Describe the different parts of the PQRS
P = the contraction of the atria QRS = the contraction of the ventricles Q if the first deflection is downwards R a deflection is upwards (whether its preceded by a Q or not) S any deflection below base line following an R T = the repolarisation of the ventricles (returning to resting electrical state)
Describe the journey the electrical activity takes in the heart
The electrical impulse starts at the SINOATRIAL NODE in the right atrium. It then spreads through the atrium. The impulse is delayed whilst it spreads through the ATRIOVENTRICULAR NODE. From the AV node it is conducted very quickly down the BUNDLE OF HIS, specialised conduction tissue found in the septum. The bundle of his divides into the right and left bundle branches (divides into two). In the muscle the impulse is conducted by PURKINJE FIBRES
Discuss the timings of the ECG
Small square = 0.04 Large square = 0.2 seconds therefore 5 large squares = 1sec R-R interval: 1 large square = 300/min 2 large squares = 150/min 3 large squares = 100/min 4 large squares = 75/min 5 large squares = 60/min 6 large squares = 50/min
PR interval is the time for the impulse to go from the SA node, through the atria and the AV node, down the BUNDLE OF HIS and into the ventricles. 0.12 - 0.2 s (3-5 small squares)
QRS is the time it takes for the impulse to spread through the ventricles. <0.12 s (3 small squares) Any abnormal conduction takes longer so the QRS will be widened. If there are normal p waves and PR value but wide QRS there is a bundle branch block.
Q waves are normal in V5, V6, AVL and I, other places may be pathological due to MI QT - beginning of QRS to end of T wave
* advanced* varies with rate, to calculate corrected QT = QT/ (square root of RR)
Which leads look at which part of the heart?
I, VL = left lateral surface II III, aVF = inferior aVR = right atrium V1, V2 = right ventricle V3, V4 = septum between ventricles and ant of left ventricle V5, V6 = ant and lateral walls of left ventricle
* upwards waves mean the depolarisation is travelling toward the lead, and downwards away from the lead*
Axis: What is normal?
The axis is the average spread of depolarisation through the ventricles as seen from the front.
Normall axis is 11 o'clock - 5 o'clock - the depolarising lead is spreading towards I, II, and III so they will all be +ve.
Axis: what causes right axis deviation and how does it look on an ECG?
Right ventricle hypertrophy - pulmonary conditions and congenital heart conditions
Axis moves from 11 o'clock - 5 o'clock to 1 o'clock - 7 o'clock.
I becomes -ve. II remains +ve, III is even more +ve.
Axis: what causes left axis deviation and what does it look like on the ECG?
Usually due to conduction defects rather than increased left ventricular bulk.
The left bundle branch has two fasciles, the anterior and posterior. If the anterior is selectively blocked then left ventricle has to be depolarised purely through the posterior fascicle, moving the axis.
Bifascicular block also gives left axis deviation with a RBB pattern.
What is first degree heart block?
What causes it?
PR interval >0.2 secs (small squares)
Normal varient, athletes, sick sinus syndrome, IHD, acute carditis, drugs (B blockers, digoxin), electrolyte imbalance
Describe the different types of second degree heart block.
Mobitz type 2: most beats conducted with a constant PR interval but occassionally there is a P with no QRS, an atrial contraction not conducted through to the ventricles.
Wenkebach: progressive lengthening of the PR interval and then the failure of the conduction of an atrial beat. A conduction beat follows and the patteren repeats.
2:1 block: alternate conducted and non conducted atrial beats. There are twice as many p waves as QRS.
*look out for p waves hiding in t waves!*
What is third degress heart block?
There is normal atrial contraction but none of the beats are conducted into the ventricle. The ventricles contract independtly at a much slower rate.
Can be due to an acute MI, fibrosis of the AV node and bundle of his, aortic valve calcification, trauma, digoxin toxicity.
NEEDS to be paced.
Describe right bundle branch block
Due to no conduction down the RBB. Means there is only conduction of the LBB, so the excitation depolaises the right side of the heart more slowly.
A second R (+ve) wave is seen the V1 due to the delayed depolarising of the right ventricle. The wave slowly comes towards the right ventricle, so towards V1. Looks like a M.
A deep S (-ve) wave is seen in V6 due to the wave slowly moving from the left ventricle to the right ventricle, away from V6. Looks like a W.
If there is RBB + LAD = bifasciular block
Describe left bundle branch block
Septum has to depolarise from right to left (abnormal).
As the excitation is moving towards the left side the there is a large R (+ve), a small S (-ve) as the right side if depolarised first, and then a large R again when the ventricle is depolaring in V6. Looks like a M.
Escape rhythms: what is the natural frequency of the different areas of the heart (when they provide the escape rhythms)
SA node: approx 70 bpm
Region around AV node (junctional) or atrial muscle: 50 bpm
Ventricle: 30 bpm
There can be a rhythm from the ventricles which is higher than the 30bpm, called accelerated idioventricular rhythm. Looks like VT but is slower, is NOT VT until >120
What is the difference between an extrasystole and an escape beat?
extrasystole = early
escape beat = late
How are extrasystoles different when they originate from different parts of the heart?
Atrial = abnormal p waves (QRS is normal)
Junctional = no p wave or it appears directly before or after the QRS (QRS is normal)
Ventricular = abnormal QRS. usually not important unless occur early in T wave when can induce VF
What is the characteristic ECG pattern for Atrial Flutter?
Saw tooth base line. Atrium contracting at 300 beats per minute, usually with a 2:1 block so the HR is 150.
Using carotid massage slows down the ventricular response so the saw tooth pattern is easier to see.
Describe the ECG pattern for SVT
Narrow complex tachycardia > 100bpm QRS <120ms
no P waves or inverted after QRS
Describe the ECG changes in Junctional Tachycardia
Narrow complex tachycardia >100bpm <120ms QRS
p wave either buried in the QRS or occuring just afterwards
Describe the ECG changes occuring with ventricular tachycardia
Broad complex tachycardia
>100 bpm, QRS >120 ms
no pwaves, QRS slightly irreguar and vary in shape
Describe the ECG changes seen in atrial fibrillation
normal shaped QRS
Describe the ECG changes seen in ventricular fibrillation
no QRS complexes can be seen and the ECG is completely disorganised
Wolff- Parkinson- White:
explain mechanism and ECG changes
There is an accessory bundle connecting the atrium to the ventricle, that bypasses the AV node so there is no delay.
This means 'pre-excitation' occurs = short PR interval
ECG shows slurred upstroke called the delta wave, normal QRS.
Can lead to a re-entry tachycardia - no pwaves seen, narrow complex QRS (cannot distinguish cause until sinus rhthym is restored.
Describe the p wave changes seen in right atrial hypertrophy
peaked p waves
tricuspid valve stenosis and pulmonary hypertension
Describe the p wave seen in left atrial hypertrophy
Describe a normal QRS complex
duration <120 ms
V1: S wave is greater than R wave
V5/ V6: height of R wave <25mm
Q waves in left ventricular leads <1mm across and 2mm deep
Describe the ECG changes seen in right ventricular hypertrophy
Best seen in V1 - QRS becomes more upright (height of R wave exceeds S wave)
Deep S in V6
Describe ECG changes in left ventricular hypertrophy
V5/6- tall R wave (>25mm), inverted t waves
V1//2 - deep S
left axis deviation
Which leads are Q waves normal in?
aVF and V6
What do Q waves signify
If they are >0.04s (One small square) in width and >2mm in depth they are significants
Indicate MI, and where they are suggets the place its been damaged.
Describe the changes seen in a MI
T wave inversion
ST elevation returns to baseline 24-48 hrs later