T Wave Amplitude as a Quantitative Index of Regional Myocardial Sympathetic Responsiveness

Alterations in T wave morphology have been quantitated in seven open chest anesthetized dogs by simultaneous recording of electrograms from 10 epicardial sites across the anterior left ventricular wall under basal conditions, following left sympathetic stimulation (LSS) at 2, 4, 8, 12, and 16 Hz and during noradrenaline infusions (NAI) of 0.125, 0.25, 0.50, and 1.0 micrograms/kg/min. Overdrive atrial pacing at 175 beats/min was employed and rate of rise of left ventricular pressure (dP/dt) monitored. Linear log dose-response relationships were found between both peak T amplitude and left ventricular dP/dt for NAI between 0.125 and 0.50 micrograms/kg/min (peak T wave amplitude 4.0 +/- 0.9 to 1.4 +/- 0.7 mV). Following LSS, T wave amplitude responses were highly variable both between animals and between electrode sites in individual studies. A linear log dose-response relationship was found at stimulation frequencies between 8 and 16 Hz (T amplitude 3.9 +/- 1.4 to 1.8 +/- 1.2 mV). Changes in QT interval were minor and inconsistent. It is concluded that changes in peak T wave amplitude may provide a useful index of regional myocardial sympathetic responsiveness following NAI, but are more variable following LSS.