A patient with resting heart rate over 100 beats per minute (bpm) displayed 1.5-2 mm ST segment depression in her ECG during daily activities. She had unprovoked further increase in her heart rate up to 145 bpm, and during these episodes, her ECG displayed further ST segment depression up to 3 mm and of 0.12 second duration.
An organic cause could not be found to explain her sinus tachycardia. Results of all laboratory investigations, including coronary angiography, were normal.
It was observed that during an episode of reflex vagotonia, when her heart rate was below 95 bpm, her previously depressed ST segments became isoelec tric. With the thought that this patient's ST segment depression was rate depen dent, carotid sinus massage was performed, and when the heart rate slowed to 95 bpm her depressed ST segments became isoelectric. The same response was accomplished with beta blockers.
Sympathetic hyperactivity was thought to be the most likely mechanism of ST segment depression in this patient. A critical increase in heart rate caused these ECG abnormalities.
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