Direction of electrical current flow by convention is from ______ to ______ charged areas
At rest the cell surface is ______ charged
How many reference axes are in the frontal plane (limb leads)?
How many in the transverse plane?
6 and 6
What are unipolar leads? Which ECG electrodes are unipolar?
Unipolar leads have no single negative pole - their negative pole is a composite reference of other leads averaged
Include aVR, aVF, aVL, V1-V6
What are bipolar leads? Which electrodes are bipolar leads?
Bipolar leads have a single electrode as the positive pole and a single electrode as the negative reference
Include leads I, II, and III
What lead is associated with each degree below from the "Circle of axes"?
0º: Lead I
60º : Lead II
120º: Lead III
Electrical force directed toward + pole of a lead generates a ______ deflection in that lead
Force directed away from + pole of a lead results in ______ deflection in that lead
Magnitude of deflection is determined by...
How parallel the electrical force is to the lead axis - the more parallel the greater the magnitude of deflection in that lead
What is the sequence of Normal Cardiac Activation?
SA node → AV node → Bundle of His → Mainstem Left bundle branch → right bundle branch and Posterioinferior left bundle branch → Purkinje fibers
Sequence of Cardiac Depolarization (6)
- Depolarize atria
- Depolarize septum from left to right
- Depolarize anteroseptal region of myocardium toward the apex
- Depolarize bulk of ventricular myocardium from endocardium to epicardium
- Depolarize posterior portion of base of the left ventricle
- Ventricles are now depolarized
Why is depolarization in the ventricles eventually directed leftward and posterior, toward V6?
As lateral wall left ventricular (LV) is depolarized, electrical forces of thick LV outweigh RV and depolarization is directed toward that side
What to look for when interpreting EKG
Intervals (PR, QRS, QT)
Calculate mean QRS axis
Evaluate P wave for abnormalities
Evaluate QRS for abnormalities
Evaluate for abnormalities of ST/T wave
EKG is recorded on a grid divided by lines space __ apart both in horizontal and vertical directions
Vertical axis measures ______
Horizontal axis measures ______
Normal heart rhythm on EKG findings include (4)
- Every P followed by QRS
- Upright P in leads I, II and III
- PR interval between 120 and 200ms
- HR between 60-100 bpm
If HR is less than ___ it is termed sinus bradycardia
If HR is more than ___ it is termed sinus tachycardia
less than 60; more than 100
Heart rate (bpm) calculation?
What if the rhythm is irregular?
HR = 1500/#small boxes between two consecutive beats
Irregular rhytm: count #QRS during 6 sec and multiply by 10
PR interval: (normal = 0.12-0.2 sec)
QRS interval: (normal < 0.1 sec)
QT corrected: (normal < .44 sec)
- PR interval: onset of P to onset of QRS
- QRS interval: beginning to end of qRS
- QT interval: onset QRS to end of T wave
- QT corrected: measured QT/square of RR interval (seconds)
What is the mean QRS axis?
What is normal?
What constitutes a left axis deviation?
What constitutes a right axis deviation?
Definition: average of instantaneous electrical forces generated during ventricular depolarization in frontal plane
Normal: -30º to +90º
Left axis deviation: negative to -30º
Right axis deviation: positive to +90º
If net QRS in leads _ and __ are positive, axis is in normal range
I and II
Geometric method of determining mean QRS axis?
- Measure magnitude of QRS
- Mark on circle of axes, + 2 units at lead with highest amp. and another unit with negative amplitude (at approximately -1 unit)
- Draw two perpendiculars
- Connect center of circle with intersection of two perpendiculars
- Estimate axis of this line
Inspection method of determining mean QRS?
- Identify lead where QRS is isoelectric (flat line)
- Identify axis perpendicular to isoelectric lead
- QRS is in line with perpendicular axis
Abnormalities of P wave are commonly measured with which leads?
Lead II (60°) and V1 (toward right heart)
Left ventricular hypertrophy increases the amplitude of electrical forces directed to the ____ and _____
Right ventricular hypertrophy can shift the QRS vector to the _____
LVH findings on EKG for V1 and aVL leads
S in V1 > 24mm
R in aVL > 13mm
RVH findings on EKG (5)
Tall R wave in V1
Right axis deviation
T wave inversion in V1-V3
Delatyed precordial transition zone in V6
Right atrial abnormality
Interruption of the left anterior fascicle or division (LAD) results in...
Right interruption of left posterior fascicle or division (LPD) results in...
- Initial inferior followed by dominant superior direction of activation
- Initial superior followed by a dominant inferior direction of activation
What condition can lead to more than one R wave in the V1 lead?
How does this condition present in the V6 lead?
Right bundle branch block (RBBB)
Causes abnormal broad S wave in V6
Recognize LBBB patterns in V1 and V6 leads
Loss of normal right to left depolarization
On EKG what is the difference between STEMI and non-STEMI ischemia?
STEMI = transmural (epicardial) injury: ST elevation
Non-STEMI = subendocardial injury: ST depression
Acute ischemia may alter ventricular action potentials by inducing...
- Lower resting membrane potential
- Decreased amplitude and velocity of phase 0
- Abbreviated action potential duration
* resulting currents of injury are reflected on ECG by deviation of the ST segment
What is the most likely cause of ST elevation or depression in one isolated lead?
Probably an error - Important that ST changes are in multiple leads that mae sense in groupings
Where do pathologic Q waves develop?
In leads overlying infarcted tissue because necrotic tissue does not generate electrical forces - Lead overlying necrotic tissue detects currents from healthy tissue on opposite regions directed away from infarct
Leads to downward deflection of Q wave
Conditions associated with ST and T wave changes
Hypercalcemia can lead to shortening of the ___ interval