Exam 2 – Cardio Ch 12 Flashcards
What do you use to calculate the electrical axis of the heart?
Lead I, II and III
What is the electrical axis of the heart based on?
Direction and voltage of tracing
What is the electrical axis of the heart used to determine?
Determine potential diagnoses for problems in people
Principles of Vectorial Analysis of EKG’s
The current in the heart flows from the area of depolarization to the polarized areas, and the electrical potential can be represented by a vector, with the arrowhead pointing in the positive direction
What is the length of the vector proportional to?
The voltage of the potential by convention
What does the instantaneous mean vector represent?
Generated potential at any instance
What is the normal mean QRS vector?
59 degrees (in people)
Define mean QRS vector
Average direction of vector during spread of depolarization wave through ventricles
What is the mean vector go through?
The partially depolarized heart
Look at axes of three bipolar and augmented leads
Look at axes of three bipolar and augmented leads
What is the mean electrical axis averaged over the entire period of depolarization?
+59 degrees
What is done with any area that is negative when determining voltage?
Any area that is negative is subtracted from the positive area
Where do you draw a perpendicular line? (mean electrical axis)
Draw perpendicular line from end of each vector, and the intersection defines mean electrical axis
In T wave (ventricular repolarization), what is the first area to repolarize?
Near the apex of heart
last areas to depolarize are first to repolarize
In T wave (ventricular repolarization), what is the normal net charge?
Normally tends to have a net positive charge in people
positive, negative, or biphasic in dogs and cats, mostly negative in horses and cattle
Where does atrial depolarization (P-wave) begin at?
Sinus node
Where does atrial depolarization spread towards?
AV node
What is the charge of the vector in atrial depolarization?
Positive vector in lead I, II and III
Can you see the atrial repolarization? (atrial T wave)
No
Why can you not see the atrial repolarization wave?
It is covered by the QRS complex
Atrial depolarization is slower than in ventricles, so first area to depolarize is also first to repolarize
If you could see the atrial repolarization wave, what would the charge of the vector be?
Negative in all 3 leads
goes from negative to positive
Why is atrial depolarization slower than in ventricles?
Atria have no purkinje system for fast conduction of depolarization signal
What does a vectorcardiogram do?
Trace vectors throughout cardiac cycle
When is the vector the largest in a vectorcardiogram?
When half of the ventricle is depolarized
What is the zero reference point?
Phase 5
When heart is fully depolarized
Is a vectorcardiogram a continuous loop?
Yes. It outlines all areas that vector went throughout the heart
What do some veterinary cardiologists use?
Cardiograms
Look at plot of mean electrical axis of heart from two electrocardiographic leads
Look at plot of mean electrical axis of heart from two electrocardiographic leads
What are factors that cause electrical axis deviations?
Changes in heart position
hypertrophy of left or right ventricle
What causes a left shift in heart position?
Expiration Lying down (abdominal contents press upward against diaphragm) Excess abdominal fat (diaphragm normally press upward against heart all the time)
What causes a right shirt in heart position?
End of deep inspiration
Standing up
Tall people
What causes hypertrophy of left ventricle?
Hypertension (high arterial blood pressure)
Aortic stenosis
Aortic regurgitation causes slightly prolonged QRS (wide) and high voltage
What type of shirt is hypertrophy of left ventricle?
Left axis shift
What causes hypertrophy of right ventricle?
Pulmonary hypertension Pulmonary valve stenosis
Interventricular septal defect
All cause slightly prolonged QRS and high voltage
What type of shirt is hypertrophy of right ventricle?
Right axis shift
How does mild ischemia cause a T wave abnormality?
Mild ischemia in particularly in the apex of the heart in people prevents apex from repolarizing first
T wave inverts
What is a T wave abnormality?
Becomes abnormal when normal sequence of repolarizatoin does not occur
How does digitalis toxicity cause a T wave abnormality?
Prolongs depolarization in certain parts of the heart and can cause a biphasic T wave
How does digitalis toxicity relate to heart failure?
Digitalis binds to sodium and potassium receptors to slow calcium from leaving the cells
Increased Ca results in stronger heartbeat
How does arrhythmias relate to digitalis toxicity?
Slows the signals that start the SA node
Fewer signals, fewer arrhythmias
What is a biphasic T wave?
Positive and negative deflection
What is a inotropic cardiac drug propertiy?
Affects strength of contraction
Mostly positive inotropic effect
Stronger contractions
What is a chronotropic drug property?
Affects rate of contraction
can have positive and negative chronotropic effects
Pacekeeping effects
What is considered to be a high voltage EKG?
If sum of voltages of Leads I-III is greater than 4 mV
What causes a high voltage EKG?
Most often causes by increased ventricular muscle mass (hypertension, marathon runner)
What causes a prolonged QRS complex?
Cause is prolonged conduction of cardiac impulse through ventricles
cardiac hypertrophy
purkinje system block
How does a prolonged QRS complex relate to a normal one?
Prolonged is wider
What does it mean if a QRS exceeds normal?
Usual cause is conduction block
What is a normal QRS time?
0.12 seconds in humans
What is current of injury?
Current flows between pathologically depolarized and normally polarized areas, even between heartbeats
What does damaged cardiac muscle remain?
Partially or totally depolarized all the times
What does injured muscle emit?
Negative charges throughout each heart beat stays polarized/does not depolarize and contract
What are causes of current of injury?
Local ischemia
Mechanical trauma
Infection
How does local ischemia cause current of injury?
Caused by local coronary occlusions
Not enough nutrients available
How does mechanical trauma cause current of injury?
Sometimes makes membranes so permeable that full repolarization cannot take place
How does infection cause current of injury?
Damage muscle membranes
What is the most common cause of current of injury?
Local ischemia
What are causes of decreased voltages in standard bipolar limb leads?
Cardiac muscle abnormalities
Conditions surrounding heart
Anterior-posterior rotation of apex of heart
What are some cardiac muscle abnormalities that cause decreased voltages in standard bipolar limb leads?
Old infarcts causing decreased muscle mass
Low voltage EKG
Prolonged QRS
What are some conditions surrounding heart that cause decrease voltages in standard bipolar limb leads?
Fluid in pericardium
Pleural effusions
Emphysema
