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

To examine whether increases in heart rate might be a commontrigger of angina at rest, changes in heart rate, blood pressureand rate-pressure product during pain were compared with theischaemic threshold (heart rate with ST segment shift >=1 mm), determined by atrial pacing, in 272 patients with unstableangina. During an average of 5.9±5.2 episodes of angina,heart rate was comparable to control values (77.0±14.5vs 75.2±11.5, beats. min^–1, ns) and significantlylower than the ischaemic threshold (147.9±22.9, P <0.00001).The rate-pressure product was also lower (955±183 vs2033±369, x 10, P <0.00001). Heart rate during restangina was lower than the ischaemic threshold even when we consideredonly patients with ST depression during pain (n: 71, 81.4±16.0 vs 132.8±21.4, P<0.00001), those with three-vesseldisease (n: 43, 79.9±15.9 vs 136.9±22.0, P <0.00001), or those with a low ischaemic threshold (= <130beats. 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 theresponse was reproducible in 137 cases (89%). Thus, heart rate barely changes during angina at rest in patientswith unstable angina and is consistently much lower than theischaemic threshold. These findings support the concept thatincreases in heart rate are an unlikely trigger of ischaemiaat rest, even in patients with markedly reduced coronary reserve.