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Canine and Feline Electrocardiography

VTHT 1491Special Topics

Reading: VTDRG: Pg: 338-345; 205-220

CTVT: Pg: 699-702; 1179-1182; 1306 All Handout

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A little history….

• In 1790, an audience of usually sedated scientists gasped in disbelief as Luigi Galvani, with a flare of showmanship, made a dead frog’s legs dance by electrical stimuli

• Galvani knew that completing a circuit connecting dissimilar metals to the legs of a recently deceased frog would create a stimulating electrical current

• The resulting current would stimulate the frog’s legs to jump, and with repeated stimulation, he could make then dance

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History cont’d

• In 1855, Kollicker and Mueller discovered that when a motor nerve to a frog’s leg was laid over its isolated, beating heart, the leg kicked with each heartbeat

• Therefore, the same electrical stimulus that causes a frog’s leg to kick must cause the heart to beat

• Logic dictated that the beating of the heart must be due to a rhythmic discharge of electrical stimuli

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History cont’d• 1880’s - Ludwig and Waller discovered that the

heart’s rhythmic electrical stimuli could be monitored from a person’s skin

• 1887-Augustus Waller invented the electrode. This device allowed the currents to be measured through intact skin

• 1901 – Dr. William Einthoven invented the EKG machine, the PQRST system and established the 3 points on the body where electrodes are placed

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History…

• Dr. Einthoven won the Nobel peace prize for his invention

• Today, the ECG machine remains one of the single most important tools in medicine used for diagnosis, monitoring, and gauging response to therapy

• We rely on ECGs daily in ambulances and hospitals and by phone line to save the lives of thousands

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Applications of the Electrocardiogram• Acute onset of dyspnea• Shock• Fainting or seizures• Monitoring during and after surgery

(monitors depth of anesthesia as well as cardiac monitoring)

• All cardiac murmurs• Cardiomegaly (enlarged heart) that

is found on thoracic radiographs • Preoperatively

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Applications of ECG continued• Cyanosis ( Blue or Purple coloration of the skin

or M Membranes)

• Evaluating effect of cardiac drugs• Pericardiocentesis (fluid is aspirated

from the sac around the heart)

• Systemic diseases– Examples?

• Electrolyte disturbances– What electrolytes do you think may be associated with ECG

abnormalities?

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Conduction System of the Heart Review

• Electrical impulses are transmitted through the heart via specialized conduction cells in a specific sequence:– Sinoatrial (SA) node– Atrioventricular (AV) node– Bundle of His– Left and right bundle branches– Purkinje fibers

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Remember Depolarization vs Repolarization?

• Depolarization- heart muscle contraction in response to electrical stimulus. (Squeeze)– Occurs when electrolytes (Na, Ca, and K) move

across the cell membrane causing a contraction• Repolarization- heart muscle relaxation occurs

when the electrolytes move back across the cell membrane rendering the cell ready for the next electrical impulse

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Five Physiologic Properties of Cardiac Muscle

1. Automaticity– SA node is the primary pacemaker of the heart, but any

cells of conduction system can initiate their own impulses under right circumstances

– The further down in the conduction system, the slower the rate of automaticity

2. Excitability– Cardiac muscle is excited when the electrical stimulus

reduces the resting potential to the threshold potential– The degree of the resting potential within the cell

determines its excitability and obeys the “all-or-none” law.

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3. Refractoriness– Heart muscle will not respond to external stimuli

during its period of contraction (Resting period in between beats)

4. Conductivity– Activation of an individual muscle cell produces activity

in the neighboring muscle cell– Conduction velocity varies in the different portions of

the specialized conduction system and muscle fibers– Velocity is greatest in the Purkinje Fibers and least in

the mid-portion of the AV node– Activation sequence is so arranged that the maximum

mechanical efficiency is provided from each corresponding contraction

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5. Contractility– Occurs in response to electrical current– The ECG only measures the stimulus for

contraction- not the actual contraction itself

*ECG is the tool of choice for measuring contractility

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Major Confusion!!!• Reading an ECG and deciphering

what it is telling you can be confusing

• ECGs provide a graphical representation of the electrical impulses of the heart

• Electrical impulses are what make the heart contract

• This contraction is a result of depolarization and repolarization

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Electrocardiogram

• Definition: Graphic recording of electrical potentials produced by heart muscle during the different phases of the cardiac cycle

• The information recorded on the EKG represents the heart’s electrical activity

• Most of the information on the EKG represents electrical activity of contraction of the myocardium

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Electrocardiogram cont’d– The voltage

variations are produced by depolarization and repolarization of individual muscle cells

– Each portion of the electrocardiogram thus arises from a specific anatomic or physiologic area of the heart

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A Little Vocabulary

• Artifact: A wave that arises from sources other than the heart• Cardiac Arrhythmia: Any electrical activity of the heart that

differs from that of a healthy, awake patient. Can be innocuous or life-threatening

• Deflection: Movement from one side to the other• Ectopic: Occurring in an abnormal location/position• Lead: A pair of electrodes, connected by an axis that provides

a particular view of the electrical activity of the heart

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Solid Ground

• Having the ECG machine well grounded is an important factor for obtaining an accurate reading

• Placing the patient on a rubber mat and ensuring that the machine is plugged into a grounded outlet as well as checking the ECG machine for loose wires or cracked insulation on wires are steps that can be taken for a good ground

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ECG CONNECTOR AND LEADS