C-AMP and ANP Levels in VVI and DDD Pacing with Different AV Delays During Daily Activity and Exercise

THEODORAKIS, G., ET AL.: C-AMP and ANP Levels in VVI and DDD Pacing with Different AV Delays During Daily Activity and Exercise. Nine patients (three males) mean age 68 ± 8 years, having complete heart block, and paced in the DDD mode were examined in VVI and DDD pacing with 100 and 150 ms atrioventricular delays (AVD) during rest and exercise. Plasma atrial natriuretic peptide (ANP) and cyclic AMP (c-AMP) were measured at rest and at peak exercise test. ANP plasma levels at rest were significantly higher in VVI pacing compared to 150 AVD (p < 0.03). On exercise, ANP release was statistically increased only in DDD with 150 ms AVD, while in WI it remained in high levels at exercise but no significant change was found (p:ns). c-AMP during rest was unchanged in any pacing mode or AVD, but on exercise DDD pacing with short AVD (100 ms) released lower c-AMP plasma levels, than at rest (p:ns). DDD pacing with long AVD (150 ms) during exercise produced statistically higher c-AMP plasma levels (p < 0.05) than at rest. Also in VVI pacing the c-AMP plasma levels were statistically higher than at rest (p < 0.02). Adrenergic activity seems to be lower during exercise in DDD pacing with shorter AVD (100 ms) than in DDD with 150 ms AVD or VVI pacing. No difference was found in c-AMP plasma levels at rest. ANP release was also found to be lower at exercise in DDD pacing with short AVD (100 ms) than in DDD with 150 ms AVD. ANP plasma levels at rest were statistically higher in VVI pacing. (PACE, Vol. 13, December, Part II 1990)     

Atrial Natriuretic Factor Release During Exercise in Patients Successively Paced in DDD and Rate Matched Ventricular Pacing

Dual chamber pacemakers were implanted in nine patients with permanent second or third degree AV block feight had complete retrograde block). Two identical exercise tests were performed after at least 1 month after implantation. During the first test (T₁) the pacemaker was programmed to the DDD mode and heart rates were recorded every 15 to 30 seconds during exercise and 30 minutes after exercise. Following 30 minutes of rest, the implanted pacemaker was programmed to the VVT mode and driven by an external pacemaker via a skin electrode. The second exercise test (T₂) was then performed and the rate of the external pacemaker was progressively changed to reproduce exactly the rate observed during T₁ at the same exercise stress. Atrial natriuretic factor (ANF) levels were determined at rest, at regular intervals during exercise, and 30 minutes after exercise. ANF levels and release were statistically higher during rate matched ventricular, than DDD pacing. It is concluded that preservation of AV synchrony reduces ANF release induced by heart rate acceleration during exercise.     

Plasma Levels of Natriuretic Peptides During Ventricular Pacing in Patients with a Dual Chamber Pacemaker

The mechanism(s) responsible for the release of brain natriuretic peptide (BNP), a cardiac hormone of ventricular origin, are still not completely understood. We measured plasma atrial natriuretic peptide (ANP) and BNP in 15 subjects (10 men, mean age 67 ± 3 years) with a dual chamber pacemaker and unimpaired heart function during ventricular pacing, which is known to induce an increase in atrial pressure and plasma ANP concentration. Under ECC monitoring, all subjects received sequential atrioventricular pacing for 30 minutes and ventricular pacing for 30 minutes, at the same rate of 80 beats/min. Arterial pressure and plasma BNP and ANP levels were measured every 10 minutes throughout the study. Ventricular pacing led to atrioventricular dissociation in eight subjects and to retrograde ventriculo-atrial conduction in seven. Arterial pressure remained unchanged in all subjects. In the group with atrioventricular dissociation, plasma ANP increased from 10.14 ± 0.58 to 16.72 ± 0.92 fmol/mL at the 60th minute (P < 0.0001), whereas plasma BNP did not change at all (fiom 1.26 ± 0.07 to 1.16 ± 0.09 fmol/mL). In the group with retrograde conduction, plasma ANP concentration doubled (fiom 10.95 ± 1.66 to 21.40 ± 1.51 fmol/mL, P < 0.0001), BNP increased 1.5-fold (from 1.16 ± 0.06 to 1.64 ± 0.14 fmol/mL, P < 0.001), and the ANP: BNP ratio augmented fiom 10:1 to 13.4:1. These results indicate that the release of ANP and BNP is regulated by different mechanisms, supporting the view that there is a dual natriuretic peptide system, comprising ANP fiom the atria and BNP fiom the ventricles.     

Effect of Dual Chamber Atrioventricular Sequential Pacing on Coronary Flow Velocity in a Patient with Hypertrophic Obstructive Cardiomyopathy

Effects of dual chamber A V sequential pacing on coronary flow velocity, especially systolic reversal flow, were tested in a patient with hypertrophic obstructive Cardiomyopathy. AV sequential pacing with shorter AV delays reduced the systolic reversal flow in the coronary artery, and improved the pressure gradient of the left ventricular outflow tract.     

Atrial Natriuretic Peptide During Different Pacing Modes in a Comparison with Hemodynamic Changes

The study investigates the response of atrial natriuretic peptide (ANP) to different cardiac pacing modes in comparison with hemodynamic changes. Ten patients underwent Swan-Ganz catheterization during pacemaker implant. Atrioventricular and ventricular pacing were performed consecutively at three pacing rate levels (80, 100, and 110 ppm). Blood samples were taken from the pulmonary artery for ANP determination, both basally and at the end of each pacing period. Concomitantly, mean pulmonary capillary wedge pressure (PCWP) and mean pulmonary artery pressure (PAP) were measured. Cardiac output (CO) was determined by thermodilution both basally and during the 110 ppm steps. During atrioventricular pacing, whereas no significant changes were observed for ANP, PCWP and PAP, CO increased significantly (P less than 0.0005). At the beginning of ventricular pacing hemodynamic parameters and ANP levels were comparable with those of baseline conditions. During subsequent ventricular pacing PCWP and ANP increased significantly at the 110 ppm rate step (P less than 0.05). PAP did not change significantly, whereas CO decreased in all cases (P less than 0.01). A positive correlation was observed between ANP and PCWP during ventricular (P less than 0.001), but not atrioventricular pacing. The results, while confirming the hemodynamic advantages of atrioventricular pacing, point to a major stimulation of ANP secretion during ventricular pacing. This fact, together with the observed drop in CO and the correlation between ANP and PCWP, suggest that the increase of ANP in ventricular pacing may be the expression of a compensatory mechanism to the hemodynamic disadvantages of atrioventricular asynchrony.     

Plasma ANP and Cyclic GMP Levels Versus Left Ventricular Performance at Different AV Delays in AV Sequential Pacing

Eleven resting patients with an implanted DDD pacemaker were studied. After 30 minutes of AV sequentiai pacing at a rate of 80 beats/min with three consecutive atrioventricular delays (AVDs; 100, 150, and 200 msec) peripheral venous blood was drawn for further analyses by specific radioimmunoassays of atrial natriuretic peptide (ANP) and the ANP second messenger, cyclic guanosine monophosphate (cGMP). Relative changes in left ventricular (LV) stroke volume following alterations of AVD were assessed by means of pulsed-Doppler echocardiography through measurement of LV outflow time-velocity integrals (TVI). The optimal AVD (oA VD) was defined in individual patients as that which was associated with the greatest TVI and with improvement over both other AVDs of more than 4%. The oA VD was found in nine patients. For these nine patients no significant differences in either plasma ANP or cGMP between various AVDs were observed. However, we found such differences with respect to values measured at oAVD; both ANP and cGMP levels were lowest at oAVD. Pooling together the data obtained in 11 patients at three AVDs, a positive correlation between ANP and cGMP levels was found (r = 0.7, P < 0.0001. n = 33). Moreover, changes of plasma ANP and cGMP induced by every A VD increment of 50 msec were also correlated (r = 0.6, P < 0.01, n = 22). It is concluded that in AV sequential pacing at rest piasma ANP reaches minimal levels at the AVD, which provides the best LV performance. Although levels of cGMP changed in parallel with those of ANP, low relative values of cGMP differences may limit the usefulness of cGMP assays in optimization of the AVD.     

Stability of the DDD Pacing Mode in Patients 80 Years of Age and Older

We reviewed the records of 119 consecutive patients aged 80 years or older (mean age 84 ± 3.7 years) in whom a dual chamber pacemaker was implanted between 1984 and 1991. Follow-up data was available up to February 1993. Immediate postimplantation complications were rare. Nine patients were lost to follow-up, all within 6 months of implantation. An additional seven patients died within 6 months of implantation. Long-term follow up for at least 6 months from implantation was available for 103 of the 119 patients (87%). Of these 89 (66%) remained in functioning DDD mode for a mean of 22 ± 15 months from implantation. Nine patients were reprogrammed to VVI mode, six due to atrial fibrillation and three due to failure of atrial sensing or pacing. One patient was programmed DVI for failure of atrial sensing; 94 of 112 patients (84%) whose status was definitely known in February 1993 remained in functioning DDD mode until death or last follow-up. Cumulative survival in DDD mode was 78% at 30 months. We conclude that DDD pacing is stable in the great majority of patients in their nineth and tenth decades who present with rhythms amenable to dual chamber pacing and who have no history of sustained atrial fibrillation.     

Acute Effects of Different Atrial Pacing Sites in Patients with Atrial Fibrillation: Comparison of Single Site and Biatrial Pacing

It has been reported that a trial single site or biatrial pacing can suppress the occurrence of AF. However, its mechanism remains unclear. The study population included 32 patients with AF (n = 20: AF group), or without paroxysmal AF (n = 12: control group). The mechanism and efficacy of atrial pacing were investigated by electrophysiological studies to determine which was more effective for suppressing AF induction; single site pacing of the right atrial appendage (RAA) or distal coronary sinus (CS-d), or biatrial (simultaneous BAA and CS-d) pacing. In the AF group, AF inducibility was significantly higher with BAA extrastimulus during RAA (12/20; P < 0.0001) or biatrial paced drive (7/20; P < 0.01) than during CS-d paced drive (0/20). In the control group, AF was not induced at any site paced. In the AF group, the conduction delay and other parameters of atrial vulnerability significantly improved during CS-d paced drive. The atrial recovery time (ART) at RAA and CS-d was measured during each basic pacing mode. ART was defined as the sum of the activation time and refractory period, and the difference between ARTs at RAA and CS-d was calculated as the ART difference (ARTD). The ARTD was significantly longer during BAA pacing in the AF group than in control group (155.0 +/- 32.8 vs 128.8 +/- 32.9 ms, P < 0.05). In the AFgroup, ARTDs during biatrial (52.0 +/- 24.2 ms) and CS-d pacing (51.7 +/- 26.0 ms) were significantly shorter than ARTD during RAA pacing. The CS-d paced drive was more effective for suppressing AF induction than biatrial or RAA paced drive by alleviating conduction delay. CS-d and biatrial pacing significantly reduced ARTD compared with RAA pacing.     

Effect of Atrial Pacing on the Frequency of Tachycardia in Patients with Recurrent Junctional Tachycardia

In order to assess whether atrial pacing reduced the frequency of tachycardia in patients with recurrent junctional tachycardias, ten patients with recurrent junctional tachycardias with atrial Intertach antitachycardia pacemakers in situ were paced in a random order in atrial demand mode at 50 ppm (AAI 50), 80 ppm (AAI 80), and 100 ppm (AAI 100) for a period of up to 1 month. The numbers of tachycardias detected by the pacemaker over this period were recorded and compared with the number seen when unpaced (000). Correct arrhythmia detection by the pacemaker was confirmed by Holter monitoring. The number of tachycardias in 000 was 44.7 +/- 19.8 (mean +/- SEM). No significant reduction in tachycardia frequency was seen in any pacing mode. Back-up atrial pacing at 50 ppm tended to reduce the frequency of tachycardias (32.3 +/- 12.8 tachycardias; P = 0.06). The higher pacing rates increased the number of tachycardias (AAI 80; 57.1 +/- 24.6 tachycardias, P = 0.20: AAI 100; 81.8 +/- 30.2 tachycardias; P = 0.31). Symptoms increased with each pacing mode and palpitations were statistically more severe in AAI 100 mode. Four patients had disabling symptoms at this rate and had to drop out. Atrial back-up pacing may be of use in some patients with junctional tachycardia, but overdrive pacing is not helpful.     

Dual Chamber Pacing in Hypertrophic Obstructive Cardiomyopathy: Beneficial Effect of Atrioventricular Junction Ablation for Optimal Left Ventricular Capture and Filling

Clinical improvement with dual chamber pacing bas largely been reported in patients suffering from hypertrophic obstructive cardiomyopathy and mainly attributed to the reduction of the subaortic pressure gradient. To be effective, pacing must induce a permanent and complete capture of the LV. In two patients of our collective, symptoms (angina and dyspnea NYHA Class III and/or syncopes) persisted or relapsed despite pacing. This was related to the inability to obtain full LV capture due to a too-short native PR interval. RF ablation of the AV junction was therefore performed in botb patients, resulting in permanent AV block in one and prolonged PR interval up to 310 ms in the second. Pacing was thereafter associated with an immediate and significant clinical improvement related to permanent LV capture, whatever the patient's activity. After RF ablation, the AV delay was set up to induce the best LV filling, as assessed by Doppler analysis of mitral flow. Our observations suggest that RF ablation or modification of the AV junction can be a successful procedure in some patients with residual or recurrent symptoms, when the latter result from a loss of capture or from the inability to program an AV delay tbat does not compromise the active component to LV filling. Doppler echocardiography is a simple and effective mean to assess the hemodynamic effect of AV interval modulation in this setting.     

Radiographic Assessment of Atrial Dipole Position in Single Pass Lead VDD and DDD Pacing

Atrial electrode position was determined by radiographic analysis in 160 patients paced in single-lead VDD for second- or third-degree A-V block, implanted > 1 year with Phymos single pass leads and Phymos 3D pacemakers. The pacing lead features an atrial dipole with a 30-mm electrode interspace. In 44% of patients, the upper atrial electrode was positioned within a band of 20 mm centered at the level of the superior vena caval insertion (junctional area) and was in the inferior vena cava or in the atrium in 35% and 21 % of cases, respectively. In spite of these different dipole locations, all patients had stable atrium-driven pacing at routine follow-up visits. With the electrode in the junctional area, unipolar stimulation of up to 5 V for 1 ms resulted in stable atrial capture in 63% and 59% of the patients in supine and upright positions, respectively. With the electrode in the atrium, corresponding success rates were 45% and 54%. In the atrium, however, the prevalence of diaphragmatic stimulation was significantly lower than at the junction (10% vs 42% in supine position; 21 % vs 47% upright). Though atrial sensing function proved adequate in a wide range of positions, these results suggest that the Phymos lead atrial dipole should be positioned within the atrium, as close as possible to the atrial wall, to maximize the number of VDD patients who might benefit from single-lead DDD pacing.     

Effects of Systolic Atrial Function on Plasma Renin Activity and Natriuretic Peptide Secretion after High Rate Atrial and Ventricular Pacing in Dogs

Rapid atrial rates cause electrical, structural remodeling, and neuro-humoral changes. This study compares the effects of mechanical remodeling on plasma renin activity (PRA) and atrial natriuretic peptide (ANP) secretion. Eight beagles were subjected to rapid atrial pacing (AP) at 400 beats/min for 16 days. After complete recovery of left ventricular function, they underwent rapid ventricular pacing (VP) at 240 beats/min of equal duration. Left atrial systolic maximal dimension (LAmax) and left atrial appendage (LAA) peak late emptying velocity (LAA-E) were assessed by echocardiography. Blood samples were taken from the right atrium and from the peripheral vein. LAmax after AP and VP enlarged significantly (2.16 ± 0.21 cm vs 2.41 ± 0.23 cm, P = 0.002). Compared with baseline, LAA-E velocities were significantly reduced (0.65 ± 0.12 m/s vs 0.26 ± 0.16 m/s, P = 0.001) after AP only. AP caused a significant elevation of PRA in right atrial (9.28 ± 4.23 nmol/L per hour) and peripheral samples compared with baseline values (4.82 ± 2.53 nmol/L per hour, P = 0.04). ANP levels increased after AP (1117.12 ± 252.21 fmol/L) with respect to baseline values (824.37 ± 159.08 fmol/L, P = 0.001). There was no difference in PRA and ANP levels between atrial and peripheral samples. Atrial size and impaired systolic appendage function play an important role in secretion of PRA and ANP. Both neuro-humoral pathways may be therapeutic targets in the treatment of patients with AF.     

生理性起搏与非生理性起搏对血浆心房利钠肽脑钠肽水平的影响

目的了解相同等级生理活动量时,生理性起搏(DDD)与非生理性起搏(VVI)之间血浆心房利钠肽(ANP)、脑钠肽(BNP)水平的差异。方法用放射免疫法测定56例起搏患者的血浆ANP、BNP浓度,其中生理性起搏17例,非生理性起搏39例。结果生理性起搏与非生理性起搏组之间血浆心房利钠肽、脑钠肽水平差异无统计学意义(P>0.05)。结论生理性起搏与非生理性起搏患者在相同等级生理活动量时血浆ANP、BNP水平差异无统计学意义。     

Ventricular Protection Against Atrial Arrhythmias in DDD Pacing Based on a Statistical Approach: Clinical Results

Atrial arrhythmias (AA) are commonly encountered in DDD paced patients. Newer dual chamber pacemakers (PM) possess mode switching functions that convert pacing to an asynchronous mode when AAs are detected. The lack of a reliable mode switch leading to rapid, irregular ventricular responses may result from AA undersensing. To avoid this, the DDDR PM Chorum 7234 Eta Medical AA diagnosis is based on a statistical approach: the PM constantly compares arrhythmic and sinus cycles and, based on "strong" and "weak" criteria, provides for rapid or slower mode switch. The aim of the study was to evaluate the efficiency and reliability of these two criteria. Thirty-one patients with a Chorum 7234 implanted for AV block (11), sinus dysfunction (10), both (5), or hypertrophic obstructive cardiomyopathy (5) were evaluated at 24 hours and 1 month using the internal memory (IM) of the PM, surface 24-hour Holter recordings, and exercise testing. Interrogation of the IM on the first day of study showed that 8 patients had mode switching episodes, based only on the strong criterion confirmed by the surface Holter recording. At I month, the IM revealed mode switching episodes in 12 patients, 6 of whom had used the weak criterion. No inappropriate mode switching episode was recorded during exercise testing at the 1-month follow-up. These results confirm the reliability and efficiency of this algorithm as well as the requirement for a specific algorithm to compensate for transient loss of sensing during AA.     

The Use of Atrial Pacing to Evaluate Patients with Definite or Suspected Wolff-Parkinson-White Syndrome

Four patients with definite or suspected WPW syndrome are presented in order to show that valuable clinical information can be obtained via simple atrial pacing. In three cases with a questionable resting ECG, atrial pacing produced pathognomonic changes in the QRS complex, establishing the diagnosis of WPW syndrome. In the fourth case, atrial pacing provoked the associated tachyarrhythmia, which had not previously been documented. In all four cases, functional properties of the accessory pathway could be assessed, and in three cases, the induction of atrial fibrillation allowed estimation of the risk of ventricular fibrillation. For evaluating patients with definite or suspected WPW syndrome, the technique of atrial pacing is recommended as an alternative to sophisticated electrophysiological studies which are costly and require special expertise and equipment. Atrial pacing is easier, cheaper, and less traumatic, and for many patients will provide most, if not all, the necessary information.     

DDD versus VVIR Pacing in Patients,Ages 70 and Over,with Complete Heart Block

Background : Dual‐chamber pacing is believed to have an advantage over single‐chamber ventricular pacing. The aim of the study was to determine whether elderly patients with implanted pacemaker for complete atrioventricular block gain significant benefit from dual‐chamber (DDD) compared with single‐chamber ventricular demand (VVIR). Methods : The study was designed as a double‐blind randomized two‐period crossover study—each pacing mode was maintained for 3 months. Thirty patients (eight men, mean age 76.5 ± 4.3 years) with implanted PM were submitted to a standard protocol, which included an interview, functional class assessment, quality of life (QoL) questionnaires, 6‐minute walk test, and transthoracic echocardiographic examinations. QoL was measured by the SF‐36. All these parameters were obtained on DDD mode pacing and VVIR mode pacing. Paired data were compared. Results : QoL was significantly different between the two groups and showed the best values in DDD. Overall, no patient preferred VVIR mode, 18 preferred DDD mode, and 12 expressed no preference. No differences in mean walking distances were observed between patients with single‐chamber and dual‐chamber pacing. VVI pacing elicited marked decrease in left ventricle ejection fraction and significant enlargement of the left atrium. DDD pacing resulted in significant increase of the peak systolic velocities in lateral mitral annulus and septal mitral annulus. Early diastolic velocities on both sides of mitral annulus did not change. Conclusion : In active elderly patients with complete heart block, DDD pacing is associated with improved quality of life and systolic ventricular function compared with VVI pacing. (PACE 2010; 583–589)     

AV Delay Optimization and Management of DDD Paced Patients with Dilated Cardiomyopathy

Ten DDD paced patients, suffering front dilated cardiomyopathy in the NYHA functional classes III or IV were studied by means of Doppler ecbocardiography at different programmed values of atrioventricular (AV) delay (200, 150, 120, 100, and 80 msec). The following variables were evaluated: LV diameter, ejection fraction, mitral and aortic flow velocity integrals, and stroke volume. During VDD pacing, a resting AV delay associated with the best diastolic filling and systolic function was identified and programmed individually. Shortening of the AV delay to about 100 msec was associated with a gradual and progressive improvement. Further decrease caused an impairment of systolic function. The patients were clinically and beinodynamically reevaluated after 2 months of follow-up. A reduction of NYHA class and an improvement of LV function were consistently found. The reported data suggest that programming of an optimal A V delay may improve myocardial function in DDD paced patients with congestive heart failure. This result may be the consequence of an optimization of left ventricular filling and a better use of the Frank-Starling law.     

Angiotensin‐converting enzyme inhibition prevents myocardial infarction‐induced increase in renal cortical cGMP and cAMP phosphodiesterase activities

We investigated whether myocardial infarction (MI) enhances renal phosphodiesterases (PDE) activities, investigating particularly the relative contribution of PDE1‐5 isozymes in total PDE activity involved in both cGMP and cAMP pathways, and whether angiotensin‐converting enzyme inhibition (ACEi) decreases such renal PDE hyperactivities. We also investigated whether ACEi might thereby improve atrial natriuretic peptide (ANP) efficiency. We studied renal cortical PDE1‐5 isozyme activities in sham (SH)‐operated, MI rats and in MI rats treated with perindopril (ACEi) 1 month after coronary artery ligation. Circulating atrial natriuretic peptide (ANP), its second intracellular messenger cyclic guanosine monophosphate (cGMP) and cGMP/ANP ratio were also determined. Cortical cGMP‐PDE2 (80.3 vs. 65.1 pmol/min/mg) and cGMP‐PDE1 (50.7 vs. 30.1 pmol/min/mg), and cAMP‐PDE2 (161 vs. 104.1 pmol/min/mg) and cAMP‐PDE4 (307.5 vs. 197.2 pmol/min/mg) activities were higher in MI than in SH rats. Despite increased ANP plasma level, ANP efficiency tended to be decreased in MI compared to SH rats. Perindopril restored PDE activities and tended to improve ANP efficiency in MI rats. One month after coronary ligation, perindopril treatment of MI rats prevents the increase in renal cortical PDE activities. This may contribute to increase renal ANP efficiency in MI rats.     

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)