RESUMEN

Background: Second-degree atrioventricular (AV) block at rest is very common in horses. The underlying molecular mechanisms are unexplored, but commonly attributed to high vagal tone. Aim: To assess whether AV block in horses is due to altered expression of the effectors of vagal signalling in the AV node, with specific emphasis on the muscarinic acetylcholine receptor (M2) and the G protein-gated inwardly rectifying K+ (GIRK4) channel that mediates the cardiac IK,ACh current. Method: Eighteen horses with a low burden of second-degree AV block (median 8 block per 20 h, IQR: 32 per 20 h) were assigned to the control group, while 17 horses with a high burden of second-degree AV block (median: 408 block per 20 h, IQR: 1,436 per 20 h) were assigned to the AV block group. Radiotelemetry ECG recordings were performed to assess PR interval and incidence of second-degree AV block episodes at baseline and on pharmacological blockade of the autonomic nervous system (ANS). Wenckebach cycle length was measured by intracardiac pacing (n = 16). Furthermore, the expression levels of the M2 receptor and the GIRK4 subunit of the IKACh channel were quantified in biopsies from the right atrium, the AV node and right ventricle using immunohistochemistry and machine learning-based automated segmentation analysis (n = 9 + 9). Results: The AV block group had a significantly longer PR interval (mean ± SD, 0.40 ± 0.05 s; p < 0.001) and a longer Wenckebach cycle length (mean ± SD, 995 ± 86 ms; p = 0.007) at baseline. After blocking the ANS, all second-degree AV block episodes were abolished, and the difference in PR interval disappered (p = 0.80). The AV block group had significantly higher expression of the M2 receptor (p = 0.02), but not the GIRK4 (p = 0.25) in the AV node compared to the control group. Both M2 and GIRK4 were highly expressed in the AV node and less expressed in the atria and the ventricles. Conclusion: Here, we demonstrate the involvement of the m2R-IK,ACh pathway in underlying second-degree AV block in horses. The high expression level of the M2 receptor may be responsible for the high burden of second-degree AV blocks seen in some horses.

RESUMEN

Heart transplantation is the treatment of choice for those with end-stage heart failure. However, despite improvements in immunosuppressive treatment, patients are at significant risk of allograft rejection, especially early after transplantation. Any changes in patient's heart condition including reduced left ventricular ejection fraction, arrhythmia and any types of blocks need attention. Herein we report on a 29-year-old man who underwent heart transplantation 5 years before due to dilated cardiomyopathy. He was on immunosuppressive therapy and was good until one week before his admission, when he felt palpitation. Electrocardiography during palpitation showed a second-degree AV-block with heart rate of 60 beats/min. Echocardiography showed good left ventricular systolic function with no regional wall motion abnormality. The patient referred for coronary angiography and endomyocardial biopsy. The angiography was normal. The biopsy showed rejection compatible with ISHLT grade 2R. After treating the patient with 1.5 g methylprednisolone, the symptoms relieved and the block resolved. Bradycardia and second-degree AV-block late after heart transplantation could be a sign of cardiac allograft rejection and need more evaluation, especially endomyocardial biopsy.

RESUMEN

One of the more common causes of bigeminy at the ventricular level is type 1 second-degree atrioventricular (AV) block with 3:2 conduction ratio. In 3:2 Wenckebach, the shorter cycles reflect the consecutively conducted impulses and the longer cycles coincide with the blocked P waves. Theoretically, however, depending on the degree of conduction delay between the first and second transmitted impulses, other types of spacing of the QRS complexes may become possible. In this retrospective study of 180 patients who underwent electrophysiologic studies for symptomatic arrhythmias, atrial pacing-induced 3:2 Wenckebach periodicity resulted in a regular ventricular rate and/or in "reverse bigeminy" in 16 cases (8.9%). Reverse bigeminy was characterized by the shorter R-R intervals including both the blocked P waves and the first conducted beats of the subsequent cycles, and the longer R-R intervals coinciding with the second conducted beats during 3:2 Wenckebach. In 14 cases, regular ventricular rate and reverse bigeminy was triggered by marked conduction delay in the AV node and in 2 cases, the conduction delay was in the His-Purkinje system. Reverse bigeminy appeared to be related to dual AV nodal physiology in 8 patients. In 2 cases, sophisticated maneuvers such as termination of atrial pacing at critical intervals during the AV Wenckebach were required to expose the true conduction pattern. This study demonstrates that during rapid atrial rhythms, one cannot always be sure which P wave is responsible for which QRS complex. Rarely, extreme conduction delays can result in P waves conducting across the subsequent ventricular beats and be responsible not for the first, but for the following QRS complexes.

Asunto(s)

RESUMEN

A 64-year-old woman presented to the hospital with a complaint of dyspnea on effort. The 12­lead electrocardiogram showed a baseline complete left bundle-branch block, in which the right bundle developed >2:1 block. The subsequent blocked sinus impulse resulted in a ventricular pause that caused the ventricular escape presenting as a QRS complex with right bundle-branch block pattern. However, some of them disclosed typical fusion beats and led to interference dissociation. Although other possibilities including Lenegre's disease or myocardial disease were not excluded, the effective refractory period of both bundle branches degenerated by myocardial ischemia might be attributed to this rhythm.

Asunto(s)

RESUMEN

BACKGROUND: Spontaneous second-degree atrioventricular block induced by exercise (Ex2AVB) is rare, but it can cause profound exercise intolerance. OBJECTIVE: We sought to determine the frequency of Ex2AVB in our exercise testing practice and to describe characteristics of patients with Ex2AVB. We hypothesized that the number of patients would be small, but they would require invasive treatment. METHODS: We reviewed the Mayo Clinic Integrated Stress Center database for nonimaging tests performed from 2006 through 2010. All exercise tests coded as "second-degree atrioventricular block" were captured and reviewed. Tests were excluded if results showed evidence of second-degree atrioventricular block at rest. RESULTS: From 40,715 tests performed, definite Ex2AVB was found in only 19 patients (0.05%; 5 women and 14 men). Ex2AVB occurred as a Mobitz type II block in 4 patients and as a Mobitz type I block in 15. In 3 patients, Ex2AVB occurred only in recovery. Ex2AVB was intermittent in 11 patients and persistent in 8. Mean peak heart rate was higher with intermittent Ex2AVB than with persistent Ex2AVB (126 ±â€¯39 vs 88 ±â€¯28 bpm, P < .01), as was mean functional aerobic capacity (87% ±â€¯20% vs 59% ±â€¯14%, P < .01). Seven patients with persistent Ex2AVB received a permanent pacemaker; 1 underwent pacemaker adjustment. CONCLUSION: Ex2AVB is uncommon but can cause exercise intolerance that requires pacemaker implantation. Structural or ischemic heart disease and resting conduction abnormalities are common findings in patients with Ex2AVB. Intervention is seldom required for intermittent Ex2AVB.

Asunto(s)

RESUMEN

Thyrotoxic periodic paralysis (TPP) occurs in 2% of the asian patients with hyperthyroidism and is characterized by bilateral flaccid paralysis of the extremity, especially lower limbs. It is well-known that hypokalemia is usually accompanied by TPP. However, hypophosphatemia is usually mild and well neglected. Although paralysis is generally recovered without treatment, in some cases, patients with TPP may die due to cardiopulmonary complications, such as cardiac arrhythmia. Therefore, proper and rapid replacement of potassium is essential. But it should be acknowledged that replacement may cause a rebound. TPP is often unrecognized and over-treated in the emergency room due to its non-specific symptoms. This is why clinicians must be familiar with this disease and its diagnostic clues such as Echocardiography change and clinical features. This is a case report of a 29-year-old male presenting with TPP accompanied by hypokalemia, hypophosphatemia and second degree atrioventricular block, who showed rebound hyperkalemia and hyperphosphatemia after rapid replacement of electrolytes. EKG changed to the normal sinus rhythm in the end after the correction of the electrolytes.

Asunto(s)