cardiac cycle/ ecg’s
TRANSCRIPT
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: