Advanced ECG Interpretation

Dr. Jeffrey Elliot Field, HBSc. DDS, Fellow, American Dental Society of AnesthesiaDiploma, the National Dental Board of Anesthesia.

04/19/231

OBJECTIVESTo get a more in depth knowledge of ECG

interpretation.

The P-Wave in DetailThe normal P-wave:

Has a smooth contourIs monophasic in lead IIIs biphasic in lead V1Has a duration 0f less than 0.12 seconds or 3

small boxes.

P-wave Abnormalities Seen in Lead IIIn lead II two types of P-wave abnormalities

can be seen.Right atrial enlargement is seen as a taller

than normal P-wave( increased amplitude)Left atrial enlargement seen as a P-wave with

a notch in it.

P-Wave Abnormalities Seen In V1Biphasic P-Waves are seen for both left and

right atrial enlargement.For right atrial enlargement the initial

portion of the P-wave is larger than the distal portion.

Alternatively for left atrial enlargement the initial portion of the P-wave is smaller than the distal portion.

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Note the larger initial portion

04/19/23 7Note the larger terminal portion

The QRS Complex in DetailAs well as showing ventricular conduction

defects, the QRS complex along with ST segment analysis is used to diagnose myocardial oxygen deficits and myocardial infarctions.

The QRS complex is also used to diagnose accessory conduction pathways in the heart.

S-T Segment AnalysisIn order to assess the S-T segment we must

first define the J-point.The J point in the ECG is the point where the

QRS complex joins the ST segment. It represents the approximate end of depolarization and the beginning of repolarization.

The Isoelectric PointS-T segments can be elevated, depressed or

isoelectric.The J-point is deemed to be isoelectric if the S-T

line/segment is not elevated or depressed with respect to the P-Q line/segment. As in the diagram below. See arrows

S-T ChangesYou can see both S-T elevation and S-T

depression on ECG’s.S-T elevation is indicative of a myocardial

infarction. So in other words myocardial cell death is occuring.

S-T depression is indicative of myocardial ischemia. The myocardial cells are not getting enough oxygen and are at risk of dying.

S-T Depression

S-T Elevation

Myocardial InfarctionMyocardial infarctions can be categorized as

follows:-Q-wave MI-Non Q-wave MI

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Q-Wave Myocardial InfarctionThis is the classic presentation for MI’s. The developing MI is seen as ST segment elevation

followed by deepening Q-waves in the leads where ST segment elevation was 1st seen.

Q waves are “significant” if they are greater than 1 box in width (longer than 0.04 msec), or are larger than 1/4 of the R wave.

Significant Q waves are indicative of myocardial infarction.

However signifigant Q-waves in lead III alone are NOT diagnostic of an infarction, even they are otherwise “significant” in size and width.

Therefore signifigant Q-waves in lead III are ignored unless other abnormalities are seen.

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Note the large Q-waves.

Non Q-Wave Myocardial InfarctionIn this case you get classic signs and

symptoms of an MI(i.e elevated cardiac enzymes and markers and of course physical signs of an MI ( chest pain ,nausea ,vomiting , etc)

But non of the usual ECG changes ( i.e. ST segment elevation and deepening Q-waves). In fact sometimes the only clue on the ECG are inverted T-waves.

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Accessory Conduction Pathways. Also Called Pre-Excitation Syndromes

Pre-excitation SyndromesThese syndromes are characterized by an

aberrant conduction pathway that enters the ventricular muscle in addition to the normal pathway. Since these aberrant pathways are shorter they cause ventricular depolarization prior to the normal pathway.

There are 2 pre-excitation syndromesWolf –Parkinson-White Lown-Ganong-Levine Both pathways show shortened P-R intervals of

less than 0.20 sec.04/19/23 19

Wolf Parkinson WhiteThe abberant pathway is the bundle of

Kent which bypasses the AV node. This gives a shortened P-R interval ( i.e. less than 0.20 seconds)

There is a shoulder on the R-wave of the QRS complex. This shouldered QRS complex is called a Delta-wave and is the result of a fused ( fusion) beat from the normal and aberant pathway.

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Wolf Parkinson White

04/19/23 21Delta Wave

Lown-Ganong-LevineThe aberant pathway is the bundle of James

which joins the normal pathway above the AV node.

Since the abnormal pathway joins the normal pathway Above the AV node rather than within it there is no delta wave but just a shortened PR interval (i.e. less than 0.20seconds)

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Importance of Pre-Excitation Syndromes

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These can lead to severe tachycardia's.

Bundle Branch Blocks

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In bundle branch blocks either the right or the left branch is partially ( hemiblocks) or totally blocked. If both right and left bundle branches are blocked, this is termed complete or third degree heart block.

Normally both bundle branches depolarize simultaneously but with bundle branch blocks the unblocked side depolarizes first and its impulse then spread to the blocked ventricle . So depolarization of the ventricles is sequential.

The major significance of a new BBB is that it may indicate the presence of a previously unknown underlying cardiovascular disease.

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Conditions that cause this are :-pulmonary embolism-chronic lung disease-cardiomyopathy-atrial and ventricular septal defects

-However in some individuals RBBB is seen in perfectly healthy individuals and is a variant of normal.

Right Bundle Branch BlockRight Bundle branch block is seen as 2 R-

waves R and R prime with an intervening S-wave in leads V1,V6 and lead 1. The s wave is deep in lead 1 and V1 . This is called R, S, R-prime.

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In Contrast to RBBB LBBB is almost always indicative of underlying cardiac pathology. There is no normal variant.

Conditions that cause this are :-dilated cardiomyopathy-hypertrophic cardiomyopathy-hypertension-aortic valve disease-coronary artery disease

Left Bundle Branch BlockLeft Bundle branch blocks are seen in

Leads 1 , V1 and V6 as 2 R-waves. R and R prime without an intervening S-wave. The wave between the R-waves is scooped. R-Rprime

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Clinical Considerations for Bundle Branch blocks

All patients with a bundle branch block should be cleared by their physician prior to any in office anesthesia.

Fortunately it is uncommon for a stable right or left bundle branch block to develop into complete heart block. Therefore with physician approval in office anesthesia can be safely performed

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Q-T Segment AbnormalitiesQ-T segment analysis is very complicated and

complete dissertation is out of the scope of this presentation.

That being said the Q-T interval is based on or corrected for the heart rate. The equation is:QT corrected=QT/the square root of the R-R

interval in seconds.

Normal Q-T The normal Q-T corrected interval is different

in males and females.< 0.430 seconds in males and < 0.450

seconds in females.

Shortened Q-T Segment Abnormalities

QT Interval AbnormalitiesDigoxin toxicity causes a shortened QT interval

with a scooping of the ST segment.

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Note the Q-T segment is only 8 small boxes wide 0.32 seconds divided by the square root of 1.16 ( 29 X 0.04)( the number of small boxes X 0.04 seconds)= 1.07. So Q-T corrected is 0.32/1.07=0.30 seconds

Although digoxin treatment toxicity is outside the scope of this lecture suffice it to say that under and over digitalization can lead to severe arrhythmia's and cardiac depression.

So if your patient is on digoxin you have to know their digoxin levels and get physician approval to proceed with in office ansthesia.As such these patients may be inappropriate for in office anesthesia.

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Other Causes of Short Q-T Intervals1) familial/genetic short Q-T syndrome2) Hypercalcemia3) Hyperthermia

Prolonged Q-T Segment Abnormalities

Causes of Prolonged Q-T Intervals1) familial/genetic prolonged Q-T syndrome2) Hypocalcemia

Although calcium abnormalities are also outside the scope of this lecture suffice it to say that they can lead to severe arythmia’s and cardiac depression.

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Potassium Induced ECG Changes Including T-Wave AbnormalitiesHyperkalemia as it evolves leads to tall

peaked T-waves, prolonged P-R interval and a widened QRS complex. Eventually the P-wave is lost and the QRS becomes biphasic

Hypokalemia leads to small biphasic flattened T-waves, S-T depression and a prominent U-waves .The U-wave is an extra wave after the T-wave.

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Potassium abnormalities are worth dwelling on for a moment as we are likely to encounter them in our day to day practice.

Either hypo or hyperkalaemia can lead to severe cardiac events and both need urgent treatment.

Normal serum potassium is 3.0-5.0 milliequivalents per litre of blood.

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Abnormalities in Serum Potassium

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The red flags to look for are:

patients on diueretics who loose potassium.Patients on exogenous potassium tablets

who can get hyperkalemia.In either of these groups it is worth getting

preop electrolytes done.

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ECG diagnosis does not have to be difficult as long as you take an orderly and well thought out approach.

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PUTTING IT ALL TOGETHER1) Look at the rhythm. Is it regular or irregular.

2) Determine the rate. Is it normal, fast or slow.

3) Determine the relationship ( if any) between the P-wave and the QRS complexes.

4) Look at the intervals , PR, QRS, QT.

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PUTTING IT ALL TOGETHER5) Finally look at recognizable patterns to

sort out a difficult diagnosis: sawtooth P-waves in atrial flutter. a missing QRS complexes in PAC’s.irregularly irregular rhythm for atrial

fibrillation. delta waves for WPW. RSR prime for RBBB. RR prime with loss of S in LBBB. Deep Q waves in MI , large T waves in

hyperkalemia etc. Missing P-waves in junctional rhythms.

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Categorizing Rhythms With Respect To An Interventional Hierarchy . Know when to worry!

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Immediate Action NeededAsystoleVentricular FibrillationPulseless Ventricular Tachycardia

Third degree heart blockTachyarythmias in which perfussion is compromised

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Action Required Within Minutes( This group is also known as pre-arrest syndromes )

Significant Bradycardia. Runs of unifocal PVC’s ( i.e. triplets ,

couplets etc.) Multifocal PVC’s.Second degree type 2 heart block

( because it often is the precursor for third degree heart block.

Tachyarrhythmia's in which perfusion is not yet compromised.

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Referral Required ( prior to dental treatment) With No immediate Action Needed

Any other abnormality noted which the patient was unaware of in their medical history.04/19/23 53

SummaryYou hopefully now understand ECG

interpretation.This can be applied in preoperative

intraoperative and post operative patient assessment.

You can utilize this knowledge in courses designed to teach arrhythmia treatment such as ILS and ALS and in your emergency simulation courses.

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END OF PRESENTATIONThank you for your commitment to

continuing education