Pacemaker

Conduction system of the heart

The heart beats regularly because there is natural pacemaker for the heart which gives out regular synchronized pulses which causes the heart to contract in a sequence. This pacemaker is known as sino-atrial node (SA node) and is situated in the right atrium (upper chamber of the heart). The pulses are conducted down the atrium to reach a relay station known as the atrio-ventricular node (AV node). In the AV node the pulse is delayed for a fraction of a second and conducted to the ventricles (lower chambers of the heart) through the bundle of His. The bundle of His gives a branch to each ventricle – the left and right bundle branches. The left bundle gives of two smaller branches. All the bundle branches subdivide further and reach very small fibres known as Purkinje fibres. The Purkinje fibres are connected to the heart muscle cells.

Funny current: pacemaker current or I_f current

The funny current or pacemaker current is predominantly a feature of the sinoatrial node. It is also seen in atrioventricular node and the Purkinje fibres. It is a mixed sodium-potassium current which is inward and gets activated on hyperpolarization. The funny current is responsible for the spontaneous diastolic depolarization which ultimately leads to the automaticity of the sinus node. Since it controls the rate of the sinus node activity, it determines the heart rate. In addition to the diastolic voltage, the funny current activation is also dependent on cyclic AMP and hence can be modulated by the autonomic nervous system. HCN channel (hyperpolarization activated cyclic nucleotide) mediates the funny current (I_f). Four types of HCN channels (HCN 1-4) are known at present. HCN4 mutations have been implicated in sinus node dysfunction.

Though the funny current has been described over a quarter of a century back, it has come into attention recently due to the availability of selective I_f current blockers like ivabradine, a pure sinus node inhibitor without any other hemodynamic effect. Zatebradine and cilobradine are two analogues. A similar I_h current has been described in different types of neurons. I_f current has also been targeted in the development of a potential biological pacemaker.

I_f pacemaker current was discovered by Professor D. DiFrancesco in 1979.

When is an artificial pacemaker needed?

When the natural pacemaker or the conduction system of the heart stops functioning properly, the heart rate may fall to dangerously low levels, producing giddiness or loss of consciousness. This situation requires an artificial pacemaker, either temporary or permanent, depending on the reversibility of the disease causing the defect. Commonest disease causing dysfunction of SA node is known as sick sinus syndrome, while the commonest defect of the AV node is known as complete heart block. These are the most common reasons for implantation of a permanent artificial pacemaker. Temporary pacemaker is most often required in a myocardial infarction (heart attack) with complete heart block.

Pacemaker on X-ray chest PA view

The pacemaker pulse generator  is seen in the right infraclavicular region, implanted in the subcutaneous plane. There is little bit of sagging of the pulse generator pocket being an elderly subject with loose subcutaneous tissue. The pacemaker lead is seen coursing up into the subclavian vein and hence into the right atrium through the superior vena cava. The tip of the lead is in the right ventricular apex asit is a single chamber (VVI) pacemaker. Additional atrial lead will be seen in case of dual chamber (DDD) pacemaker. A left ventricular lead will be there for biventricular pacemakers used for cardiac resynchronisation therapy.

Permanent pacemaker and dilated aorta

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Permanent pacaemaker with its lead in situ. It is a single chamber pacemaker implanted in the right pectoral region with the lead implanted by a subclavian route. The pulse generator (PM) is connected to the lead which passes through the subcutaneous tissue into the right subclavian vein and hence into the superior vena cava. The lead can be further traced into the right atrium and the right ventricle (the ventricular part is not clear in this picture and needs a penetrated view). Tha aorta is dilated in this elderly person.

This is a single chamber ventricular pacemaker (VVI), though the ventricular portion of the lead is not seen here. If it was an atrial pacemaker (AAI) the lead would not have come down the lateral aspect of the right atrium, but would have been curving up into the right atrial appendage, the usual site for right atrial pacing.

Dual chamber pacemaker – X-ray chest PA

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Dual chamber pacemaker with leads seen on a chest x-ray PA view. The pulse generator consists of battery and circuitry. The circuitry controls and monitors the function of the pacemaker. It can also transmit the data using radiofrequency signals to a programmer head kept over the pacemaker. Two leads are connected to the pacemaker (pulse generator) and firmly fixed in situ by screws. The pulse generator position is a little down because of sagging within the pacemaker pocket as the subcutaneous tissue is lax. The two leads fixed to the connectors can be seen coursing up to enter the subclavian vein and hence into the innominate vein, superior vena cava and the right atrium. The atrial lead terminates in the right atrium while the ventricular lead can be traced further down. Usually the atrial lead is kept in the right atrial appendage while the ventricular lead is kept at the right ventricular apex. If there is difficulty in anchoring, a screw in lead can be used. A more penetrated view will be needed (or image intensifier fluroscopy) to see the actual lead tips and the entire intra cardiac course of the leads. Magnified fluroscopic views are useful to look for lead fractures.

Scout scan of an implanted pacemaker and lead

The initial image obtained during CT scanning is usually called a scout image, pilot or topogram. Sometimes it is also called a scanograph. The scout image is used to determine the exent of slices to be taken during axial scan. The gantry is kept in a fixed position and the table is moved as the x-ray beam is delivered to record the scout scan. The scout image is similar to radiograph in appearance. The image below shows an implanted pacemaker and the lead very well unlike in a conventional radiograph.

Pacemaker is seen in the right pectoral position, a single chamber device used for ventricular pacing. The lead entering the subclavian vein and tracking down through the superior vena cava into the right atrium and across the tricuspid valve (more or less in the midline) into the right ventricle is seen well. Both the proximal ring electrode and the distal electrode of the lead are seen fairly well. A penetrated view of x ray chest PA view can also show the lead fairly well, but it will darken out the lung fields very much.

Temporary pacing electrode

Tip of a bipolar temporary pacing electrode showing a distal tip electrode and a proximal ring elecrode. It is customary to give negative polarity to the tip and positive polarity to the ring electrode, as cathodal pacing has a lower threshold. Threshold is the minimum voltage needed for capturing the chamber paced. The proximal connectors of the lead are connected to an external pacemaker.

Unipolar VVI Pacing

Unipolar VVI Pacing: Leads L1-L3

Unipolar VVI Pacing: Leads aVr, aVl, aVf

The sharp vertical deflection preceding each QRS complex is the pacing spike or artefact. The ECG shows a large amplitude pacing spike indicating that the pacing mode is unipolar. In unipolar mode, the electrode at the tip of the pacing lead acts as the cathode and the pacemaker can as the anode. Cathodal pacing has better threshold than anodal pacing. That is why the lead tip is programmed as cathode. In bipolar pacing, the lead tip still acts as the cathode while a proximal ring electrode acts as the anode. In that situation, as the current circuit is completed within the ventricle itself, the spillover to the surface ECG recording electrode is small so that pacing spikes are quite small and hardly visible in some leads. Temporary pacing is always bipolar and the pacemaker is not implanted within the body and hence cannot act as an anode. Permanent pacing can be programmed in unipolar or bipolar modes. Similarly the sensing of the lead can also be programmed either unipolar or bipolar. Unipolar pacing can sometimes cause local pacing in the region of the pacemaker can. This is more likely if the active side of the can is kept in contact with the pectoral muscle. Usually the pacemaker is placed within the pocket in such a way that the active side faces the posterior aspect of the skin. This will decrease the chance of local pacing in case programming in unipolar mode is required.

Even though this ECG is from a person implanted with a VVI pacemaker, it is not possible to make out that from this ECG as all the QRS complexes are paced complexes. Demand function can be identified only when there are spontaneous QRS complexes, in which case the pacemaker will wait for the programmed interval before giving out the pacing spike once it senses a QRS complex. If the sensing is defective, the pacemaker will function as a fixed rate pacemaker ignoring spontaneous QRS complexes. That mode will be termed VOO mode. Spontaneous P waves and AV dissociation are evident on close scrutiny of the ECG, especially in aVf. Lead I shows an LBBB type pattern due to the right ventricular location of the pacing lead. Inferior leads show negative QRS complexes, indicating an activation proceeding from below upwards, suggesting the location of the lead tip in the right ventricular apex. The axis will be downward in outflow pacing. Left ventricular pacing will give a right bundle branch block pattern. Pacing of the septal aspect of the right ventricular outflow tract is being evaluated as a possible way of more physiological pacing in terms of activation sequence of the two ventricles, mimicking the natural sequence.

Unipolar VVI Pacing: LeadsV1 – V3

Unipolar VVI Pacing: LeadsV4 – V6

Though we would expect a dominant positive QRS in V5, V6 in a left bundle branch block (LBBB) pattern with right ventricular pacing, when the pacing is done from the right ventricular apex, V5 , V6 shows a predominantly negative QRS complex as the activation proceeds away from the apical region. This is evident in the tracing shown above. The rS pattern in V6 is likely to be thought of as an RBBB pattern and hence indicating left ventricular pacing. But lead I and aVl show the LBBB like pattern of right ventricular pacing and so does V1, showing a negative QRS, though of smaller amplitude in this case.

Pacemaker tips

Rheobase: Minimum voltage which will capture the myocardium at any pulse width.
Chronaxie: Pulse width needed for capture at twice the rheobase voltage
Electrogram (EGM; egram): intracardiac signal picked up by the intracardiac electrode
Slew rate: slope of the electrogram
Current of injury: ST elevation occurring in the ECG monitored from the electrode tip as the lead makes contact with the myocardium

Pacemaker blanking period: To prevent damage to the sensing circuit by the pacing spike, the sensing circuit is turned off for a short period immediately after the spike is delivered.

Refractory period is the period after the blanking period during which a sensed event does not reset the lower rate interval. But an event sensed in this period will extend the ventricular refractory period. A signal sensed during the refractory period is designated Vr and a signal sensed beyond the refractory period is designated Vs.

Noise mode response: Sensing multiple noise signals causes reversion to interference mode and pacing will occur in asynchronous mode. This prevents asystole due to oversensing.

T wave oversensing need not be associated with tall T wave on surface ECG. But the T wave of the EGM will be sufficient for resetting the lower rate interval – both QRS and T wave will be designated as Vs in the report.

Sensed AV interval is always kept lower than the paced AV interval because the P wave is sensed only later due to the low slew rate of the P wave.

Atrial based versus ventricular based DDD pacemakers: In atrial based pacemakers, AA interval is constant while VA interval is constant for ventricular based pacemakers.

Intrinsic deflection and intrinsicoid deflection

Intrinsicoid deflection is recorded in the surface ECG and intrinsic deflection is recorded directly from the heart. Intrinsic deflection can be recorded in experimental settings or by the intracardiac electrodes in pacemakers.

Ventricular blanking period

Ventricular blanking period is the period after the atrial stimulus during which the ventricular sensing circuit does not sense. This is programmed to prevent far field sensing of the atrial stimulus by the ventricular circuit. A spontaneous ventricular impulse occuring during the blanking period is not sensed and hence the pacemaker will deliver a ventricular stimulus, which does not capture the ventricle as the stimulus occurs during ventricular refractory period of the spontaneous beat.

Functional non capture

Capture does not occur as the stimulus is delivered during the refractory period. This can cause pseudofusion or loss of capture depending on the timing of the stimulus. The slew rate will be low in bundle branch block and can cause pseudofusion. A spontaneous impulse occurring during the ventricular blanking period can cause pacing stimulus delivery during the refractory period of the ventricle following the beat causing loss of capture.

Slew rate

Slew rate is the rate of rise of the spontaneous impulse. A slow rising impulse like P wave has a low slew rate which QRS complex which is a rapidly rising wave has a high slew rate. Slew rate of QRS can be low in bundle branch block.

DDI mode in dual chamber pacemakers

Atrium and ventricle are sensed as well as paced, but there is no tracking of atrium. This is used as a fall back mode in atrial tachycardias. The upper rate and lower rate intervals are equal for both atrium and ventricle. The pacemaker will maintain a minimum atrial and ventricular rate. Pacing never occurs above the lower rate interval. If the spontaneous atrial rate goes up, the ventricular paced rate will not go up. But if the spontaneous atrial impulses get conducted to the ventricles, the ventricular rate will go up. DDI mode can be considered as a DDD – VAT mode. DDD pacemaker is supposed to have the functions of AAI, VVI and VAT. In DDI mode, VAT function is not there, but AAI and VVI functions remain.

Upper tracking rate

Upper tracking rate is the maximum rate at which P synchronous pacing is delivered. Above this rate either a second degree block pattern occurs. The type of block depends on the programmed type of conduction ratio algorithm of the pacemaker.

Upper sensing rate

Upper sensing rate is the maximum rate at which a rate responsive pacemaker is programmed to pace in the appropriate chamber depending on the input from the sensor.

Automatic mode switching

When there is a paroxysmal atrial tachycardia which is above the upper rate interval, the pacemaker switches to either DDI/DDIR or VVI/VVIR mode. It switches back to DDD/DDDR mode once the tachycardia subsides.

What are the situations where the impedance is elevated?

Any discontinuity in the lead or any loose contact at the lead – pacemaker interface can increase the lead impedance.

MRI compatible pacemakers

Magnetic resonace imaging (MRI) compatible pacemakers have been launched recently. The can continues to be titanium. The internal circuitry has been altered to become MRI compatible. A scanning algorithm has been incorporated to check all the functions of the pacemaker after an MRI scan.

Magnetic resonance imaging of pacemakers and implantable defibrillators – new study

Conventionally, magnetic resonance imaging (MRI) is contraindicated in patients with implanted rhythm devices (pacemaker, implantable cardioverter defibrillator, insertable loop recorder). Earlier studies had shown that scanning at 0.5 – 1.5 Tesla field strength could be done cautiously in those with an absolute life saving requirement for an MRI scan. Now a recent study in PACE (Pacing Clin Electrophysiol.  2008;31(7):795-801.) has shown that it is possible to use even 3 Tesla MRI scanners with good safety. But extensive monitoring, pre and post programming and supervision by an electrophysiologist are mandatory. Moreover, the information to be gained by MRI should be so important in the management of the patient and not obtainable from other modalities. In this study on 14 patients, no significant change in the programmed parameters, pacing thresholds, sensing, impedance, or battery parameters was noted. Prolonge atefactual asystole was recorded by the insertable loop recorder during the MRI scan. Free full text of this article is available from MEDSCAPE to registered users (free registration): http://www.medscape.com/viewarticle/579155?src=rss

Cross talk in dual chamber pacemakers

Cross talk is sensing of the far field signal, either the ventricular signal in the atrial circuit or atrial signal in the ventricular circuit. Usually it is the sensing of atrial signal in the ventricular channel.

Cross talk detection in the ventricular channel can cause asystole. Following conditions predispose to cross talk inhibition of the ventricular channel:

Short ventricular blanking period
High atrial output
High ventricular sensitivity

Atrial cross talk is known as far field R wave oversensing. This can cause automatic mode switching because the sensed atrial rate is high. This can be prevented by programming the post ventricular atrial refractory period (PVARP) to a higher value.

Endless loop tachycardia

Endless loop tachycardia is the commoner of the two types of pacemaker mediated tachycardia, the latter being repititive non-rentrant ventriculoatrial synchrony (RNRVAS). In pacemaker mediated endless loop tachycardia, the pacemaker actively participates in the tachycardia circuit. This occurs only in dual chamber pacemakers. The ventricular depolarisation is conducted retrogradely and is sensed by the pacemaker circuitry. This triggers a ventricular pacing stimulus at the end of the programmed AV delay. The ensuing ventricular beat is again conducted back into the atrium and the vicious cycle continues. Final result is a tachycardia at the upper rate interval of the pacemaker. The tachycardia is usually triggered by a ventricular ectopic beat which fortuitously has an appropriate timing for initiating the vicious cycle. Sometimes endless loop tachycardia can also be initiated by an atrial ectopic beat occurring within the post ventricular atrial refractory period (PVARP). Pacemaker mediated endless loop tachycardia is usually terminated by magnet application and it can be prevented by programming the PVARP appropriately. Though usually, the tachycardia is terminated by application of a magnet one unusual case in which it was induced by magnet application and persisted despite repeated removal and reapplication of magnet has also been reported. If a pacemaker programmer device is not available, pacemaker mediated endless loop tachycardia can also be terminated by chest wall stimulation to inhibit the ventricular channel of the dual chamber pacemaker. Occasionally pacemaker mediated endless loop tachycardia gets converted to RNRVAS and is one of the reasons for magnet unresponsiveness.

Pacemaker syndrome

Mitsui et al was the first to describe pacemaker syndrome in 1969 as a symptom complex associated with right ventricular pacing [Mitsui T, Hori M, Suma K, Wanibuchi Y, Saigusa M. The “pacemaking syndrome.” In: Jacobs JE, editor. Proceedings of the Eighth Annual International Conference on Medical and Biological Engineering. Chicago, Illinois: Association for the Advancement of Medical Instrumentation; 1969:29-33]. In general it is due to loss of AV synchrony and it is relieved by AV sequential pacing. But of late, the role of V-V synchrony (loss of) has also been highlighted so that efforts to minimise right ventricular pacing have been promoted.

Patients with pacemaker syndrome may present with exertional dyspnoea, hypotension, syncope or even syncope. Syncope occurs in the setting of a drop of systolic blood pressure more than 20 mm Hg at the onset of ventricular pacing. Easy fatigability, sensation of fullness and pulsations in the head and neck are also features of pacemaker syndrome. When an intact V-A conduction is present, the pacemaker syndrome may be more severe due to the reverse atrial kick associated with every cardiac cycle. Venous pressure elevation as a result of atrial contraction against a close A-V valve causes reflex peripheral vasodilation and hypotension by stimulating the vagal afferents.

The reported incidence of pacemaker syndrome varies widely. In those trials in which a dual chamber pacemaker was implanted and programmed to single chamber mode for the study purpose, the change over rates to dual chamber mode for suspected pacemaker syndrome was high. But in those trials in which a surgical revision was required for a mode change, the reported incidence was low. This may indicate a lower threshold for the diagnosis of pacemaker syndrome when surgical revision is not required.

Potential link between atrial undersensing and calcium channel blockers

Single lead VDD pacemakers are convenient to implant and less expensive than DDD pacemakers. The advantage of single lead VDD system is that only a single lead need be implanted and thereby reducing procedure time, fluroscopy time and the cost. There is also a chance for lower perioperative complications with single lead VDD systems compared to DDD systems. But there are a few concerns regarding the single lead VDD system. Though a single lead VDD system is a cheaper and simpler method to provide AV sequential pacing in those with high grade AV block and good sinus node function, there is a chance for loss of atrial sensing over time. This is due to the floating atrial lead which may become less reliable over a period of time. Another potential problem is the delayed onset of sinus node dysfunction which will lead to VVI pacing or need the implantation of an additional atrial lead. A recent study involving over one hundred and fifty patients [Europace (2010) 12(9): 1251-1255] evaluated the potential risk factors for loss of atrial sensing in the long term performance of single lead VDD systems. They found that the chance of inappropriate atrial sensing was higher in the elderly and in those with atrial potentials less than 3 millivolts. An interesting finding was the association with the concomitant use of non-dihydropyridine calcium channel blockers (P < 0.003). This finding needs to be confirmed in larger studies and by other researchers. Atrial fibrillation was another obvious reason for lack of sensing in this study (P < 0.001). Inappropriate atrial sensing in this study was defined as an AV synchrony ratio of less than 90% of the paced beats.