Atrial pacing for rhythm control of atrial fibrillation

After atrial fibrillation (AF) develops, the first step is to search for and treat underlying (heart) s. Thereafter, AF should be treated. This includes prevention of cardiovascular morbidity and mortality, especially vascular events, and reduction of symptoms.1 The latter may be obtained by two treatment strategies: rhythm-control and/or rate-control treatment. Recent randomised trials have shown that rate control is not inferior to rhythm control with regard to cardiovascular morbidity and mortality.2 In these studies, predominantly elderly patients with underlying heart s (especially hypertension) were included. Patients with (severely) symptomatic AF and advanced heart failure were excluded. Since then, rate-control treatment has been adopted more frequently, even as first-choice therapy, especially in the elderly.     

Termination of atrial flutter by directed transesophageal atrial pacing during transesophageal echocardiography.

INTRODUCTION: The purpose of this study was to evaluate termination of atrial flutter (AFL) by directed rapid transesophageal atrial pacing (TAP) with and without simultaneous transesophageal echocardiography (TEE) performed using a novel TEE tube electrode. MATERIALS AND METHODS, AND RESULTS: A total of 16 AFL patients (age 63+/-12 years; 13 males) with mean AFL cycle length of 224+/-24 ms (n=12) and mean ventricular cycle length of 448+/-47 ms (n=12) were analyzed using either an esophageal TO electrode (n=10) or a novel TEE tube electrode consisting of a tube with four hemispherical electrodes that is pulled over the echo probe (n=6). AFL could be terminated by directed rapid TAP using an esophageal TO electrode, leading to induction of atrial fibrillation (AF) (n=6), induction of AF and spontaneous conversion to sinus rhythm (SR) (n=3), and with conversion to SR (n=1). AFL could also be terminated by directed rapid TAP using the TEE tube electrode, with induction of AF (n=3) or induction of AF and spontaneous conversion to SR (n=3). CONCLUSION: AFL can be terminated by directed rapid TAP with hemispherical electrodes with and without simultaneous TEE. TAP with the directed TEE tube electrode is a safe, simple, and useful method for terminating AFL.     

Differential impact of short periods of rapid atrial pacing on left and right atrial mechanical function

Current techniques to describe atrial function are limited by their load dependency and hence do not accurately reflect intrinsic mechanical properties. To assess the impact of atrial fibrillation on atrial function, combined pressure-volume relationships (PVR) measured by conductance catheters were used to evaluate the right (RA) and left (LA) atrium in 12 isoflurane-anesthetized pigs. Biatrial PVR were recorded over a wide range of volumes during transient caval occlusion at baseline sinus rhythm (SR), after onset of rapid atrial pacing (RAP), after 1 h of RAP, after conversion to SR, and after 1 h of recovery. Cardiac output decreased by 16% (P = 0.008) with onset of RAP. Mean LA and RA pressures increased by 21 and 40% (P < 0.001), respectively, and remained elevated during the entire recovery period. RA reservoir function increased from 51 to 58% and significantly dropped to 43% after resumption of SR (P = 0.017). Immediately after RAP, a right shift of LA end-systolic PVR-intercept for end-systolic volume required to generate an atrial end-systolic pressure of 10 mmHg (24.4 ± 4.9 to 28.1 ± 5.2 ml, P = 0.005) indicated impaired contractility compared with baseline. Active LA emptying fraction dropped from 17.6 ± 7.5 to 11.7 ± 3.7% (P < 0.001), LA stroke volume and ΔP/Δt(max)/P declined by 22% (P = 0.038 and 0.026, respectively), while there was only a trend to impaired RA systolic function. Stiffness quantified by the ratio of pressure to volume at end-diastole was increased immediately after RAP only in the RA (P = 0.020), but end-diastolic PVR shifted rightward in both atria (P = 0.011 LA, P = 0.045 RA). These data suggest that even short periods of RAP have a differential impact on RA and LA function, which was sustained for 1 h after conversion to SR.     

Echocardiographic [correction of Ehocardiographic] evaluation of regional left atrial function after rapid atrial pacing

Rapid regular atrial pacing (RAP) produces changes in atrial function similar to those caused by atrial fibrillation in animal models. Left atrial appendage (LAA) function represents regional atrial function. The aim of our study was to investigate the influence of RAP on left atrial regional function and to evaluate the reversibility of changes after termination of pacing in a canine model. Eight dogs were subjected to RAP (400 bpm) for 16 days. Transesophageal echocardiography was performed at baseline, immediately after RAP and 4 weeks after the termination of RAP. The LAA peak late emptying velocity (LAA-E) and filling wave (LAA-f) were measured. LAA-E velocities were significantly reduced and filling wave velocities (LAA-f) were significantly less negative after RAP compared with the baseline values. Four weeks after termination of pacing, the LAA-E and LAA-f velocities were normal. RAP results in impaired regional atrial systolic and diastolic function. The changes were completely reversible 4 weeks after termination of pacing. These results suggest that the LAA is mechanically stunned after RAP.     

Effect of c‐Ski on atrial remodelling in a rapid atrial pacing canine model

Atrial fibrosis is an important factor in the initiation and maintenance of atrial fibrillation (AF); therefore, understanding the pathogenesis of atrial fibrosis may reveal promising therapeutic targets for AF. In this study, we successfully established a rapid atrial pacing canine model and found that the inducibility and duration of AF were significantly reduced by the overexpression of c‐Ski, suggesting that this approach may have therapeutic effects. c‐Ski was found to be down‐regulated in the atrial tissues of the rapid atrial pacing canine model. We artificially up‐regulated c‐Ski expression with a c‐Ski–overexpressing adenovirus. Haematoxylin and eosin, Masson's trichrome and picrosirius red staining showed that c‐Ski overexpression alleviated atrial fibrosis. Furthermore, we found that the expression levels of collagen III and α‐SMA were higher in the groups of dogs subjected to right‐atrial pacing, and this increase was attenuated by c‐Ski overexpression. In addition, c‐Ski overexpression decreased the phosphorylation of smad2, smad3 and p38 MAPK (p38α and p38β) as well as the expression of TGF‐β1 in atrial tissues, as shown by a comparison of the right‐atrial pacing + c‐Ski‐overexpression group to the control group with right‐atrial pacing only. These results suggest that c‐Ski overexpression improves atrial remodelling in a rapid atrial pacing canine model by suppressing TGF‐β1–Smad signalling and p38 MAPK activation.     

Impaired sinus node function and global atrial conduction time after high rate atrial pacing in dogs

In animal and human studies, it has been shown that atrial fibrillation shortens the atrial refractory period and impairs its rate adaptation. The objective of this study was to evaluate the effects of high-rate pacing on sinus node function and intra-atrial conduction. Eight dogs were subjected to rapid atrial pacing (AP) at 400 bpm for 16 days. Sinus node recovery time (SNRT) and P-wave duration were measured at baseline, immediately after AP and four weeks after the termination of AP. SNRT immediately after AP was significantly prolonged at all pacing rates compared to the baseline values. P-wave duration was significantly longer after AP relative to the baseline values. All the variables were completely reversible four weeks after the termination of pacing. Rapid AP induces sinus node dysfunction and prolongs the intra-atrial conduction time. It is possible that the electrical remodelling extends to the sinus node as well.     

Fast pacing facilitates discontinuous action potential propagation between rabbit atrial cells

We examined the critical coupling conductance (G(C)) for propagation at different pacing cycle lengths (CLs) (1,000 and 400 ms). As G(C) was progressively reduced, propagation failed at a CL of 1,000 ms, whereas propagation succeeded at a CL of 400 ms over a range of G(C) values before failing at a CL of 400 ms at a lower G(C), showing facilitation of propagation at the shorter CL. Critical G(C) was (means +/- SE) 0.8 +/- 0.1 nS for a CL of 400 ms and 1.3 +/- 0.1 nS for a CL of 1,000 ms (a 63% increase, P < 0.002, n = 9 cell pairs). In 14 uncoupled cells, action potential duration at 30% repolarization (APD(30)) increased from 19.9 +/- 2.5 to 41.8 +/- 2.6 ms (P < 0.001) as CL decreased from 1,000 to 400 ms. In five cell pairs, critical G(C) with 4-aminopyridine (4-AP) was reduced to 0.4 +/- 0.1 nS at a CL of 1,000 ms (P < 0.05 compared with control solution), and critical G(C) in 4-AP was unchanged by decreasing CL to 400 ms. It is possible that the "remodeling" of atrial cells due to atrial fibrillation or tachycardia, which has been shown to produce a decrease in the transient outward current, may result in an enhanced ability to propagate, possibly facilitating further development of fibrillation under conditions of decreased cellular coupling.     

Dual chamber pacing mode in an atrial antitachycardia pacing device without a ventricular lead – A necessary evil

We present a case of a single chamber atrial pacemaker implanted for sinus node dysfunction and treatment of macroreentrant atrial tachycardias with atrial antitachycardia pacing. The patient presented with sustained atrial tachycardia above the detection rate, however, the device was unable to detect the tachycardia and did not deliver the programmed therapy. We discuss the nuances of the atrial tachyarrhythmia detection algorithms, and the programming strategies to maximize detection of atrial arrhythmias in a single chamber atrial pacemaker.     

Systematic nocturnal atrial demand pacing results in high-output heart failure.

Beat-to-beat parameters of heart rate (HR), intra-arterial blood pressure (BP), central venous pressure, and derived indexes of cardiac output and total peripheral resistance were recorded 18 h/day (from 1800 to 1200 h the following day) in four monkeys (Macaca mulatta) during 20 control days followed by 20 days of atrial demand pacing. The pacing rate was set at approximately 10 beats/min above the fastest hourly average HR recorded during the control period, i.e., sufficient to prevent the normal nocturnal fall in HR. Nocturnal pacing resulted in progressive weekly increases in central venous BP and arterial BP. Analyses of levels and diurnal trends in hemodynamic parameters and cardiac function curves across consecutive 5-day periods of nocturnal pacing revealed a hemodynamic pattern characteristic of high-output heart failure, which progressively increased (week by week) during the early morning hours (0500-0700). Sustained elevated left ventricular work resulting from the prevention of a nocturnal fall in HR may have been responsible for the reduction in cardiac function seen in this experimental model.     

Neutral endopeptidase inhibitor suppresses the early phase of atrial electrical remodeling in a canine rapid atrial pacing model

Introduction

We examined the acute effects of neutral endopeptidase inhibitor on the hemodynamics and electrical properties of dogs subjected to rapid atrial pacing.

Methods

Ten beagle dogs were used and divided into two groups with and without candoxatril, a neutral endopeptidase inhibitor preadministration. Before and after the 6 hours rapid atrial pacing from the right atrial appendage, the hemodynamics, atrial effective refractory period, and monophasic action potential duration of the right atrial appendage were measured and blood samples were collected. Atrial tissue was also excised after the experiment.

Results

Candoxatril significantly increased plasma ANP levels (Control: 88.4 ± 50.25 vs. Candoxatril: 197.1 ± 32.09 pg/ml, p = 0.004) and prevented reductions in atrial effective refractory period and monophasic action potential duration. We further demonstrated that the treated animals exhibited significantly higher levels of atrial tissue cyclic GMP (Control: 28.1 ± 1.60 fmol/mg vs. Candoxatril: 44.5 ± 12.28 fmol/mg, p = 0.034) as well as that of plasma cyclic GMP (Control: 32 ± 5.5 vs. Candoxatril: 42 ± 7.1 pg/ml, p = 0.028).

Conclusion

Candoxatril suppressed the shortening of atrial effective refractory period and monophasic action potential duration in the rapid atrial pacing model. As plasma ANP and the atrial tissue levels of cyclic GMP were higher in the Candoxatril group than the control, this effect was considered to appear through the reduction of calcium overload caused by ANP and cyclic GMP.     

Effect of nocturnal atrial demand cardiac pacing on diurnal hemodynamic patterns.

Heart rate (HR), stroke volume (SV), intra-arterial blood pressure, and central venous pressure were recorded on a beat-to-beat basis, 18 h/day (1800-1200 h the following day), for approximately 2 mo in four monkeys (Macaca mulatta). Cardiac output, left ventricular work, and total peripheral resistance were derived from these primary measurements. During the 1st mo we measured these parameters under control conditions, and during the 2nd mo the animals were studied while HR was paced by atrial demand pacing sufficient to prevent the normal nocturnal fall in HR (approximately 10 beats/min above the fastest hourly average rate recorded during the control condition). The main hypothesis of this study was that when HR is prevented from falling, SV, which normally does not fall overnight, would fall; this hypothesis was confirmed. In addition, we observed that, during the period of pacing, relative to the control period, SV was approximately 14% greater during the early evening and 4% lower during the early morning; total peripheral resistance was similar during the early evening but was 13% higher by morning. Throughout the night, systolic pressure was approximately 4% greater, diastolic pressure was 17% higher, central venous pressure was 43% greater, and left ventricular work was 27% higher. These findings show that when HR is prevented from falling overnight by atrial demand pacing, even to a relatively modest degree, there can be very significant sustained changes in cardiovascular function.     

Attenuation of liver ischemia-reperfusion-induced atrial dysfunction by external pacing but not by isoproterenol

Remote ischemia-reperfusion detrimentally affects myocardial function by initially interfering with the rate of contraction. We investigated the usefulness of isoproterenol versus external electrical pacing in attenuating secondary functional damage of isolated Wistar rat atria. Atrial strips (n = 10/group) were bathed within oxygenated Krebs-Henseleit solution that exited from isolated livers that had been either perfused normally (controls) or underwent no flow (ischemia) for 2 h. In addition to one noninterventional ischemia-exposed strip group, a second group was externally paced at a fixed rate (55 pulses.min-1, 6 V) and a third "ischemia" group was treated with isoproterenol (0.1 mM), both interventions commencing upon the strips' exposure to the hepatic effluents. Control strips displayed unaltered contraction rate and systolic-generated tension during the 2-h exposure. Nontreated strips exposed to ischemic reperfusate experienced bradycardia compared with baseline values (7 +/- 2 vs. 50 +/- 12 beats.min-1, p < 0.05), followed <1-min later by a fall in the generated tension (11 +/- 4 vs. 20 +/- 6 mmHg, p < 0.05). The paced-ischemic strips displayed unaltered rate and force of contraction, whereas the addition of isoproterenol did not prevent deterioration in the rate and force of contraction (8 +/- 3 beats.min-1, 12 +/- 4 mmHg, respectively; p < 0.05 vs. baseline control ischemia-paced strips). Thus, external electrical pacing prevented liver ischemia-reperfusion-induced atrial strips' bradycardia and loss of contractility, while isoproterenol did not.     

Use of a novel pacing mode to achieve biventricular pacing in a patient with recurrent atrial lead dislodgement after CRT-D implantation

Cardiac resynchronization therapy device (CRT-P and CRT-D) implantation has increased tremendously with increasing operator experience, eligible patients and expansion of indications. Refinements in devices and algorithms now aid physicians to improve biventricular pacing and optimize CRT. We report a case in which an interesting device program was used to achieve biventricular pacing after repeated dislodgement of the atrial lead in a patient implanted with CRT-D.     

Plasma atrial natriuretic factor and atrial wall stress in hypertensive and normotensive rabbits after pacing and volume expansion.

Atrial natriuretic factor (ANF) is present in high concentration in atria but in very low concentration in the ventricles. Under conditions of haemodynamic overload ventricular gene expression may become activated, but it is not clear if ventricular ANF can be released through a regulated or constitutive pathway. The purpose of this study was to determine whether basal and stimulated release of ANF are increased in perinephritic rabbits with mild hypertension. Six rabbits were rendered hypertensive by wrapping both kidneys in cellophane, and six sham-operated rabbits were used as controls. Eight weeks after renal wrapping, mean arterial pressure was approximately 20 mmHg higher in the experimental group. After anaesthesia, the renal-wrapped group had a higher vascular resistance. Right and left atrial wall stress was measured using sonomicrometry. Volume expansion by 30% of blood volume, using donor blood, caused a small increase in right and left atrial diastolic and systolic wall stress but did not significantly increase plasma ANF. Pacing the heart at 6 Hz caused increases in systolic but not diastolic wall stress and caused a significant increase in plasma ANF; the increase was larger after volume expansion. There were no significant differences between the responses of the experimental and control groups. It is concluded that mild hypertension, in the rabbit, does not lead to changes in atrial wall stress or either basal or stimulated release of ANF.