Cardiac Cycle/ ECG’s

Session Aims

•Cardiac Monitoring

•Basic Physiology of Heart

•How to read an ECG

•Look at Rhythm Strips

Reminder from Day One

• 3 Lead ECG placement

• 5 lead placement

12 lead ECG

• Ten leads- four limb leads that can be placed on limbs or bony points of pelvis and shoulders

• Allow for better view of the heart • Rhythm strip at base is set at lead 2 • Limb leads 1-3 look at Inferior side

of heart • Chest leads 1-3 look at anterior

aspect of heart • Leads 4-6 look at lateral aspect of

heart

What is it monitoring?

• Measure the rate and regularity of heart beats. It is an interpretation of the electrical activity of the heart over a period of time.

• It detects and amplifies the electrical charge that are caused when the heart muscle depolarises (muscle contracts) during each heartbeat

Basic Electrocardiography Rest=cardiac conducting system and

myocardium POLARISED

Negative charge exists between the inside and outside of all cardiac cells

Shift in calcium and /or sodium

across cell membranes-

DEPOLARISATION

Generates electrical signal

that travels through

conduction system

Contraction of myocardial cells

followed by repolarisation

Normal Conduction through the heart- atrial activity

• Depolarisation begins in Sino atrial (SA node)

• This is followed by wave of depolarisation through the atrium and equates to the P wave on the ECG

• Atrial contraction is the mechanical response to this electrical activity

Venticular activity • This electrical impulse then spreads through to the ventricles

• AV Node (slow conduction)

• Purkinje fibres (rapid conduction)

• Bundles of His through to right and left bundle branch

• Depolarisation of the Bundles of His form the QRS complex on the ECG

• Ventricular contraction is the mechanical response to the electrical activity

• T wave formed by repolarisation of the ventricles

PQRST complex • P wave- depolarisation from SA node through atria

• QRS wave- depolarisation of the ventricles- usually 0.12 seconds or less secondary to rapid conduction

• T wave indicates recovery- ventricular repolarisation

To recap

How to read an rhythm strip (ALS)

• Six stage system:

• Is there any electrical activity?

• What is the QRS rate?

• Is the QRS rhythm regular or irregular?

• Is the QRS complex width normal or prolonged?

• Is atrial activity present?

• Is atrial activity related to ventricular activity and if so, how?

Is there any electrical activity? • No- check patient, leads- no pulse asystole, straight line lead is

disconnected

• If P waves (atrial activity) still present but no QRS complex this is referred to as ventricular asystole

• If electrical activity but still no pulse determine whether you can recognise QRS complex- if bizarre broad complexes- VF

• If recognisable rhythm but still no pulse then PEA and start CPR while investigating cause (4 H’s, 4 T’s)

What is the ventricular (QRS) rate? • Normal rate 60- 100 bpm

• ECG paper set at 25mm sec, 1 second= 5 large squares (25 small squares)

• Quick way to work out QRS rate-

• Count the number of large squares between two complexes and divide into 300

• If not constant then can divide by small squares and divide into 1500

Is the QRS rhythm regular or irregular?

• Measure from spike (r wave) to spike

• Decide is this totally irregular with no pattern to the R to R interval ? (ie atrial fibrillation)

• Is the basic rhythm regular with intermittent irregularity? (ie ectopic beats)

• Is there recurring cyclical variation in the R-R rate? (this will occur when normal pacemaker-SA node too slow or absent) Escape beats

Is the QRS complex width normal or prolonged?

• The upper limit of normal for the QRS interval is 0.12 sec (3 small squares)

• If less than this (narrow) the rhythm originates from the SA node, AV node or atria, not from the ventricular myocardium.

• If more than this (broad) then the activity originates from a supraventricular source ie bundle branch block

Is atrial activity present? • Lack of p waves- atrial fibrillation whereby waves of depolarisation

travel randomly through both atria

• AF most commonly seen arrythmia in clinical practice. Characterised by disorganised atrial activity in the atria. There is an irregular R to R rate.

• P waves seen as saw tooth appearance- atrial flutter

Is atrial activity related to ventricular activity and if so, how?

• If consistent interval between P wave and QRS complex then likely that normal electrical activity occurring.

• If no relationship at all then consider complete heart block

• Prolonged PR interval (normal is 0.12 to 0.20 seconds) = delay in conduction through AV junction= first degree block

Heart block • P wave missing in relation to some QRS complexes-

• Mobitz type 1- PR interval increases in length until P wave occurs without QRS complex

• Mobitz type 2- PR interval constant but some lack QRS complex ie 2:1 block or 3:1 block

• Third degree block- complete- no relationship between P and QRS complex- need pacemaker as at present depolarisation occurring from different pacemaker sites

Rhythm Strips- Sinus rhythm

VF (ventricular fibrillation)

• No recognisable QRS complexes

• Bizarre irregular waveform

• Random frequency and amplitude

• Uncoordinated electrical activity

• Coarse / fine

• Exclude artefact

– movement

– electrical interference

• Monomorphic VT

– broad complex rhythm

– rapid rate

– constant QRS morphology

Asystole

• Absent ventricular (QRS) activity

• Atrial activity (P waves) may persist

• Rarely a straight line trace

PEA

• Clinical features of cardiac arrest

• ECG normally associated with an output

MI

Others you may see-

• AF

• SVT

To conclude: • Physiology- Consider normal physiology passing from SA node,

through AV node and through ventricles.

• Although you cannot sign of on ECG’s consider a systematic approach to reading it and assess whether normal- who will you get to review it?

• Any questions?

In summary: