Permanent pacing

Bradydysrhythmias such as third-degree AV block and sick sinus syndrome are the major indications for permanent pacing, requiring implantation of an artificial pacemaker.⁷ The pacemaker is a multi-programmable and implantable battery-powered pacing device. It is connected with one or more leads towards the right atrium or ventricle (single chamber pacemaker) or towards both (dual chamber pacemaker) to stimulate a specific chamber at a specific rate (e.g. 35 bpm) so that symptomatic bradycardia is avoided. However, because an intrinsic atrial and/or ventricular rhythm might still be present, pacemakers have the ability to sense this intrinsic rate via the implanted leads and can be programmed to respond in a specific way to an intrinsic beat. Besides the treatment of bradydysrhythmias, in humans, pacemaker implantation is also used to prevent induction of certain tachydysrhythmias.⁸ However, this technique has never been used in horses.

Pacemaker types are described using a three-letter code, with an optional fourth or even fifth letter. The first and second position indicate the chamber being paced and sensed, respectively: A (atrium), V (ventricle), D (double, both) or O (none). Manufacturers occasionally use the S to indicate that the device is capable of pacing/sensing only a single chamber. The third position denotes the reaction of the pacemaker when an intrinsic beat is sensed: I (inhibited) when pacing is inhibited by a sensed beat, T (triggered) when pacing is triggered, D (dual, both) for a dual mode of response, or O (none) when the pacemaker does not react on a sensed signal. The fourth position of the code may reflect the programmability of the device but is most frequently used to indicate that the pacemaker incorporates a sensor for rate modulation (R). Such a sensor detects changes in pressure, temperature, oxygen saturation, ventilation or detects patient activity and is capable of adapting the pacing rate towards an upper or lower limit. The fifth letter is rarely used and is restricted to antitachycardia functions: P (antitachycardia pacing), S (direct current shock), D (dual, P+S), or O (none). After implantation multiple pacing and sensing parameters can be adapted by telemetric (transcutaneous) programming of the pacemaker. Telemetric access requires the pacemaker to be implanted at a short distance from the skin at a location where it does not get damaged or hamper the animal to move or lie down.

Depending on the kind of dysrhythmia, different types of pacemakers can be applied. If the cause of bradycardia is a depressed or absent AV node conduction, a “simple”, single chamber (ventricular) pacemaker can be implanted. When this pacemaker is programmed to the VVI mode, with a minimal ventricular rate of, for example, 35 beats per minute, syncope will be prevented by preserving this minimal heart rate. However, no adaptation of heart rate to the physical needs (e.g. exercise) will occur. Rate-modulation, for example, based upon an activity sensor, is achieved with a VVIR pacemaker. However, much more efficient in case of a third-degree AV block, is to implant a dual chamber pacemaker. Such a pacemaker can be programmed to prevent a too slow atrial and ventricular rate but also to “look for” (sense) an intrinsic atrial depolarisation and to deliver (trigger) a ventricular stimulus for each sensed atrial signal, which is achieved in the DDD function (Fig. 14.3). As sinus node function in an animal with third-degree AV block is usually intact, a fairly normal paced ventricular rate, which adapts with stress or exercise, can then be obtained. If bradycardia is caused by an abnormal sinus node function, adaptation of pacing rate with exercise can only be achieved by using a pacemaker with an incorporated sensor (e.g. VVIR, DDIR).

Figure 14.3 During DDD pacing, every atrial spontaneous depolarization is sensed (AS), which triggers a ventricular paced beat (VP).

In horses, both epicardial and transvenous lead placement has been described. The epicardial lead placement requires general anaesthesia and thoracotomy, which implies a risk, especially in an animal with a compromised cardiovascular system.^9,10 This method has successfully been applied in a healthy pony and in two donkeys with third-degree AV block for implantation of a single chamber or dual chamber pacemaker.^11,12,13 The pacemaker device was inserted underneath the pectoral muscle or, subcutaneously, caudal to the left elbow. In the donkeys, successful pacing was obtained for 6 weeks in one and more than 1 year in the other. During an attempt to place an epicardial lead in a mature horse with third-degree AV block, the horse developed ventricular fibrillation during general anaesthesia and died.¹⁴

Transvenous lead placement is a much safer and simpler procedure to achieve cardiac pacing and is widely used in human and small animal medicine. It requires a vein that is located relatively close to the heart (maximal lead length is around 110 cm), that is large enough to introduce one or two leads and that can be ligated permanently. A standardized approach has been described to implant a dual chamber pacemaker via the cephalic vein.⁶ As the whole procedure can be performed in the standing, sedated horse, the additional risk of anaesthesia can be avoided. During the implantation procedure, an atrial and a ventricular active fixation lead are inserted through a surgically exposed cephalic vein. Lead placement is guided by both echocardiography and by measuring the electrical characteristics of the lead. After permanent ligation of the vein a pacemaker pocket is created between the lateral pectoral groove and the manubrium sterni. Successful clinical application of transvenous lead placement was first reported by Reef et al.,¹⁵ implanting a single chamber (ventricular) pacemaker in a horse with third-degree AV block. Under general anaesthesia, a passive fixation lead was inserted in the jugular vein and placed in the right ventricular apex under echocardiographic guidance. The pacemaker pocket was created dorsal to the jugular vein. Although the horse remained asymptomatic, 16 months later, a second lead with active fixation was implanted in the right atrium and the pacemaker was updated to a dual chamber model, allowing AV sequential pacing of the ventricles. Consequently, a physiological rate response to exercise and stress occurred. With the dual chamber pacemaker the horse was able to perform exercise with a maximal achievable heart rate of 150 beats per minute. About 3 years after the initial implantation, however, the horse suddenly died due to a suppurative endocarditis and suspected terminal bacteraemia.¹⁶ Implantation of a dual chamber rate-adaptive pacemaker has also been used in a horse with post-exercise bradycardia and syncope due to sinus node dysfunction.¹⁷ At 7-year follow-up, proper dual chamber pacing was still achieved.

Temporary pacing

Short-term therapeutic pacing is generally applied as a temporary solution for bradydysrhythmias, e.g. prior to the implantation of a permanent pacemaker and during neonatal cardiopulmonary resuscitation. On rare occasions, it can also be used to terminate certain tachydysrhythmias. Temporary pacing is achieved with an external, temporary pacing device, connected to two electrodes (bipolar pacing) which are positioned near the heart, usually near the ventricles, in order to accelerate ventricular rate. The closer the electrodes are positioned to the heart, the smaller the pulse intensity required to elicit a ventricular depolarization, and thus the better pacing will be tolerated. Although little information is available in equine medicine, transcutaneous and transoesophageal pacing are likely to be poorly tolerated in the conscious adult horse and therefore only applicable during general anaesthesia, but transcutaneous temporary pacing can be used successfully in sick, obtunded foals. ( SBR)

A transvenously inserted catheter, electrode or lead within the right atrium or right ventricle can directly stimulate the endomyocardium, requiring only low pulse intensity, and is a safe and effective means to obtain consistent cardiac pacing. As the pacing stimuli are not sensed by the horse the technique can be used without sedation or anaesthesia. Temporary transvenous pacing has been used in horses with symptomatic bradycardia to avoid syncope prior to the permanent implantation of a pacemaker.^{13–15,17,18} Transvenous temporary pacing has also been used to treat transient asystole following electrical cardioversion of atrial fibrillation (AF) in a horse.¹⁹

In human beings, temporary pacing is also used to terminate tachydysrhythmias such as atrial flutter, AV node tachycardia and sustained ventricular tachycardia.⁷ During such a transvenous overdrive pacing procedure, a train of electrical pulses is repeatedly given at a fixed rate in excess of the basic natural rhythm. The purpose is to entrain and interrupt the tachydysrhythmia by depolarizing regions of the excitable gap of the re-entry cycle.^8,20 This technique has been successfully applied in a horse with persistent, medically resistant atrial flutter.²¹ In this horse, right atrial pacing at a rate slightly higher (300 ms cycle length) than the atrial flutter rate (365 ms cycle length) terminated the re-entry phenomenon and re-established sinus rhythm. The procedure was performed in the standing unsedated horse and was well tolerated. However, because overdrive pacing can accelerate tachycardia and result in fibrillation, the technique is not recommended for ventricular tachycardia in horses.