16 analyzing ekg vectors and mea (1)
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Vectors and EKGs
Dr. Byron Reyes R3 Medicina
Dr. Enrique Tllez R2 Medicina
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Electrocardiogram
(ECG) Depolarization wave passes through
the heart and the electrical currentspass into surrounding tissues.
Small part of the extracellularcurrent reaches the surface of the
body.
The electric potential generated canbe recorded from electrodes placedon the skin
An EKG is a comparison oftwovectors
It compares the heart vectorshowing current flow on the heartwith the reference, recording leadvector on the body.
(Non-invasive)
Heart Rate
Signal conduction
Heart tissue (enlarged)
Conditions (MI)
electrolyte and hormone
imbalances
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Vector diagrams
Vectors are used to describe depolarization andrepolarization 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
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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 vectoris in area of positive charge; tail is in area of negativecharge.
+++++++++++ ------------------
+++++++++++ ------------------
+++++++++++------------------
+++++++++++------------------
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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
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= depol SA
nodal fibers,spread of neg
charge over
atria
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- +
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+
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+
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The atria would
start to repolarizedown and to the
left, as the current
continues
downward to the
ventricles
We dont detect
this on the EKG,
but what would the
repolarizing vectorlook like?
(review your
specialized
cells/contractilecells lecture!)
+
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+
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Atria now have
repolarized and
now have positivesurface charge
again.
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Meanwhile, as
the atria are
repolarizing......We turn to the
Depolarizing
AV node
These are smalldiameter fibers
with few gap
junctions; little
or no detectablecurrent flow
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IV Septal
Depolarization
Moving down
bundle of His;
Current moves
down R and L
bundle branches
from L toward
Rwhy?
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Apex then Lateral
walls
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Through the
thickness of theheart, from
endo- , to myo-,
to epicardium
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Ventricles completely
depolarized, negative
surface chargeNo current
No vector
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Begin
Ventricular
Repolarization
Spread of
positive charge
+
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Rest
End of
cycle;
No currentflow, no
vector.
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Recording from Lead II
Standard limb lead
II
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I
II III
Bipolar
Limb
Leads
+-
++
--
Einthovens
Triangle
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II
Atrial
depolarization
V
T
Pen here
The heart
vector is
parallel to the
lead, but how
can you
confirm?
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II
Atrial
depolarization
+
-
1. Draw a
perpendicular line
to the lead vector
2. Draw a line towardfrom the
perpendicular
vector toward your
cardiac vector
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II
Atrial
depolarization
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II
AV nodal
depolarization
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II
IV septal
depol, from Lto R
Draw it!Anti-parallel!
Pen deflects down
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II
IV septal
depol, frombase to apex
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II
Lateral walls
depol
Draw it!
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II
Depolarization
complete; no
current flow; penreturns to
baseline
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II
Waiting to begin
repolarization;
no current flow
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II
Ventricular
Repolarization
begins
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II
Ventricular
Repolarization
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II
Ventricular
Repolarization
complete; no
current flow;
pen on
baseline
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II
Ventricular
Repolarization
complete;waiting to
start all over
again
End of one
cardiac cycle
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12 Lead EKGs 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)
Heart
Body Cross-sectionat Heart Level
V1V2
V3
V4V5
V6
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6 Leads- bipolar and augmented; all of these
are in flat planeAugmented- Obtained by using the average voltage of any two points on skin as ground (neg
pole) and reading from the third electrode (pos pole.)
di h h i
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Cardiac Arrhythmias
Tachycardia: abnormally fastheart rate
Bradycardia: Abnormally
slow heart rate
Incomplete AtrioventricularBlock: Prolonged P-R interval
(1st degree)
Complete Atrioventricular
Block: P waves and QRScomplexes become
dissociated (3rd degree)
Fibrillation: Complete lack of
coordination
No P waves
Arrhythmia: conduction failure at AV node
No pumping action occurs
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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 increasedchronotropy?
Hypokalemia:
Lowers RMP (makes itmore negative)
Decreases heart rate
U waves