Ischaemic threshold and haemodynamic changes during angina at rest in patients with unstable angina

To examine whether increases in heart rate might be a common trigger of angina at rest, changes in heart rate, blood pressure and rate-pressure product during pain were compared with the ischaemic threshold (heart rate with ST segment shift > = 1 mm), determined by atrial pacing, in 272 patients with unstable angina. During an average of 5.9 +/- 5.2 episodes of angina, heart rate was comparable to control values (77.0 +/- 14.5 vs 75.2 +/- 11.5, beats.min-1, ns) and significantly lower than the ischaemic threshold (147.9 +/- 22.9, P < 0.00001). The rate-pressure product was also lower (955 +/- 183 vs 2033 +/- 369, x 10, P < 0.00001). Heart rate during rest angina was lower than the ischaemic threshold even when we considered only patients with ST depression during pain (n: 71, 81.4 +/- 16.0 vs 132.8 +/- 21.4, P < 0.00001), those with three-vessel disease (n: 43, 79.9 +/- 15.9 vs 136.9 +/- 22.0, P < 0.00001), or those with a low ischaemic threshold (= < 130 beats.min, n: 78, 77.0 +/- 14.9 vs 118.3 +/- 10.7, P < 0.00001). In 154 patients in whom a second pacing test was performed the response was reproducible in 137 cases (89%). Thus, heart rate barely changes during angina at rest in patients with unstable angina and is consistently much lower than the ischaemic threshold. These findings support the concept that increases in heart rate are an unlikely trigger of ischaemia at rest, even in patients with markedly reduced coronary reserve.