12-Leads Advanced Cardiology: AXIS, HEMI-BLOCK, and BBB.
Determining Axis and Hemiblock:
Axis is the term used to describe the general direction of electrical travel through the heart. In a healthy heart this occurs from the base (SA node) to the apex (left ventricle), in a right-to-left fashion. This means that impulses are generated at the SA node, travel through the AV node, down the bundle branches, and reach the purkinje fibres uninhibited. Determination of the axis is relatively easy.
Axis deviation is significant as it applies to Hemiblocks. The bundle branch system of the heart is divided into left and right bundle branches. The right bundle branch is a single circuit, while the left bundle branch is made up of an anterior and a posterior portion (fascicles). These 3 pathways make up the tri-fascicular system. Blocks in this system can be precursors to a heart block. These areas of the heart are living tissue, and, as such require blood supply to function. Blocks are indicative of impaired blood flow to the affected fascicle(s). While not the level of ischemia that raises concerns seen in ACS, they are indicators of some level of CAD.
Hemiblocks are useful in determining cardiac perfusion and in evaluating for possible NSTEMI:
Anterior Hemiblock: This is indicated by left axis deviation. It is typically asymptomatic, but can be a predictor of increased mortality in the presence of AMI.
Posterior Hemiblock: This is indicated by right axis deviation. The posterior hemifascicle is much larger and requires redundant circulation from both the RCA and the LCX. A block here, in the setting of AMI, indicates massive occlusion and a poor outcome.
A complete Hemiblock of both fascicles on the left side is a LBBB.
Bundle branch blocks:
A bundle branch block can be confirmed when there is a wide QRS (>0.12s) in V1, as well as widening in Lead !, and Lead V6. RBBB has the characteristic rSR format, while LBBB is typically only a wide complex. Determination is as easy as finding the J point and then looking at the direction of the terminal deflection to the left of it. Positive deflection is a RBBB, and negative deflection is a LBBB (turn-signal method).
Patho:
Impulses generated in the SA node travel through the AV node and down left and right bundle branches. In order for a QRS to be narrow, the impulses must travel down both branches at the same time. In a BBB, one of the branches is damaged, usually by ischemia. The impulse travelling down that side has to be carried by the surrounding cardiac cells. It still goes through, but it takes longer. This means that one ventricle depolarizes slightly slower than the other. The result is a wide QRS.
This does affect cardiac output because it results in less efficient contraction, and a decrease in SV. This reduces the ejection fraction of the left ventricle.
Patients with a BBB, with a complex that is wider than 170ms (0.17s) have and ejection fraction <50%. This is significant in that, perfusion is largely dependant upon the afterload provided by compensatory vasoconstriction. Giving these patients vasodilators, such as nitroglycerine or morphine, can be higher risk.
The new onset LBBB and STEMI equivalence:
Although LBBB is commonly listed among the many STEMI mimics, new onset LBBB can occur in the presence of major LCA/LAD occlusion. Furthermore, LBBB can hide a STEMI and should be considered a major red flag. For this reason, the Scarbossa Criteria are used to evaluate LBBB in the prehospital setting.
Scarbossa Criteria:
Concordant STE >1mm in at least one lead..
Concordant ST Depression >1mm in at least one lead, V1-V4.
Discordant STE ANYWHERE in at least one lead, >25% the size of the preceding S-wave.
If any of the above criteria are met, the interventional cardiologist should be contacted, as it is high probability that this IS as STEMI.
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