Is Dual Site Better Than Single Site Atrial Pacing in the Prevention of Atrial Fibrillation?

Long-term prevention of atrial fibrillation is not constantly realized by single-site right atrial pacing, and the beneficial role of multisite atrial pacing is still being studied. Accordingly, we compared the effectiveness of dual site and single site atrial pacing in 83 patients (50 men, 33 women, aged 69 +/- 10 years), who received a DDD device for primary sinus node dysfunction or bradycardia with documented atrial fibrillation. Inclusion criteria for dual site pacing were a sinus P wave > or = 120 ms and at least two episodes of documented paroxysmal AF in the 6 months preceding implantation. Dual site atrial pacing (high right atrium-coronary sinus ostium) was performed in 30 cases, and was compared to 53 single site paced patients, 21 with a P wave > or = 120 ms and 32 with a P wave < 120 ms. The basic pacing rate was programmed at 68 +/- 4 beats/min (range 60-75 beats/min). Sinus P wave (133 +/- 20 vs 95 +/- 9 ms; P < 0.001), paced P wave (107 +/- 14 vs 99 +/- 15; P < 0.05), number of antiarrhythmic drugs used (2.4 +/- 1.2 vs 1.6 +/- 1.5, P < 0.05), and the duration of symptoms (8.1 +/- 4.5 vs 3.8 +/- 2.4 years; P < 0.001) were significantly higher in dual site patients. The other characteristics were similar. During the follow-up of 18 +/- 15 months (range 3-30 months), paroxysmal AF was documented in 33 patients. Among these patients, 13 developed permanent AF following at least one episode of paroxysmal AF. When comparing dual site patients and single site patients with a P wave duration > or = 120 ms, paroxysmal AF incidence was lower in the dual site group (9/30 patients vs 15/21 patients, P < 0.01), as well as permanent AF (1/30 patients vs 8/21 patients, P < 0.01). By contrast, comparison between dual site patients and the group of single site patients with a P wave duration < 120 ms did not evidence any significant differences in paroxysmal (9/30 patients vs 9/32 patients) and permanent (1/30 patients vs 4/32 patients) AF incidences. Dual site seems better able than single site atrial pacing to improve the natural history of patients with a prolonged P wave, reducing the incidence of paroxysmal and permanent AF. No benefit could be expected in patients with a normal P wave duration.     

Hemodynamic effect of rapid atrial pacing in fetal lambs

To determine the threshold at which rapid atrial pacing brings on fetal circulatory failure, we made a fetal supraventricular tachyarrhythmia model and measured the central venous pressure, aortic pressure, and right and left ventricular outputs in five fetal lambs. Under maternal anesthesia, the uterus was opened, and under local anesthesia, polyvinyl catheters were inserted into the fetal superior vena cava and ascending aorta through a neck incision. Pacing leads (Medtronic model 6492) were then sutured onto the fetal right atrial appendage via right thoracotomy. Ventricular output was estimated using a Toshiba SSH-65A echocardiography by a transuterine approach. Fetal hemodynamics were observed without pacing (control), and at the atrial pacing rates of 200, 300, 350, and 400/min. Central venous pressure (CVP) increased and the aortic pressure decreased when the right atrium was paced at 350/min or more. Right ventricular output decreased when the right atrium was paced at 300/min or more. The left ventricular output, however, remained constant. The right ventricular output was 382 +/- 106 mL/kg/min at control, and 391 +/- 117 mL/kg/min when paced at 200/min, but decreased to 210 +/- 138 mL/kg/min when paced at 300/min, to 223 +/- 102 mL/kg/min when paced at 350/min, and to 186 +/- 86 mL/kg/min when paced at 400/min. Fetal circulatory failure occurred when the right atrium was paced at 300/min or more.     

Is Automatic Mode Switching Effective for Atrial Arrhythmias Occurring at Different Rates? A Study of the Efficacy of Automatic Mode and Rate Switching to Simulated Atrial Arrhythmias by Chest Wall Stimulation

Automatic mode switching (AMS) is a useful means to avoid rapid ventricular response during atrial fibrillation (AF), but AMS cannot occur if the detected atrial rate during AF is below the mode switching criteria. This may be the result of antiarrhythmic medications, or when the atrial events fall within the atrial blanking period, or if the atrial amplitudes during AF are too small to be sensed. We hypothesize that the addition of an automatic rate switching (ARS) algorithm may complement AMS response during AF with different detected atrial rates. We studied the Marathon DDDR pacemaker (Model 294-09, Intermedics Inc.) with the AMS and ARS algorithms that are independently programmable but can also operate in combination. AF sensed above the AMS rate (160 beats/min) will lead to VDIR pacing, whereas AF below AMS rate will be tracked at an interim rate as dictate by the ARS, at a ventricular response that is 20 beats/min above the sensor indicated rate. Atrial tachyarrhythmias were simulated by chest wall stimulation (CWS). CWS was applied to 33 patients (16 men, 17 women, mean age 69 +/- 11 years) with a Marathon DDDR pacemaker using an external pacer to simulate AF occurring at two rate levels: above the AMS rate (programmed at 160 beats/min) at 180 beats/min and below the AMS rate at 120 beats/min. The maximum, minimum, and mean ventricular rates during CWS in DDDR mode with AMS alone, ARS alone, and their combination were compared. During CWS at 120 beats/min, the AMS plus ARS setting showed a mean ventricular rate of 79 +/- 3 beats/min and 124 +/- 14 beats/min in the AMS setting alone (P < 0.01). With CWS at 180 beats/min, the mean ventricular rate in the AMS plus ARS setting compared to the AMS setting alone was not significantly different. However, the variation in ventricular pacing rate was 7 +/- 14 beats/min in the AMS plus ARS setting and 40 +/- 42 beats/min in the AMS setting (P < 0.05). In conclusion, AMS is effective for simulated atrial tachyarrhythmias sensed above the AMS rate. Combined AMS with ARS is useful to handle simulated atrial tachyarrhythmia at a slower rate and to avoid rate fluctuation during AMS. There is also a possibility that this can be applied to the naturally occurring atrial tachyarrhythmias.     

Left atrial tachyarrhythmia in fetal lambs

To evaluate the hemodynamic effect of rapid left atrial pacing on fetal circulation, a fetal supraventricular tachyarrhythmia model was made and the aortic pressure, central venous pressure, and left and right ventricular outputs were measured in ten fetal lambs. Under maternal anesthesia, the uterus was opened, and under local anesthesia, catheters were inserted into thefetal superior vena cava and ascending aorta through a neck incision. Pacing leads were then sutured onto the fetal left atrial appendage via left thoracotomy. Ventricular output was estimated using echocardiography by a transuterine approach. Fetal hemodynamics were observed without pacing (control), and at the pacing rates of 200, 300, 350, and 400/min. The aortic pressure decreased when the left atrium was paced at 300/min or more and the central venous pressure increased when the left atrium was paced at 350/min or more. The left and right ventricular outputs decreased when the left atrium was paced at 350/min or more. The left ventricular output was 215+/-54 mL/kg per minute at control, 205+/-60 mL/kg minute when paced at 200/min, 178+/-58 mL/kg per minute when paced at 300/min, but decreased to 164+/-44 mL/kg per minute when paced at 350/min and to 149+/-57 mL/kg per minute when paced at 400/min. The right ventricular output was 338+/-66 mL/kg per minute at control, 336+/-95 mL/kg per minute when paced at 200/min, 273+/-91 mL/kg per minute when paced at 300/min, but decreased to 256+/-80 mL/kg per minute when paced at 350/min and to 202+/-76 mL/kg per minute when paced at 400/min. Fetal circulatory failure was initially confirmed when the left atrium was paced at 300/min in this left atrial tachyarrhythmia model.     

European clinical experience with a dual chamber single pass sensing and pacing defibrillation lead

Dual chamber ICDs are increasingly implanted nowadays, mainly to improve discrimination between supraventricular and ventricular arrhythmias but also to maintain AV synchrony in patients with bradycardia. The aim of this study was to investigate a new single pass right ventricular defibrillation lead capable of true bipolar sensing and pacing in the right atrium and integrated bipolar sensing and pacing in the right ventricle. The performance of the lead was evaluated in 57 patients (age 61 +/- 12 years; New York Heart Association 1.9 +/- 0.6, left ventricular ejection fraction 0.38 +/- 0.15) at implant, at prehospital discharge, and during a 1-year follow-up. Sensing and pacing behavior of the lead was evaluated in six different body positions. In four patients, no stable position of the atrial electrode could intraoperatively be found. The intraoperative atrial sensing was 2.3 +/- 1.6 mV and the atrial pacing threshold 0.8 +/- 0.5 V at 0.5 ms. At follow-up, the atrial sensing ranged from 1.5 mV to 2.2 mV and the atrial pacing threshold product from 0.8 to 1.7 V/ms. In 11 patients, an intermittent atrial sensing problem and in 24 patients an atrial pacing dysfunction were observed in at least one body position. In 565 episodes, a sensitivity of 100% and a specificity of 96.5% were found for ventricular arrhythmias. In conclusion, this single pass defibrillation lead performed well as a VDD lead and for dual chamber arrhythmia discrimination. However, loss of atrial capture in 45% of patients preclude its use in patients depending on atrial pacing.     

Effects of An Acute Increase in Atrial Pressure on Atrial Refractoriness in Humans

Contraction-excitation feedback has been studied extensively in mammalian ventricles. In contrast, little is known about contraction-excitation feedback in mammalian atria. The objective of this study was to investigate the effect of acute alterations in atrial pressure, induced by varying the atrioventricular (AV) interval, on atrial refractoriness. Twenty patients without structural heart disease participated in the study. In each patient the atrial effective (ERP) and absolute refractory periods (ARP) were measured during AV pacing at a cycle length of 500 msec and an AV interval of 120 msec. Acute increases in atrial pressure were induced by pacing the atrium and ventricle simultaneously for the final two beats of the drive train. The ERP was defined as the longest extrastimulus coupling interval that failed to capture with an extrastimulus current strength of twice the stimulation threshold. The ARP was defined in a similar manner with an extrastimulus current strength of 10 mA. The ERP and ARP were determined using the incremental extrastimulus technique. A subset of patients had the pacing protocol performed during autonomic blockade. As the AV interval of the final two beats of the drive train was shortened from 120 msec to 0 msec, the peak right atrial pressure increased from 7 ± 3 mmHg to 15 ± 5 mmHg (P < 0.001). The increase in atrial pressure associated with simultaneous pacing of the atrium and ventricle resulted in shortening of the atrial ERP and ARP by 7.3 ± 5.2 and 6.2 ± 3.5 msec, respectively (P < 0.0011). Similar results were obtained during autonomic blockade. These findings confirm the presence of contraction-excitation feedback in normal human atria.     

Effect of right atrial overdrive pacing in the prevention of symptomatic paroxysmal atrial fibrillation: a multicenter randomized study,the PAF-PACE study

The aim of this study was to assess if right atrial overdrive pacing can suppress symptomatic episodes of paroxysmal atrial fibrillation (PAF) in patients without bradyarrhythmias. Forty-two patients with frequent and symptomatic PAF without other pacing indication had a pacemaker implanted after a 4-week run-in period, during which the frequency of symptomatic PAF episodes and the mean heart rate were objectively documented. Depending on the mean heart rate recorded during run-in, the pacemaker was programmed in random order to right atrial AAI pacing at 10-19 beats/min > mean heart rate (medium overdrive [MO]), at 20-29 beats/min > mean heart rate (high overdrive [HO]) and to no pacing (OAO mode) for 4-12 weeks each using a crossover design. In the 35 patients who completed the protocol, the number of symptomatic episodes of PAF (>30-second duration) per week was significantly lower during MO pacing (median 0.88, P = 0.001, n = 35) and during HO pacing (median 0.75, P = 0.002, n = 20) than during OAO (median 2.02 and 2.04, respectively). There was no difference between MO and HO pacing in the 20 patients paced at both rates (0.97 vs 0.75, P = 0.33). Seven patients did not complete the protocol due to persistent atrial fibrillation (n = 4), angina pectoris requiring surgery (n = 1), and unwillingness to continue the study due to improvement (n = 1) or worsening (n = 1) of symptoms during the study periods. Right atrial overdrive pacing can reduce the number of symptomatic PAF episodes in patients with frequent and drug refractory PAF but without bradyarrhythmias.     

The effect of the atrial pacing site on the total atrial activation time

The effect of dual site pacing for prevention of atrial fibrillation may be due to synchronization of right and left atrial activation. Little is known, however, about the effect of pacing from single right atrial sites on differences in interatrial conduction. Twenty-eight patients without structural heart disease were studied following radiofrequency catheter ablation of supraventricular arrhythmias. Pacing was performed using standard multipolar catheters from the presumed insertion site of Bachmann's bundle, the coronary sinus ostium, the high lateral right atrium, and the right atrial appendage (n = 8 patients). Bipolar recording was performed from the distal coronary sinus, the high and low lateral right atrium, and the posterolateral left atrium (n = 13 patients). The longest conduction time from each pacing to each recording site was considered the total atrial activation time for the respective pacing site. During high right atrial pacing, the total atrial activation time was determined by the conduction to the distal coronary sinus (118 +/- 18 ms), during coronary sinus ostium pacing by the conduction to the high right atrium (94 +/- 18 ms), and during Bachmann's bundle pacing by the conduction to the distal coronary sinus (74 +/- 18 ms). The total atrial activation time was significantly shorter during pacing from Bachmann's bundle, as compared to pacing from other right atrial sites. Thus, in normal atria, pacing from the insertion of Bachmann's bundle causes a shorter total atrial activation time and less interatrial conduction delay, as compared to pacing from other right atrial sites. These findings may have implications for alternative pacing sites for prevention of atrial fibrillation.     

Capture of atrial fibrillation reduces the atrial defibrillation threshold

The effect of the atrial activity synchronization by single site right atrial pacing on atrial defibrillation threshold (ADFT) was investigated in patients with AF. Two series of randomized incremental cardioversion tests, with increasing energy levels from 0.5 to 10 J, were performed in 15 patients with recurrent episodes of idiopathic paroxysmal AF using two 7 Fr "single coil" catheters for internal cardioversion. After induction of sustained AF (> 10 minutes), shocks were delivered, preceded or not by 10 seconds of overdrive local atrial pacing, according to the randomization, using an external cardioverter defibrillator. A total of 187 shocks was delivered to the study population. ADFT was reduced when overdrive atrial stimulation preceded the cardioversion (3.6 +/- 1.6 vs 2.9 +/- 1.7 J, P = 0.02). Local atrial capture was considered on the basis of 1:1 phase locking between stimulus and atrial activation wave, and constant morphology of atrial wave criteria. Effective atrial capture was obtained in 8 of 15 patients. There was not significant difference in the mean of FF intervals of patients in which atrial capture was or was not stable (209 +/- 22 vs 208 +/- 28 ms). Patients were then considered according to the outcome of atrial pacing before direct current shock. A marked ADFTreduction was observed in patients with stable capture (3.8 +/- 1.7 vs 2.5 +/- 1.7 J, P = 0.0003), while no significant difference in ADFT was found when capture was not achieved (3.4 +/- 1.6 vs 3.6 +/- 1.5 J, P = NS). In conclusion, regularization of atrial electrical activity by atrial capture reduces the ADFT. A constant pacing entrainment seems to lower the energy required for electrical cardioversion by reducing the amount of fibrillating tissue.     

Atrial and Ventricular Stimulation Threshold Development: A Comparative Study in Patients with a DDD Pacemaker and Two Identical Carbon-Tip Leads

The atrial and ventricular pacing threshold development during the first postoperative year was studied in a group of patients receiving DDD pacemakers. Identical carbon-tip endocardial leads were implanted in atrium and ventricle. Atrial and ventricular voltage stimulation thresholds were measured at implantation, and noninvasively at 1 and 12 months thereafter. The atrial amplifier sensitivity required for adequate P wave sensing during follow-up was also determined. The possible influence of a number of factors upon atrial and ventricular threshold evolution was statistically assessed. The threshold data were complete in 57 patients (mean age +/- SD, 65.2 +/- 12.4 years). Thirteen patients had a diagnosis of sinus node disease, whereas 44 had not. Patient age and diagnosis did not significantly influence atrial or ventricular stimulation threshold development. Atrial sensing thresholds were not related to atrial stimulation thresholds during follow-up. Atrial pacing thresholds were higher than ventricular thresholds at pacemaker implantation (P less than 0.00005), but the postoperative threshold rise and thresholds at 1 and 12 months postoperatively did not differ significantly between the atrium and ventricle. The ratio of chronic to acute stimulation thresholds was higher on the ventricular than on the atrial level (0.001 greater than P greater than 0.0005). The chronic atrial threshold showed a logarithmic relation to the threshold at implantation (P = 0.0006); postoperative threshold rise was not a significant determinant of the chronic atrial threshold (P = NS). On the ventricular level, the reverse was seen: The chronic threshold was related to the postoperative threshold rise (P = 0.0015, logarithmic relation), but not to the implantation threshold (P = NS).(ABSTRACT TRUNCATED AT 250 WORDS)     

Importance of Heart Rate Response During Exercise in Patients Using Atrioventricular Synchronous and Ventricular Pacemakers

Atrioventricular synchronous pacing offers advantages over fixed-rate ventricular (VVI) pacing both at rest and during exercise. This study compared the hemodynamic effects at rest and exercise of ventricular pacing at a rate of 70 beats/min, ventricular pacing where the rate was increased during exercise and dual chamber pacing. Ten patients, age 63 +/- 8 years, with multiprogrammable DDD pacemakers were studied using supine bicycle radionuclide ventriculography. Radionuclide data during dual chamber pacing was acquired at rest and during a submaximal workload of 200-400 kpm/min. The pacemakers were then programmed to VVI pacing at a rate of 70 beats/min, and 1 week later, studies were repeated in the VVI mode at rest, during exercise at a rate of 70 beats/min, and during exercise with the VVI pacemaker programmed to a rate adapted to the DDD pacing exercise rate. At rest, the cardiac output was lower in the VVI compared with the AV sequential mode (4.1 +/- 1.1 vs 5.7 +/- 1.1 1/min, P less than 0.01). During exercise, the cardiac output increased from resting values in the DDD and VVI pacing modes, however cardiac output in the rate-adapted VVI mode was higher than in the VVI mode with the rate maintained at 70 beats/min (8.1 +/- 1.5 vs 6.3 +/- 1.1 1/min, P = 0.02). Three patients completed lower workloads with VVI pacing at 70 beats/min compared with AV synchronous pacing. At rest, AV sequential pacing was superior to VVI pacing, suggesting the importance of the atrial contribution to ventricular filling. With VVI pacing during exercise, cardiac output was improved with an increased pacemaker rate, suggesting that the heart rate response during exercise was the major determinant of the higher cardiac output.     

Single lead DDD pacing using electrodes with longitudinal and diagonal atrial floating dipoles

In this study, an assessment was made of the possibilities of single lead DDD pacing in two groups: a group of 15 patients in whom a lead with a longitudinal atrial "floating" dipole was implanted, and another group of 10 patients with a lead with a diagonal atrial "floating" dipole. In both groups, the electrodes were connected to a SLD generator. At discharge, atrial capture was achieved with the unipolar mode in 17 of 25 patients, whereas in the group carrying the longitudinal atrial dipole, atrial capture was achieved with the overlapping biphasic impulses (OLBI) system in 12 of 15 patients and in all 10 patients in the group with the diagonal atrial dipole. At 3 months, atrial capture was achieved with the unipolar mode in 13 of 22 patients (5.75 +/- 1.77 V/0.5 ms), whereas with the OLBI system atrial capture was achieved in 8 of 13 patients carrying the longitudinal atrial dipole (3.11 +/- 1.13 V/0.5 ms), and in 8 of the 9 patients carrying the diagonal atrial dipole (2.80 +/- 0.69 V/0.5 ms). In this study, the use of the OLBI system led to a significant reduction of atrial threshold (P < 0.0001). Phrenic stimulation is the main untoward effect reported during single lead DDD pacing, a lower incidence being detected in the group carrying the diagonal atrial dipole (10 vs 35.7%, P = NS). Other limitations of this form of pacing could result from a crossed stimulation phenomenon detected in a patient during single lead DDD pacing.     

Vector mapping in localizing the transverse conduction site of the crista terminalis in patients with typical atrial flutter

BACKGROUND: The difference in the conduction properties of the crista terminalis (CT) along its course, has not been fully clarified. Using the vector mapping method, we localized the transverse conduction (TC) site of the CT and elucidated its conduction capabilities in patients with typical atrial flutter (AF). METHODS: The TC site of the CT was localized by the analysis of the polarity reversal of the double potentials recorded at 10 sites along the CT using a 20-pole deflectable catheter in 17 patients. The conduction capabilities of the TC site were analyzed during incremental pacing delivered from 100 beats/min to 2-to-1 local capture at the low anterior (LARA) and posterior (LPRA) right atrium. RESULTS: At a pacing rate of 100 beats/min, TC at a single site was observed in 15 patients during LARA pacing and 7 patients during LPRA pacing, respectively. TC sites were distributed from superior to middle third of the CT in all patients. TC was bidirectional in 4 sites, but was unidirectional in the remaining 14 sites. Following an increase in the pacing rate, TC was blocked in all 7 sites during LPRA pacing and 11 of 15 sites during LARA pacing. Shift in the location of the TC site was not observed in any of the patients before TC block. The conduction block rate during pacing from LARA was significantly higher than that from LPRA (211 +/- 59 beats/min vs 145 +/- 66 beats/min, P < 0.01). CONCLUSIONS: The superior to middle third of the CT provides TC capabilities. The TC across the CT was caused by a preferential conduction site and most of these TC were unidirectional, and stable in location irrespective of the change in the conduction rate.     

Impedance to Transesophageal Atrial Pacing: Significance Regarding Power Sources

Transesophageal stimulation is an expeditious method of atrial pacing. Using pulse widths of 10 msec results in reduction of current requirement to values that are usually less than 15 mA. An unknown variable in transesophageal atrial pacing has been impedance. In this study, we investigated the impedance to transesophageal atrial pacing in ten patients using a stimulator with a 63 V power source capable of delivering constant current to 20 mA against an impedance of 2,000 ohms. A bipolar electrode was used to deliver stimuli with a current of 15 mA. Voltage across a known resistance and current were measured on an oscilloscope and the impedance was calculated. Pacing thresholds were also performed and ranged from 6.2 to 16.5 mA (mean 9.4 +/- 2.9 mA, SD). Impedance varied between 720 and 2,670 ohms (mean 1,750 +/- 540 ohms). The stimulator used to measure impedance in man and two other commercially available stimulators were bench tested against known resistances of 500 to 2,000 ohms. The other stimulators with power sources of 12.5 and 15 V had attenuation of the delivered current at resistances of between 1,000 and 2,000 ohms. Thus, this study has demonstrated that transesophageal atrial pacing incurs impedances two to five times greater than incurred with intracardiac pacing leads. Therefore stimulators with high power sources are required to deliver the programmed current against these impedances.     

Epicardial, Biatrial Synchronous Pacing for Prevention of Atrial Fibrillation after Cardiac Surgery

About 30% of patients develop AF after open heart surgery. Biatrial synchronous pacing (BSP) has been shown to promote sinus rhythm in patients with paroxysmal AF refractory to drug therapy. We conducted a prospective, randomized study to test the effect of BSP via epicardial electrodes on the incidence of AF after heart surgery, as compared to conventional therapy. To apply BSP, we attached two epicardial electrodes to the right and one to the left atrium. Immediately following surgery, BSP was initiated in the AAI-Mode at a rate of 10 beats/min above the underlying rhythm (maximum 110 beats/min) and continued for 3 days, during which the rhythm was continually monitored. After 21 (age 63 +/- 9 years) of the planned 200 patients, the study was prematurely aborted because of the proarrhythmic effect of BSP: 6 of the 12 patients treated with BSP developed sensing failure (P amplitude < 1 mV), which provoked AF in 5 of these 6 patients. BSP was discontinued due to diaphragmal stimulation in two patients and due to ventricular stimulation by a dislocated left atrial electrode in one patient. Two patients in the control group (n = 9) developed AF. Using the available standard technology, BSP via epicardial electrodes is not suitable to suppress AF after heart surgery, primarily due to postoperative deterioration of atrial sensing and its profibrillatory effect. In patients requiring atrial pacing after heart surgery, sensing thresholds must be closely monitored to prevent induction of AF.     

Comparison of the effects of right atrial, right ventricular apex and atrioventricular sequential pacing on myocardial oxygen consumption and cardiac efficiency: a laboratory investigation

As the impact of cardiac pacing on myocardial energetics has not yet been established, this laboratory investigation was undertaken to evaluate the effects of right atrial (AP), right ventricular apex (VP) and atrioventricular sequential pacing (AVP) on cardiac energetics in a closed-chest model. Ninety-two pacing interventions were performed in ten anesthetized mongrel dogs with normal loading conditions and contractile states. The energetic effects of pacing were assessed in terms of myocardial oxygen consumption (MVO2), its hemodynamic determinants and cardiac efficiency. Efficiency was calculated as the ratio of O2-equivalent of external cardiac work to MVO2, using standard definitions. In the first series of experiments 36 intra-individual comparisons were made between AP and VP at identical rates (95-210 beats/min). In the second series AVP was compared to VP in 10 intra-individual comparisons at identical rates (109-190 beats/min). MVO2 was lower (p less than 0.001) during AP (8.30 +/- 2.14 ml O2/min.100 g) compared to VP (10.16 +/- 3.15 ml O2/min.100 g) at the same rate (158 +/- 32 beats/min). Efficiency (p less than 0.001) was considerably higher during AP (21.6 +/- 5.7%) compared to VP (12.8 +/- 5.9%). During AVP, MVO2 (10.85 +/- 1.76 ml O2/min.100 g) was not significantly different from VP (10.57 +/- 1.34 ml O2/min.100 g) at the same rate (146 +/- 25 beats/min). Hemodynamics were superior with AVP compared to VP. Efficiency was significantly higher (p less than 0.01) with sequential (15.4 +/- 3.9%) as compared to ventricular pacing (12.0 +/- 3.2%). In conclusion, this study indicated that VP exerts disadvantageous effects on MVO2 and cardiac efficiency. AP has beneficial effects on cardiac energetics because it improves the relationship between mechanical performance of the heart and its energy requirements. AVP results in a higher efficiency than VP due to superior hemodynamics, despite MVO2 levels comparable to those of VP. The mechanism of energy waste with right ventricular apex pacing is probably related to an asynchronous contraction in the ventricular myocardium due to a nonphysiological spread of excitation.     

Right Atrial-ventricular Dissociation and Entrainment while Pacing from High Right Atrium and Coronary Sinus during Circus Movement Tachycardias

Thirteen patients with circus movement tachycardias (CMT) were studied. Twelve had left-sided, and one, right-sided, accessory pathways. Entrainment was possible during overdrive high right atrial stimulation in 13/13 patients, and during coronary sinus pacing in 10/12 patients. The minimal pacing rates required for this to occur were 10 to 31 beats/min faster than those of the tachycardias. Short episodes of right atrial-ventricular dissociation occurred while pacing from the high right atrium (6/13 patients), but not from the coronary sinus (0/13 patients). It is possible to explain this phenomenon by postulating the existence of two distinct atrioventricular (AV) nodal inputs (one for right-sided and the other for left-sided impulses); it could also have been an expression of the close distance existing between the AV node and the coronary sinus. Entrainment, by defining a range of pacing rates followed by resumption of the tachycardia upon the cessation of stimulation, indicated that faster rates were needed for the CMT to be interrupted. However, regardless of the pacing rate and pacing site, tachycardia termination occurred when an anterograde impulse was blocked at the AV node. The information obtained from this study suggests that some patients with drug-resistant CMT may benefit from pacing modes capable of terminating the arrhythmia "through entrainment" at the slowest atrial rate at which this is possible.     

The acute effects of biatrial pacing on atrial depolarization and repolarization

Permanent biatrial and/or multisite atrial pacing may prevent atrial fibrillation (AF), but the effects on atrial electrophysiology remain incompletely understood. Acute biatrial pacing was studied in 20 patients with and 28 without (controls) a history of atrial fibrillation and/or flutter. Twelve-lead electrocardiograms were recorded during pacing from the high right atrium (RA), from the distal coronary sinus (LA), and biatrial pacing. P wave duration was measured in each lead and the difference between maximum and minimum P duration was termed P wave dispersion. Effective refractory periods (ERPs) were measured during each pacing mode. The dispersion of P wave duration was 35 +/- 14 ms in controls and 40 +/- 29 ms in AF patients (P = 0.17). Compared to RA pacing, LA pacing shortened P duration in controls (127 +/- 18 to 107 +/- 16 ms, P < 0.05) and biatrial pacing markedly shortened P duration in controls (127 +/- 18 to 93 +/- 14 ms, P < 0.05) and AF patients (114 +/- 43 to 97 +/- 21 ms, P < 0.05). P wave dispersion was unaffected. In controls, the LA ERP was longer than the RA ERP. This phenomenon was not present in AF patients, whose LA ERP was shorter than that of controls. Biatrial pacing had no effect on atrial ERPs or the dispersion of atrial refractoriness. In conclusion, acute biatrial pacing does not affect atrial repolarization but it does cause a marked shortening of global biatrial depolarization. Distal coronary sinus pacing produces a shorter P wave than RA pacing. There is substantial dispersion in the surface P wave of the electrocardiogram, the significance of which awaits further study.     

AAIR Versus DDDR Pacing in Patients with Impaired Sinus Node Chronotropy: An Echocardiographic and Cardiopulmonary Study

The aim of this study was to compare AAIR and DDDR pacing at rest and during exercise. We studied 15 patients (10 men, age 65 ± 6 years) who had been paced for at least 3 months with activity sensor rate modulated dual chamber pacemakers. All had sick sinus syndrome (SSS) with impaired sinus node chronotropy. The patients underwent a resting echocardiographic evaluation of systolic and diastolic LV function at 60 beats/min during AAIR and DDDR pacing with an AV delay, which ensured complete ventricular activation capture. Cardiac output (CO) was also measured during pacing at 100 beats/min in both pacing modes. Subsequently, the oxygen consumption (VO₂at) and VO₂at pulse at the anaerobic threshold were measured during exercise in AAIR mode and in DDDR mode with an AV delay of 120 ms. The indices of diastolic function showed no significant differences between the two pacing modes, except for patients with a stimulus-R interval > 220 ms, for whom the time velocity integral of LV filling and LV inflow time were significantly lower under AAI than under DDD pacing. At 60 beats/min, CO was higher under AAI than under DDD mode only when the stimulus-R interval was below 220 ms. For stimulus-R intervals longer than 220 ms, and also during pacing at 100 beats/min, the CO was higher in DDD mode. The stimulus-R interval decreased in all patients during exercise. The time to anaerobic threshold, VO₂at ond VO₂at pulse showed no significant differences between the two pacing modes. Our results indicate that, at rest, although AAIR pacing does not improve diastolic function in patients with SSS, it maintains a higher CO than does DDDR pacing in cases where the stimulus-R interval is not excessively prolonged. On exertion, the two pacing modes appear to be equally effective, at least in cases where the stimulus-R interval decreases in AAIR mode.     

Assessment of atrial regional wall motion using strain Doppler imaging during biatrial pacing in the bradycardia-tachycardia syndrome

INTRODUCTION: Biatrial pacing is expected to have preventive effects on atrial fibrillation. METHODS: We evaluated atrial regional wall motion by strain Doppler imaging (SDI) in 6 patients (62.5 +/- 11.3 [SD] years), who suffered from atrial fibrillation, with an implanted biatrial pacemaker. SDI was performed and atrial regional wall motion was estimated during biatrial (BiA) and right atrial appendage (RAA) pacing. RESULTS: There was no significant difference in the interval from the pacing spike to the peak strain of the atrium in the lateral right atrium (LRA) between BiA and RAA pacing. However, there was a significant difference in the septal atrium (SA) between BiA and RAA pacing (225.0 +/- 19.9 vs 267.2 +/- 15.7 ms, P < 0.0001) and in the lateral left atrium (LLA) between BiA and RAA pacing (216.7 +/- 21.6 vs 275.0 +/- 16.2 ms, P < 0.0001). There were significant differences in the time difference of peak strain between BiA and RAA pacing in each atrial segment (LRA-AS: 2.2 +/- 5.9 vs 45.0 +/- 11.9 ms, P = 0.0016, SA-LLA: -8.3 +/- 5.5 vs 7.8 +/- 2.7 ms, P < 0.0011, LRA-LLA: -6.1 +/- 3.9 vs 52.8 +/- 13.2 ms, P = 0.0002). There was no significant difference in the interval from the pacing spike to the inflection point of atrial strain (S-I) of LRA. However, there were significant differences in S-I of SA (83.9 +/- 24.1 vs 129.9 +/- 30.6 ms, P = 0.0086) and LLA (102.2 +/- 37.9 vs 166.1 +/- 13.4 ms, P = 0.0028). CONCLUSION: BiA pacing improved the synchronicity of regional wall motion of both atrium.