Right bundle branch block during the acute phase of myocardial infarction: modern redefinitions of old concepts.

Many investigators have dealt with the aspects of this problem. Both historical and contemporary registries and randomized clinical trials (RCTs) help us to understand the clinical relevance of BBB in the presence of an acute MI. The prevalence of BBB on arrival in patients with MI varies between 1.6 and 10.9%, without marked difference between RBBB and LBBB. The true incidence of BBBs of new or probable new onset is a more difficult task to assess and thus this information is often missing in RCTs and registries. When reported, it varies between 15 and 55%, with a higher incidence of RBBB, despite the fact that LBBB is more prevalent in patients with chronic ischaemic heart disease. The timing of onset of BBB, either left or right, might indicate the pathophysiology of the present acute coronary syndrome and in the case of LBBB, helps to overcome the limitations inherent to the masking of the repolarization phase. In the presence of a patient with an acute coronary syndrome and an LBBB of undetermined onset, all efforts must be directed to the underlying pathophysiology because the electrocardiogram is not readable in terms of repolarization phase and extension of the acute injury. Despite a similar outlook, the clinical background of these patients is heterogeneous; it ranges from a pre-existing LBBB in the setting of an acute coronary syndrome without ST-segment elevation to an STEMI with a pre-existing LBBB, an anterior STEMI, and an LBBB of new onset. The treatment of course varies greatly. Nonetheless, although an RBBB does not theoretically mask the repolarization phase, nor a pre-existing Q-wave, minor ST-segment elevation in the anterior leads (i.e. V1–V4) can be missed because these are ‘compensated’ by thepseudo-normalization of the negative T-waves. The difficulty in interpreting the clinical picture of an MI in the presence of any complete BBB is clearly evident in NMRI-2. In this large registry, among patients for whom thrombolytic therapy was clearly indicated, fewer patients with LBBB or RBBB (16.6 and 32.0%, respectively) received this therapy than those with no BBB (66.5%). In addition, the presence of an RBBB is rarely considered, as opposed to LBBB, among the criteria of the STEMI scores. Many studies, especially in the pre-thrombolytic era, associated both RBBB and LBBB in the presence of MI with a higher mortality. However, results were limited by the small numbers and the lack of strict definitions. Wong et al. analysed the HERO-2 data bank to assess the prognostic value of BBB in the acute phase of an MI. This large RCT is particularly suitable for the purpose because patients with LBBB were included and, by protocol, electrocardiograms were collected at randomization and at 60 min. In this way, the authors could discriminate patients with BBB of definite new onset (i.e. those who developed BBB after randomization) from those with BBB already present on admission (including new, presumably new, and new onset BBBs). In other words, they could separate the prognosis of BBB possibly associated with a pre-existing cardiac disease from that of BBB as a consequence of a large acute cardiac damage.