ECG 2 Flashcards
What does a standard lead look like?
A standard ECG recording is a 12 lead record
A “lead” is a pair of electrodes (one connected to the +ve input of the ECG machine and the other to the -ve input
ECG leads
Bipolar limb leads leads 1,2,3
Augmented (unipolar) limb leads-
aVL
aVR
aVL
Precordial chest leads V1 to V6
Although the wave of depolarization that goes through the heart is “fixed” the 12 ECG…
leads record this wave from different angles and directions. Right leg electrode is the “ground/earth”
The ECG recording set up
Why are there 12 leads in ECGs?
View the same QRS depolarization from different angles/electrodes or leads
-Each lead used to get whole picture/information about the heart
Describe ECG leads
Non-invasive, view heart from 2 planes:
Frontal plane-leads 1, 2 and 3, aVR, aVL and aVF (frontal plane with extremity leads/limb leads )
Transverse plane- chest leads V 1-6 (horizontal plane with precordial leads/chest leads )
What is the function of limbs in bipolar limb leads?
Limbs act as “volume” conductors
What is the potential difference between bipolar limb leads?
Lead 1- right arm(-) and left arm (+)
Lead 2- right arm(-) and left leg(+)
Lead 3- left arm (-) and left leg (+)
What is the function of ECG bipolar limb leads?
DCG leads compare voltage differences between two skin electrodes. Positive and negative poles are ordained by convention
Note: arrow heads of leads denote the +ve electrode
What is the function of lead 1 in detecting QRS complex?
Mean QRS (—> ) is directed down and towards left side of the heart
QRS complex : largely upward(depolarization is heading towards the +ve electrode on LA)
Portion of mean QRS vector (—>) is in direction of lead 1
What is the function of lead 2 in detecting the QRS complex?
QRS complex is upward
Depolarization moving towards the +ve electrode on LL
QRS complex is relatively large upward deflection since most of the QRS vector (—>) is in direction of lead 2
What is the function of lead 3 in detecting the QRS complex?
QRS vector is now nearly perpendicular to the recording axis (LA to LL). LL is the +ve electrode
QRS complex is biphasic with significant downward and upward deflections giving a net deflection of practically zero.. this type of QRS complex is callled “isoelectric complex”
Mean QRS vector (—> ) nearly perpendicular to recording axis of lead 3
What is the potential of the unipolar limb/augmented lead aVR(r= right)?
aVR
Potential difference = RA and a “V” lead made by connecting-LA and LL electrodes
“Zero” - V
What is the potential difference in unipolar lead aVL(l=left)?
aVL
Potential difference- LA and a “V” lead made by connecting -RA and and LL electrodes
“Zero”- V
What is the potential diffference between aVF (f=foot)?
LL and a “V” lead made by connecting -LA and RA electrodes
“Zero”-V
Hat are augmented/ unipolar limb leads do?
Augmented leads measure voltage differences between the same skin electrodes and a central reference point
Basically, it is the voltage difference between an exploring (+ve) electrode placed on one limb (RA, LA or LL) compared with an indifferent electrode (V) which is at zero potential - central reference terminal. (Hence the name unipolar)
Note: the exploring electrode on the limb is always the positive electrode
What is the function of aVR?
Record the electrical activity between an “exploring electrod” (X) on RA and an indifferent electrode “V”. RA is +ve.
The recording axis is as shown with the +ve electrode on the right arm
The mean QRS vector is in the opposite direction to lead a VR, thus QRS complex is largely negative with a relatively large downward deflection
What is function of aVL?
Record activity between the +ve “exploring electrode” on LA with an indifferent electrode “V”
The mean QRS vector - -> is partially in direction of lead aVL, thus QRS complex is biphasic with usually a net upward deflection
What is the function of aVF?
Record electrical activity between the +ve “exploring electrode”(X) on the LL (foot) and an indifferent electrode “V”
The mean QRS vector—> is mostly in direction of lead aVF, thus QRS complex is largely upward
What aVL stand for?
a= augmented
V= indifferent electrode
L=left arm(+ve electrode)
What does aVR stand for?
a=augmented
V=indifferent electrode
R=right arm(+ve electrode)
What does aVF stand for?
a=augmented
V=indifferent electrode
F= left foot (+ve electrode)
What are the precordial chest leads/transverse(horizontal) plane?
V1 to V6 are all +ve electrodes I.e. exploring electrodes
The negative (indifferent) electrode is the 3 bipolar leads connected together which is at zero potential
What are the displays of the precordial chest leads(transverse horizontal)?
V1/2- looks at RV
V3/4- “looks at” septum
V5/6 looks at LV
Note change in QRS complex:
V1/2 downward
V3 equiphasi
V4-6= upward
What does a 12 lead ECG record look like?
QRS in I and II is net positive
aVR is net negative; aVL is net +ve
Note the polarity changes from V1 to. V6
Explain how the shape of the QRS complex is determined by the relationship 9f the mean QRS vector with a particular recording lead
- As mean QRS vector travels towards the negative electrode, waveform is mostly downwards
- As mean QRS vector travels perpendicular to the lead, the waveform may be small or go in both directions (biphasic)
- As mean QRS vector travels towards the +ve electrode, waveform 8s mostly upwards
What is the Q in the QRS waveform?
Any initial downward deflection followed by an upward deflection (a single downward deflection is called QS)
What is the R in the QRS waveform?
Any upward deflection whether or not it is preceded by a Q wave
What is the S in the QRS wave form?
Any downward deflection preceded by an R wave
What are the variations in QRS waveforms?
Can often be seen in different leads
qRs
rS
QS
RS
rsR’
qR
Qr
Upper and lower case denotes size of waves
Describe Einthoven’s triangle
Imaginary formation of the leads: I, II, & III using as vertices the shoulders and the pelvis, as direct extensions from the right arm (RA), left arm (LA), and left leg (LL)
What are the characteristics of lead I in Einthoven’s triangle?
Between 2 shoulders
(-) electrode right shoulder (RA)
(+) electrode left shoulder(LA)
0 degree angle orientation
I=LA-RA
What are the characteristics of lead II in Einthoven’s triangle?
Between the right shoulder and the pelvis
(-) electrode right shoulder
(+) pelvis (LL)
+60 degree angle orientation
I= LL-RA
What are the characteristics of lead III of the Einthoven’s triangle?
Between the 2 shoulders
(-) electrode left shoulder (RA)
(+) electrode pelvis (LL)
0 degree angle orientation
III= LL-LA
Whaat is the triaxial system?
Rearranged leads of Einthoven’s triangle,
Lead I is horizontal at 0 degrees
Lead II is 60 degrees to lead I
Lead III is 120 degree to lead I
What is eithenovens law?
In an ECG, at any given instant, the potential of any wave in lead III is equal to the sum of the potentials in leads I and lead III
If the electrical potentials of any two of the three bipolar limb electrocardiographic leads are known at any given instant, the third one can be determined mathematically by simply summing the first two
II = I + III
Explain eithenovens law
-If RA electrode= -20 mV and the LA electrode= +30mV, then lead I = LA - RA= 0.3mV -(-0.2 mV)= +0.5mV
Lead I will have a deflection of 0.5 mV
- If LL electrode = 1.0 mV and LA electrode= +0.3 mV, then Lead III = LL-LA=1.0 mV - 0.3 mV = +0.7 mV
- Lead III will have a deflection of 1.0 mV-0.3 mV = 0.7mV
-Since LL electrode = 1.0 mV and RA electrode= -0.2 mV, then lead II= LL -RA = 1.0 mV -(-0.2mV)=+1.2 mV)
Lead I (+0.5 mV)+lead III (+0.7mV)= lead II (1.2 mV)
Describe axes formed by the augmented unipolar limb leads
Leads bisect each other at 60 degrees angles
The 3 leads are referenced to center of heart which is at zero potential
Explain How the hexaxial reference circle is derived
The 3 pairs of bipolar limb leads form the Einthenkven triangle
If the 3 leads are connected together (represented by the center of the triangle) the sum voltage of the 3 leads is zero
How is the hexiaxial circle formed?
Augmented leads super8mpksed on bipolar leads
Angles are now 30 degrees to each other
Note direction of arrows: arrowhead denotes +ve electrode
What is the mean electrical ( cardiac) axis of the heart?
MEA is the net direction of electrical conduction during ventricular depolarization
MEA can be calculated (expressed in degrees)
What are the characteristics of the mean electrical axis?
Calculated by measuring QRS complexes in the frontal plane leads (I, II, III, aVL, aVR, and aVF)
The precordial leads (V1-6) are NOT used
Ascribed a numerical value in degrees
Abnormal values associated with pathological conditions e.g. conduction defects, ventricular hypertrophy
Wave of excitation produces Electrical vectors produced in various directions as it moves through the heart
Vectors can be averaged to produce an MEA
Describe abnormal MEA
Note: between zero degrees and -30 degrees can be normal or abnormal depending on existing pathology.
An MEA between zero degrees and 90 degrees is definitely normal.
Normal MEA axis: lies between 0 and 90 degrees. However 0 degrees to -30 degrees can regarded as normal if no evidence of pathology
Left axis deviation(LAD): MEA is more -ve than -30 degrees
Right axis deviation(RAD): MEA is more +ve than +90 degrees
Describe the clinical significance of the MEA
Right axis deviation
- right ventricle hyoertrophy
- right-bundle branch block
Left axis deviation
- left ventricular hypertrophy
- left bundle bunch block
How to calculate MEA?
Mapping(geometric) method-using the hexiaxial reference system
Quadrant method
Isoelectric QRS complex
The thumb method
Describe the mapping(geometric) method of calculate
Measure net amplitude of QRS in at least two of the six limb leads. (Usually select the 0argest QRS complex to minimize measuring system)
How to measure QRS amplitude?
Amplitude is determined by summating the squares above (R) and below the (S) the baseline
- Positive: deflection: R=8, S=-4 amplitude= 8-(-4)= 4
- Equiphastic: deflection- R=8, S=-7 amplitude= 8+(-7)= +1
- Negative: R=1, S=-7 amplitude= 1+(-7)= -6
Comeback fir worked example
Summarize the quadrant method
By looking at an ECG recording you can tell straight away if the axis is normal. HOW?
If the net deflection of the QRS complex is upward in leads I AND aVF then the MEA will always fall between 0 degrees and 90 degrees
An upward deflection in lead I could be represented anywhere in blue section (+QRS)
An upward deflection in aVF could be represented anywhere in red section (+QRS)
However an upward deflection in lead I and aVF can only be represented by the dark blue segment (
What is the cautionary of the quadrant method?
A cardiac axis between 0 degrees and 90 degrees is normal
However, in many instances a cardiac axis between 0 degrees and -30 degrees is also normal. Some cardiologist advocate looking at the QRS complexes in leads I and II. If they are both upright then the axis lies between -30 degrees and +90 Degrees . But if the QRS complex in either lead I or Lead II is not primarily upward then the axis is abnormal and the axis should then be determined by more accurate means.
Describe the isoelectric QRS complex
Inspect all 6 limb leads and find the one with the most equiphastic QRS complex
QRS is considered isoelectric in lead I
This means the electrical axis is perpendicular to lead I
When an ECG limb lead inscribed an isoelectric QRS complex, it indicates that the axis of the ventricles is perpendicular to that particular lead
Explain the isoelectric QRS complex
MEA is perpendicular to lead I. But at this stage it is not known in which direction the axis is +. 90 degrees or -90 degrees
Which one is correct?
Inspect the ECG lead that is perpendicular to lead I I.e. aVF. If QRS is +ve in aVF then the axis points toward the +ve pole of that lead I.e. +90 degrees. If the QRS is negative in aVF then the axis is -90 degrees
Describe the thumb method -2 leads for calculating MEA
QRS deflection (up/down) modeled using thumbs
Left thumb lead 1 (up) right thumb (lead II) =MEA or axis deviation= normal all up
Left thumb lead 1(up) right thumb(lead 2) = MEA or axis deviation = leafy (L up)
Left thumb lead 1(down) right thumb(lead 2)= MEA or axis deviation Right (R up)
Left axis deviation (LAD)= only left thumb up
Right axis deviation (RAD)= only right thumb up
A common error!
You MUST remember the direction of the arrows in each leaf!
If QRS is a net downward deflection in e.g. aVR- take measurement from the center (0) towards X not towards Y
Net +ve QRSare measured from zero t8wards the arrowhead and net negative measurements from zero towards X
