ECG Flashcards
THE CONDUCTING SYSTEM: Normal durations, how recordings are obtained, Einhovens triangle, cardiac axis, systematic approach to interpretation, common abnormalities vs normal sinus rhythm PRACTICAL CARDIOVASCULAR PHYSIOLOGY: Ability to perform an ECG and recognise normal ranges, cardiac investigations
What is the normal duration of the P wave?
80ms
What is the normal duration of the QRS complex and the QRS interval?
80-120ms
What is the normal duration of the T wave?
160ms
What is the normal duration of the PR segment?
50-120ms
What is the normal duration of the ST segment?
80-120ms
What is the normal duration of the PR interval?
120-200ms
What is the normal duration of the ST interval?
320ms
What is the normal duration of the QT interval?
350-450ms
How is an interval measured on an ECG?
From the start of a wave to the end of the last letter
e.g. QT interval from start of Q to end of T
How is a segment measured on an ECG?
From the end of the wave to the start of the next
e.g. ST segment from end of S to start of T
What does the gradient on an ECG represent?
Velocity of the action potential
What does the width of a wave on an ECG represent?
Duration of the event
What does the P wave represent?
SAN myocytes depolarise causing atrial depolarisation
Wave moves across from right to left ventricle via internodal fibres slightly towards the cathode
What does the PR segment represent?
Depolarisation of the AVN
Isoelectric to delay impulse and allow ventricular filling
What does the Q wave represent?
@Start of Q wave - bundle of His rapidly conducts wave of depolarisation down septum
rest of Q wave - septum depolarises away from cathode
What does the R wave represent?
Ventricular depolarisaton due to Purkinje fibres that carry wave UP towards the cathode
What does the S wave represent?
Purkinje fibres carry wave up myocardium for late ventricular depolarisation, moving away from the cathode
What does the ST segment represent?
Depolarised ventricles produce isoelectric ECG
What does the T wave represent?
Ventricular repolarisation moving towards the cathode
How are ECG recordings obtained?
3 bipolar leads that measure potential difference between the limbs I RA to LA - to + II RA to LL III LA to LL 3 augmented limb leads aVF aVL aVR 6 chest leads
How are augmented limb leads formed?
Comparison of a single +ve electrode with a combination of the other two
aVR RA-(LA+LF)
aVL LA-(LF+RA)
aVF LF-(RA+LA)
Where are the 6 chest leads placed?
V1 - right 4th intercostal space, parasternal
V2 - left 4th intercostal space, parasternal
V3 - Left midway between V2 and V4
V4 - Left 5th intercostal space @ midclavicular line
V5 - Same horizontal level as V4 but along anterior axillary line
V6 - same horizontal level as V4, but along mid axillary line
What is the systematic approach to reading and analysing an ECG, and the expected readings for a normal ECG?
1) Ensure it is correct recording
2) Check signal and quality of leads
3) Voltage and paper speed
4) Patient background
THEN
1) Rate and rhythm (regular R-R intervals, 60/rr interval between 60 and 100)
2)P wave and PR interval (80ms, 120-200ms)
3) QRS duration (80-100ms)
4) QRS axis (between 30° and 120°)
5) ST segment (80-120ms, not isoelectric)
6) QT interval (420ms)
7) T wave (160ms)
DOUBLE CHECK EVERYTHING
What is Einhoven’s triangle?
Imaginary triangle formed by the bipolar limb leads
Used to measure movement of electrical activity away from the heart
Augmented leads measure the potential difference from centre of triangle out to each corner of triangle (LL RA LA)
Chest leads act as cathodes
What are the lead axes angles?
Centre of Einhoven's triangle is centre ∴ Lead I 0° Lead II 60° Lead III 120° aVL -30° aVF 90° aVR -150°
How can the cardiac axis be calculated and what does it show?
The mean electrical axis of the heart during ventricular depolarisation
Helps us recognise when a patient has a deviation of the cardiac axis
1) Calculate net QRS deflection for lead I and for aVF
2)Mark these on the respective axes
3)intercept perpendicular lines to get the cardiac axis vector
4) calculate cardiac axis w tan
What is the normal direction of the cardiac axis?
between 0° and 90°
What value of cardiac axis would be classed as left axis deviation and what could this be caused by?
Axis lies between -90° and 0° Left ventricular hypertrophy Left bundle branch block Hyperkalemia Inferior myocardial infarction Conditions that elevate diaphragm e.g. COPD, pregnancy, ascites, abdominal tumours
What value of cardiac axis would be classed as right axis deviation and what could this be caused by?
Axis lies between +90° and 180°
Right bundle branch block
Right ventricular hypertrophy
Lateral wall myocardial infarction
What value of cardiac axis would be classed as the indeterminant region and what could this be caused by?
Axis lies between -90° and -180°
Very rare
most commonly caused by leads wrong way round
What is normal sinus rhythm?
each P followed by QRS wave
regular rate and normal HR
otherwise unremarkable
How can sinus bradycardia be recognised and what can it be caused by?
Each P wave followed by QRS wave
regular rate, SLOW HR
Can be healthy, caused by medication or vagal stimulation
How can sinus tachycardia be recognised and what could it be caused by?
Each P wave followed by a QRS wave
Regular rate, FAST HR
Often is a physiological response
How can sinus arrhythmia be recognised?
Each P wave followed by a QRS wave
Rate IRREGULAR, normal -ish HR
R-R interval varies with breathing cycle
How can atrial fibrillation be recognised?
Oscillating baseline - atria contracting asynchronously
Rhythm can be irregular and slow rate
How can atrial flutter be recognised?
Regular saw tooth pattern in baseline of II,III, aVF
Atrial to ventricular beats at a 2:1 ratio or higher
(Saw tooth not always visible in all leads)
How can 1st degree heart block be recognised and what could it be caused by?
Prolonged ST segment/interval caused by slower AV conduction
Regular rhythm 1:1 ratio of P waves to QRS complexes
Most benign heart block, but a progressive disease of ageing
How can 2nd degree heart block Mobitz I (Wenckebach) be recognised and what could it be caused by?
Gradual prolongation of the PR interval until beat skipped
Most P waves followed by QRS, some not
Regularly irregular
Caused by diseased AV node
How can 2nd degree heart block Mobitz II be recognised?
Regular P waves, only some followed by QRS
No PR prolongation like in Mobitz I
Regularly irregular
Can rapidly deteriorate into 3rd degree heart block
How can 3rd degree heart block be recognised?
P waves regular, QRS regular, but no relationship
P waves can be hidden by bigger vectors
Truly non-sinus rhythm
How can ventricular tachycardia be recognised?
P waves hidden - dissociated atrial rhythm
Rate regular and fast
High risk of deteriorating into fibrillation
Shockable rhythm
How can ventricular fibrillation be recognised?
Irregular HR 250bpm and above
Heart unable to generate an output
Cardiac arrest
Shockable rhythm
How can ST elevation be recognised and what can it be caused by?
P waves visible and always followed by QRS
Rhythm regular, rate normal
ST segment elevated >2mm above isoelectric line
Caused by infarction (tissue death due to hypoperfusion)
How can ST depression be recognised and what can it be caused by?
P waves visible and always followed by QRS
Regular rhythm, normal rate
ST segment depressed >2mm below isoelectric line
Caused by myocardial ischaemia (coronary insufficiency)
