Cardiac pacing

Pacemy stimulation is the use of a pulsed electric current to impose a certain rhythm of cardiac contractions on the heart. Such an external pacemaker driver is needed when internal pacemakers (heart cells with special properties generate electrical impulses that cause heart contractions) and the conduction system can not provide normal heart function.

[1]

Indications for pacing

Temporal pacemaking is indicated for high-grade atrioventricular blockades with a number of ventricular contractions of less than 40-45 per minute, accompanied by severe hemodynamic disorders, concomitant rhythm disturbances (paroxysms of ventricular tachycardia), Adams-Stokes-Morgagni attacks, progressive circulatory insufficiency, and the like.

Do not perform defibrillation with established asystole (previously described the damaging effect of a defibrillating discharge on the myocardium). In this case, against a background of massage and artificial ventilation of the heart, one should resort to external, endocardial or intra-esophageal electrical stimulation of the heart. Sometimes this is the only way to save a patient's life in cases where drug therapy is absolutely ineffective.

Pacemy is rarely effective with complete asystole with no P teeth on the electrocardiogram (therefore not recommended as a routine method).

It must be remembered that pacemaking will be effective only if the myocardium is still able to respond to stimulating impulses.

The development 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 conduction system of the heart to independently generate impulses that induce the myocardium to contract.

The first-order automatism center is a sinus node located in the right atrium at the confluence of the hollow veins. The rhythm coming from this node is called the sinus rhythm. It is this rhythm that is the norm for all healthy people.

In the presence of pathological changes in the myocardium, the source of rhythm can be the atrioventricular node - the center of automatism of the second order (produces 40-60 pulses per minute). With the inability of the atrioventricular node to generate pulses that can cause myocardial contraction (or impairment of impulses from it), a third-order center of automatism, a ventricular system that can produce 20-50 impurities per minute, is included in the work.

Carrying out pulses on the myocardium

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

Depending on the level at which impulse conduction has occurred, distinguish:

• violation of atrial atrial conductivity (blockade of the sinus pulse in the atria);
• violation of atrioventricular conduction (atrioventricular block);
• violation of intraventricular conduction (intraventricular blockades).

[2], [3], [4]

Atrioventricular blockade

Atrioventricular blockades are characterized by a delay or discontinuation of impulses from the atria through the atrioventricular node. A bundle of His and his legs to the ventricles. Atrioventricular blockades are incomplete (I and II degrees) and complete (grade III or complete transverse blockage). Atrioventricular blockade often develops in myocarditis, ischemic heart disease, cardiac glycoside overdose, and the like.

A partial atrioventricular block of degree I is characterized only by an elongation of the PQ interval above 0.20 s and has no clinical manifestations.

Incomplete atrioventricular blockade of degree II is characterized by more severe cardiac conduction abnormalities, resulting in one or more ventricular contractions.

There are three types of atrioventricular blockade of the II degree. In type I (Mobitz I), the electrocardiogram is gradually lengthened with periodic falls of ventricular complexes (Wenkebach-Samoilov periods).

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

At the time of the transition of an incomplete blockade, ventricular fibrillation and sudden death may occur.

With blockade III degree, one of the atrial complexes does not reach the ventricles, as a result of which the ventricles and atria contract independently. The frequency of contractions of the ventricles may be below 40-50 beats per minute. Full transverse blockage sometimes occurs asymptomatically, but more often the heartbeats, dizziness, fainting, may be convulsions (Adams-Stokes-Morgagni syndrome).

Special attention should be paid to atrioventricular blockades of a high degree in patients with myocardial infarction. A complete atrioventricular block (atrioventricular blockade of grade III) occurs in 5-7% of patients.

Prognostically more favorable is its development in patients with a coronary artery of the posterior wall of the left ventricle. The pacemaker is most often located in the atrioventricular node. On the electrocardiogram, the QRS complex is not expanded, the heart rate is more than 40 per minute. The blockade passes independently for several days.

With a full transverse blockade in patients with anterior infarction, the prognosis is significantly worse. This is due to the fact that there is extensive necrosis of the anteroposterior 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 broadened, the heart rate is less than 40 per minute.

[5], [6], [7], [8]

What is the pacemaker?

Methods of electrocardiostimulation are subdivided:

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

The procedure for electrical stimulation

Most often due to the severity of the condition and the threat of complete cessation of circulation, pacemaking is usually performed in two stages. First, as an interim measure, external electrical stimulation begins, on which very little time is required. Later, after stabilization of hemodynamic parameters, the central vein is punctured and through it an endocardial electrode is placed in the region of the apex of the right ventricle.

External Pacing

Temporary external pacing is a relatively simple method that allows saving a patient's life in an emergency situation. For its implementation, the same multifunctional resuscitation complexes are used as for the defibrillation, which include the modules of an artificial pacemaker (Zoll M-Series, Defigard 5000 Schiller, etc.).

External cardiac stimulation causes painful contractions of the skeletal muscles, which causes the patient uncomfortable or painful sensations.

Modern universal adhesive defibrillation electrodes can reduce these negative manifestations, since they provide good contact with the skin and, when using 40 ms of rectangular pulse, reduce painful muscle contractions caused by high current density.

Preparation. It is necessary to remove hair from the places of application of electrodes with the help of 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 a pacemaker device).

[9], [10], [11], [12], [13], [14]

Position of electrodes

Optimal is the anterior-posterior application of electrodes, in which the dorsal electrode (+) is applied to the region of the left scapula, and the precordial electrode (-) - near the lower end of the sternum on the left. Such an arrangement of electrodes is more often used when a "periarest arrythmia" occurs.

If the pacemaker is carried out during resuscitation, the standard position of the electrodes is more clearly shown: one of the electrodes is located on the front surface of the thorax below the clavicle at the right edge of the sternum, and the other at the V intercostal space in the anterior axillary line (electrocardiography electrodes in the lead V5-V6). This is done in order not to interrupt the resuscitation measures and that the electrodes do not interfere with their carrying out.

Modes of pacing

As a rule, when using pacemaking, the demand (on demand) and fixed rate modes are used.

In the "fixed" mode, the module delivers stimulating pulses with current and frequency settings set up by the conductive pacemaker. The chosen heart rate remains constant and is not affected by the patient's own heart activity. The use of this mode is preferable when cardiac arrest is stopped.

In the "on-demand" mode, the stimulator does not pulse until the heart rate exceeds the predetermined rhythm frequency.

If the frequency of self-contractions decreases below the stimulation frequency, the pacemaker begins to send stimulating pulses.

To achieve adequate stimulation of the myocardium, use switches of stimulation frequency and adjustment of the stimulation current (usually factory settings are 70 stm / min and 0 tA, respectively). The achievement of "electric capture" is indicated by the accompaniment of each electric stimulus with a subsequent wider QRS-complex, which indicates a reduction in the ventricles. The presence of "mechanical capture" is evidenced by the appearance of a palpable pulse on the background of electric capture. After the presence of electric and mechanical grippers is installed, it is recommended to increase the current strength by 10% more current capture (safe limit).

Endocardial Pacing

Temporary endocardial pacing can be performed by carrying an endocardial electrode through a catheter for central venous catheterization. The most technically simple and convenient way of puncture the probe through the subclavian veins, especially the left.

[15], [16], [17], [18], [19], [20],

Method of electrode installation

The electrode through the veins is carried out in the right heart, where it contacts the endocardium of the atrium or ventricle. The most commonly used access is through a 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 catheter catheter into the cavity of the right ventricle is determined by a sharp increase in venous pressure and the appearance of pulsation. Through the lumen of the catheter, a temporary endocardial electrode is inserted, the catheter is removed. Stimulation is carried out through an electrode from an external stimulant.

Controlling the correctness of the situation

The correctness of the position of the electrode is confirmed by X-ray monitoring or changes in the electrocardiographic pattern during trial stimulation (the electrocardiographic picture of the blockade of the left leg of the bundle of the Hyis indicates the stimulation of the right ventricle myocardium).

Modes of pacing

The magnitude of the impulses is chosen individually. First, the minimum pulse strength that causes heart contraction is selected (that is, the individual sensitivity threshold). As a rule, the value of the operating pulses is set above the threshold value by 150-200%. Optimal positioning of the distal part of the electrode in the trabecular muscles of the tip of the right ventricle is considered. The threshold strength of the pulses is usually from 0.8 to 1 mA, and the working power does not exceed 1.5-2 mA. The improper arrangement of the electrodes leads to an increase in the threshold current. This method is quite simple and can be applied (if appropriate equipment is available) at the pre-hospital stage.

[21], [22], [23]

Duration of the event

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

[24], [25], [26]

Intrapidesophageal pacemaking

The electrode is guided through the esophagus and is positioned in the position providing the best "capture" of the cardiac activity. This method is rarely used in resuscitation.

Pacemy with certain disorders of rhythm and conduction of the heart

Pacemy is appropriate not only for asystole, but for a heart rate of less than 50 per minute. As a rule, it is performed with complete atrioventricular blockade, bradycardia and bradyarrhythmia (syndrome of weakness of the sinus node, incomplete atrioventricular blockade of a high degree). With severe hemodynamic disturbances, it is also necessary at 50-60 beats per minute.

Pacemy is used to arrest paroxysmal tachyarrhythmias in the case of ineffective drug therapy. The following electrostimulation options are used:

• superfluous pacemaking (suppression of the ectopic foci of excitation by super-rapid transesophageal heart stimulation with a frequency of 500-1000 pulses per minute);
• programmed pacemaking by a single electric pulse (stimulation is performed by a single pulse, the timing of application of which is synchronized with the tooth R, and the interval between this tooth and the electric stimulus automatically increases until the next pulse breaks the paroxysm of tachycardia);
• pacing pacemaking (application of paired stimuli, every second impulse, not accompanied by a contraction of the heart, prolongs the refractory period after the previous independent excitation, reducing the number of ventricular contractions).

Pacemaking in patients with myocardial infarction

Temporary pacing in patients with myocardial infarction has its own peculiarities, conditioned by the transient nature of the arising disorders. Therefore, pacemaking should be carried out taking into account the available electrical activity of the patient's heart. We should not allow the situation that the heart was simultaneously influenced by the natural pacemaker (sinus node) and pacemaker. It is believed that this situation is fraught with the development of serious violations of the heart rhythm (up to ventricular fibrillation).

Pacemaking in patients with myocardial infarction is indicated when:

• Attacks of Adams-Stokes-Morgagni;
• marked, especially progressive circulatory failure;
• atrioventricular blockade, accompanied by other rhythm disturbances (paroxysms of ventricular tachycardia);
• number of ventricular contractions less than 40-45 per minute.

The duration of temporary endocardial electrostimulation depends on the duration of rhythm disturbances. Usually, with acute myocardial infarction, atrioventricular conduction disorders are temporary. Most often, the blockades that occur in the acute period pass independently or under the influence of drug treatment. Less often, the atrioventricular conduction is not restored.

If after disabling the electrostimulator there are signs of circulatory failure or other impairment of the pumping function of the heart, you should think about implanting a constant pacemaker driver.

In case of temporary pacing, it is recommended to periodically stop it to assess the patient's own rhythm of the heart.

Typically, the electrode remains in place of the initial administration for 3-5 days (up to two weeks) after the normalization of the rhythm (depending on the existing disorders).

Cardiostimulation and drug therapy

Pacemy stimulation in most cases allows you to quickly "impose" on the heart of almost any frequency, and therefore has significant advantages over drug treatment in patients with emergency conditions. It can be quickly started and immediately discontinued if necessary.

Pacemaking in no way interferes with drug therapy. Conversely, against the background of electrostimulation, one can not be afraid of aggravation of atrioventricular blockade caused by the use of antiarrhythmic and other drugs.

[27], [28], [29], [30]

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 (contains an energy source, capacitors, electronic circuits and memory) and a system of electrodes in contact with the heart. Electrodes provide diagnostics of tachyarrhythmias by constant monitoring of the heart rhythm, defibrillation and cardioversion, and in some models also frequent ECS for cupping tachyarrhythmias and increasing with bradyarrhythmias.

The devices are administered transvenously. In the tunnels created under the skin, the electrodes are led to a generator that is implanted in the pocket created for it under the skin or under the muscle in the upper abdomen or, if the dimensions permit, 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 pacing

With pronounced bradycardia, which determines the clinical manifestations of circulatory arrest, mechanical stimulation of the myocardium is shown by piercing the chest. This method will be most effective in patients with a stop of ventricular activity against the background of conservation of atrial activity.

Mechanical pacing (fist pacing) is performed by applying accurate strokes to the precordial area to the left of the sternum. Beatings are applied from a height of about 10 cm and should be tolerated satisfactorily by patients who are conscious. If the first strokes do not lead to the appearance of QRS complexes on the electrocardiogram, the point of attack should be changed, focusing on the appearance of electrocardiographic signs of ventricular contraction.When performing a "mechanical grip" and obvious signs of ventricular contractions, the impact strength should be reduced to a minimum, at which the contractile activity of the ventricles still persists.

Mechanical pacemaking is less effective than electrical pacing. If the perfusion rhythm does not occur during its implementation, chest compressions and lung ventilation should be started immediately.

Usually, the method of mechanical stimulation is used during the delivery and preparation of equipment for pacing.

Complications of pacing

Complications of pacing are few. The main obstructions of transvenous endocardial pacing are phlebitis. In severe cases, phlebitis and thrombophlebitis develops a few days after the probe is inserted (especially through the peripheral veins of the extremities), even if the entire procedure was performed in compliance with the rules of aseptic and antiseptic. In rare cases, sepsis may occur.

Mechanical irritation of the heart wall with the introduction of the probe can cause extrasystole. This rarely provokes other arrhythmias up to ventricular fibrillation.

[31], [32], [33], [34]