Cardio Review and Intro to ECGs

Special Properties of Cardiac Cells: Excitable

• Each cardiac cell has a resting potential to “fire” or depolarize

  • Purpose: For the cell to contract

Special Properties of Cardiac Cells: Conductive

• Activated cardiac cells can stimulate adjacent cells to contract

  • Purpose: For the heart to pump as a unit

Special Properties of Cardiac Cells: Automatic

• Certain cells have capacity to “fire” spontaneously with an inherant rhythm without external stimulus

  • Purpose: Pacemaker cells

Special Properties of Cardiac Cells: Refractory

• Cells cannot be stimulated to depolarize during rest phase

  • Purpose: allows heart chambers to rest/refill

Cardiac innervation - What do the nerves innervating the heart control?

1. Heart rate and blood pressure

2. Conduction of electrical impulses through the heart muscle

3. Myocardial contraction

Cardiac Innervation: Parasympathetic Nervous System

• Works to slow the heart rate

• Vagus Nerve

Cardiac Innervation: Sympathetic Nervous System

• Works to increase heart rate and blood pressure

• Norepinephrine

Cardiac Action Potential

• Rapid sequence of changes in the voltage across a cell membrane

• The membrane voltage (or potential) is determined by the relative ratio of extracellular to intracellular ions

• Depolarization and Repolarization of the cell

  

How electrical impulses travel thorugh the heart

SA Node

• Stimulates atrial contraction (main pacemaker)

• Sends Impulse to AV Node

AV Node

• Slows conduction, allowing atria to fully contract

• Controls length of diastole

Bundle of His

• Signals beginning of systole

R/L Bundle Branches

Purkinje Fibers

Cardiac Action Potential - Photo

6 Ways to evaluate heart function

1. Pulse rate and quality

2. Heart rate and rhythm

3. Heart sounds

4. Mucous membrane color

5. Capillary refill time (CRT)

6. Electrocardiogram

What does and ECG tell us?

• Only tells us about the electrical function of the heart

• It does not tell us anything about how the heart is physically functioning.

• Electrical activity can be present without a physical heartbeat.

What we learn from ECG’s

• Heart rate and rhythm

• Chamber enlargement

• Visual representation of how the electrical impulses move through the heart

• Can help determine best course of treatment

ECG Indications

• Evaluation of arrythmias and heart rate

• History of syncope, weakness, or trauma

• Pre-anesthetic workup

• Cardiac monitoring during anesthesia

• Critical illness

• Concurrent drug administration

Lead Polarity

• Patient attached to the ECG machine by 4 cables with clips

• Lead selection knob on the ECG machine causes 2 of the 4 cables to become charged

• One limb becomes the positive pole

• A different limb become the negative pole

• Run a standard 6 lead ECG

• 6 different viewa of heart’s electrical activity

• Each lead reads different on machine

• Lead II is most important for evaluation and interpretation

Lead Polarity: Different leads

• Change in leads = change in polarity of electrodes

• Changes appearance and amplitude of the ECG waves

• DIfferences help with diagnosis

Lead Polarity: Limb Leads

• Lead I: RA 9-) to LA (+)

• Lead II RA (-) to LL (+)

• Lead III: LA (-) to LL (+)

Lead Polarity

Augmented leads

• aVR: ½ between LA-LL(-) to RA(+)

• aVL: ½ between RA-LL(-) to LA(+)

• aVF: ½ between RA-LA(-) to LL(+)

Einthoven’s Triangle

• Leads I-III form an inverted triangle around the heart

• Each lead = heart at another angle

• Electrical impulses traveling toward the “+” pole = positive deflection on ECG

  • Moving away = “-” deflection

    

When electrical impulses move toward the positive pole of a lead…

An upward deflection is recorded

When electrical impulses move toward the negative pole of a lead…

A downward deflection is recorded

When an electrical impulse moves perpendicular to the lead…

No deflection is recorded

When there is an absence of electrical activity…

there is no deflection recorded

The magnitude of the deflection is…

proportional to the mass of the muscle

Patient positioning

• Right lateral recumbency

  • Decreases muscle tremor artifact

  • Alters the amplitude of PRS waves

  • Shifts mean electrical axis to the right relative to standing Legs 90 degrees to the body

  • Clips should be attached near the elbows, stifles

  • Clips should not touch each other or the table (metal)

  • Patient and restrainer must remain still

ECG Cable Placement

White - Right FL

Green - Right HL

Black - Left FL

Red - Left HL

What is Happening in the Heart - P Wave

• Atrial Depolarization

• Deflection of ECG - Small positive deflection

What is Happening in the Heart - PR Segment

• Pause at the AV node

• Deflection of ECG - no deflection

What is Happening in the Heart - PR Interval

• Atrial contraction and pause at AV node

• Deflection of ECG - Small positive deflection (P wave) and no deflection (PR segment)

• What is Happening in the Heart - QRS Complex

• Ventricular depolarization and atrial repolarization (hidden)

• Deflection of ECG - Small negative deflection, followed by large positive deflection, and small negative deflection

• What is Happening in the Heart - St Segment

• Pause between ventricular depolarization and repolarization

• Deflection of ECG - No deflection

• What is Happening in the Heart - T wave

• Ventricular repolarization

• Deflection of ECG - small positive deflection

• What is Happening in the Heart - QT interval

• Ventricular depolarization

• Small positive deflection

• What is Happening in the Heart - QT interval

• Ventricular depolarization and repolarization

• Deflection of ECG - Small negative, then large positive, small negative, no deflection, sm. +

ECG Waveforms

U Wave?

• Small positive deflection after the T wave

• Difficult to identify

• It is thought to represent repolarization of the Purkinje fibers, but its exact source is unknown

  

Paper speed and amplitude

• Amplitude is measured on the vertical axis in millivolts (mV)

• Time is measured on the horizontal axis in seconds (s)

• Each small box measures 1 mm by 1 mm

• Each large box measures 5 mm by 5 mm

• 
  

  

• Used to calculate heart rate and durations

• Two Paper Speeds

  • 25 mm/sec

    • 1 mm= 0.04 sec

    • Standard speed

  • 50 mm/sec

    • 1 mm = 0.02 sec

    • Best for arrhythmias

Paper Speed 25 mm/sec

1500 small boxes/min

300 large boxes/min

Paper speed 50 mm/sec

3000 small boxes/min

600 large boxes/min

Heart rate calculation methods

Calibration

Calibration is used to calculate the amplitude (voltage) of the waves on an ECG tracing

Calibration Settings

• 5 mm/mV - often used for large animals (horses, cattle) due to large complex size

• 10 mm/mV - Often used for dogs

• 20 mm/mV - Often used for cats due to small complex size

Calibration Box

• The calibration box should print out at the beginning of every ECG tracing

• You can calculate the calibration from the calibration box

• Most ECGs will also have the calibration printed on the bottom of the paper

ECG Interpretation - Time

• Time (Duration) - Measured on horizontal axis (seconds)

  • This is the time it takes for something to happen

  • Atria to contract, pause at the AV node, Ventricles to contract/atria repolarization, ventricular repolarization

• Paper speed affects only horizontal measurements

ECG Interpretation (Amplitude)

• Amplitude (Mass) - Measured on vertical axis (height of waves) (milivolts)

• Calibration affects only vertical measurements

Time interpretation

Abnormally long time means

• One or more chambers is/are enlarged

• ± faculty electrical conduction

Shorter (faster) than normal times mean

• SA node is being driven to contract faster by sympathetic nervous system (damaged heat tissue/muscle cells) usually in the ventricles

Amplitude Interpretation

• Amplitude of ECG waveform indicates the amount of muscle mass present

  • Amplitude of P wave = size of atrial mass

  • Amplitude of QRS complex = size of ventricular mass

Mass Interpretation

Larger than normal amplitudes mean

• The corresponding muscle mass is larger than normal

  • P wave = atrial enlaregment

  • QRS complex = ventricular enlargement

Smaller than normal amplitudes mean

• The corresponding muscle mass is smaller than normal

Abnormal Examples

P wave

• too tall = right atrial enlargement

• too wide = left atrial enlargement

PR interval

• too short = tachycardia from drugs, fear, or ectopic beat

• too long = increased vagal tone, AV block (1st, 2nd, or 3rd degree)

QRS complex

• too tall = left ventricular enlargement (LV has more mass than RV)

• too wide = ventricular enlargement, bundle branch block, or ectopic beat

• Deep S wave = right ventricular enlargement

ST segment

• Evaluation or depression from baseline may include myocardial ischemia