Rhythm Interpretation

Conduction System of the Heart

Heart is a pump mechanism

Pump action is brought about by electrical changes in the heart

Depolarisation and repolarisation

When cardiac cells are at rest their surface is more positively charged than the interior of the cells.

Depolorisation occurs forcing NA+ into cells and K+ out. Once neutrality is reached Ca++ can enter cell which stimulates cells contraction.

Repolarisation occurs when Na+ returns to outside cell and K+ returns inside cell. Cells relax.

Depolarisation

Na+ Ca++Na+Na+

K+ Ca+K+ Na+

K+K+K+

Cell Contractionstimulated by Ca++ = Depolarisation

K+K+K+

Na+Na+K+

Ca++Na+Na+Na+

Normal Conduction Pathway

NormalSignal moves rapidly through the ventricles

Path of Conduction

There is an “electrical gate” at the junction of the atria and ventricles.

Conduction is funnelled along the intraventricular septum

AV node slows conduction in order for atria to finish contraction prior to ventricles contracting.

This mechanism allows complete emptying of the atria into the ventricles

Rhythm interpretation

The ECG Paper

HorizontallyOne small box - 0.04 sOne large box - 0.20 s

P Wave & PR Interval P Wave= Atrial Depolarisation

PR Interval =Impulse from sino atrial node to venticular myocardium

Normal = 3-5 small boxes (0.12-0.2secs)

QRS

Depolarisation of both ventricles Normal = < 3 small boxes (0.12sec)

T Wave

Ventricular repolarisation

HR Calculation

Count the number of large squares between two R waves ÷ 300 by that number

Count number of small squares between two R waves ÷ 1500 by that number

Count number of R waves in a 6 second strip X 10 = 60 seconds

Rhythm analysis Rate- normal, fast or slow Rhythm – Regular/irregular/pattern P waves – normal/ constant/ before QRS/

absent PR interval – 0.12 – 0.20 secs or 3-5 small

boxes, constant QRS – normal/narrow/wide Ratio – P:QRS =1:1???

Quiz

What is depolarisation? What is repolarisation? How do you calculate Rate? What is normal PR interval? What is normal QRS interval?

12 Lead ECG Bipolar leads: The leads are termed I, II,

and III. Unipolar leads: AVR, AVL, AVF

The V leads, which extend across the precordium, V1 in the fourth right interspace, V2 4th left, V4 at the apex (5th interspace, midclavicular line), V3 halfway in between V2 and V4, and V5 & V6 in the 5th interspace at the anterior and mid axillary lines respectively.

It's usual to group the leads according to which part of the left ventricle (LV) they look at.

AVL and I, as well as V5 and V6 are lateral, while II, III and AVF are inferior.

V1 through V4 tend to look at the anterior aspect of the LV (some refer to V1 and V2 `septal’).

Changes in depolarisation in the posterior aspect of the heart are not directly seen in any of the conventional leads, although "mirror image" changes will tend to be picked up in V1 and V2.

ST Segment Analysis

Indicates myocardial tissue ischaemia, injury or infarction.

Myocardial Injury. ST elevation on ECG over area of tissue damaged in early stages.

Injured myocardial tissue can be saved if blood flow is restored promptly – Thrombolysis or PTCA necessary

ST Elevation

Occurs in the early stages

Occurs in the leads facing the infarction

Slight ST elevation may be normal in V1 or V2

Deep Q Wave

Only diagnostic change of myocardial infarction

At least 0.04 seconds in duration

Depth of more than 25% of ensuing R wave

T Wave Changes

Late change Occurs as ST

elevation is returning to normal

Apparent in many leads

Sequence of changes in evolving AMI

1 minute after onset 1 hour or so after onset A few hours after onset

A day or so after onset Later changes A few months after AMI

Q

R

P

QT

STR

P

Q

ST

P

QT

ST

R

P

S

T

P

QT

ST

R

P

Q

T

References:

Hampton, J.R. (2003) the ECG Made easy. 6th edition. Edinburgh: Churchill Livingstone.

Thaler, M.S. (1999) The only EKG Book You’ll Ever need. 3rd edition. Lippincott, Williams and Wilkins, London.

ACLS Provider Manual. American Heart Association

1. Is there any electrical activity? 2. Rate 3. Is the Rhythm regular/irregular/pattern? 4. P waves – Normal/ Constant/Before QRS/Absent? 5. PR interval. Is it normal/prolonged/constant? 6. Is the QRS normal/narrow/prolonged? 7. Ratio- 1:1?

1. Is there any electrical activity?2. Rate3. Is the Rhythm regular/irregular/pattern?4. P waves – Normal/ Constant/Before QRS/Absent?5. PR interval. Is it normal/prolonged/constant?6. Is the QRS normal/narrow/prolonged?7. Ratio- 1:1?

1. Is there any electrical activity? 2. Rate 3. Is the Rhythm regular/irregular/pattern? 4. P waves – Normal/ Constant/Before QRS/Absent? 5. PR interval. Is it normal/prolonged/constant? 6. Is the QRS normal/narrow/prolonged? 7. Ratio- 1:1?

1. Is there any electrical activity? 2. Rate? 3. Is the Rhythm regular/irregular/pattern? 4. P waves – Normal/ Constant/Before QRS/Absent? 5. PR interval. Is it normal/prolonged/constant? 6. Is the QRS normal/narrow/prolonged? 7. Ratio- 1:1?

1. Is there any electrical activity?2. Rate3. Is the Rhythm regular/irregular/pattern?4. P waves – Normal/ Constant/Before QRS/Absent?5. PR interval. Is it normal/prolonged/constant?6. Is the QRS normal/narrow/prolonged?7. Ratio- 1:1?

1. Is there any electrical activity? 2. Rate 3. Is the Rhythm regular/irregular/pattern? 4. P waves – Normal/ Constant/Before

QRS/Absent? 5. PR interval. Is it normal/prolonged/constant? 6. Is the QRS normal/narrow/prolonged? 7. Ratio- 1:1?

1. Is there any electrical activity?2. Rate3. Is the Rhythm regular/irregular/pattern?4. P waves – Normal/ Constant/Before QRS/Absent?5. PR interval. Is it normal/prolonged/constant?6. Is the QRS normal/narrow/prolonged?7. Ratio- 1:1?

1. Is there any electrical activity? 2. Rate 3. Is the Rhythm regular/irregular/pattern? 4. P waves – Normal/ Constant/Before QRS/Absent? 5. PR interval. Is it normal/prolonged/constant? 6. Is the QRS normal/narrow/prolonged? 7. Ratio- 1:1?

1. Is there any electrical activity?2. Rate3. Is the Rhythm regular/irregular/pattern?4. P waves – Normal/ Constant/Before QRS/Absent?5. PR interval. Is it normal/prolonged/constant?6. Is the QRS normal/narrow/prolonged?7. Ratio- 1:1?

1. Is there any electrical activity? 2. Rate 3. Is the Rhythm regular/irregular/pattern? 4. P waves – Normal/ Constant/Before QRS/Absent? 5. PR interval. Is it normal/prolonged/constant? 6. Is the QRS normal/narrow/prolonged? 7. Ratio- 1:1?