Intro to EKG Interpretation - Myocardial Infarctions (Part 2 of 2)

Summary

This video focuses on the EKG diagnosis of acute myocardial infarctions (MIs), covering special situations such as new conduction system diseases, identifying the culprit vessel in inferior MIs, diagnosing right ventricular and posterior infarcts, and handling cases with left bundle branch blocks. It delves into the blood supply to the conduction system and discusses various diagnostic criteria and EKG patterns for recognizing different types of MIs. Special techniques, including the use of right-sided and posterior EKG leads, are introduced to enhance diagnostic accuracy, especially in complex or atypical cases of MI.

Takeaways

• 😀 The blood supply to the conduction system is variable and generally not as well established as the blood supply to myocardial walls.
• 😀 In about 55% of people, the RCA supplies the SA node, while in 45%, it is supplied by the circumflex artery.
• 😀 Sinus node dysfunction, such as sinus bradycardia or arrest, is often caused by obstruction in the proximal RCA or circumflex artery.
• 😀 Type 2 second-degree AV block is usually due to obstruction in the LAD or RCA, rarely the circumflex.
• 😀 New right or left bundle branch blocks are most suggestive of LAD disease.
• 😀 To hypothesize the culprit vessel in an inferior MI, concurrent ST elevation in Lead 1 indicates circumflex involvement, while ST depression in Lead 1 suggests RCA obstruction.
• 😀 RV infarcts are relatively rare but can be suggested by hypotension and conduction system abnormalities in the setting of an inferior MI.
• 😀 Right-sided EKG leads (V3R, V4R, V5R, V6R) are used to confirm RV infarction, with V4R being the most commonly used lead.
• 😀 Posterior MIs can be identified using reciprocal changes such as horizontal ST depression and upright T-waves in V1 and V2, along with an R/S ratio greater than 1.
• 😀 Diagnosing STEMI in the presence of a left bundle branch block is challenging, but can be aided by Scosa criteria, which focus on ST elevation and depression patterns.

Q & A

What is the primary focus of this video on myocardial infarctions (MIs)?

The video focuses on understanding the EKG findings related to various types of myocardial infarctions (MIs), particularly how to diagnose acute MIs in special circumstances, such as new conduction system disease, right ventricular infarction, posterior infarction, and acute MI with a left bundle branch block.

How does new conduction system disease relate to the diagnosis of acute MI?

New conduction system disease, such as sinus node dysfunction, AV block, or bundle branch blocks, can indicate the location of the MI. For instance, sinus bradycardia or sinus arrest suggests proximal RCA or circumflex obstruction, while type 2 AV block often points to LAD or RCA obstruction.

How can you hypothesize the culprit vessel in an inferior MI based on EKG findings?

In an inferior MI, if there is ST elevation in lead 1, the circumflex artery is likely the culprit. If ST depression occurs in lead 1, the RCA is more likely. If the ST elevation in lead 2 is greater than in lead 3, the circumflex is more likely; otherwise, the RCA is the probable cause.

Why are right ventricular infarctions (RV infarcts) less common?

RV infarcts are less common because they require a proximal RCA occlusion, and the RV is less prone to ischemia and infarction due to its thinner wall and reduced workload compared to the left ventricle.

What EKG findings suggest a right ventricular infarction?

In addition to ST elevation in lead V1 (greater than V2), the presence of hypotension and conduction system problems accompanying an inferior MI can suggest an RV infarct. Confirmatory evidence includes ST elevation in the right-sided EKG leads (V3R, V4R, V5R, V6R).

How can you diagnose a posterior myocardial infarction using EKG?

Posterior MI is not directly visible on conventional EKG leads. However, reciprocal changes such as horizontal ST depressions and prominent T-wave inversions in V1 and V2, along with a R/S ratio greater than 1, can suggest posterior involvement. Using posterior leads (V7-V9) can confirm this diagnosis.

What is the significance of using V7, V8, and V9 leads in diagnosing a posterior MI?

Leads V7, V8, and V9 are placed on the patient's back to capture the electrical activity of the posterior wall, which is not directly visible in standard EKG leads. These leads replace V4-V6 on the printout to confirm the presence of a posterior infarction.

What challenges arise when diagnosing an acute MI in the presence of a left bundle branch block?

Diagnosing an acute MI in the presence of a left bundle branch block is challenging due to the widespread secondary ST-T changes caused by the block. These changes typically oppose the QRS complex and can obscure the primary repolarization abnormalities indicative of a STEMI.

What are the key elements of the SCoSA criteria for diagnosing a STEMI in the presence of a left bundle branch block?

The SCoSA criteria include two main signs: 1) ST elevation greater than or equal to 1 mm in a lead with a positive QRS complex, and 2) ST depression greater than or equal to 1 mm in leads V1, V2, or V3. These criteria help differentiate primary MI from changes due to the left bundle branch block.

What are the limitations of the SCoSA criteria for diagnosing a STEMI in patients with a left bundle branch block?

The SCoSA criteria have a low sensitivity (20-35%) but high specificity (over 90%). This means that while the criteria are reliable when positive, they are not very sensitive and might miss some cases of acute MI in patients with left bundle branch block.