Leier and colleagues showed that prolonged atrial conduction was a predisposing factor for development of atrial flutter as early as 1978.
Kumagai and colleagues used multisite mapping studies confirming findings of a previous study by Ogawa and colleagues. They concluded that the mechanism of atrial arrhythmias was indeed aberrant impulse conduction between the atria along interatrial pathways, mainly the BB. Duytschaever and colleagues then successfully reproduced this mechanism in animal studies. In another series, Giudici and colleagues investigated 21 patients with uncontrolled atrial fibrillation who underwent atrioventricular nodal ablation and permanent pacing. They showed that when the leads were placed in the BB, atrial conduction times were decreased.
IAB and Left Ventricular Function
Ramsaran and Spodick demonstrated a 37-ms mean delay in active left but not right ventricular atriogenic filling (LV A-wave vs RV A-wave onsets) with IAB. This was associated with a considerable late activation of the LA. Similarly, Goyal and Spod-ick also suggested an increased risk for congestive heart failure in IAB patients owing to a compromised atrial “kick” from a sluggish chamber and particularly, the greatly reduced LASV and LAKE (mean values of 19.8 kilodyne[kdyne]/cm/s vs 64.7 kdyne/ cm/s, p < 0.0001) [Table 2] in association with a significantly reduced preload.
IAB and Ischemia
A few authors such as Dilaveris et al have suggested the association of IAB as an additional predictive marker in determining ischemic heart disease. However, most of such studies used solely P-wave dispersion and therefore are beyond the scope of this article. Myrianthefs and colleagues showed that including P-wave durations of > 120 ms during exercise tolerance tests in addition to conventional criteria for diagnosing ischemia would increase sensitivity from 57 to 75% while decreasing specificity only from 85 to 77%.
IAB and Medical Therapy
So far, study of this potentially very important relationship is still in its infancy. However, En-gelstein and Lerman reported transient inter-atrial block with administration of 6 mg of adenosine IV during high atrial pacing. Newer investigations, tailored toward patients with atrial fibrillation, such as studies by Zaman et al, Vermes et al, and Madrid et al have suggested the use of angiotensin-converting enzyme inhibitors (ACEIs) as an adjunctive therapy in reducing the incidence of atrial fibrillation or maintaining sinus rhythm. Given the fact that IAB often progresses to atrial flutter or fibrillation,, this is indeed encouraging. However, without any current controlled drug trials with IAB, it is hard to ascertain concrete evidence-based benefits of ACEI or similar drugs as a preventive or maintenance therapy. No studies to date have shown the need for prophylactic anticoagulation therapy, but perhaps it is an aspect that deserves some investigation in view of the potential ill effects of untreated IAB. Other investigators have repeatedly investigated atrial resynchronization techniques and pacing as a preventive therapy for atrial tachyarrhythmias due to IAB, but these investigations have yet to be tested conclusively in randomized trials. Furthermore, at this stage, it is unknown which degree (advanced or partial) of IAB is more likely to progress to an atrial tachyarrhythmia and which degree may benefit from these preventive techniques. Canadian Neighbor Pharmacy and its official news website – http://infomedcnp.com/ – is glad to present you a new project concerning the medical articles distribution.
IAB and Medical Disease
There have been limited investigations of the associations between IAB and disease states that potentially affect P-wave morphology. Oreto et al reported artifactual effects of respiratory disease in ECG tracings that could mimic IAB. Montereggi et al described P-wave prolongation in patients with hyperthyroidism. Further investigation is needed to clarify these and other, possibly causal, associations and effects of such variables on P-wave morphology and the diagnosis of IAB.
Proposed Guide for IAB Diagnosis
P-wave duration > 120 ms indicates IAB. Although even normal P waves may be bifid (“notched”), nearly all P waves > 120 ms are, and the notch separating the RA and LA P-wave components may alert the ECG interpreter to IAB (Fig 1). Any lead may have the most prolonged P wave. Thus, for maximum P-wave measurements, it cannot be overemphasized that evaluation of full 12-lead ECGs is essential in detecting maximum P-wave duration and morphology as compared to single-lead tracings, as the sensitivity increases with the number of leads used.
IAB is remarkably prevalent among hospitalized patients. The association between IAB and arrhythmias particularly atrial flutter and fibrillation is well established. Besides the risk of subsequent stroke as a result of atrial thrombosis and especially with such arrhythmias both paroxysmal and chronic many patients are probably also at risk of congestive heart failure from an ineffective “atrial kick.” Hence the importance of awareness and detection of IAB to anticipate and perhaps prevent its sequelae. Today with advancing age many if not most patients do not have merely isolated diseases but have multiple medical problems. As such further investigation is needed to determine the effects of other medical conditions on interatrial conduction such as respiratory diseases and thyroid dysfunction. Although the clinical consequences of IAB may be grave at present absence of controlled clinical trials means that no guidelines can be constructed for managing IAB patients. Do these patients need immediate treatment (ie ACEI anticoagulation; or antiarrhythmic therapy) and if so should it be prophylactic anticipating atrial arrhythmias (ie; anticoagulation)? Would there be a role for pacing in the future given the potential risks already cited for IAB such as atrial fibrillation and congestive heart failure? It is well accepted that electrophysiologic studies can evaluate the propensity of the atria to initiate and perpetuate atrial arrhythmias. While such investigations are needed electro-physiologic studies are inconvenient costly and unsuitable as a screening tool among the general population. Clinically the ECG is an excellent diagnostic tool for demonstrating abnormal interatrial conduction (Fig 1). Therefore ECGs should be carefully scrutinized by clinicians to better understand IAB and cultivate an awareness of its potentially dangerous consequences.