Download Physio Lecture 16 Analyzing EKG vectors and MEA

Lecture 16 Analyzing EKG vectors and MEA
Vectors are arrows that point you in a direction, and their size tells you how much current is traveleing
through the heart. An EKG is a monitor of electrical events occurring on the surface of the heart.
EKG records these events: 5 cardiac vectors
1. Atrial depolarization: Down ant to left
2. Right side of IV septum, up and to the right
3. Big vector for IV septum
4. Left ventricular wall, goes lateral
5. Ventricular Repolarization: IV septum is last to repolarize
Electrocardiogram (ECG)
• Depolarization wave passes through the heart and the electrical currents pass into surrounding
tissues.
• Small part of the extracellular current reaches the surface of the body.
• The electric potential generated can be recorded from electrodes placed on the skin
• An EKG is a comparison of two vectors
• It compares the “heart vector” showing current flow on the heart with the reference,
“recording lead vector” on the body.
Vector diagrams
• Vectors are used to describe depolarization and repolarization events
• Vectors are arrows which show two things:
– Direction or pathway (of charge spread)
– Magnitude or size (amt of charge)
• Vector analysis explains the waves on an EKG
GPS tells you where you are and where you want to go. The EKG shows the same thing.
EKG is Extracellular Recording
• Only looks at the charge on the outside of fibers!
– Resting cell: outside positive
– Depolarizing cell: outside negative
– Repolarizing cell: outside positive
• Depolarization: spread of surface negative charge
• Repolarization: spread of surface positive charge
Vectors will always be positioned so that head of vector is in area of positive charge; tail is in area of
negative charge. Top right atria will repolarize first. Even though electrical current is spreading from top
down, the arrow has to point to the positive charge, so it points up and to the right
What does that tell you about the recording you obtain from each lead?
• Each lead describes the events on the heart from “it’s own point of view”
• Reading from several leads gives you different points of view about the same set of repeating
events (depol, repol)
• What if the recording lead was oriented this way?
12 Lead EKG’s
• Read from each lead independently; one at a time over several heartbeats.
• See what each lead shows.
• 12 leads
– 3 bipolar limb leads (I, II, III)
– 3 augmented unipolar limb leads
• (aVR, aVL, aVF)
– 6 precordial leads (chest leads, V1-V6)
6 Leads- bipolar and augmented; all of these are in flat plane
Bipolar Leads and Einthoven’s Law
• Lead I - The negative terminal of the electrocardiograph is connected to the right arm, and the
positive terminal is connected to the left arm.
• Lead II - The negative terminal of the electrocardiograph is connected to the right arm, and the
positive terminal is connected to the left leg.
• Lead III - The negative terminal of the electrocardiograph is connected to the left arm, and the
positive terminal is connected to the left leg.
• Einthoven’s Law states that the electrical potential of any limb equals the sum of the other two
(+ and - signs of leads must be observed).
Lead I
LA – RA
Lead III
LL- LA
Lead II
LL- RA
Summary of Events
• P wave
– atrial depolarization- SA node to the AV node
– (mechanical event that will result: atrial systole)
• QRS complex- depolarization of ventricles
– Q wave- due to left to right depolarization at bundle branch (right has “detour”)
– atrial repolarization and diastole (signal obscured)
– AV node fires, ventricular depolarization
– (mechanical event that will result: ventricular systole)
• T wave
– ventricular repolarization
– (mechanical event that will result: ventricular diastole. ventricles remain somewhat
contracted until a few milliseconds after the end of the T repolarization wave.)
Intervals & Segments
• Segments are flat lines, do not include waves: PR segment, ST segment.
• Intervals include at least one wave
• P-R interval- from beginning of P to the Q wave. Is time for atrial depolarization plus delay from
AV node. Also, time of atrial contraction (more than .2 sec could be 1st degree block)
• P-R segment- delay in impulse through AV node.
• Phases of EKG
• Q-T interval- includes Q and T waves, total time for ventricular depolarization and
repolarization; this approximates the time of total ventricular contraction.
• T-P segment - end of one cycle to beginning of next
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P-P interval - time for one complete cycle (could also use R-R or T-T, etc.)
S-T segment: time between ventricular depolarization and repolarization; time of peak
ventricular contraction (maximum tension)
Cardiac Arrhythmias
Tachycardia: abnormally fast heart rate
Bradycardia: Abnormally slow heart rate
Incomplete Atrioventricular Block: Prolonged P-R interval (1st degree)
Complete Atrioventricular Block: P waves and QRS complexes become dissociated (3rd degree)
Fibrillation: Complete lack of coordination
Electrolyte imbalance
Hypernatremia:
– Inhibits calcium entry into the cell
– Depresses overall heart activity and becomes flaccid; (negative inotropy)
Hypercalcemia:
– (-, +)
– Increased heart irritability
– More calcium into cytoplasm
– What reflex could augment the decreased chronotropy?
Hyperkalemia:
– Peaked T waves.
Determining the MEA Vector
This presentation aims to teach you the trick to visually determine the position of the MEA from the
EKG
• Know the Orientation of All Leads: it creates a compass (circle) which we can separate into
quadrants. This will allow you to estimate the mean conductance.
• Use any two leads on an EKG
• Given: limb leads I and avF
• Goal: to find the MEA vector
• Draw 6 circles, with arrows in them showing the vector for each lead. Superimpose lead 1 and
lead avF, will get a quadrant.
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Step 1:
Visually, examine the profiles of leads I and aVF
Step 2:
Superimpose the two diagrams of the heart, and see where the hatched areas overlap.
This will be the area which must contain the MEA vector
• Step 1:
Visually, examine the profiles of leads I and aVF
• Step 2:
• Superimpose the two diagrams of the heart, and see where the hatched areas overlap.
• This will be the area which must contain the MEA vector
Mean Axial Shift
• Left axis deviations
– endomorph- short stature
– Pregnancy
– Left ventricle hypertrophy
– LBBB
• Right axis deviations
– Ectomorph- tall /thin
– Hypertrophy of right ventricle
– RBBB