AV Block or Atrioventricular block
AV Block – Atrioventricular block is partial or total disruption of the electrical impulse from the Atria to the ventricles. Atrioventricular block is classified by degrees: isolated atrioventricular blockade of the I degree,
atrioventricular block II degree type I and type II, and atrioventricular blockade III degree (complete); and by level: supra nodal and infra nodal.
Atrioventricular block of I degree is manifested by slowing of the pulse from the Atria to the ventricles. There is no clinic signs.
ECG signs of atrioventricular block I degree:
1) the presence of the P wave and QRS complex in all cycles
2) lengthening of the interval P-Q(R) over 0.20;
3) normal form and duration of the QRS complex.
II. Atrioventricular block II degree is intermittent discontinuation of the separate impulses from the Atria to the ventricles. There are two main types of atrioventricular block II degree type I Mobitz (with periods Semyonov-Wenckebach) and type II Mobitz.
In this variant of blockade, there can be no clinical manifestations in some patients, or may be unmotivated weakness, dizziness up to syncope when there is loss of several ventricular contractions, impaired cognitive functions (impaired memory, attention).
ECG signs of atrioventricular block of the II degree (type Mobitz I):
1) gradually from cycle to cycle elongation of interval P-Q(R) with the subsequent loss of ventricular QRST complex (period Samoilov – Wenkebach);
2) long pauses are not equal to twice the RR interval (the P wave, no QRS);
3) after loss of ventricular complex recorded normal interval P-Q(R), then the whole cycle repeats;
4) before the pause may be some shortening of the RR;
5) QRS usually narrow (supranadal form).
Clinical manifestations Mobitz II same as type I.
ECG signs of atrioventricular block of the II degree (type Mobitz II):
1) no progressive lengthening of the interval P-Q(R) before blocking pulse (the stability of interval P-Q(R);
2) long pauses equal to twice the RR interval, with P, no QRS;
3) the PQ intervals before loss QRS are normal or long (more than 0.22 C);
4) PP regular;
5) PQ equal to PQ after it;
6) QRS may be less than 0.12 s, but usually enlarged more than 0.12 s (infra nodal form).
Atrioventricular block II degree 2:1. Until recently, this rhythm disturbance called advanced blockade and was seen as the first step to a complete AV blockade. Specific clinical signs this violation does not exist.
ECG signs of AV block II 2:1:
1) the right (regular) ventricular rhythm (all RR intervals are equal);
2) there is one ventricular contraction to the 2 atrial (1 QRS complex to 2 atrial (2 P waves);
3) the PQ interval in all the QRS complexes with the same (either normal or elongate);
4) pauses do not exist;
5) it should be remembered that the Atria pump in the sinus rhythm (60 – 80 per minute), and the ventricles, depending on the source of excitation have different heart rates: when supraventricular form 40 – 45, ventricular – less than 40. The variation of AV blockade II degree. of rhythmic form is far advanced or recurrent blockage. ECG criteria are identical to the above, but for one ventricular contraction there are more than 2 atrial contractions (usually an odd number: 5 – 7).
Atrioventricular block III (complete atrioventricular block) is the complete cessation of impulse conduction from the Atria to the ventricles, causing the Atria and ventricles are excited and contract independently from each other.
Clinic: Morgagni – Edems`s – Stokes attacks: syncopes with seizures due to severe brain ischemia, observed in the decrease in cardiac index less than 2 l. per minute that occur during asystole or sudden slowing of the heart rate is less than 15 – 20 per minute. Patients also may experience involuntary discharge of urine and feces.
ECG signs of atrioventricular block III degree:
1) the presence of P wave with the frequency of the atrial rhythm of 60 – 80 per minute;
2) the intervals R – R constant;
3) the presence of a permanent ventricular rhythm (RR equal to each other), but less frequently than atrial (less than 60 per minute);
4) no relationship between P and QRS (absent clearly primarily the PQ interval), occasionally the P wave superimposed on the QRS complex and the T wave, their deformity.
It should be remembered that the complete AV blockade depending on the source of pacemaker for the ventricles is proximal (supra nodal) with a narrow QRS and a ventricular rate number 50 to 55 per minute.
The second kind is distal blockade (infra nodal) with wide QRS complexes and a ventricular frequency of responses is less than 40. Prognosis in the last form worse.
The blockade of His bundle
Blockade legs and bundle branch’s of His is a slowing or complete cessation of the excitation of one, two or three branches of bundle of His. When complete cessation of the excitation of one or another branch or the stem of the His bundle they suggest a complete blockade.
In this case, the QRS complex duration is more than 0.12 seconds. Partial conduction slowing indicates incomplete blockade bundle branch’s of His. In this case, the duration of the QRS complex is 0.12 seconds or less.
ECG signs of blockade of the right bundle branch:
1) in right precordial (chest) leads (V1,2) the ventricular complexes of the type rSR” RR or”, with M-shaped form;
2) the presence in the left precordial (chest) leads (V5, V6) and in leads I, aVL enlarged, often jagged S wave;
3) increase the duration of the ventricular complex (QRS 0.12 or more);
4) in lead V1 of depression of segment S-T and negative or biphasic (-/+) asymmetric T wave
The appearance of a complete blockade of the right bundle branch is not an urgent situation. Most often this can be without organic changes in the heart, is less commonly associated with chronic forms of ischemic heart disease, pulmonary heart, the manifestation of right ventricular hypertrophy and pulmonary embolism.
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ECG signs of blockade of the left bundle branch:
1) in the left precordial (chest) leads (V5, V6), I, aVl widened deformed ventricular complexes, R type split or a wide top;
2) the presence of in leads V5,6, I, aVL segment displacement R(S)-T which is discordant to QRS and a negative or biphasic (-/+) asymmetrical T waves;
3) absence of Q in I, aVL, V5-6 leads;
4) in leads V1, V2, widened ventricular complexes of the type QS or rS with a split or a wide tooth top S, ST elevation and + high T;
5) in III, aVF broad, low-amplitude QRS type “M-shaped”.
It should be noted that the sudden appearance of a complete blockade of the left bundle branch is regarded as urgent situation, requiring hospitalization, because it is one of the manifestations of acute coronary syndrome.
It is believed that the appearance of complete blockade of the left bundle branch always has an organic nature (myocarditis, cardiomyopathy, hypertension, valvular). So patients with this ECG syndrome require careful examination.
As the left bundle branch consists of 2 branches, which can be isolated blockade of both the anterior and posterior branches. The ECG identification of these violations is the prerogative of doctors of functional diagnostics, so this tool is not discussed here.
VIOLATION OF CONDUCTIVITY
A slowing or complete cessation of the electric impulse in any division of the conducting system called heart block.
Reasons. In its origin heart block (especially I and II stages) can be functional (vagal) – in athletes, young men with vegetative dystonia on the background of sinus bradycardia and in other similar cases.
They disappear during exercise or with intravenous 0.5-1.0 mg of atropine sulfate. The second type of the block (IIIst.) – organic, with the defeat of the myocardium.
Clinical signs (disruption of the heart until asystole with syncopations) appear when hemodynamically significant blockades (blockade).
Sinoaricular the blockade is a violation of excitation from the sinus node to the atrial myocardium. We can`t see the process of depolarization of the sinus node on the standard ECG, except the depolarization of the Atria (the P wave).
Therefore, not all violations of sinoaricular conductivity can be registered on a standard ECG. Depending on the violation of the speed of the pulse in this blockade they traditionally distinguish III degree of sinoaricular blockades ,
and the ECG criteria of I and III are missing (they may be suspected by indirect signs ECG or electrophysiological study of the conduction system of the heart).
The main reasons for this violation are organic and vegetative lesions of the sinus node, so sinoaricular blockade is included in the ECG manifestations of the syndrome of weakness of sinus node.
On the ECG we see the manifestation of sinoaricular blockade II stage (degree), which in turn can be of several types: Type 1 – progressive slowing of conduction of excitation between the sinus node and the Atria until the complete loss of cardiac activity.
Clinic: possible dizziness and rarely syncope.
1) the appearance of the pauses on the background of the right (regular) sinus rhythm (without the P wave and QRS) with duration less than two PP intervals;
2) the PP intervals are shortened before the pause, after the pause lengthened;
3) the interval PQ is everywhere stable.
Type 2 is the transient interruption of conduction of impulses from the sinus node to the Atria, with loss of one or more cycles. The clinical picture is similar to the previous one.
1) The sudden appearance of a pause on the background of sinus rhythm (no P and QRS). Pause duration is equal to two PP intervals
2) The PP intervals before and after the pause are identical to each other.
3) A variation sinoaricular II stage 2 type is rhythmic block 2:1 when one impulse is conducted to the Atria, and every second is blocked. On the ECG we see a rare correct (regular) sinus rhythm with a ventricular rate less than 46 in a minute.