients with stable angina underwent treadmill exercise tests twice within a day, performed at 8-9 AM for the first test and at 3-4 PM for the second one. Ischemic threshold was defined as the heart rate at the onset of 1 mm ST segment depression during exercise tests. Blood samples were taken at 5 minutes before each exercise test, and plasma ET-1 was measured for determining the possible relation to ischemic threshold in patients with CHD. The results showed that the heart rate-ischemic threshold in individual patients varied by 10 ± 1% (range, 2-15%) in the morning and 9 ± 1% (range, 2-14%) in the afternoon, while there was a mean (11.2%) reduction in the ischemic threshold between 2 time points, with the ischemic threshold being significantly lower in the morning compared with that in the afternoon (115 ±22 bpm vs 128 ±31 bpm p<0.04). ET-1 values were 6.20 ±2.44 ng/L in the morning hours and 4.02 ±1.61 ng/L in the afternoon hours, with a statistical significant difference (p<0.01). In conclusion, the present study indicated that circadian variation of plasma levels of ET-1 was likely to be one of the most likely mech anisms involved in reduction in the ischemic threshold in the morning hours.