Ventricular Stimulation Threshold Research Articles

Cellular electrophysiologic effects of dexamethasone sodium phosphate: implications for cardiac stimulation with steroid-eluting electrodes

Impregnation of implantable cardiac pacemaker electrodes with dexamethasone sodium phosphate (dexamethasone) has been associated with reduced energy requirements for both atrial and ventricular stimulation. To determine whether cardiac cellular electrophysiologic effects of dexamethasone could in part account for lower stimulation thresholds, conventional microelectrode recording and stimulation techniques were used to assess both the immediate (acute) effects of dexamethasone (10 −6 and 10 −4 M) in superfused isolated rabbit right atrial and right ventricular preparations, and chronic effects in rabbit right ventricular tissue following 2 weeks of either daily parenteral dexamethasone (5 mg/kg, plasma concentration approximately 1 to 5 × 10 −5 M) or saline placebo injections. In acute superfusion studies, dexamethasone resulted in a concentration dependent prolongation of spontaneous right atrial cycle length, but did not significantly affect right atrial transmembrane action potential characteristics or refractoriness. However, acute dexamethasone superfusion tended to increase right ventricular resting membrane potential and diminish stimulation threshold. On the other hand, compared to findings in saline-injected control rabbits, chronic dexamethasone injection had little effect on right ventricular stimulation threshold transmembrane action potential characteristics, or right ventricular refractoriness. Thus, the acute direct electrophysiologic effects of high-dose dexamethasone are compatible with the early reduction of cardiac stimulation thresholds associated with dexamethasone impregnated pacing electrodes. On the other hand, electrophysiologic findings in the presence of chronic dexamethasone exposure do not fully account for long-term reduction of stimulation energy requirements.

Acute and long-term atrial and ventricular stimulation thresholds with a steroid-eluting electrode.

Thirty leads with a steroid-eluting electrode (Medtronic 4003 and 4503) have been implanted in 24 patients, 11 in the atrium and 19 in the ventricle. Six patients received the steroid lead in both atrium and ventricle. The stimulation thresholds were followed using Elema pulse generators with Vario function in 15 patients (11 atrial leads and 10 ventricular leads) during 11 +/- 3 months (mean +/- SD). At 0.5 ms pulse duration the mean atrial and ventricular thresholds were 0.55 Volt +/- 0.22 and 0.39 Volt +/- 0.22 respectively at implant, 0.94 Volt +/- 0.13 and 0.82 Volt +/- 0.16 after 12 months. During the entire follow-up period both atrial and ventricular mean thresholds never exceeded 1 V.

A new transvenous lead for both atrial and ventricular pacing.

A new transvenous tined unipolar lead (Medtronic SP 342) has been implanted in 56 patients; 35 leads were positioned in the atrium and 34 in the ventricle. The lead comprises a trifilar DBS-conductor within an insulation of polyurethane, four symetrically placed polyurethane tines furnished with scallops and a platinum-iridium ring-tipped electrode. The scallops in the tines ensure flexibility and a small effective diameter of the lead which makes the choice of vein less critical than with most previous tined leads. It allows use of the same normally sized vein for a second lead for dual-chamber sensing pacing. Mean atrial and ventricular electrogram amplitudes (+/-SD) were 4.6 +/- 1.2 mV and 8.8 +/- 4.4 mV; mean atrial and ventricular stimulation thresholds were 0.8 +/- 0.3 V and 0.4 +/- 0.2 V; and mean atrial and ventricular lead impendances were 415 +/- 114 ohms and 456 +/- 105 ohms, respectively. The results of this study show that the SP 342 lead is useful especially in dual chamber systems.

THE COMPARATIVE ANTIARRHYTHMIC EFFECTS OF MEPIVACAINE AND LIGNOCAINE

SUMMARY A comparison of the antiarrhythmic effects of 2 mg/kg of mepivacaine and lignocaine, as measured by elevations in the end-diastolic ventricular stimulation threshold (EDVST), was made in open-chest dogs. Both mepivacaine and lignocaine increased the EDVST significantly. The peak elevation of EDVST occurred within 2 minutes after injection and averaged 0.013 mA (13 μA) with mepivacaine and 0.022 mA (22 μA) with lignocaine. The greater early response to lignocaine as compared with mepivacaine was significant. After 4 minutes the rate of decreasing effect was approximately 0.001 mA/min for both mepivacaine and lignocaine. The studies provide new evidence demonstrating the antiarrhythmic properties of mepivacaine and support the continued evaluation of related compounds in the search for better antiarrhythmic agents.