Right ventricular radiofrequency ablation of ventricular tachycardia after myocardial infarction.

Radiofrequency transcatheter ablation of ventricular tachycardia in the setting of a prior myocardial infarction is typically performed with application of energy to the left ventricular endocardium. In this article, two cases are described in which successful radiofrequency transcatheter ablation of ventricular tachycardia occurred with energy delivery to the right ventricular septum after failed ablation attempts from the left ventricle. Both patients had tachycardias with a left bundle branch block morphology and markedly presystolic activity recorded from the right ventricular septum. Right ventricular septal activation mapping during ventricular tachycardia should be performed in patients with left bundle branch block tachycardia morphology and coronary artery disease to maximize efficacy of the catheter ablation procedure.

Contractile properties of single isolated feline ventriculocytes.

Isolated single cardiac myocytes are becoming an increasingly popular experimental preparation. To date, however, the mechanical properties of these isolated adult mammalian cardiac myocyte preparations have not been thoroughly evaluated. The aims of this research were to 1) define the basic contractile properties of externally unloaded single feline ventriculocytes, 2) determine their response to standard inotropic stimuli, and 3) determine whether contractile properties vary as a function of cell size. Cell contractions were elicited by field stimulation (1 Hz) and recorded with the use of a photodiode array technique. In the 121 myocytes studied (2 mM Ca2+; 37 degrees C) the maximum extent of shortening averaged 7.5 +/- 0.2% resting cell length (L). The maximum rates of shortening and relengthening were 92 +/- 2.7 and 103 +/- 2.0% L/s, respectively. Elevating the extracellular Ca2+ and paired pulse stimulation increased the magnitude of twitch contractions as well as the rates of shortening and relengthening. The relationship between cell size and contractile performance was assessed in this research either by comparing the contractile properties of large and small myocytes or by plotting specific contractile parameters as a function of myocyte surface area. The results of this research support the idea that single feline ventricular myocytes retain normal contractile capabilities following isolation and respond to inotropic maneuvers in a similar fashion to that observed in multicellular preparations. In addition the present experiments showed that the contractile properties of the myocytes isolated from normal feline hearts are not cell size dependent.

Comparison of slow response action potentials from normal and hypertrophied myocardium.

Previous studies have shown that papillary muscles from hypertrophied cat right ventricles (RVH) exhibit altered mechanical properties which may be associated with defects in the excitation-contraction coupling process. Since calcium influx [as slow inward current (Isi) during the cardiac action potential is thought to be a major determinant of contractile state, we compared Isi-mediated slow response action potentials ( SRAPs ) in papillary muscles from cats with RVH, induced by chronic pulmonary artery constriction, to SRAPs from sham-operated controls. The results show that 1) when depolarized by elevated extracellular potassium (K+o, 22 mM), RVH muscles became inexcitable (as defined here) significantly faster than control muscles; 2) inexcitable RVH muscles required significantly more isoproterenol than controls to restore slow response activity; 3) at all isoproterenol concentrations tested, SRAPs from RVH muscles were reduced in amplitude and duration compared with controls; 4) SRAPs evoked by long duration stimulus pulses in the absence of isoproterenol were also markedly reduced in RVH; and 5) the relationship between resting potential and K+o was the same in both groups. If the alterations in SRAPs observed in RVH are produced by a smaller Isi, this change may be associated with the diminished inotropic state of cardiac muscle caused to hypertrophy due to pressure overload.

Tension development of normal and hypertrophied-failing papillary muscles following rapid stimulation.

Recent studies have shown that numerous cellular alterations exist in hypertrophied-failing (HF) cardiac muscle. Of particular interest is the finding of an altered ability of the Na-K pump to regulate membrane potential in this tissue during periods of transient stimulation. The present study was designed to determine if this altered Na-K pump function is in any way related to the ability of this tissue to develop force. Along these lines the rate of stimulation (6/min) of normal and hypertrophied-failing right ventricular papillary muscles from cats was increased to 60/min for 90 sec. This procedure was repeated in solutions with low Na+, low Na+ and Ca++, and Ouabain. These solutions were utilized to vary the ionic load on the Na-K pump and the Na-Ca exchanger. The results demonstrate that the pattern of changes in tension in HF papillary muscles seen following periods of rapid stimulation are significantly different from those of normal muscles. The pattern of changes in mechanical performance were found to be similar to the membrane potential changes described in previous studies. In addition, lowering the Na+ load presented to HF muscles returned the characteristic pattern of changes in tension, following drive, toward normal. Ouabain was found to inhibit the changes in tension development following increased rates of stimulation that are thought to be produced by activation of the Na-K pump. The results suggest that the ability of the Na-K pump to maintain normal transmembrane ionic gradients may be altered in HF muscles. This alteration appears to be capable of affecting cellular Ca++ possibly through the Na-Ca exchange system.