ECG Review:
The cardiac conduction system:
Impulses are generated in the SA node. From here it travels down to the AV node where it is slowed down to allow for full contraction of the Atria. The impulse then moves down the bundle of HIS, and along each bundle branch, and finally to the Purkinje fibres.
The Intrinsic rates are:
SA node: 60-100 bpm
AV node: 40-60 bpm
Purkinje fibres: 25-40 bpm
In order to have a narrow QRS (,0.12) the impulse must travel down both bundle branches. If the QRS is wide, you likely have a bundle branch block.
Components of the ECG strip:
P wave: Represents atrial depolarization.
QRS: Represents depolarization of the ventricles.
T wave: Represents repolarization of the ventricles.
Sinus Rhythms:
In order for a rhythm to be sinus, impulses must travel unimpeded throughout the cardiac conduction system. This results in a rhythm that presents with a P wave, a narrow QRS, and a T wave that follows with a normal QT interval.
Normal Sinus Rhythm (NSR), Sinus Tachycardia, and Sinus Bradycardia:
The Rhythm will have regular P-P rhythm, P waves preceding each QRS, a narrow QRS, and T waves following each QRS. If the rate is 60-100, it is a NSR. A rate greater than 100 is considered Sinus Tachycardia. A rate less than 60 is considered sinus bradycardia (Some sources consider <50 bpm to be bradycardia).
Irregularities in Sinus rhythm:
Premature atrial contractions (PACs): This occurs when an impulse is generated in the atria more quickly than it should be, leading to depolarization that is ahead of the normal rhythm. The result is a single beat that will be out of sync, usually followed by a compensatory pause.
Junctional Rhythms:
These rhythms result when the initial impulse is not generated at the SA node, but at the AV junction. They present the same as a sinus rhythm, except that the P wave will be inverted, and may occur after the QRS, or even be hidden within the QRS itself. These rhythms will usually be 60bpm or less, but can occur as accelerated rhythms up to 100bpm. They are susceptible to pre-excitation just like sinus rhythms, and can present with PJCs.
Re-entry dysthythmias:
Supraventricular Tachycardia: This occurs when an accessory pathway forms between the SA node and the AV node. As a result there is circular electrical activity, causing the atria and ventricles to continue to fire rapidly (140-250bpm). The result is a rhythm that presents with narrow QRS, followed by prominent T waves, and absent P waves. This rhythm may be continuous, or intermittent (Paroxysmal SVT).
Atrial Rhythms: There are 2 major dysrhythmias associated with the atria.
Atrial fibrillation: This occurs when multiple foci in the atria generate impulses in a disorganized fashion. This presents with irregularly irregular P-P intervals, and a lack of discernible P waves.
Atrial Flutter: This occurs when multiple foci fire in a single atrium. The AV node protects the ventricles and impulses are generated normally to them. The result is regular P-P intervals, and saw tooth P waves.
AV Blocks: These occur when there is an abnormally long delay in conducting impulses through the AV node.
1st degree AV block: This presents with a prolonged PR interval >0.20 seconds
2nd degree AV block Type 1: The PR gradually lengthens until a QRS is dropped.
2nd degree AV block Type 2: The PR remains normal with sudden drop of a QRS.
3rd degree AV block: There is a complete block at the AV node, resulting in independent contraction of the atria and ventricles. This presents with a rhythm in which there is no relation at all between the P waves and QRS complexes.
Ventricular Rhythms:
Premature Ventricular contractions (PVCs): Occur when an impulse is generated in the ventricles, leading to unexpected depolarization. This presents with a sudden wide QRS. These may occur as a single event, or in a repeating pattern.
Idioventricular rhythm: When higher pacemaker sites fail, cells in the ventricles take over. The ventricles can generate a slow, regular, wide complex rhythm without P waves. If the rate is >40 and <100 it is called an accelerated idioventricular rhythm.
Ventricular Tachycardia: V-Tach occurs when ectopic foci in the ventricles generate a repeating impulse to fire. This results in a wide complex, tachycardia that is usually >120bmp. V-Tach may be monomorphic, polymorphic, or a combination (Torsades).
Ventricular Fibrillation (V-Fib): This rhythm represents chaotic electrical impulse generation in the ventricles and is not compatible with life. The rhythm will have not clear pattern at all, and the patient will be pulseless.
Pulseless Electrical Activity (PEA): This occurs when the electrical system of the heart continues to function, even though the heart does not provide any mechanical activity. It will appear as a sinus rhythm, but no carotid pulse will be palpable.
12-Lead ECG:
This method of ECG looks at the heart from 12 views, and is useful in examining ischemia or electrical abnormalities in one or more areas.
The leads as they relate to the heart:
The primary reason to use a 12 lead is to look for ischemia, as seen in a MI. This will present with ST segment elevation in the leads of the affected region(s).
Other things commonly seen in 12 lead: The bundle branch blocks.
In order for a QRS to be narrow, the impulse from the SA node must pass the AV node and travel down both bundle branches. If there is a blockage, the impulse down one of the branches will be delayed, and this is why our QRS segment will be wider than normal. This is relevant because these blocks can be misinterpreted as ST elevation and lead to improper treatement.
Left Bundle Branch Block:
All bundle branch blocks will be seen in the septal leads (V1 and V2). In the case of a LBBB lead V1 will have a QRS >0.12 and will have a negative deflection. To confirm LBBB, look at leads I and V6. If both are wide, its a bundle branch block.
Right Bundle Branch Block: A RBBB will also present wide in V1, but will have a positive deflection and have a “Bunny Ear” formation.
This is a basic overview of ECG, 3 and 12 leads. I will attach a PDF including common STEMI mimics to the facebook page, and will cover the more common ones in future posts.
Cheers.
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