Stimulation of unidirectional pulses in excitable systems.

Using a judicious spatial shape of input current pulses (and electrodes), responses of an excitable system (FitzHugh-Nagumo) appear as unidirectional pulses (UDP's) instead of bidirectional ones (in one dimension) or circular ones (in two dimensions). The importance of the UDP's for a possible mechanism for pinpointing the reentry cycle position and for a possible use in tachycardia suppression is discussed.

Cardiac output-based versus empirically programmed AV interval--how different are they?

AIMS: To compare empirically programmed and cardiac output-based programming of atrioventricular (AV) interval in patients with dual chamber pacemakers. METHODS AND RESULTS: In 19 patients with implanted dual chamber pacemakers due to AV block but otherwise normal hearts, cardiac output was assessed using an impedance cardiography device. In all patients, the AV interval had been previously programmed empirically by an experienced cardiologist. Cardiac output was estimated at AV intervals from 50 to 250 ms during VDD pacing. AV intervals adjusted by serial cardiac output estimations caused a rise in cardiac output in 84% of patients. The maximal achievable cardiac output was greater by 12% +/- 8% (range 0-32%), P < 0.001, than was observed with empirically programmed AV intervals. CONCLUSIONS: In patients with dual chamber pacemakers due to AV block and otherwise normal hearts, empirically selected AV intervals may lead to compromise of cardiac haemodynamics. Optimal AV intervals may be selected by serial cardiac output measurements.

Single pass lead VDD pacing in children and adolescents.

Implantation of permanent pacemakers in children and adolescents is possible but usually is limited to single chamber generators. The natural growth of these patients may require physiological pacing, but until recently two leads were required for this type of pacing. The single pass lead VDD pacing mode makes possible physiological pacing by using only one lead, for both atrial sensing and ventricular sensing and pacing. The feasibility of VDD pacing using endocardial lead was evaluated in 16 children and adolescents with congenital or postsurgical atrioventricular block. Their mean age was 7.9 +/- 4.9 years (range 1-16 years) and the smallest patient's weight was 8.2 kg. In all the patients, a single pass pacing lead with atrial sensing rings and bipolar ventricular pacing and sensing capability was implanted through the left or right subclavian vein. The pacemaker generator was implanted in a rectopectoral position. The mean atrial electrogram during the implantation was 4.2 +/- 2.1 mV and 2.6 +/- 1.9 mV after a mean of 1 week. The ventricular pacing threshold was 0.5 +/- 0.2 V; the ventricular pacing impedance was 560 +/- 95 omega; and the ventricular electrogram amplitude was 9.9 +/- 2.1 mV. This is a first report to demonstrate the feasibility of atrial synchronous ventricular endocardial pacing using a single pass lead in a relatively large group of children and adolescents.

Selecionando o marcapasso ideal para cada paciente: há necessidade de um ensaio clínico em grande escala para a escolha do modo de estimulaçäo ?

Nas últimas três décadas houve grandes avanços na área de estimulaçäo cardíaca artificial. O implante de marcapasso cardíaco é hoje um procedimento clínico comum, com uma taxa anual de implante primário de 218 por milhäo no Reino Unido, 238 por milhäo em Israel, 410 por milhäo nos Estados Unidos, 522 por milhäo na Alemanha e 600 por milhäo na Bélgica. Cerca de 90 por cento dos pacientes com marcapassos implantados têm o rtmo sinusal preservado, sugerindo um amplo uso de marcapassos que preservam o ncronismo atrioventricular (AV) (AAI/R, DDD/R e VDD). Além disso, notou-se que a preservaçäo da sequência normal da contraçäo cardíaca associa-se a uma menor incidência de fibrilaçäo atrial (FA) e consequentemente, menor incidência de tromboembolismo e de acidente vascular cerebral (AVC), com a preservaçäo da contratilidade miocárdica e com a reduçäo da insuficiência cardíaca congestiva (ICC) e da mortalidade. Portanto, näo é surpresa que a utilizaçäo desses marcapassos (principalmente DDD e DDDR) tenhacescido de maneira significativa nos Estados Unidos na última década, representadno aproximadamente 9 por cento, 32 por cento, 45 por cento e 55 por cento do implantes nos anos de 1981, 1989, 1991 e 1993, respectivamente. A utilizaçäo mais frequente desses marcapassos sofisticados e caros levanta a seguinte questäo: o custo adicional associado ao implante de um marcapasso bicameral (DDD/R e VDD) é compatível com benefício hemodinâmico e com a melhora na qualidade de vida que esse tipo de marcapasso proprcona ? ...

Clinical evaluation of a new single pass lead VDD pacing system.

UNLABELLED: Twenty-five patients with second- to third-degree AV block and normal sinus function (16 males, mean age 60 +/- 18; range 15-78 years) underwent implantation of VVD pacemakers (THERA VDD, Medtronic, Inc.) with a single pass (SP) lead. RESULTS: During implantation the mean amplitude of the atrial (A) signal was 3.9 +/- 1.4 mV (range 2.0-7.8 mV). Stable, acceptable A-signals during implantation were usually observed in the mid- or lower part of the right atrium. The lead tip electrical parameters were not compromised in any patient in order to obtain an acceptable A-signal. To verify VDD device function, patients underwent pacing system analysis on the second day and again 1, 3, and 6 months after implantation. Acute and chronic electrical measurements in the ventricle were similar to those with regular steroid leads. During follow-up tests, stable atrial sensing (A > or = 0.7 mV) was found in all but one patient (in whom A was 0.25-0.5 mV and an intermittent loss of atrial sensing occurred). There was no difference between serial measurements of A-signal amplitudes on the second day or 1, 3, and 6 months after implantation: 1.9 +/- 1.3 mV, 1.5 +/- 0.6 mV, 1.3 +/- 0.8 mV, and 1.5 +/- 1.1 mV, respectively. The mean implantation time was 54.0 +/- 17 minutes and the mean fluoroscopy time was 3.2 +/- 1.3 minutes. CONCLUSIONS: SP lead VDD pacing is reliable and easy to manage with dependable atrial sensing and ventricular pacing.(ABSTRACT TRUNCATED AT 250 WORDS)

Initial experience with a new algorithm for automatic mode switching from DDDR to DDIR mode.

UNLABELLED: Implantation of dual chamber devices in patients with paroxysmal atrial tachyarrhythmias who require permanent pacemakers may lead to significant complications due to an inappropriately triggered ventricular response. VVI/VVIR units cause loss of AV synchrony in the presence of sinus activity. A new DDDR device (THERA DR, model 7940), with an automatic mode switching (AMS) algorithm, was evaluated. When the mean atrial rate is > 182 beats/min, atrial tachyarrhythmia is detected, and AMS is activated. Twenty-three patients (12 males, mean age 71 +/- 7 years) underwent implantation of a THERA DDDR device with the AMS algorithm. Seventeen patients had AV block and/or sick sinus syndrome (SSS) and atrial arrhythmias, and 6 patients (2 with hypertrophic obstructive cardiomyopathy) had SSS and paroxysmal atrial fibrillation (PAF). The follow-up period was from 1-9 months. During follow-up, Holter monitoring and treadmill tests were performed. RESULTS: Eighty-seven episodes of AMS were recorded. Telemetered AMS recordings demonstrated episodes in which the DDDR mode switched to the DDIR mode in the presence of PAF, and reverted to DDDR when sinus rhythm returned. Paroxysmal supraventricular arrhythmias with a heart rate < 182 beats/min did not activate the mode switch. CONCLUSIONS: This early, short-term clinical experience with a DDDR device capable of AMS from DDDR to DDIR demonstrated appropriate clinical function and response to PAF. These preliminary results suggest that DDDR pacemakers with AMS to DDIR may significantly extend the current indications for dual chamber pacing.

Assessment of hemodynamic changes in the early phase of uncomplicated acute myocardial infarction.

The purpose of our study was to evaluate the early phase hemodynamic changes in patients with clinically uncomplicated acute myocardial infarction. Detailed sequential hemodynamic evaluation is hardly available in the early phase of acute myocardial infarction in this group of patients as opposed to patients with complicated acute myocardial infarction. We evaluated sequentially the hemodynamics of 17 patients with uncomplicated acute myocardial infarction (Kilip class I). Cardiac output and ventricular ejection period were determined by impedance cardiography. All patients had an uneventful recovery, without clinical evidence of derangement of myocardial function as assessed by physical examination, chest X-ray and blood gases. Despite the uniformly uncomplicated convalescence, different hemodynamic patterns were found. Based on the hemodynamic data, patients were divided into two groups. In 10 patients no significant change in hemodynamic parameters was observed, while in 7 patients a significant decrease in cardiac output was found during the early post myocardial infarction period. Total peripheral resistance was significantly elevated in the group with decreased cardiac output. No relationship was found among location of infarction, creatine phosphokinase levels and hemodynamic outcome. A considerable proportion of patients with asymptomatic and uneventful convalescence after acute myocardial infarction have a decrease in cardiac output and a significant increase in total peripheral resistance which is not detected by routine clinical evaluation.