The Electrocardiogram Flashcards
Lead I
Placed on the left hand.
If current travels towards Lead I, the deflection is upward.
If it travels away from Lead I, the deflection is downward.
If it travels perpindicular, there is no deflection.
P wave
Atrial deflection. When the SA node fires and sends its signal to the AV node, also resulting in contraction of the atria.
QRS complex
Represents firing of AV node, depolarization of septa, and depolarization of the ventricles (left dominant)
Why does depolarization of the left ventricle dominate the ventricular electrical signal?
Because there is more muscle in the left ventricle! So consequently, there is more electrical activity, since all of those muscles are firing at once. In effect, the right ventricle signal is being hidden and the net vector is left.
T wave
Repolarization of the ventricles. Also left dominant.
P QRS T diagram
Why is Q negative on Lead I?
Because Q represents depolarization of the interventricular septum, which is to the right of the left ventricle (especially given the bulge due to higher left ventricular hydrostatic pressure)
x and y axis on an ECG
x = time
y = voltage
Ca2+ resting potential
+123 mV
Na+ resting potential
+67 mV
K+ resting potential
-92 mV
Depolarization potential
Usually -70 mV
(The potential necessary for fast sodium channels to open)
Gating potential for voltage-gated potassium channels
+20 mV
Depolarization stages
V1 location
Right 4th intercostal space, on the sternal border
V2 location
Left 4th intercostal space, on sternal border
V4 location
Left 5th intercostal space, on midclavicular line
V3 location
Halfway between V2 and V4
V5 location
Left anterior axillary line, in the same axial plane as V4
V6 location
Left midaxillary line, same axial plane as V4
Underneath the armpit
Appendage leads
Two on the arms, in the same axial plane as V4 when the patient is supine, should be ~2/3 down the brachium
Two on the legs, same axial plane as one another
Upward deflections are always going towards the ___ electrode.
Upward deflections are always going towards the + electrode.
myocardial action potential durations are more prolonged in cells near the ___ than ___
myocardial action potential durations are more prolonged in cells near the inner endocardium (the first cells stimulated by Purkinje fibers) than in myocytes near the outer epicardium
The cells close to the endocardium are the ____
The cells close to the endocardium are the first to depolarize but are the last to repolarize
ECG placement diagram
The right-leg electrode
The right-leg electrode is not used for measurement but serves as an electrical ground.
aVR
Right arm + with respsect to other electrodes
unipolar lead
Unipolar leads
There is no single (−) pole; rather, the other limb electrodes are averaged to create a composite (−) reference.
Ex: aVR, aVL, aVF
aVF
Recorded by setting the left leg as the (+) pole, such that a positive deflection is recorded when forces are directed toward the feet.
aVL
Lead aVL is selected when the left-arm electrode is made the (+) pole and it records an upward deflection when electrical activity is aimed in that direction
Bipolar leads
Bipolar indicates that one limb electrode is the (+) pole and another single electrode provides the (−) reference.
Limb lead diagram
Axial reference system for limb leads
The magnitude of the deflection, either upward or downward, relative to other deflections reflects . . .
The magnitude of the deflection, either upward or downward, relative to other deflections reflects how parallel the electrical force is to the axis of the lead being examined
Precordial leads in relation to ventricles
The first region to depolarize is . . .
The first region to depolarize is the midportion of the interventricular septum on the left side
Current vectors of a standard myocardial depolarization
Standard depolarization as seen from different precordial leads
Horizontal axis of an ECG
The horizontal axis represents time. Because the standard recording speed is 25 mm/sec, each 1 mm division represents 0.04 sec and each heavy line (5 mm) represents 0.2 sec.
ECG machines routinely inscribe a ____ at the beginning or end of each 12-lead tracing to document the voltage calibration of the machine.
ECG machines routinely inscribe a 1.0-mV vertical signal at the beginning or end of each 12-lead tracing to document the voltage calibration of the machine.
Each 1-mm vertical box on the ECG paper represents ___
each 1-mm vertical box on the ECG paper represents 0.1 mV
So the calibration signal records a 10-mm deflection
Sinus rhythm
Normal cardiac rhythm:
- every P wave is followed by a QRS
- every QRS is preceded by a P wave
- the P wave is upright in leads I, II, and III
- the PR interval is greater than 0.12 sec
normal sinus rhythm
If the heart rate in sinus rhythm is between 60 and 100 bpm and the rhythm is sinus
sinus bradycardia
If the heart rate is less than 60 bpm and rhythm is sinus
sinus tachycardia
If the heart rate is greater than 100 bpm and the rhythm is sinus
Calculating HR from an ECG
When measuring irregular heart rates on ECG, it is most convenient to . . .
. . . count the # of QRS complexes between the first and third markers on the strip (6 seconds) and multiply by 10.
When measuring regular heart rates on ECG, it is most accurate to . . .
. . . divide 1,500 over the number of mm between beats
(1,500 comes from 25 mm/sec and 60 sec/min)
1500 / (mm between QRS)
corrected QT interval
Since QT varies with HR, this should be corrected for by dividing the measured QT by the square root of the R—R interval.
Angle of a normal QRS
Leads __ and __ are best used to classify a normal vs abnormal current vector
Leads I and II are best used to classify a normal vs abnormal current vector
isoelectric complex
When the upward and downard deflections of QRS are of equal magnitude
When an ECG limb lead inscribes an isoelectric QRS complex, it indicates that the mean electrical axis of the ventricles is perpendicular to that particular lead.
How to calculate QRS axis
- Inspect limb leads I and II. If the QRS is primarily upward in both, then the axis is normal and you are done. If not, then proceed to the next step
- Inspect the six limb leads and determine which one contains the QRS that is most isoelectric. The mean axis is perpendicular to that lead.
- Inspect the lead that is perpendicular to the lead containing the isoelectric complex. If the QRS in that perpendicular lead is primarily upward, then the mean axis points to the (+) pole of that lead. If primarily negative, then the mean QRS points to the (−) pole of that lead.
Bundle Branch Block summary
Normal vs path Q waves
Left lateral wall infarction on ECG
Conditions detectable on ECG
