L5 Physiology: clinical introduction to ECG Flashcards
We have 12 leads but _____ is called the rhythm strip.
Lead 2
Lead 2 gives us the best look at the ______?
P wave
Holter monitor is worn for ___ hours and it can pick up any arrythmias.
24 hours
How do you calculate heart rate using leads?
Use lead 2 (rhythm strip) and look at the interval between two R waves (R-R interval).
Count big boxes between the two waves: each big box = 200 millisecond, so 5 big boxes is 1 second.
Heart rate is 300/number of big boxes.
Tachycardia
Heart rate greater than 100
Bradycardia
HR less than 60
ECG is a surface representation of?
Electrical activity of the heart
Describe the conduction of the heart
Heart has its own conduction.
- Wave of depolarization propagates from the SA node through intermodal pathways and through Bachman’s bundle.
- Pauses at the AV node
- Propagates through Bundle of His, then the bundle branches.
- Bundle branches terminate in purkinje fibres.
SA node depolarisation is controlled by?
Controlled by hormones and the SNS (vagus) -> if you are fit with high vagal tone the HR will be slow
P wave
Action potentials that result from the depolarization wave from the SA node as it spreads across the atria (atrial depolarisation).
PR segment
- It then pauses for a second
- 120-200 ms (3-5 small squares)
QRS complex
- Ventricular depolarization begins.
- The Q wave is due to septal depolarization which actually moves from left to right, and then propagates back to form the QRS complex.
- Biphasic because electricity moves in 2 directions.
- LV thicker with more conduciveness so its contribution is greater.
- <120 ms (2-3 small squares)
ST segment
Once vetricular depolarization is complete, there is a pause.
T wave
Ventricular repolarization begins at the apex -> then repolarization is complete
QT segment
- From the Q wave to the end of the T wave
- Normally <440-450 ms
- This allows for both ventricular depolarization and repolarization.
- Genetic abnormalities and drug interactions that can cause a long QT interval.
What indicates normal sinus rhythm in the heart?
P waves before every QRS complex
A positive deflection is elicited by?
A wave of depolarization travelling towards a particular electrode on the chest surface.
Negative deflection is elicited by?
If it is moving away it will be a negative deflection, but if its perpendicular then you get a biphasic response.
How many leads on chest and limbs?
6 on chest, 3 on limbs
Describe this image
V1 is the right pre-cordial lead across to V6 in the axilla.
Normal complex will look different depending on where they are placed.
Chest leads look at a ______ axis, and limb leads look in a _________.
Horizontal axis; frontal plane.
Limb leads are referred to as ________ because?
Bipolar leads; because they have an electrode attached to one limb, but the amplifier measures the difference between one limb and the other.
Unipolar derived leads
Uses limb leads, so we can imagine a central point in the torso which is called wilsons central terminal.
Wilsons central terminal
Imaginary central point in torso in limb leads context.
Einthoven’s triangle
Allows heart to be split up into segments, and see where the conduction is.
Cardiac axis
Cardiac axis is average (or net) direction of spread of depolarization estimated from the frontal (limb) leads 1, 2 and 3.
Normal axis is usually positive in leads?
Leads 1, 2 and 3.
What should occur in normal axis?
- Lead 1 and aVF (augmented vector foot) should both be positive
- Lead 2 positive because the direction of conduction is down this way
- aVR (augmented vector right) negative because it is looking up towards the right arm, therefore conduction is away from it.
What should occur in leftward axis?
- Lead 1 positive, aVF negative
- Common examples: left ventricular hypertrophy; left bundle branch or fascicular block delayed conduction
What should occur in rightward axis
- Lead 1 negative, aVF positive
- Common examples: right ventricular hypertrophy; RV “strain” e.g. pulmonary embolus
Atrial fibrillation
- Irregularly irregular rhythm
- No clear P waves
- There is chaotic electrical activity.
- No organized atrial depolarization from the SA node across the atria.
Consequence of atrial fibrillation
Because it potentially occurs 600 times a minute (very quickly), some waves progress to the ventricles.
If the cardiac cells are not ready to accept the atrial signal what happens?
Nothing will happen but at odd times, the signal does get through and we get a QRS segment.
It can be fast or slow, but atrial fibrillation is commonly?
Tachy
Heart block causes what?
Delay in conduction at the AV node
How many various types of heart block exists?
1st, 2nd and 3rd degree
Describe 1st degree heart block
- A long PR interval of more than 1 big box.
- This is benign
- Can be caused by drugs, or just by ageing, generally not dangerous
Describe 2nd degree AV block
Two types of 2nd degree AV block: Mobitz 1 and Mobitz 2
What is observed in Mobitz 1 (2nd degree AV block)?
- Long PR interval, but with each subsequent beat, the PR interval gets a bit longer, until eventually, there is no QRS and conduction has gone.
- Wait for intrinsic SA activity to kick in again before we get another QRS wave.
Wenckebach AV block is?
It is progressive PR prolongation
Observe Mobitz 2
Rather than progressive PR prolongation, person has normal PR intervals.
However, random lost conduction and random blockage occurs.
PP interval is the same, and randomly it is blocked.
P waves progress at a constant rate.
This is a marker of more serious conduction disease.
Mobitz 2 can progress into?
3rd degree AV
What happens in 3rd Degree AV block
- This is where there is a complete disconnect between the atria and the ventricles at the AV node in terms of electrical conduction.
P waves at normal rate.
Cardiomyocytes have their own intrinsic activity, so there is still a QRS complex, but they will have an escape rhythm that is usually at a slower rate.
In 3rd degree AV block, there is no?
No relation between P wave and QRS, and AV conduction is not occurring.
3rd degree AV block typically causes?
Typically causes bradycardia and needs a pacemaker.
Danger of 3rd degree AV block?
Intrinsic ventricular activity can be very slow, and these people can faint and injure themselves and not wake up.
What will left ventricular hypertrophy indicate?
Strong positive deflection compared to V1 and V2 where amplitude is moving away.
Criteria of left ventricular hypertrophy
S (negative deflection) in V1 + R (positive deflection) in V5 or V6 of greater or equal to 35 mm.
R in a VL greater or equal to 11mm.
Right bundle branch block
- Conduction in the right bundle to the right ventricle is slow.
- Common in healthy normal.
Criteria of right bundle branch block
QRS is wide i.e. greater than 120ms.
V1 where there is an RSR’ (R wave, followed by S, then second more dominant R wave) what is markedly positive.
Left bundle branch block
Conduction in the left bundle to the left ventricle is slow.
Criteria of left bundle branch block
QRS is wide, i.e. > 120ms.
There is a dominant S wave (which may be a ‘W’ shape).
Last electrical activity is from the left ventricle (away from V1), which is dominant and unopposed by the right ventricle.
In V6 there may be a broad QRS with a notched ‘M’.
