Atrial pacing

In healthy volunteers, tocainide produced a slight depression in His-Purkinje conduction as well as a slightly delayed enhancement of A-V node conduction during atrial pacing. No significant alterations in heart rate, right ventricular ERP or the excitation thresholds of atrial or ventricular muscle were observed in these subjects. 

Carnes CA, Chung MK, Nakayama T, Nakayama H, Baliga RS, Piao S, Kan-derian A, Pavia S, Hamlin RL, McCarthy PM, Bauer JA, Van Wagoner DR. Ascorbate attenuates atrial pacing-induced peroxynitrite formation and electrical remodeling and decreases the incidence of postoperative atrial fibrillation. Circ Res 2001 89 E32-E38. 

Gk, the activator of ligand-gated K+ channels mechanism of muscarinic regulation of atrial pacing... 

Clonidine causes sinus bradycardia and atrioventricular block, as illustrated by two cases, one a 10-year-old boy (6) and the other a 71-year-old woman (7), who developed Wenckebach s phenomenon. Clonidine was also studied in seven patients subjected to electrophysiological studies after 5 weeks of therapy (8). It slowed the sinus rate and increased the atrial pacing rate, producing Wenckebach s phenomenon, indicating depressed function of the sinus and AV nodes. 

In 24 patients with atrial fibrillation who underwent elective transvenous cardioversion for atrial fibrillation, flecainide reduced the energy requirements for further defibrillation after induction of atrial fibrillation by atrial pacing (27). There were no ventricular dysrhythmias, but transient bradycardia requiring ventricular pacing occurred in two patients. Two patients had transient asymptomatic hjrpotension after flecainide and one reported transient dizziness and some hght-headedness. 

Ventricular pace/sense Atrial pace/sense... 

FIGURE 15.6 Record stored by an ICD of successful delivery of a defibrillation. An atrial EGM, (top tracing), ventricular electrogram (middle tracing), and ICD marker channel are shown. Markers include ventricular sense (VS), atrial sense (AS), ventricular pace (VP), atrial pace (AP), end of ICD capacitor charge (CE), and delivery of shock (CD). (Reproduced with permission of Medtronic, Inc.)... 

Changes in blood volume are sensed by an intratho-racic volume receptor which stimulates vasopressin secretion. The most direct evidence for the existence of a volume receptor was obtained by distending the left atrium, an operation that leads to increased diuresis. Vagotomy abolishes the diuretic response to atrial distension. Increased atrial pressure can also be produced in anesthetized dogs by atrial pacing. This is normally accompanied by increased diuresis. Both va-... 

Mond HG, Hua W and Wang CC Atrial pacing leads the clinical contribution of steroid elution. PACE 1995 18 1601-1608. 

Hua W, Mond H and Sparks P The clinical performance of three designs of atrial pacing leads from a single manufacturer the value of steroid elution. Eur J.C.P.E... 

Fig. 2.15 Interrogation screen from Intermedics Marathon DR pacemaker showing percentages of cycles in five categories by rate range (ApVp, atrial pace followed by ventricular pace AsVp, atrial sense followed by ventricular pace, etc. PVE, premature ventricular event).

In the AAI mode, atrial pacing will occur when the atrial rate falls below the programmed atrial rate (Fig. 3.1). When no intrinsic atrial activity has occurred within a specified interval (lower rate limit, LRL), atrial pacing occurs. The letter P is commonly used to indicate an atrial sensed event and the letter A is commonly used to indicate an atrial paced event. The intervals used in the AAI timing intervals are PP, AA, AP, and PA. Ventricular events such as a conducted QRS complex or a premature ventricular beat are not sensed in the AAI mode. The AAI mode is only appropriate in patients with intact... 

Fig. 3.1 Normal function in the AAI mode. P refers to a sensed atrial event. A refers to a paced atrial event. Two successive events may be described as one of the following intervals AA, PP, PA, and AP. The LRL (also called the low rate timer) in the example shown is 60bpm. Thus, the PA and AA intervals are 1,000ms. The P wave is sensed and after 1000ms the low rate timer expires. An atrial paced event occurs (A). A second atrial paced event occur after 1000ms. Sensing atrial activity inhibits the pacemaker output and resets the low rate timer. In the AAI mode, timing is based on the time between atrial events.

Fig. 3.3 Normal function in the DDD mode. R refers to a sensed ventricular event. V refers to a paced ventricular event. P refers to a sensed atrial event. A refers to a paced atrial event. The atrioventricular relationship is described by the intervals PR, AR, AV, PV. P waves (sensed atrial events) may he tracked and followed by ventricular-paced events (PV) or P waves may be conducted to the ventricle (PR). Atrial-paced events may be followed by a ventricular-paced event (AV) or may be conducted to the ventricle (AR).

Fig. 3.4 VDD mode. Sensing occurs in both the atrium and the ventricle but pacing only occurs in the ventricle. Atrial events may be tracked and followed by ventricular-paced events (PV). When there is a sinus pause or a PVC not followed by a P wave, the next event will be a ventricular-paced event rather than an atrial-paced event in DDD mode. The AV and PV intervals can be programmed separately in some pulse generators.

The DVI mode provides pacing in the atrium and the ventricle, but sensing occurs only in the ventricle (Fig. 3.8). Therefore, like the DDI mode, atrial tracking does not occur. However, unlike DDI, atrial events do not inhibit atrial output. Because atrial output occurs without regard to intrinsic atrial activity, in the DVI mode atrial pacing after an intrinsic atrial event may precipitate an atrial tachyarrhythmia. This pacing mode is essentially obsolete. 

Fig. 3.15 AAI mode with hysteresis. In this example, the LRL is 1,000ms with a hysteresis interval of 1,200ms. Pacing occurs after the hysteresis interval of 1,200ms. The next atrial paced event (third P wave) occurs after 1000ms.

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