16 analyzing ekg vectors and mea (1)

7/30/2019 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

16 analyzing ekg vectors and mea (1)

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