The influence of basal heart rate (HR) on the effects of inotropic (dobutamine infusion) and chronotropic (atrial pacing) stimuli during acute myocardial ischemia was assessed by measurement of intramural carbon dioxide partial pressure (PCO2) in open-chest dogs undergoing transient 10-minute left anterior descending coronary artery occlusions. In protocol I, in 5 dogs anesthetized with pentobarbital alone, HR increased from 153 ± 10 to 182 ± 7 beats/min between experimental coronary occlusions, but the increase in ischemic zone intramural carbon dioxide tension (ΔPmCO2) was not altered by this significant increase in HR. In protocols II to V, dogs were anesthetized with combinations of morphine, thiamylal and pentobarbital and had a basal average HR of 81 beats/min. Atrial pacing in protocol II (13 dogs) increased HR from 76 ± 21 to 134 ± 19 beats/min (p < 0.001); left ventricular (LV) myocardial oxygen consumption (MVO2) rose from 3.9 ± 1.6 to 4.9 ± 1.4 ml/min/100 g (p < 0.05), and (ΔPmCO2) rose from 42 ± 14 to 50 ± 15 mm Hg (p < 0.01), indicating more severe ischemic injury during the second experimental coronary occlusion. In protocol III, 11 dogs received 20 μg/kg/min of dobutamine before the second experimental occlusion, which significantly (p < 0.02) increased HR, LV dPdt and MVO2; (ΔPmCO2) increased from 46 ±13 to 63 ± 18 mm Hg (p < 0.01). The 7 dogs in protocol IV received 3.9 ± 1.9 μg/kg/min of dobutamine, titrated such that HR was unchanged (84 ±10 vs 81 ± 15 beats/min). but LV dPdt increased by 92% (p < 0.001). The (ΔPmCO2) was unchanged between experimental occlusions in this group. In protocol V, 9 dogs had atrial pacing before both experimental coronary occlusions (HR 133 ± 7 and 174 ± 7 beats/min, respectively, p < 0.001). The (ΔPmCO2) was similar during both periods of ischemia. In 4 control dogs anesthetized with morphine, thiamylal and pentobarbital, there was no significant change in (ΔPmCO2) during 3 successive coronary occlusions performed without interventions.
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