Subthreshold electrotonic depolarizations have been shown to exert inhibitory actions on impulse conduction and generation in isolated cardiac tissues. The authors performed this study to determine whether inhibition occurs in human myocardium, and to investigate the effects of time and voltage, as well as distance, on myocardial inhibition. Sixteen subjects were studied in the clinical electrophysiology laboratory by standard techniques. Atrial and ventricular pacing were performed with the use of a quadripolar catheter. The basic drive train (S1) and premature stimulus (S2) were introduced at the distal bipolar electrode pair through one current output generator and the subthreshold conditioning stimulus S(c)] was introduced before S2 at the distal or proximal bipolar pair through a separate current output generator. When S(c) was initiated at the distal electrodes 40 msec before S2, inhibition of S2 could always be demonstrated (atrium or ventricle). Since S(c) was introduced progressively earlier than S2, S(c) inhibited the response to S2 according to a curvilinear strength-interval relationship; increasing milliamperes of S(c) from less than 1.0 to 10.0 increased the interval at which S(c) preceded S2 and still inhibited S2. With currents of S(c) of 10.0 mA or less, S(c) inhibited S2 in the ventricle (n = 11) and atrium (n = 5) when S(c) preceded S2 by 40 to 160 msec (mean 85 msec) and 80 to 190 msec (mean 116 msec), respectively. Ventricular inhibition attempted with S(c) at the proximal bipolar pair and S2 at the distal pair was successful in three of nine patients. The effect of S(c) on ventricular excitability threshold of S2 was determined in three patients. For all three patients the current threshold of S2 varied directly as a function of the magnitude of current used for S(c). These data demonstrate that (1) subthreshold stimuli can prevent subsequent threshold stimuli from depolarizing human atrium and ventricle, (2) inhibition is both time and voltage dependent, and (3) inhibition is more effective if the inhibitory stimulus is applied close to the site of the threshold stimulus. Inhibition most likely occurs by S(c) electronically affecting the response of the tissue to S2, possibly in part by modifying myocardial excitability threshold, thereby preventing S2 from initiating an active response.
ASJC Scopus subject areas
- Cardiology and Cardiovascular Medicine
- Physiology (medical)