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Surgery is a recent approach in the treatment of arrhythmias. The first successful surgery of this type was the interruption of the accessory pathway in a patient suffering from the Wolff Parkinson White syndrome. In 25 consecutive patients who underwent surgery, 28 accessory AV pathways were identified with classical endocardial and epicardial mapping. Only three patients experienced recurrence of preexcitation without symptoms; one patient had AV block; and another patient died. In all cases we successfully used either an endocardial or an epicardial approach. The purpose of this paper is to report the Cuban experience with this procedure.

The influence of moderate hypoxia (p02 : 70 mmHg) on action potential and contraction of isolated rabbit papillary muscles was analyzed. 1. Action potential duration, maximum twitch tension and time-to-peak tension decreased during 60 min of hypoxic perfusion, while resting tension increased and resting potential, overshoot and fast depolarization of the action potential remained constant. 2. MnCl2 (5 mM), after 60 min of hypoxia, abolished overshoot increased resting potential, and further reduced action potential duration. "Slow"action potentials induced by isoproterenol (2.5 X 10(-6) M) in KCl (20 mM) depolarized muscles, were highly resistant to hypoxic perfusion. This resistance indicates that the inflow of cations through slow channels persists under these hypoxic conditions. 3. CsCl (2-4 mM) increased action potential duration in hypoxic fibers to control (O2) values. These results suggest that an increase in membrane background K conductance could account for the reduction in action potential duration under hypoxic incubation. A possible relationship between changes in internal free calcium concentration and background K conductance during hypoxia is considered.

The influence of 1 and 5 mg/l of droperidol on rabbit sinus node intracellular recordings was evaluated. At 1 mg/l the drug merely prolonged the spontaneous basic cycle by reducing phase 4 slope of the action potential. At the higher concentration, droperidol induced a more pronounced effect of these variables and after 20 min perfusion, it also decreased the upstroke velocity and the threshold potential. Further exposure continued to reduce spontaneous rate, until irregular failure of impulse initiation and subthreshold oscillations appeared. After 45 min exposure to droperidol, only subthreshold oscillations could be recorded. Isoproterenol (0.2 mg/l) rapidly restored normal action potentials, while acetylcholine (0.5 mg/l) caused an immediate arrest leaving a stable level of membrane potential. The activity of droperidol in 1 mg/l concentration could be dependent on a partial blockade of the sodium leakage current, while at higher concentrations of 5 mg/l the drug seems to provoke a total blockade of Na--Ca slow channels. Subthreshold oscillations could be explained by changes in K conductance together with a remanent of the sodium leakage current.

Experiments in isolated left atria from rat hearts were performed in order to study the effects of stimulation rate on the transmembrane action potential. 1. Two components (fast and slow) of the action potential upstroke could be differentiated by adding MnCl2 to the perfusion solution. 2. With the increase in rate of stimulation over the control cycle length (500 msec), amplitude, Vmax and action potential duration at 80% of repolarization (D80) diminished in normal Krebs. In Mn-containing Krebs, only a slight reduction in amplitude was recorded. Resting potential and action potential duration at 20% (D20) and 50% (D50) of repolarization were only slightly affected in normal Krebs and not at all in Mn-Krebs. 3. Low rates of stimulation in normal Krebs increased D50 only slightly; however, D80 increased significantly while other parameters remained constant. No effects were seen in Mn-Krebs. 4. The results with Mn-Krebs indicate the importance of slow inward current on changes induced by stimulation rates. A possible mechanism relating intracellular calcium concentration and the outward K current, depending on the rate of stimulation, is discussed.