Pacemaker

Cardiac pacing is the use of pulsed electrical current to force the heart into a specific rhythm of contractions. Such an external pacemaker is needed when the internal pacemakers (heart cells with special properties to generate electrical impulses that cause heart contractions) and the conduction system cannot ensure normal heart function.

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Indications for cardiac pacing

Temporary cardiac stimulation is indicated for high-grade atrioventricular blocks with a ventricular contraction rate of less than 40-45 per minute, accompanied by severe hemodynamic disturbances, concomitant rhythm disturbances (paroxysms of ventricular tachycardia), Adams-Stokes-Morgagni attacks, progressive circulatory failure, etc.

Defibrillation should not be performed in established asystole (the damaging effect of a defibrillating discharge on the myocardium has been described previously). In this case, external, endocardial or intraesophageal electrical stimulation of the heart should be used against the background of massage and artificial ventilation of the heart. Sometimes this is the only way to save the patient's life in cases where drug therapy is absolutely ineffective.

Cardiac pacing is rarely effective in complete asystole with absent P waves on the electrocardiogram (therefore it is not recommended as a routine method).

It is important to remember that cardiac pacing will only be effective if the myocardium is still able to respond to stimulating impulses.

Production of electrical impulses in the heart

The human heart has the functions of automatism, excitability, conductivity and contractility. Automatism is understood as the ability of the cardiac conduction system to independently generate impulses that induce the myocardium to contract.

The first-order automaticity center is the sinus node, located in the right atrium at the confluence of the vena cavae. The rhythm emanating from this node is called the sinus rhythm. This rhythm is the norm for all healthy people.

In the presence of pathological changes in the myocardium, the atrioventricular node, the second-order automatism center (produces 40-60 impulses per minute), can become the source of rhythm. If the atrioventricular node is unable to generate impulses capable of causing myocardial contraction (or if the conduction of impulses from it is disrupted), the third-order automatism center, the ventricular conduction system, capable of producing 20-50 impulses per minute, is activated.

Conduction of impulses through the myocardium

From the sinus node, the impulse spreads through the myocardium of the atria, then passes through the atrioventricular node, the bundle of His, and the conduction system of the ventricles. The intraventricular conduction system is divided into the right leg of the bundle of His, the main trunk of the left leg of the bundle of His and its two branches (anterior and posterior) and the Purkinje fibers, which transmit the impulse to the muscle fibers of the ventricles. The most vulnerable areas of the conduction system are the atrioventricular node, the right leg of the bundle of His, and the left anterior branch. Violation of the normal conductivity of the sinus impulse through the conduction system of the heart can be observed along its entire path.

Depending on the level at which the conduction disturbance of the impulse occurred, a distinction is made between:

• disturbance of intra-atrial conduction (sinus impulse block in the atria);
• disturbance of atrioventricular conduction (atrioventricular block);
• disturbance of intraventricular conduction (intraventricular blocks).

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Atrioventricular blocks

Atrioventricular blocks are characterized by a delay or cessation of impulse conduction from the atria through the atrioventricular node. the bundle of His and its legs to the ventricles. Atrioventricular blocks are incomplete (I and II degrees) and complete (III degree or complete transverse block). Atrioventricular block often develops with myocarditis, ischemic heart disease, overdose of cardiac glycosides, etc.

Partial atrioventricular block of the first degree is characterized only by an extension of the PQ interval over 0.20 s and has no clinical manifestations.

Incomplete atrioventricular block of the second degree is characterized by more pronounced disturbances of cardiac conduction, as a result of which one or more ventricular contractions are lost.

There are three types of second-degree atrioventricular block. In type I (Mobitz I), the electrocardiogram shows a gradual lengthening of the PQ interval with periodic loss of ventricular complexes (Wenckebach-Samoylov periods).

In the second type (Mobitz II), periodic loss of ventricular complexes is observed without an increase in the length of the PQ interval.

At the moment when incomplete blockade turns into complete blockade, ventricular fibrillation and sudden death may occur.

In a third-degree block, one of the atrial complexes does not reach the ventricles, causing the ventricles and atria to contract independently of each other. The ventricular rate may be lower than 40-50 beats per minute. Complete transverse block is sometimes asymptomatic, but more often palpitations, dizziness, fainting, and seizures (Adams-Stokes-Morgagni syndrome) are a concern.

High-degree atrioventricular blocks deserve special attention in patients with myocardial infarction. Complete atrioventricular block (third-degree atrioventricular block) occurs in 5-7% of patients.

Its development is more prognostically favorable in patients with infarction of the posterior inferior wall of the left ventricle. The pacemaker is most often located in the atrioventricular node. On the electrocardiogram, the QRS complex is not widened, the heart rate exceeds 40 per 1 minute. The blockade resolves on its own within a few days.

In patients with complete transverse block, the prognosis is significantly worse with anterior infarction. This is due to the fact that there is extensive necrosis of the anterior septal region of the left ventricle with severe left ventricular failure or cardiogenic shock. The pacemaker is often located below the atrioventricular node. The QRS complex is deformed and widened, the heart rate is less than 40 per 1 minute.

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What types of cardiac pacing are there?

Methods of cardiac stimulation are divided into:

• by nature of application:
  • therapeutic cardiac stimulation;
  • diagnostic cardiac pacing;
• by localization:
  • external cardiac pacing (percutaneous);
  • transesophageal (the electrode is located in the esophagus);
  • myocardial cardiac pacing (the electrode is located in the wall of the heart);
  • endocardial (the electrode is located inside the heart);
• by duration of the event:
  • temporary cardiac pacing;
  • permanent cardiac pacing.

Procedure for conducting electrical stimulation

Most often, due to the severity of the condition and the threat of complete cessation of blood circulation, cardiac stimulation is usually performed in two stages. First, external electrical stimulation is started as a temporary measure, which takes very little time. Later, after the hemodynamic parameters have stabilized, the central vein is punctured and an endocardial electrode is installed through it in the area of the apex of the right ventricle.

External cardiac pacing

Temporary external cardiac stimulation is a relatively simple method that allows saving a patient's life in an emergency. The same multifunctional resuscitation systems are used for its implementation as for defibrillation, which have artificial pacemaker modules (Zoll M-Series, Defigard 5000 Schiller, etc.).

External cardiac pacing causes painful contractions of skeletal muscles, which causes discomfort or pain in the patient.

Modern universal adhesive defibrillation electrodes can reduce these negative effects by providing good contact with the skin and, when using a 40 ms rectangular pulse, reducing painful muscle contractions caused by high current density.

Preparation. It is necessary to remove hair from the electrode application sites using a razor or scissors. Remove excess moisture from the patient's skin. Attach electrodes for electrocardiographic monitoring (if this function is not provided automatically by the pacemaker).

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Position of electrodes

The optimal placement of electrodes is considered to be anterior-posterior, in which the dorsal electrode (+) is placed on the area of the left shoulder blade, and the precordial electrode (-) is placed near the lower edge of the sternum on the left. This placement of electrodes is more often used when "periastal arrhythmia" occurs.

If cardiac stimulation is performed during resuscitation, then the standard position of the electrodes is more indicated: one of the electrodes is placed on the anterior surface of the chest below the clavicle at the right edge of the sternum, and the other at the level of the 5th intercostal space along the anterior axillary line (the attachment sites of the ECG electrodes in lead V5-V6). This is done so as not to interrupt resuscitation measures and so that the electrodes do not interfere with their implementation.

Cardiac pacing modes

Typically, demand and fixed rate modes are used for cardiac pacing.

In the "fixed" mode, the module delivers stimulating pulses with the current and frequency settings set by the person performing the pacing. The selected heart rate remains constant and is not affected by the patient's own heart activity. This mode is preferable when cardiac activity has stopped.

In on-demand mode, the pacemaker does not deliver pulses until the pacemaker's intrinsic heart rate exceeds the pacemaker's set rate.

If the heart's own contraction rate drops below the stimulation rate, the pacemaker begins to send stimulating impulses.

To achieve adequate stimulation of the myocardium, the stimulation frequency and stimulation current intensity settings are used (usually the factory settings are 70 stim/min and 0 mA, respectively). The achievement of "electrical capture" is indicated by the accompaniment of each electrical stimulus by a subsequent wider QRS complex, which indicates ventricular contraction. The presence of "mechanical capture" is indicated by the appearance of a palpable pulse against the background of electrical capture. After the presence of electrical and mechanical captures is established, it is recommended to increase the current by 10% more than the capture current (safe limit).

Endocardial pacing

Temporary endocardial cardiac stimulation can be performed by inserting an endocardial electrode through a central venous catheter. The most technically simple and convenient method is puncture insertion of the probe through the subclavian veins, especially on the left.

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Electrode installation technique

The electrode is inserted through the veins into the right chambers of the heart, where it contacts the endocardium of the atrium or ventricle. The most commonly used access is through the subclavian vein. The subclavian vein is catheterized and a catheter with an internal diameter of 3 mm and a length of 40 cm is inserted. The entry of the catheter into the cavity of the right ventricle is determined by a sharp increase in venous pressure and the appearance of pulsation. A temporary endocardial electrode is inserted through the lumen of the catheter, the catheter is removed. Stimulation is performed through the electrode from the external pacemaker.

Checking the correct position

The correct position of the electrode is confirmed by X-ray control or changes in the electrocardiographic picture during test stimulation (stimulation of the myocardium of the right ventricle is indicated by the electrocardiographic picture of the left bundle branch block).

Cardiac pacing modes

The pulse strength is selected individually. First, the minimum pulse strength causing heart contraction is selected (i.e., the individual sensitivity threshold). As a rule, the working pulse strength is set 150-200% higher than the threshold. The optimal location is considered to be the distal part of the electrode in the trabecular muscles of the right ventricle apex. The threshold pulse strength is usually from 0.8 to 1 mA, and the working one does not exceed 1.5-2 mA. Incorrect placement of the electrodes leads to an increase in the threshold current strength. This method is quite simple and can be used (if the appropriate equipment is available) at the pre-hospital stage.

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Duration of the event

The duration of electrical stimulation depends on the nature and duration of the rhythm disturbances. After the heart rhythm is restored, the electrode should remain in place for 2-3 days (in case of relapse). If after the cessation of electrical stimulation, pronounced signs of circulatory failure appear, it is necessary to decide on the implantation of a permanent pacemaker.

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Intraesophageal cardiac pacing

The electrode is passed through the esophagus and placed in a position that provides the best "capture" of cardiac activity. This method is rarely used in resuscitation.

Cardiac pacing for certain cardiac rhythm and conduction disorders

Cardiac stimulation is advisable not only in asystole, but also when the heart rate is less than 50 per minute. As a rule, it is performed in cases of complete atrioventricular block, bradycardia and bradyarrhythmia (sick sinus syndrome, incomplete high-grade atrioventricular block). In cases of severe hemodynamic disturbances, it is also necessary at 50-60 beats per minute.

Cardiac stimulation is used to stop paroxysmal tachyarrhythmias when drug therapy is ineffective. The following types of electrical stimulation are used:

• ultra-frequent cardiac pacing (suppression of the ectopic excitation focus by ultra-frequent transesophageal cardiac stimulation at a frequency of 500-1000 impulses per minute);
• programmed cardiac stimulation with a single electrical impulse (stimulation is performed with a single impulse, the time of application of which is synchronized with the R wave, and the interval between this wave and the electrical stimulus is automatically increased until the next impulse terminates the paroxysm of tachycardia);
• slowing cardiac stimulation (application of paired stimuli, every second impulse, not accompanied by a contraction of the heart, lengthens the refractory period after the previous spontaneous excitation, reducing the number of ventricular contractions).

Cardiac pacing in patients with myocardial infarction

Temporary cardiac stimulation in patients with myocardial infarction has its own characteristics, due to the transient nature of the disorders that arise. Therefore, cardiac stimulation should be performed taking into account the existing electrical activity of the patient's heart. It is not necessary to allow a situation where the heart is simultaneously under the influence of the natural pacemaker (sinus node) and the pacemaker. It is believed that such a situation is fraught with the development of serious cardiac rhythm disorders (up to ventricular fibrillation).

Cardiac pacing in patients with myocardial infarction is indicated for:

• Adams-Stokes-Morgagni attacks;
• severe, especially progressive, circulatory failure;
• atrioventricular block, accompanied by other rhythm disturbances (paroxysms of ventricular tachycardia);
• the number of contractions of the ventricles of the heart is less than 40-45 per minute.

The duration of temporary endocardial electrical stimulation depends on the duration of the rhythm disturbances. Usually, in acute myocardial infarction, disturbances of atrioventricular conduction are temporary. Most often, the blocks that occur in the acute period pass on their own or under the influence of drug treatment. Less often, atrioventricular conduction is not restored.

If signs of circulatory failure or other disturbances in the heart's pumping function appear after the pacemaker is turned off, implantation of a permanent pacemaker should be considered.

When pacing is temporary, it is recommended to periodically discontinue it to assess the patient's own heart rhythm.

Typically, the electrode remains at the site of initial insertion for 3-5 days (up to two weeks) after the rhythm has returned to normal (depending on the disturbances present).

Cardiac pacing and drug therapy

Cardiac stimulation in most cases allows the heart to be quickly "imposed" on almost any frequency, which is why it has significant advantages over drug treatment in patients with emergency conditions. It can be started quickly and stopped immediately if necessary.

Cardiac stimulation does not interfere with drug therapy in any way. On the contrary, against the background of electrical stimulation, there is no need to fear the aggravation of atrioventricular block caused by the use of antiarrhythmic and other drugs.

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Implantable cardioverter defibrillators

Implantable cardioverter-defibrillators are the most effective means of preventing sudden death in patients with life-threatening ventricular arrhythmias.

These devices consist of two main parts: a pulse generator (containing a power source, capacitors, electronic circuits and memory) and a system of electrodes in contact with the heart. The electrodes provide diagnostics of tachyarrhythmias by continuously monitoring the heart rhythm, defibrillation and cardioversion, and in some models, the implementation of frequent ECS to stop tachyarrhythmias and accelerate them in bradyarrhythmias.

The devices are inserted transvenously. In tunnels created under the skin, electrodes are fed to the generator, which is implanted in a pocket created for it under the skin or under the muscle in the upper abdomen or, if the size allows, under the large pectoral muscle on the left.

The presence of an implanted cardioverter-defibrillator in a patient does not interfere with cardiopulmonary resuscitation.

Mechanical cardiac pacing

In cases of severe bradycardia, which causes clinical manifestations of circulatory arrest, mechanical stimulation of the myocardium by tapping the chest is indicated. This method will be most effective in patients with ventricular arrest while maintaining atrial activity.

Mechanical cardiac stimulation (fist pacing) is performed by applying gentle blows to the precordial area to the left of the sternum. The tapping is applied from a height of about 10 cm and should be tolerated satisfactorily by conscious patients. If the first blows do not result in the appearance of QRS complexes on the electrocardiogram, then the point of application of the blows should be changed, focusing on the appearance of electrocardiographic signs of ventricular contraction. When performing "mechanical capture" and there are clear signs of ventricular contractions, the force of the blows should be reduced to the minimum, at which contractile activity of the ventricles is still preserved.

Mechanical pacing is less effective than electrical pacing. If it does not produce a perfusing rhythm, chest compressions and ventilation should be started immediately.

Typically, the mechanical stimulation method is used during the delivery and preparation of equipment for cardiac stimulation.

Complications of cardiac pacing

There are few complications with cardiac pacing. The main complications of transvenous endocardial cardiac pacing are phlebitis. In severe patients, phlebitis and thrombophlebitis develop several days after the placement of the probe (especially through the peripheral veins of the extremities), even if the entire procedure was performed in compliance with the rules of asepsis and antisepsis. In rare cases, sepsis may occur.

Mechanical irritation of the heart wall when inserting the probe can cause extrasystole. Very rarely, this provokes other arrhythmias, including ventricular fibrillation.

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