بسم الله الرحمن الرحيمElectrocardiography (ECG)

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QUICK REVIEW

It is the process of recording the electrical activity of the heart. We are concerned with learning the normal ECG, and how to use it to know the basic abnormalities that can be detected on it.

First, we have to understand one important thing. How the impulse travels through the heart? See this video.

https://www.youtube.com/watch?v=te_SY3MeWys

As you see, the impulse travels from the atria to the ventricles in this order: Atrium a Delay in the AV node to allow the atria to fully contract and fill the ventricles before ventricular depolarization and contraction Ventricles.

In the ventricle there are 3 stages of depolarization and contraction: Depolarization starts at the septum, then goes down as far as the apex and then back to the base (posterior aspect of the ventricle).

So, the three stages of ventricular depolarization are: 1- Septal Depolarization (Q wave) 2- Major Ventricular Depolarization (R wave) 3- Basal Depolarization (S wave)

There are 4 main electrical events that occur in the heart: 1- Atrial depolarization 2- Atrial repolarization 3- Ventricular depolarization 4- Ventricular repolarization

Atrial repolarization and ventricular depolarization occur at the same time, so atrial repolarization doesn't show up on ECG, as it's masked by the stronger ventricular depolarization (QRS comp.).

Standardized ECG:

- X-axis: Time - Y-axis: Voltage - The speed of the machine is 25 mm/sec. - Each square is 1 mm. 1 second is represented by 25 square on ECG paper. Each square represents 0.04 sec. - Last lecture, we talked about calculation of the heart rate from ECG paper. Typically, we look at two successive R waves (R-R interval). R-R intervals are the duration of the cardiac cycle, and the number of cardiac cycles per minute equals the heart rate. ex: Cardiac cycle = 0.8 seconds à Heart Rate = 75 Beats/min - When the duration of the cardiac cycle decreases, heart rate increases and vice versa. - Normal range of heart rate (60-100) beats/min. Below this is bradycardia and above it is tachycardia. Intervals: 1- P-R interval: represents the duration of atrial depolarization and repolarization. - Normal duration: 0.16 sec - P-R interval shouldn't exceed 0.20 sec. If it exceeds this range, this indicates that there's a delay in AV conduction. - Also known as P-Q interval, but because Q may be absent on some ECGs, we prefer to use P-R to standardize the terminology. 2- Q-T interval: represents the duration of ventricular depolarization and repolarization.

- Normal duration: 0.35 sec - Can be lengthened by electrolyte disturbances, conduction problems; coronary ischemia, or myocardial damage. 3- R-R interval: represent the time between two successive ventricular depolarizations (i.e. the duration of the cardiac cycle). - Intervals always include waves.

Segments

Segments on ECG represent isoelectric lines (no waves). The electrical potential is zero at times of complete depolarization or complete repolarization (because at these times, all cells have the same charge, and there would be no current moving to be recorded by the Galvanometer). 1- P-R segment: this is very small. 2- S-T segment: This represents the time between the end of depolarization and the start of repolarization in the ventricles (i.e. the plateau of phase 2 of ventricular action potential). Here, we don't care about the duration of the segment but rather we look at It if it's elevated or depressed from the isoelectric line. Both elevation and depression of the S-T segment indicate ischemia. - Ischemia may develop into infarction. So, a patient with S-T elevation or depression on ECG should be kept under control to avoid the development of MI

Times:

- P wave: 0.12 sec = 3 small squares on ECG trace

- QRS complex = 0.12sec = 3 small squares on ECG trace

- PR interval = between 0.12-0.21 sec (equivalent to 3-5 small squares)

Heart Rate Calculation The heart rate can be calculated from the EKG using the R-R interval.

The R-R interval represents one complete cardiac cycle.

The R-R interval is calculated through multiplying the number of small squares on the grid of the EKG between 2 consecutive R waves by 0.04 seconds (which is the value of each small square).

Suppose the R-R interval was 0.83 seconds, the heart rate would be:

Heart Rate = 60 seconds / (time b/w R-R interval)

= 60/ 0.83 = 72 beat per minute

OR

Suppose R – R wave is 8.5 big boxes

Rate = 300 / number of big boxes

Rate = 300 / 8.5 = 35 beats per minute

The rhythm of the heart and its abnormalities:

- The keys to rhythm abnormalities are:

The P waves – can you find them? Look for the lead in which they are most obvious.

The relationship between the P waves and the QRS complexes – there should be one P wave per

QRS complex.

The width of the QRS complexes (should be 120 ms or less).

supraventricular ventricular rhythms

Site - Sinus rhythm, atrial rhythm and junctional rhythm together constitute the ‘supraventricular’ rhythms.

- Ventricles

Description -In the supraventricular rhythms, the depolarization wave spreads to the ventricles in the normal way via the His bundle and its branches. The QRS complex is therefore normal, and is the same whether depolarization was initiated by the SA node, the atrial muscle, or the junctional region.

-In ventricular rhythms, on the other hand, the depolarization wave spreads through the ventricles by an abnormal and slower pathway, via the Purkinje fibers. The QRS complex is therefore wide and is abnormally shaped. Repolarization is also abnormal, so the T wave is also of abnormal shape.

Remember: - Supraventricular rhythms have narrow QRS complexes.

- Ventricular rhythms have wide QRS complexes

The only exception to this rule occurs when there is a supraventricular rhythm with right or

left bundle branch block, or the Wolff–Parkinson–White (WPW) syndrome, when the QRS

complex will be wide.

Sinus arrhythmias:

- Means that the electrical impulses are coming from the SA node. If there is no SA node, no impulses will be transmitted to the atria and consequently there will be no P wave

Sinus tachycardia:

- SA node is depolarizing faster than normal, impulse is conducted normally

Sinus Bradycardia: - SA node is depolarizing slower than normal, impulse is conducted

normally - When the R-R interval is more than 1 second, then there is Bradycardia.

Atrial tachyarrhythmias:

1. Atrial tachycardia (100 – 250 beat/min)

Atrial rate > 100 bpm. P wave morphology is abnormal when compared with sinus P wave due to

ectopic origin. There is usually an abnormal P-wave axis (e.g. inverted in the inferior leads

II, III and aVF) At least three consecutive identical ectopic p waves. QRS complexes usually normal morphology.

2. Atrial flutter (250 – 350 beat/min)

Narrow complex tachycardia Regular atrial activity at ~300 bpm Flutter waves (“saw-tooth” pattern) best seen in leads II, III, aVF. Ventricular rate is determined by the AV conduction ratio (“degree of AV

block”). Loss of the isoelectric baseline

Ectopic atrial tachycardia:

There is a narrow complex tachycardia at 120 bpm. Each QRS complex is preceded by an abnormal P wave —inverted in the inferior

leads II, III and aVF.

3. Atrial fibrillation ( >350 beat/min)

Irregularly irregular rhythm. No P waves. Absence of an isoelectric baseline. Variable ventricular rate. QRS complexes usually < 120 ms unless pre-existing bundle branch block,

accessory pathway, or rate related aberrant conduction. Fibrillatory waves may be present and can be either fine (amplitude <

0.5mm) or coarse (amplitude >0.5mm). Atrial fibrillation can occur in up to 20% of patients with WPW watch this

nice video https://youtu.be/qrhWH2_KKOY

The following picture may be helpful.

Atrial Flutter with 2:1 Block

There are inverted flutter waves in II, III + aVF at a rate of 300 bpm (one per big square)

There are upright flutter waves in V1 simulating P waves There is a 2:1 AV block resulting in a ventricular rate of 150 bpm

Ventricular tachyarrhythmia:

1. Ventricular tachycardia:

Rapid heart rate (> 100 bpm).

Broad QRS complexes (> 120 ms)

Extreme axis deviation (“northwest axis”) — QRS is positive in aVR and negative in lead I + aVF.

AV dissociation (P and QRS complexes at different rates). Positive or negative concordance throughout the chest leads, i.e. leads V1-6 show

entirely positive (R) or entirely negative (QS) complexes, with no RS complexes seen.

The distance from the onset of the QRS complex to the nadir of the S-wave is > 100ms.

Notching near the nadir of the S-wave. RSR’ complexes with a taller “left rabbit ear”. This is the most specific finding in

favour of VT. The cardiac output is often strongly reduced during VT resulting in hypotension and loss of

consciousness. VT is a medical emergency as it can deteriorate into Ventricular fibrillation and thus mechanical

cardiac arrest.

Broad complexes (~ 200 ms in V5-6). The distance from the onset of the QRS complex to the nadir of the S-wave is >

100ms. Notching near the nadir of the S wave is seen in leads II, III, aVF.

2. Ventricular Flutter:

Extreme form of VT with loss of organized electrical activity

Associated with rapid and profound hemodynamic compromise

Usually short lived due to progression to ventricular fibrillation

Continuous Sine Wave

No identifiable P waves, QRS complexes, or T waves

Rate usually > 200 beats / min

The ECG looks identical when viewed upside down!

3. Ventricular Fibrillation:

Ventricular fibrillation (VF) is the most important shockable cardiac arrest rhythm.

The ventricles suddenly attempt to contract at rates of up to 500 bpm.

This rapid and irregular electrical activity renders the ventricles unable to contract in a synchronized manner, resulting in immediate loss of cardiac output.

Chaotic irregular deflections of varying amplitude

No identifiable P waves, QRS complexes, or T waves

Rate 150 to 500 per minute

Amplitude decreases with duration (coarse --> fine)

Classic ECG patterns in Acute MI: a. LAD coronary artery anterior wall infarction • Q waves in leads V1–V2 b. Anteroseptal infarction due to proximal LAD coronary artery occlusion • Q waves in leads V1–V2 c. Anterolateral infarction due to mid-LAD or circumflex coronary artery occlusion • Q waves in leads V4–V6, I, aVL d. Lateral wall infarction due to left circumflex coronary artery occlusion • Q waves in leads I, aVL e. Inferior wall infarction due to RCA occlusion

• Q waves in leads II, III, aVF

Correlation of ECG changes with pathologic changes (Fig. 11-10) a. Inverted T waves • Correlates with areas of ischemia at the periphery of the infarct b. Elevated ST segments • Correlates with injured myocardial cells surrounding the area of necrosis c. New Q waves

• Correlates with the area of coagulation necrosis

AV block:

AV node block prevents the normal conduction of the AV node. This abnormality could be partial or complete. Partial means that the AV node sometimes conducts the impulse from the atria but not always.

First Degree Heart Block:

- PR interval > 200ms (five small squares)

- ‘Marked’ first degree block if PR interval > 300ms

- If every P wave is followed by QRS interval and the PR interval is longer than 0.2 seconds, and then this is a first degree AV block

AV Block 2nd degree:

In the second degree AV block, the PR interval is longer than 0.2 seconds but not all P waves are followed by QRS interval. There are 2 types of second degree AV blocks.

Mobitz I:

Progressive prolongation of the PR interval culminating in a non-conducted P wave

The PR interval is longest immediately before the dropped beat The PR interval is shortest immediately after the dropped beat The greatest increase in PR interval duration is typically between the first and

second beats of the cycle. The RR interval progressively shortens with each beat of the cycle

First degree heart block (PR interval > 200 ms)

Mobitz II:

Intermittent non-conducted P waves without progressive prolongation of the PR interval (compare this to Mobitz I).

The PR interval in the conducted beats remains constant. The P waves ‘march through’ at a constant rate. The RR interval surrounding the dropped beat(s) is an exact multiple of the

preceding RR interval (e.g. double the preceding RR interval for a single dropped beat, treble for two dropped beats, etc).

In around 75% of cases, the conduction block is located distal to the Bundle of His, producing broad QRS complexes

There may be no pattern to the conduction blockade, or alternatively there may be a fixed relationship between the P waves and QRS complexes, e.g. 2:1 block, 3:1 block.

QRS complexes cluster in groups, separated by non-conducted P waves. The P:QRS conduction ratio varies from 5:4 to 6:5. Note the difference in PR interval between the first and last QRS complex of

each group

Left Bundle Branch Block

Normally the septum is activated from left to right, producing small Q waves in the lateral leads.

In LBBB, the normal direction of septal depolarization is reversed (becomes right to left), as the impulse spreads first to the RV via the right bundle branch and then to the LV via the septum.

This sequence of activation extends the QRS duration to > 120 ms and eliminates the normal septal Q waves in the lateral leads.

The overall direction of depolarization (from right to left) produces tall R waves in the lateral leads (I, V5-6) and deep S waves in the right precordial leads (V1-3), and usually leads to left axis deviation.

As the ventricles are activated sequentially (right, then left) rather than simultaneously, this produces a broad or notched (‘M’-shaped) R wave in the lateral leads.

Diagnostic Criteria

QRS duration of > 120 ms Dominant S wave in V1 Broad monophasic R wave in lateral leads (I, aVL, V5-V6) Absence of Q waves in lateral leads (I, V5-V6; small Q waves are still allowed in

aVL) Prolonged R wave peak time > 60ms in left precordial leads (V5-6)

The R wave in the lateral leads may be either:

‘M’-shaped Notched Monophasic RS complex

‘M’-shaped R wave

The QRS complex in V1 may be either:

rS complex (small R wave, deep S wave) QS complex (deep Q/S wave with no preceding R wave)

rS complex

Right Bundle Branch Block:

In RBBB, activation of the right ventricle is delayed as depolarisation has to spread across the septum from the left ventricle.

The left ventricle is activated normally, meaning that the early part of the QRS complex is unchanged.

The delayed right ventricular activation produces a secondary R wave (R’) in the right precordial leads (V1-3) and a wide, slurred S wave in the lateral leads.

Delayed activation of the right ventricle also gives rise to secondary repolarization abnormalities, with ST depression and T wave inversion in the right precordial leads.

In isolated RBBB the cardiac axis is unchanged, as left ventricular activation proceeds normally via the left bundle branch.

Diagnostic Criteria

Broad QRS > 120 ms RSR’ pattern in V1-3 (‘M-shaped’ QRS complex) Wide, slurred S wave in the lateral leads (I, aVL, V5-6) ST depression and T wave inversion in the right precordial leads (V1-3)

Tall R’ wave in V1 (“M” pattern) with wide, slurred S wave in V6 (“W” pattern)

Typical RSR’ pattern (‘M’-shaped QRS) in V1

Typical pattern of T-wave inversion in V1-3 with RBBB

Draw the hexagonal axis of QRS.

In this sheet we will discuss how to draw the Hexagonal axis of the ECG then try to find the mean electrical axis of QRS (Ventricular depolarization).

We knew that the current of the heart will move from the depolarized area to the still-polarized area, the vector has direction “the angle“ and value "the length of the vector“.

The normal mean QRS vector is 60o ,,, the normal range is (-30◦ - 110◦). how to draw the hexagonal axis:

- We draw the trigonal axis then add the Augmented unipolar leads

aVL: from the heart to the left arm, intersects (bisects) the angle between the

lead III and lead I the resultant angle would be -30

aVR: from the heart to the right arm, intersects (bisects) the angle between

the lead II and lead I the resultant angle would be 210 ( 180 +30 )

aVF: from the heart to the left foot, intersects (bisects) the angle between the

lead II and lead III , the resultant angle would be 90

There are some things should be taken inconsideration:

I. Lead I and aVF should be perpendicular to each other. II. Lead II and aVL should be perpendicular to each other. III. Lead III and aVR should be perpendicular to each other.

We can choose any two leads and draw the mean electrical axis. The doctor prefers Lead I and aVF (because they are perpendicular to each other).

Now if we have the ECG and want to draw the mean electrical axis: 1) Take any two leads you want, let’s take Lead I and aVF , then measure the length of the R , Q and S waves by counting the number of small squares

and do algebraic summation { counting the number of small squares of R , S and Q foreach QRS complex in each lead and taking the direction in consideration if it’s down deflection then negative and if it’s positive deflection then it’s positive and sum them together. Let’s say that the R is 14 ss positive deflection and the S is 4 ss down deflection the Q is 3 ss down deflection then the totla is 7 ss positive }. Let’s say that the mean of QRS in lead I is 7 small square and the mean of QRS in the aVF is also 7 small squares. 2) Count 7 small squares from the center and Draw a line perpendicular

to the lead I axis. 3) Count 7 small squares from the center and Draw another line

perpendicular to the aVF axis “perpendicular to the first line” . 4) At the site of intersection, draw a line extending to the center. 5) Measure the angle by using Tan {opposite / adjacent }, by using the calculator find the angle !in this case it’s 45 . 6) the mean electrical vector is 45 then it’s Normal . watch this video https://www.youtube.com/watch?v=jg5X3V5IPS4

If the algebraic summation of lead I is positive and the algebraic summation of aVF is also positive, then the mean electrical axis is between 0 and 90, which is the normal range!

If the algebraic summation of lead I is positive and the algebraic summation of aVF is negative, then the mean electrical axis between 0 and -90, then it’s left axis deviation!

If the algebraic summation of lead I is negative and the algebraic summation of aVF is positive then the mean electrical axis between 90 to 180 then Right axis Deviation!.

If the algebraic summation for both is negative then between the -90 and -

180 then extreme axis deviation, don’t know is it right or left?! so we have to look to the patient ; if he has right heart disease then it’s Extreme

RIGHT Axis Deviation but ; if he has left heart disease then it’s Extreme

LEFT Axis Deviation . https://www.youtube.com/watch?v=_CCUWdAaQoA, watch it to make surethat you understand the idea .

Important note: the normal range is (-30◦ - 110◦), but clinically normal range is ( 0-90).