Transcript Vectors and EKG ’s

Vectors and EKG’s
Chapters 11, 12, and 13
1
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.
•(Non-invasive)
•Heart Rate
•Signal conduction
•Heart tissue (enlarged)
•Conditions (MI)
•electrolyte and hormone
imbalances
2
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
Q S
3
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 neg 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.
4
Rest
No current
flow, no
vector.
The following
vectors represent
the spread of
negative charge
during
depolarization;
Then the spread
of positive charge
during
repolarization
5
= depol SA
nodal fibers,
spread of neg
charge over
atria
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- +
7
+
8
+
9
The atria would
start to repolarize
down and to the
left, as the current
continues
downward to the
ventricles
We don’t detect
this on the EKG,
but what would the
repolarizing vector
look like?
(review your
specialized
cells/contractile
cells lecture!)
+
10
+
11
Atria now have
repolarized and
now have positive
surface charge
again.
12
Meanwhile, as
the atria are
repolarizing......
We turn to the
Depolarizing
AV node
These are small
diameter fibers
with few gap
junctions; little
or no detectable
current flow
13
IV Septal
Depolarization
Moving down
bundle of His;
Current moves
down R and L
bundle branches
from L toward
R…why?
14
15
16
Apex then Lateral
walls
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18
Through the
thickness of the
heart, from
endo- , to myo-,
to epicardium
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20
Ventricles completely
depolarized, negative
surface charge
No current
No vector
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Begin
Ventricular
Repolarization
Spread of
positive charge
+
22
23
24
25
26
27
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Rest
End of
cycle;
No current
flow, no
vector.
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Recording from Lead II
Standard limb lead
II
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The Rules of Vectors Analysis
•
•
•
•
•
•
•
•
An EKG is a comparison of two vectors
It compares the “heart vector” with the
reference “recording lead vector” on the
body.
If the vectors run parallel (same
direction) the pen moves upward from
baseline
If the vectors run antiparallel (opposite
direction) then the pen moves downward
from baseline.
If the vectors are perpendicular, the pen
remains on baseline.
If there is no current flow, the pen
remains on baseline.
Each lead consists of two electrodes
placed on the skin, with a voltmeter
between them.
The voltmeter is attached to a pen, which
travels over paper running at 25 mm/sec.
This produces waves called an
electrocardiogram.
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-
I
+
-
-
III
II
Einthoven’s
Triangle
+
+
Bipolar
Limb
Leads 33
Atrial
depolarization
Pen here
II
V
T
The heart
vector is
parallel to the
lead, but how
can you
confirm?34
Atrial
depolarization
II
1.
2.
+
Draw a
perpendicular line
to the lead vector
Draw a line toward
from the
perpendicular
vector toward your
cardiac vector
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Atrial
depolarization
II
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AV nodal
depolarization
II
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IV septal
depol, from L
to R
II
Anti-parallel!
Pen deflects down
Draw it!
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IV septal
depol, from
base to apex
II
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Lateral walls
depol
II
Draw it!
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Depolarization
complete; no
current flow; pen
returns to
baseline
II
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Waiting to begin
repolarization;
no current flow
II
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Ventricular
Repolarization
begins
II
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Ventricular
Repolarization
II
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Ventricular
Repolarization
complete; no
current flow;
pen on
baseline
II
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Ventricular
Repolarization
complete;
waiting to
start all over
again
II
End of one
cardiac cycle
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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?
Use the words “down” or “up” to note the deflection compared
to the five cardiac vectors above
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Body Cross-section
at Heart Level
Heart
V6
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, V1V6)
V5
V1
V2
V3
V4
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6 Leads- bipolar and augmented; all of these
are in flat plane
Augmented- Obtained by using the average voltage of any two points on skin as ground (neg
pole) and reading from the third electrode (pos pole.)
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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
Lead III
Lead II
LA – RA
LL- LA
LL- RA
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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.)
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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.
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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
• 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
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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
Arrhythmia: conduction failure at AV node
No pumping action occurs
No P waves
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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.
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Electrolyte imbalance
• Hyponatremia:
– Depolarization delay
– Decreased heart rate
• Hypocalcemia:
– (+,-)
– Less heart contractility
– What reflex could
augment the increased
chronotropy?
• Hypokalemia:
– Lowers RMP (makes it
more negative)
– Decreases heart rate
– U waves
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Determining the MEA Vector
This presentation aims to teach you
the trick to visually determine the
position of the MEA from the EKG
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Know the Orientation of All Leads
• Use any two leads on
an EKG
• Given: limb leads I 210o
and avF
180o
• Goal: to find the MEA
vector
+ 120 o
- 90 o
aVR
aVL
- 30 o
I
III
0o
II
aVF
+60 o
-90 o
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Step 1:
Visually,
Lead I
examine
the
profiles of
leads I and
aVF
aVF
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Lead I: Make note
of the SIGN of the
net deflection
+ 10 mm
Lead I
- 2mm
Lead I Net deflection (+ 8) is POSITIVE.
Where would this fall in the graph?
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-
+
-90o
180o
0
+90o
Lead I
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Lead aVF: Make
note of the SIGN
of the net
deflection
+ 1mm
- 8 mm
aVF
Lead aVF Net deflection (- 7) is NEGATIVE.
Where would this fall in the graph?
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-90o
180o
0
-
+90o
+
Lead aVF
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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
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-
+
-90o
180o
0
-
+90o
+
MEA vector must lie in the zone of overlap
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-
+
-90o
180o
0
-
+90o
+
MEA vector must lie in the zone of overlap
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-
+
-90o
180o
0
-
+90o
+
Conclusion: LAD
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Here’s
Step 1:
Visually,
Lead I
examine
the
profiles of
leads I and
aVF
another example:
aVF
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Lead I: Make note
of the SIGN of the
net deflection
Lead I
Lead I Net deflection is VERY NEGATIVE.
Where would this fall in the graph?
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-
+
-90o
180o
0
+90o
Lead I
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Lead aVF: Make
note of the SIGN of
the net deflection
aVF
Lead aVF Net deflection is a SOMEWHAT POSITIVE.
Where would this fall in the graph?
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-90o
180o
0
-
+90o
+
Lead aVF
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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
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-
+
-90o
180o
0
-
+90o
+
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-
+
-90o
180o
0
-
+90o
+
MEA vector must lie in the zone of overlap
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-
+
-90o
180o
0
-
+90o
+
Conclusion: RAD
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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
How does the current normally flow down the IV septum? Left to right? OR
Right to Left? How would this change if there was a LBBB? RBBB?
Why does a LBBB cause a LAD? (think about the vector!)
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