Adaptation of human atrial muscle repolarisation after high rate stimulation

We analysed the effect of a sudden change from a high rate of stimulation to a "physiological" rate upon the repolarisation of human atrial muscle. Microelectrode technique was used to study the effect upon the action potential (AP) recorded from myocardial specimens obtained during open heart surgery in nine patients. Suction electrodes were used to record monophasic action potentials (MAPs) in 12 patients undergoing electroconversion of atrial fibrillation. The abrupt change from 30 min of high-rate stimulation of the atrial specimen to a pacing rate of 60 per min resulted in a successive prolongation of the atrial myocardial AP duration so that 50% of the prolongation was reached after 3 min according to an exponential analysis. A similar prolongation of repolarisation was seen in the MAP recordings after conversion of atrial fibrillation to sinus rhythm and during regular atrial stimulation at a rate of 100 per min. In these recordings, the time needed to reach 50% of the prolongation of the MAP after DC conversion was about 7 min. The findings demonstrate that human atrial muscle undergoes an adaptation of repolarisation after abrupt slowing from a fast stimulation rate. A steady-state level of the AP or MAP duration is reached 10 to 15 min after the change of rate. Together with earlier studies, these experiments indicate, that when right atrial MAP recording is done for assessment of the likelihood of the patient's remaining in sinus rhythm after conversion of atrial fibrillation, the recording must be made within a few minutes of the conversion.