Ebook Basic electrocardiography: Part 2
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Ebook Basic electrocardiography: Part 2
Atrioventricular (AV) Block4There are three types of atrioventricular (AV) node block: first-degree, second-degree. and third-degree. These are someti Ebook Basic electrocardiography: Part 2imes abbreviated I®. 2°. and 3". respectively (Table 4.1). These abbreviations should not be confused with "primary, secondary, ami tertiary." which can carry the same annotation. While AV block may be a transient phenomenon (e.g.. associated with ischemia, infarction, or drug intoxication), the blo Ebook Basic electrocardiography: Part 2ck may be permanent.First-degree AV block is simply a prolongation of the PR interval above the normal range, i.e.. >0.20 s (Fig. 4.1). At slow heartEbook Basic electrocardiography: Part 2
rates the normal PR interval may extend up to 0.21 s. but for simplicity it is reasonable to read any prolongation of the PR interval over0.20 s as fiAtrioventricular (AV) Block4There are three types of atrioventricular (AV) node block: first-degree, second-degree. and third-degree. These are someti Ebook Basic electrocardiography: Part 2AV block is essentially a benign condition and is very unlikely to be associated with progression to a higher degree of AV block 11 J.Second-degree AV block is of two subtypes: Mobitz 1 and Mobitz II. Mobitz I is also commonly known as Wenckebach block. In Mobitz I (Wenckebach), there is gradual pro Ebook Basic electrocardiography: Part 2longation of the PR interval duration until finally one p wave is not conducted through the AV node to the ventricles (Fig. 4.2). As a consequence, aEbook Basic electrocardiography: Part 2
p wave is not followed by a QRS complex. Following the "dropped beat" (the missing ỌRS after a p wave), the PR interval is once again relatively shortAtrioventricular (AV) Block4There are three types of atrioventricular (AV) node block: first-degree, second-degree. and third-degree. These are someti Ebook Basic electrocardiography: Part 2efore the nonconducted beat) may vary. The extent of block is given in a ratio with the number of atrial beats observed in the cycle followed by a colon and then the number of ventricular beats in the cycle. For Mobitz 1. the number of atrial beats in the cycle is always just one greater than the nu Ebook Basic electrocardiography: Part 2mber of ventricular beats (e.g., 3:2. 4:3). This repetitive prolongation of PR intervals until a p wave is not conducted continues as long as the factEbook Basic electrocardiography: Part 2
or responsible for the block persists without improvement to a lower degree of block (first degree) or deterioration into a higher degree of block.WhiAtrioventricular (AV) Block4There are three types of atrioventricular (AV) node block: first-degree, second-degree. and third-degree. These are someti Ebook Basic electrocardiography: Part 2ontradictory, at first glance. Yet the apparent paradox IS possible because the increment of change in successive PR intervals gradually decreases, causing the R-R interval to shorten. This phenomenon is diagrammatically demonstrated in Fig. 4.3.Even though in classic cases of Mobitz I the consecuti Ebook Basic electrocardiography: Part 2ve R-R intervals decrease, it is clear that this is not always true (Fig. 4.4).Mobitz II is a higher degree of AV block than Mobitz I wherein a relatiEbook Basic electrocardiography: Part 2
vely constant fraction of p waves are conducted through the AV node to cause ventricular depolarization (Fig. 4.5). The PR intervals of the conducted Atrioventricular (AV) Block4There are three types of atrioventricular (AV) node block: first-degree, second-degree. and third-degree. These are someti Ebook Basic electrocardiography: Part 2 to QRS complexes is often constant, the ratio also may vary somewhat.k should be noted that Mobitz II with a 2:1 block cannot be distinguished from Mobitz I block with a cycle length of only 2. In cases of 2:1 block, it is appropriate to simply call the phenomenon "second-degree AV block w ith 2:1 Ebook Basic electrocardiography: Part 2block" and not specify Mobitz I or 11. unless other cycles clearly demonstrate the presence of one or the other (Fig. 4.6).Mobitz I is generally consiEbook Basic electrocardiography: Part 2
dered to be a benign problem, primarily because it is usually transient in the setting of inferior myocardial infarction, and because in most cases onAtrioventricular (AV) Block4There are three types of atrioventricular (AV) node block: first-degree, second-degree. and third-degree. These are someti Ebook Basic electrocardiography: Part 2f chronic Mobitz I. however, the prognosis appears to be just as poor as with chronic Mobitz II. w ith a 5-year survival of only about (SO'.t |2|.Third-degree AV block is also know n as complete heart block. It means that none of the p w aves are being conducted through the AV node to the ventricles Ebook Basic electrocardiography: Part 2. As a consequence, the heart would stop depolarizing if "subsidiary pacemakers” below the point of block in the AV node did not take over the initiatEbook Basic electrocardiography: Part 2
ion of electrical activity (Fig. 4.7). These subsidiary pacemakers are in the node below the point of block, or in the65B.G. Petty. Basic EleeimcaniioAtrioventricular (AV) Block4There are three types of atrioventricular (AV) node block: first-degree, second-degree. and third-degree. These are someti Ebook Basic electrocardiography: Part 2First degree r 1*5: PR >0.20 sSecond decree ("ỉ*1")Mobitz I (Wenckebach): Gradual prolongation of PR interval until leal droppedMobilz II: Pn>|xwt>onal L'lKiducliofi usually al a cinislaiit ratio and equal (usually normal) PR intervals for conducted beatsThird degree (“3*"): Also known as “complete Ebook Basic electrocardiography: Part 2heart block"No relationship between p waxes and QRS ciHnpleses. each with independent rateconduction bundles. or in the ventricles themselves. When coEbook Basic electrocardiography: Part 2
mplete heart block is present and a subsidiary pacemaker takes over the initiation or electrical activity, the rhythm is called an “escape” rhytlun. aAtrioventricular (AV) Block4There are three types of atrioventricular (AV) node block: first-degree, second-degree. and third-degree. These are someti Ebook Basic electrocardiography: Part 2 of the nature of all cardiac cells to automatically depolarize. The rate at which this automatic depolarization occurs is fastest in the sino-alrial (SA) node, which llierefore acts as the normal pacemaker for the heart. The automatic depolarization rate is slower in the node (about 45-55 beats per Ebook Basic electrocardiography: Part 2 minute) and even slower in the ventricles (about 35-45 beats per minute). The rate of the escape rhythm, in addition to the QRS complex duration, canEbook Basic electrocardiography: Part 2
therefore give a clue as to where the escape rhythm originates. If the QRS complex IS normal induration (<0.12 s). the escape focus (point where elecAtrioventricular (AV) Block4There are three types of atrioventricular (AV) node block: first-degree, second-degree. and third-degree. These are someti Ebook Basic electrocardiography: Part 2mal or near normal conduction pattern. If the QRS complex is prolonged (>0.12 s). the activation focus is either in the ventricles or in the bundle system below (he bifurcation of (he bundle of His. and the QRS complexes have the configuration of a bundle branch block. In these instances where the Q Ebook Basic electrocardiography: Part 2RS is prolonged, a slower rate (35—15) favors a ventricular focus, while a relatively taster rate (45—55) favors a nodal or bundle focus. OccasionallyEbook Basic electrocardiography: Part 2
there is what might be called an ‘‘iatrogenic escape rhythm.” otherwise known as a transvenous pacemaker (big. 4.8).With third-degree AV block, the pAtrioventricular (AV) Block4There are three types of atrioventricular (AV) node block: first-degree, second-degree. and third-degree. These are someti Ebook Basic electrocardiography: Part 2rom the gradual prolongation of the varying PR intervals in Mobitz I (Wenckebach). It is usually possible in third-degree AV block to determine both the atrial rate (the frequency of p w aves) and the ventricular rate (the frequency of QRS complexes) (Fig. 4.9). Since the electrical activity of the Ebook Basic electrocardiography: Part 2atria and ventricles in complete heart block is unrelated, or "dissociated," it is clear that complete heart block is an example of "AV dissociation,"Ebook Basic electrocardiography: Part 2
where the atria and ventricles beat at different rates and are totally independent. Yet AV dissociation is a general term and not synonymous with comAtrioventricular (AV) Block4There are three types of atrioventricular (AV) node block: first-degree, second-degree. and third-degree. These are someti Ebook Basic electrocardiography: Part 2ourth p waves arc conducted with it PR interval of 0.22 s. The second and fifth p waves arc conductedwilh a PR interval of 0.30 s. and the third and sixth p waves are not conducted. This is a 3:2 block. Arrows indicate nonconducted p wavesFig. 4.3 Mobitz 1 (Wenckebach). I he diagram shows how the R- Ebook Basic electrocardiography: Part 2R interval may decrease while the PR interval gradually increases. The P-P interval is constant at 0.8 s (heart rate of 73 beats per minute). The PR gEbook Basic electrocardiography: Part 2
radually increases until the sixth p wave is not conducted into the ventricles (arrow). The RP interval is simply the P-P interval minus the PR intervAtrioventricular (AV) Block4There are three types of atrioventricular (AV) node block: first-degree, second-degree. and third-degree. These are someti Ebook Basic electrocardiography: Part 2r an increment of 0.08 s. Ute third PR interval is 0.30 s. so the increment of increase in PR between the second and third p wave is 0.06 s. In this example, the increment of increase between consecutive PR intervals IS 0.08.0.06.0.04. and 0.02 s. The R-R interval decreases, then, because the increm Ebook Basic electrocardiography: Part 2ent of change in consecutive PR intervals decreasesFig. 4.4 Mobitz I (Wenckebach). The PR interval gradually increases until a beat is dropped, rhe drEbook Basic electrocardiography: Part 2
opped beats actually fall in the middle of ventricular repolarization, and so they distort the normal configuration of the T waves (arrows) In this exGọi ngay
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