Flashcards in Cardiology - ECG basics Deck (29):
What is the basis of the ECG? How does the direction of current flow determines the deflection in ECG leads?
ECG records patterns of spread of electricity in the various phases of the cardiac cycle. Electrons carry current and flow from areas of negative to areas of positive charge. When current moves towards an electrode (i.e. a lead), there is a positive deflection.
The basis of current flow around the heart starts off as current flow within individual heart cells, which induces current flow between cells. Depolarisation causes positive Na+ ions to move into the cell; repolarisation causes K+ ions to move out of cells - this leaves excess negative charge outside the cell at the start of the cardiac cycle, and excess positive charge at the end.
Current flows from areas that have just depolarised (excess extracellular negative charge) into areas yet to depolarise. This current flow depolarises neighbouring cells until the whole heart has been depolarised.
Does the heart depolarise uniformly?
The heart does not depolarise or repolarize simultaneously - some areas de/repolarize before other areas so that in depolarisation, current flows into areas about to depolarise and with repolarization current flows from areas about to repolarize.
Describe the sequence of depolarization in the heart?
The impulse starts in the sino-atrial node (SAN) which spreads the wave of depolarisation throughout the atria via interNODAL and interATRIAL (bundle of Bachmann) pathways. This allows for right and left atrial depolarisation. The impulse then moves down into the AVN where there is a delay of approximately 150-200ms to allow for ventricular filling. Depolarisation then spreads in the specialised conducting tissue of the ventricles - bundle of His, bundle branches and their divisions.
Specialised conducting tissue quickly distributes the electrical impulse throughout the ventricle and into the myocytes, causing contraction.
Where is the specialised conducting tissue of the heart located?
This is sub-endocardial, so the wave of depolarisation spreads endocardially to epicardially. This explains why most leads observing the left ventricle have a positive deflection.
What segment of the ECG is caused by atrial systole?
Atrial systole results in the P wave. Atrial repolarization results in a small current flow that is overshadowed by ventricular contraction.
What is a QRS complex?
The QRS complex is the segment of the ECG that is caused by ventricular systole.
- a Q wave is defined as the initial negative deflection of the QRS complex
- an R wave is defined as the first positive deflection of the ventricular complex
- an S wave is a negative deflection of the QRS complex following an R wave - it CANNOT be the first deflection of the QRS complex
- an R' wave (pronounced R prime) is a second positive deflection - i.e. an R wave followed by an S wave, followed by a second R wave
What causes the T wave?
During repolarization, current flow results in the T wave (and possibly the U wave).
What are the "limb leads"?
There are 4 limb leads that are attached to the 2 upper and 2 lower limbs. They are coloured, red, yellow, green and black and have the following configuration:
- Red = Right arm
- Yellow = Left arm
- Green = Left leg
= Black = Right leg
(Think - Ride Your Green Bike)
Using these position, leads I, II, III, aVR, aVL and aVF are derived.
What are the positions of the chest leads?
There are 6 chest leads that are attached as follows:
V1 - 4th intercostal space, right sternal edge
V2 - 4th intercostal space, left sternal edge
V3 - midway between V2 and V4
V4 - 5th intercostal space, left mid clavicular line
V5 - level with V4, left anterior axillary line
V6 - level with V4, left mid axillary line
What is the usual paper speed that ECGs are recorded at? What is the sensitivity and why is this important?
ECG is usually recorded at a paper speed of 50mm/s. Each long square is 1cm long and records 200ms of activity. It is divided into 5 smaller squares of 40ms duration.
The sensitivity of the ECG is usually 10mm/mV. The size of the deflection on the paper relates to the sensitivity setting, if it is increased then complexes are larger and vice versa.
What information does the P wave provide?
The P wave reflects electrical activation of the atria. It can give information about:
- where atrial depolarization started and whether the atria are enlarged, as P wave shape relates to where depolarization starts and the route it takes
- assess sinus node, as P wave reflects sinus node function
What are the best leads to look at the P wave?
The best leads to look at the P wave are those directly in or away from the path of atrial depolarization - i.e. lead II and lead V1
What is the normal duration of the P wave? In what conditions can this be altered?
Normal P wave duration is 110 ms. The duration of the P wave reflects how long atrial depolarization lasts. The duration is increased if the wave of electricity travels slower than normal (i.e. certain cardiomyopathies) or if the wave travels at the normal speed but the atria are enlarged. In the former the P wave is usually diminished, whereas in the latter the P wave is often of a good size or normal.
Do both atria depolarize at the same time?
No. Depolarization starts in the right atria and is distributed to myocytes via internodal tracts. It is transmitted to the left atria by a specialised interatrial conducting system called the bundle of Bachmann. The time taken for electricity to travel down the bundle of Bachmann means that left atrial depolarization starts a little while after right atrial depolarization, and also goes on for a little while after right atrial depolarization has finished. In health, both contribute equally to the size of the P wave.
What determines the size of the P wave?
The size of the P wave reflects both the volume of electrically active tissue AND the insulation between the atria and observing electrode. If the atria have more/ larger myocytes then the size of the P wave increases.
On the other hand, if the number/ size of the myocytes decreases or there is more insulation between the heart and the ECG electrode (e.g. pericardial effusion, obesity) then the P wave size diminishes.
What is the best measure of sinus node function?
24 ECG recordings give the most information regarding sinus node function, and rather than measuring the PP interval (i.e. the interval between 2 adjacent p waves) it is easier to measure the RR interval (assuming that the PR interval is constant).
What is the path of depolarization from the atria to the ventricles?
Depolarization starts in the sino-atrial node. From here it moves into:
- the AVN, which slows down depolarization to ensure that atrial contraction is over before ventricular contraction starts
- then onto the bundle of His
- then into the left side of the septum, with the current passing down the septum from the left side to the right side (this is important to remember as it accounts for the small septal Q waves in left sided ECG leads)
- then via the Purkinje cells into the sub-endocardium of the ventricles and through the myocardium towards the epicardium, so producing a co-ordinated contraction
How does the pattern of ventricular depolarization produce different shapes of QRS complexes in different leads?
In LEFT sided heart leads there is often a small Q wave, reflecting septal depolarization (which is directed left --> right, i.e. away from the left sided leads), followed by a large R wave as the bulk of the left ventricle depolarizes towards the left sided electrodes.
In RIGHT sided heart leads there is a small R wave, followed by a large S wave as the later QRS complex is dominated by depolarization of the large bulk of the LV rather than the small right ventricle. The LV depolarization wave moves away from right sided chest leads leading to a negative deflection (S wave) in these leads.
Why are there no large Q waves in the ECG?
Current passes from the endocardium to the epicardium - i.e. always initially towards an observing electrode. The exception to this is aVR which looks through the AV valve into the ventricle resulting in a large Q wave.
What is the normal duration of a QRS complex?
The duration is short, normally <120ms, though more often like 100ms.
What is the vector of depolarization and how can it be calculated?
The vector of depolarization, termed the QRS axis, represents the overall direction that electrical depolarization passes through the heart. It is determined by plotting the largest R waves in two leads (usually leads I and aVF)against each other. A "quick" way to determine whether the axis is normal is to look at leads I and II; if both QRS complexes are positive the axis is normal. Left axis deviation results in a negative deflection in lead II and lead III (positive in lead I). Right axis deviation results in negative QRS in lead I; lead II is usually positive but may be negative; lead III is positive.
A handy way to remember this is to look at leadsI and II. If they aRrive (right axis deviation) - i.e. negative deflection in lead I, positive in lead II there is right axis deviation. In left axis deviation they Leave - i.e. positive in lead I and negative in lead II.
What determines the size of the QRS complex?
1) Size of the patient (fatter patients have smaller complexes)
2) Ventricular muscle mass - the lead directly opposite the largest mass of ventricular tissue has the largest QRS complex
3) Age of the patient (older patients have smaller QRS complexes for any given mass of muscle)
What does the T wave represent?
Current movement during repolarization.
How is the direction of depolarizing current, different to the direction of repolarizing current in making the T wave shape?
Depolarizing current is endocardial to epicardial (from inside the heart to outside), as the first tissue to depolarize is closest to the specialised conducting system (Purkinje fibres) which lies just under the endocardium. Depolarization moves positively charged ions into the cell, leaving excess negative charge (electrons) outside which flows into areas of more positive charge (i.e. those areas yet to depolarize).
The action potentials of epicardial cells are shorter than those of endocardial cells. These cells, despite being the last to depolarize, repolarise first. Thus, although the direction of repolarization is epicardial to endocardial, in the opposite direction to depolarization, the current flow associated with this is endocardial to epicardial. This current flow moves towards an electrode causing a positive deflection.
Is there any relationship between the R wave and T wave?
An important relationship is that if there is an R wave present then there should be an upright T wave. This is in reference to the T wave axis - the T wave axis should be within 60 degrees of the QRS axis. So if one sees a QRS complex with a good R wave but a flattened or inverted T wave, the ECG is abnormal.
What is unique about the inferior lead T waves?
These leads (especially lead III and aVF) often show variable T wave polarity, as the bulk of other parts of the heart balance out any local inferior wall repolarizing current flow. Posture also affects the inferior lead T waves - so exercise testing ECGs should initially be done lying down then standing.
What are U waves?
These are positive deflections occuring after the T wave, sometimes merging with it. The origin of the U wave is uncertain. It may be due to repolarization of papillary muscle.
Prominent U waves are normal in youth (<35-40) but are rare in the elderly. Certain diseases are associated with U waves, including:
- those on class I antiarrhythmic drugs
- mitral valve prolapse
- long QT syndrome (very rare)
What is the normal duration of the PR interval?
>3 small squares