Biomedical subjects

R L Mitra

Publications and source records attributed to R L Mitra.

8 recordsLinked to original sources

PubMed1997

Ventricular output failure in a DDD permanent pacemaker associated with increased atrial output.

Previous reports have described the occurrence of ventricular output failure in a permanent DDD pacemaker system related to an increase in the atrial output in the presence of low atrial lead impedance (Medtronic Synergyst/Synergyst II). This phenomenon is seen exclusively following atrial paced events, and may potentially lead to significant bradyarrhythmia or ventricular asystole in a pacemaker dependent patient. We describe the occurrence of analogous behavior in a Medtronic Symbios 7006 generator.

Aged↗

PubMed1997

Ventricular output failure in a DDD permanent pacemaker associated with increased atrial output.

Previous reports have described the occurrence of ventricular output failure in a permanent DDD pacemaker system related to an increase in the atrial output in the presence of low atrial lead impedance (Medtronic Synergyst/Synergyst II). This phenomenon is seen exclusively following atrial paced events and may potentially lead to significant bradyarrhythmia or ventricular asystole in a pacemaker dependent patient. We describe the occurrence of analogous behavior in a Medtronic Symbios 7006 generator.

Aged↗

PubMed1995

Characterization of return cycle responses predictive of successful pacing-mediated termination of ventricular tachycardia.

OBJECTIVES: The purpose of this study was to characterize response patterns during overdrive pacing that predict successful termination of ventricular tachycardia. BACKGROUND: Overdrive pacing during ventricular tachycardia typically results in entrainment at slow pacing rates and in termination or acceleration at faster rates. The factors that determine the critical paced cycle length that results in tachycardia termination have not been extensively studied. METHODS: Ventricular tachycardias in 14 patients with coronary artery disease were studied with overdrive pacing at several cycle lengths. Return cycles were measured after each additional paced beat at each paced cycle length. The return cycle responses during pacing trials that resulted in tachycardia termination and those that resulted in entrainment were compared. RESULTS: Three return cycle responses were identified: flat, plateau and increasing. Twenty trials of overdrive pacing resulted in tachycardia termination; all were characterized by an increase in the return cycle with the delivery of each successive beat in the pacing drive until the tachycardia terminated (increasing response). Thirty-four pacing trials resulted in entrainment and not termination; these were characterized either by a constant return cycle (flat response) or an initial increase in return cycle followed by a longer, constant return cycle (plateau response) with the delivery of additional paced beats. The longest paced cycle length that resulted in tachycardia termination correlated with the relative refractory period of the circuit, defined as the tachycardia cycle length minus the fully excitable gap (r2 = 0.764, p = 0.0001). Tachycardia termination was not observed unless the paced cycle length was shorter than the relative refractory period of the circuit. CONCLUSIONS: The critical paced cycle length that causes termination of ventricular tachycardia depends on the relative refractory period of the circuit because this factor determines whether the nth + 1 beat of the pacing drive will encounter partially recovered tissue. These data provide insights into the mechanism of pacing-mediated tachycardia termination and entrainment and are applicable to the development of improved antitachycardia pacing algorithms.

Aged↗

PubMed1995

Efficacy of antitachycardia pacing in patients presenting with cardiac arrest.

The efficacy of antitachycardia pacing (ATP) incorporated into implantable cardioverter defibrillators (ICDs) was assessed in 29 consecutive survivors of cardiac arrest, not attributable to acute myocardial infarction, ischemia, or drug and electrolyte effects. The cohort included 25 men and 4 women with a mean age of 65 years and a mean left ventricular ejection fraction of 29%. Seventeen patients had coronary artery disease, 11 had nonischemic dilated cardiomyopathy, and 1 had long QT syndrome. Programmed stimulation yielded monomorphic ventricular tachycardia (VT) in 17 patients, polymorphic VT in 6, and no inducible VT in 6. During a mean follow-up of 22 months, a total of 91 episodes of monomorphic VT occurred, 73 of which were successfully pace terminated (83%). Monomorphic VT amenable to pace termination recurred only in the group that had this arrhythmia inducible. The recurrent arrhythmias in the 12 patients having either no inducible VT or polymorphic VT were all rapid VTs, having a cycle length < 220 ms; and therefore, not amenable to pace termination. These results suggest that ATP incorporated into ICDs is useful in survivors of cardiac arrest and may significantly reduce the number of shocks that these patients would otherwise receive. Programmed stimulation may also help to define those patients who would receive the maximum benefit from ATP.

Adult↗

PubMed1994

Early postoperative increase in defibrillation threshold with nonthoracotomy system in humans.

The stability of the defibrillation threshold (DFT) early after implantation of an implantable cardioverter defibrillator was evaluated in 15 patients. All but one patient had a three lead nonthoracotomy system using a subcutaneous patch, a right ventricular endocardial lead, and a lead in coronary sinus (n = 5) or superior vena cava (n = 9). Shocks were delivered using simultaneous in nine, sequential in three, and single pathway (coronary sinus not used) in one patient. DFTs were measured at implant (n = 15), 2-8 days postoperation (postop, n = 15), and 4-6 weeks later (n = 8). The DFT was defined as the lowest energy shock that resulted in successful defibrillation. The DFT was assessed with output beginning at 18 joules or 2-4 joules above the implant DFT. All shocks were delivered in 2- to 4-joule increments or decrements. DFTs were significantly higher postoperatively than DFTs at implant (22.7 +/- 7.0 J vs 16.9 +/- 3.9 J; P < 0.05). Eight of 15 patients had DFT determined at all three study periods. In these patients, DFT increased at postop (22.8 +/- 8.3 J vs 16.4 +/- 3.9 J at implant; P < 0.05) and returned to baseline at 4-6 weeks (16 +/- 7.1 J vs 16.4 +/- 3.9 J at implant; P = N.S.). Thus, in patients with a multilead nonthoracotomy system, a DFT rise was observed early after implant. The DFT appears to return to baseline in 4-6 weeks. These results have important implications for programming energy output after implantable cardioverter defibrillator implantation.

Adult↗

PubMed1993

Unique sensing errors in third-generation implantable cardioverter-defibrillators.

OBJECTIVES: Third-generation cardioverter-defibrillators appear to be susceptible to unique sensing errors. This study was performed to determine the incidence and types of sensing errors in combination therapy implantable devices. BACKGROUND: One of the advantages offered by third-generation implantable cardioverter-defibrillators is the combination of bradycardia and antitachycardia pacing and cardioversion-defibrillation capabilities in a single device. The potential for unique sensing errors, those caused by the conflicts presented by combining bradycardia and tachycardia sensing and therapy algorithms in the same device, has not been previously addressed. METHODS: To determine the incidence of important sensing errors, 61 patients with a combination therapy device (Cadence [Ventritex] and PCD [Medtronic]) were studied for a 25-month period. In addition to surface electrocardiographic recordings during implantation and routine device testing, real-time and stored electrograms recorded from the rate-sensing leads (Cadence) and real-time marker channel recordings (PCD) were reviewed to diagnose sensing errors that resulted in symptoms, device inefficacy or delivery of inappropriate therapy. After recognition, specific reprogramming steps were performed in an attempt to avoid recurrent sensing errors. RESULTS: A total of 13 sensing errors were diagnosed in 12 patients (19.7%); the incidence was similar in both devices. Five distinct categories of sensing errors were identified. After device reprogramming, only one recurrent error occurred in 98 patient-months of follow-up. CONCLUSIONS: Important sensing errors occur in approximately 20% of patients with third-generation combination therapy cardioverter-defibrillators. Prompt diagnosis of sensing errors can lead to specific reprogramming steps to avoid recurrent errors.

Algorithms↗

PubMed1993

The clinical significance of nonsustained ventricular tachycardia.

Nonsustained ventricular tachycardia (NSVT) is an arrhythmia not often associated with symptoms; however, its occurrence in patients with structural heart disease is a prognostic indicator of an increased risk of mortality and sudden death. The management of asymptomatic patients with NSVT should first attempt to identify which patients are at highest risk for cardiac arrest, and second, devise a treatment that can reduce the incidence and/or mortality of cardiac arrest in this group. In patients with chronic coronary artery disease (CAD) and NSVT, programmed electrical stimulation identifies both a low and high risk group with respect to occurrence of ventricular arrhythmias. The negative predictive value of programmed electrical stimulation in patients with CAD and NSVT has been well established; however, uncertainty remains as to the optimal therapy for CAD patients with inducible ventricular arrhythmias. A number of reports suggest that patients whose inducible ventricular arrhythmias are rendered noninducible with antiarrhythmic drugs have a much lower risk of sudden death. It is yet to be resolved whether arrhythmias rendered noninducible identify a subgroup at low risk for cardiac arrest, independent of treatment. There is some evidence to suggest that the frequency of NSVT in patients with nonischemic dilated cardiomyopathy identifies a group at higher risk of sudden death. Programmed electrical stimulation adds little in helping to identify which of these patients are most likely to have cardiac arrest. The presence of NSVT in asymptomatic patients with hypertrophic cardiomyopathy may identify a group at higher risk for cardiac arrest.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiomyopathy, Dilated↗

PubMed1991

Permeance of Cs+ and Rb+ through the inwardly rectifying K+ channel in guinea pig ventricular myocytes.

Inward currents carried by external Cs, Rb, NH4 and K through the IK1 channel were studied using a whole-cell voltage clamp technique. Cs, NH4, and Rb currents could be recorded negative to -40 mV following depolarizing prepulses (greater than or equal to 0 mV and 200-1000 msec in duration). The current activation displayed an instantaneous component followed by a monoexponential increase (tau a) to a peak amplitude. Subsequent inactivation was fit by a single exponential, tau ia. With hyperpolarization, tau a and tau ia decreased e-fold per 36 and 25 mV, respectively. In Ca-free external solutions (pipette [Mg] approximately 0.3 mM), inactivation was absent, consistent with the hypothesis that inactivation represents time- and voltage-dependent block of Cs, NH4, and Rb currents by external Ca. The inactivation and degree of steady-state block was greatest when Cs was the charge carrier, followed by NH4, and then Rb. K currents, however, did not inactivate in the presence of Ca. Na and Li did not carry any significant current within the resolution of our recordings. Comparison of peak inward current ratios (Ix/IK) as an index of permeability revealed a higher permeance of Cs (0.15), NH4 (0.30), and Rb (0.51) relative to K (1.0) than that obtained by comparing the steady-state current ratios (Cs:NH4:Rb:K approximately 0.01:0.06:0.21:1.0). At any given potential, tau a was smaller the more permeant the cation. In the absence of depolarizing prepulses, the amplitude of tau a was reduced. Divalent-free solutions did not significantly affect activation in the presence of 0.3 mM pipette [Mg]. When pipette [Mg] was buffered to approximately 50 microM, however, removal of external Ca and Mg lead to a four- to fivefold increase in Cs currents and loss of both time-dependent activation and inactivation (reversible upon repletion of external Ca). These results suggest that (i) permeability ratios for IK1 should account for differences in the degree to which monovalent currents are blocked by extracellular Ca and (ii) extracellular or intracellular divalent cations contribute to the slow phase of activation which may represent either (a) the actual rate of Mg or Ca extrusion from the channel into the cell, a process which may be enhanced by repulsive interaction with the incoming permeant monovalent cation or (b) an intrinsic gating process that is strongly modulated by the permeant monovalent ion and divalent cations.

Ammonia↗