Licensed books on medicine
<< Previous || Next >> |
Advanced atrioventricular block of the second degree and block of the third degree of type A1
1 According to the authors' classification, type A denotes a variant of AV blockade with a normal form of the QRS complex, and type B indicates a variant of AV blockade with an expanded QRS complex. - Note. translator.
The term “advanced grade II AV block” is used when more than two consecutive supraventricular impulses are blocked . Although it is difficult to distinguish this variant of blockade from AV blockade of the third degree (if any), nevertheless, with blockade of the third degree, most ventricular complexes are the result of impulses arising in the secondary source of automatism, whereas with advanced blockade of the second degree, ventricular activation is controlled mainly by unblocked supraventricular impulses. The type of conduction in which one sinus impulse is conducted into the ventricles and followed by two blocked impulses is called 3: 1 conduction, while the successful conduction of only one of the four atrial impulses is defined as 4: 1 conduction. A few examples of this type of blockade are discussed below.
In fig. 1.13 shows the experimental recording of transmembrane action potentials in the region of the AV node, as well as an electrocardiogram and ladder diagram for analysis of the activation sequence . In the initial part of Fig. 1.13 there are several periods of AV-conducting 2: 1, in which the AV-interval detects the alternation of short and long cycles of 202, 138, 190 and 150 ms. Violation of the conduction always occurs above the NH fiber, and the conduction time according to the His – Purkinje system (or subnode time) remains constant and almost corresponds to the norm. Judging by the ventricular complexes on the ECG, there are no disorders of the intraventricular conduction. Thus, here, apparently, there is only one block level. However, fluctuations in the amplitude of local responses of NH fibers suggest that the alternation of AV conduction time is due to the difference in the depth of penetration of blocked atrial impulses into the AV node. For example, the second and sixth atrial excitation waves (low-amplitude two-phase oscillations on the electrocardiogram) are hardly accompanied by any changes in the membrane resting potential of the NH fiber, while the fourth atrial impulse causes a certain change in potential (local response), which indicates a somewhat deeper penetration of excitement. This increases the time of AV conduction of the fifth atrial impulse in comparison with other non-blocked impulses - the effect of hidden conduction. As noted above, the period of AB-holding 2: 1 is followed by a period of 4: 1. During 4: 1, the second of three consecutive blocked pulses causes a slightly larger change in potential, which implies a deeper penetration of this pulse into the node. A similar phenomenon is usually observed in the presence of 4: 1.
Fig. 1.13. Advanced AV block II degree with a variable depth of penetration of the atrial impulse into the AV node (AVU).
On the presented in fig. 1.13, B of the record obtained 3 minutes after the registration shown in Fig. 1.13, A, the three periods of AB-holding 2: 1 are followed by answers 3: 1 and 4: 1. Registration of the transmembrane potential in the N-region of the AV node during 2: 1 again demonstrates fluctuations in the level of conduction disturbance, causing the alternation of time periods of the AV conduction. When a blocked pulse penetrates deeper into the AV node, the subsequent action potential of the fiber detects a decrease in the rate of rise of the front when smearing or memorizing the zero phase. Obtaining such data implies the presence of either decremental or uneven conduct.
Based on the sequence of alternation of the degree of excitation penetration - during periods of 2: 1, it would be expected that the sixth atrial impulse will be blocked somewhere in the proximal part of the AV node, however, it apparently penetrates deeper, judging by the amplitude of the corresponding local response. The intra-nodal blockage of the subsequent (seventh) atrial impulse and the occurrence of a 3: 1 conduct ratio are most likely due to this deeper hidden conduct. Thus, with advanced AV blockade of the II degree, variations in the depth of excitation penetration are apparently the rule, even if violation of the conduction always occurs in the tissue of the AV node.
Other mechanisms, such as latent circulation of excitation, contribute to the development of AV blockade .
When conditions similar to atrial flutter are created experimentally by increasing the frequency of atrial stimulation in an isolated rabbit heart, AV 2: 1 is usually observed at a sufficiently high frequency of stimulation. In such cases, every second atrial impulse is blocked inside the AV node, although phenomena similar to those shown in Fig. 1.12 are also sometimes observed.
With AV blockade of the III (or high) degree, ventricular activation is controlled mainly by the secondary pacemaker, and sinus or atrial impulses are rarely carried out in the ventricles. Obviously, the classification of types I and II of the AV blockade of the II degree is not applicable for this variant of the blockade.
In the example of the AV blockade of the III degree presented in fig. 1.14, A, sinus bradycardia with moderate sinus arrhythmia is observed. However, P waves and QRS complexes are independent of each other with a constant ventricular rhythm of 30 beats / min. The width of the QRS complex does not go beyond the norm (0.07 s); therefore, the automatic focus is most likely located above the bifurcation of the His bundle. This suggests the localization of the block above the secondary pacemaker, probably inside the AV node [17.30, 33].
In fig. 1.14, B small waves at the level of the contour line indicate atrial fibrillation, while ventricular complexes appear very regularly at a frequency of 40 beats / min. Thus, the presence of a secondary pacemaker is quite obvious [15-17]. In order for such a mechanism of escape (escape) of the rhythm to remain active, it is necessary to postulate a very high degree of impaired conduction with the blocking of most impulses coming from the blinking atria. The normal width of the QRS complexes in this case involves the localization of the conduction block in the atrioventricular junction, most likely in the AV node. With the introduction of high doses of cardiac glycosides and the presence of impaired conduction, digitalis intoxication as a mechanism of such disorders should be suspected first of all. On the other hand, if the ventricular cycles against the background of atrial fibrillation are not absolutely regular, as in this case, and many R – R intervals are the same, we can assume the frequent occurrence of ejection impulses as a result of degree II AV block. However, in the absence of clear criteria for assessing the percentage of identical R – R intervals (suggesting a slipping rhythm in some long piece of recording), a blockade of the second degree should be suspected. Thus, the diagnosis in such cases may be subjective rather than empirical, in contrast to cases of grade III blockade presented in Fig. 1.14, B. Nevertheless, in such situations, the clinician should not forget about the possible effects of digitalis preparations.
Fig. 1.14. Grade III AV block with narrow QRS complexes with sinus rhythm (A) and atrial fibrillation (B).
| << Previous || Next >> |
| = Skip to textbook content = |
Advanced atrioventricular block of the second degree and block of the third degree of type A1
- Advanced atrioventricular block of the second degree and block of the third degree of type B
Advanced AV block II degree and AV block III degree can also be determined in the presence of wide QRS complexes . The experimental record shown in Fig. 1.15 was obtained a few minutes after the recording shown in Fig. 1.12. In fig. 1.15 registration of transmembrane potentials in the atrial fiber adjacent to the AV node (II), as well as in the region of the NH AV node (NH)
- Atrioventricular block of the second degree
Grade II AV block is usually divided into two types: Wenckebach I (Mobitz I) and Wenckebach II (Mobitz II) [35, 36]. A high degree AV block with a higher conduction ratio (2: 1, 3: 1) may be a block of type I or II. Block type Wenckebach I (Mobitz I). Classical blockade of type I is characterized by a progressive increase in the P – R interval until the P wave is blocked (Fig. 2.3).
- Atrioventricular block of the second degree with expanded QRS complexes
Grade II AV block, combined with delayed intraventricular conduction (QRS ^ 0.12 s), usually proceeds like Mobitz II and has a less favorable prognosis than AV block with normal QRS complexes. Therefore, it is important that the features and principles of the diagnosis of this type of blockade are fully understood [6,7,9,32,33]. A typical example of grade II AV block (type II) is shown in Fig. 1.7;
- Atrioventricular block of the second degree with normal QRS complexes
In fig. 1.2 in the middle part of the II lead three contractions are visible with a progressive increase in the P – R interval; the fourth P-wave (P4) cannot pass into the ventricles, which causes a long pause. The pause ends with a wave P (P5), which is carried out in the ventricles (again with a shorter interval P — R). Since three of the four sinus impulses are transmitted to the ventricles, this is called the “holding ratio
- Spontaneous atrioventricular block Atrioventricular block of the first degree
Grade I AV block (P — R> 0.21 s) may be the result of delayed conduction in the atrium, AV node, His bundle or in its legs (Fig. 2.2) . In 79% of our patients with an increased P – R interval, delayed conduction occurred in two (or more) places, although the AV node was the dominant place of delay (in 83% of patients). Delay in only one place was noted in 21% of patients: in 11%
- The degree of atrioventricular block
From the point of view of the severity of the blockade, it is considered complete if signs of excitation in the ventricles are not found anywhere on the ECG. It should be noted, however, that in very large fragments of the rhythmogram and with 24-hour Holter monitoring, random unblocked excitations are often detected in patients presumably having a “complete” AV block. This situation should be distinguished from
- Atrioventricular block of the first degree
Since the normal time range of atrioventricular conduction (P – R interval) in adults is believed to be 0.12–0.21 s, the determination of P – R intervals exceeding 0.22 s indicates degree I AV block. This criterion can only be used if there is a regular sinus (or atrial) rhythm. When atrial extrasystole is administered to the ventricles at intervals of R — R
- 2nd degree atrioventricular block - incomplete
For atrioventricular blockade of the 2nd degree it is characteristic that part of the impulses emerging from the sinus node do not pass the atrioventricular connection and do not enter the ventricles. Therefore, this part of the sinus impulses blocked by the atrioventricular connection cannot cause ventricular excitation. Therefore, on the electrocardiogram after the P wave (atrial excitation) of the ventricular
- 3rd degree atrioventricular block - complete
With complete atrioventricular block, the atria are excited from the main driver of the heart rhythm - from the sinus node. Therefore, a P wave recorded at a certain constant frequency (for example, 90 per minute) will take place on the electrocardiogram, and the P – P intervals measured on different sections of the ECG tape will be the same (in our example, 0.67 s). And what will be the pacemaker for
- Atrial ventricular block of the second degree
ICD-10 code I44.1 Diagnostics Diagnosis Mandatory Level of consciousness, respiratory rate and effectiveness, heart rate, pulse, blood pressure, ECG, history if possible Laboratory tests: hemoglobin, blood gases, CBS, electrolytes (K, Na, Mg, Ca, Cl), blood glucose, white blood cells, blood counts, KFK, AlAT, AsAT enzymes During treatment Monitoring
- Anterograde AV blockade of the I degree, or incomplete AV blockade with lengthening of time of AV carrying out
This form of blockade was described at the end of the 19th century. K. Wenckebach (1899), who showed that when AV conductivity worsens, the a – c interval elongates on the jugular venous pulse curve. The same phenomenon on the ECG (more in the II lead) is reflected in the lengthening of the P – R (Q) interval, with each P wave followed by a QRS complex associated with it, that is, conducted. Total AV block I degree
- Anterograd AV block of the III degree, or full AV block
The loss of connection between the excitation of the atria and ventricles is the most important feature of the transverse block of the heart. Atrial rhythm (P waves) is more often than ventricular (QRS complexes), in contrast to AV dissociation, in which the independent ventricular rhythm exceeds or equal to the atrial rhythm. Electrocardiographic and clinical signs of complete AV blockade are largely dependent on the level
- Anterograde in blockade of the II degree, or incomplete AV blockade with the absence of one or more consecutive sinus (atrial) pulses
In 1894, T. Engelman reported that he observed on the frog’s heart a progressive lengthening of conduction from the atria to the ventricles, ending with the disappearance of one contraction. After 5 years, K. Wenckebach (1899) described the same phenomenon in humans and called it the Luciani periods by the name of the researcher, who as early as 1872 pointed to a similar phenomenon. In 1906, J. Hay discovered that with