Electrocardiogram and cardiac function tests Flashcards
How does an echocardiogram generate an image of the heart?
A transducer sendsd a pulse of ultrasound into the tissue. the sound reflects and echoes off a part of the tissue. the amount of time it takes for the sound to echo back reveals the depth of the tissue. the computer than takes this information and generates an image.
What additional information can doppler echocardiography give you?
speed and direction of blood flow
How can you tell the direction of blood flow on an echo?
it uses the doppler effect - the apparent chance in the frequency of a wave caused by relative motion between the source of the wave and the observer - wave frequency increases with blood coming towards the probe, decreases with blood going away from the probe
What do the red and blue colors represent as far as blood direction?
red - blood moving toward the prove
blue - blood moving away from the probe
What is coronary angiography used for?
visualization of the interior heart chambers and blood vessels - looking for a blackage or narrowing of the lumen
you inject radio-opaque contrast into the vessel and use fluoroscopy to generate a video fo the vessels filling
What does radionuclide ventriculography (multigated acquisition scan = MUGA) use to view the heart pumping blood?
IV injeciton of a radioactive isotope like technetium to label RBCs.
then take pictures with a gamma camera
What is the Fick Principle?
it calculates the rate at which a substance is being added to or removed from the blood as it passes through an organ.
What is the equation for the fick principle if you’re solving for flow, or cardiac output?
Flow = Xtc (the amount of substance that crossed the capillaries) divided by (Xarterial - Xvenous)
Q = Xtc / ([X]a – [X]v)
How do you check a cardiac index?
You use a nomogram to determine the body surface area from height and weight
CI = CO/body surface area
What is the normal conduction time through the atria and AV node?
120-200 msec
What is the normal length of time for ventricular depolarization (QRS)?
60-100 msec
What is the normal length of a duration of ventricular systole (QT interval)?
less than 380 msec at normal heart rate
On EKG paper, 1 small block is worth how many seconds? 1 large box? 5 large boxes?
1 small = 0.04 sec
1 large = 0.2 sec
5 large = 1 sec
What’s the easiest way to determin heart rate on EKG paper?
count how mnay QRS complexes there are in 6 seconds (2 of the seconds marked by the 3 seconds ticks) and multiple by 10
What are the three EKG regions of no voltage, or isoelectric regions?
- end of the PR interval
- ST segment
- Between the T wave and the next P wave
Why is there no voltage at the end of of the PR interval?
- depolarization wave reaches the non-muscular border between the atria and ventricles - no voltage
- atrial muscle cells are completely depolarized and in phase 2 (no voltage change)
- the ventricular cells are still resting
(note the electrical field created during passage thorugh the AV node is too small to see)
WHy is there no voltage during the ST segment?
- you don’t have any rapid changes in the membrane potential anywhere - atrial cells are resting now and ventricular cells are in the stage 2 plateau phase
Why is there no voltage between the T wave and the P wave
there’s zero conduction happening anywhere - just waiting for the next depolarization in the SA node
What is Einthoven’s Triangle?
a hypothetical triangle created around the heart when electrodes are placed on both arms and the left leg
What is the EKG trace recording?
the the net dipole of voltage differences measured between any given two points on the Einthoven triangle
What are the leads between the two arms and left leg called?
bipolar limb leads
Why bipolar?
Because one end is a negative node and one is a positive node.
Right arm - to left leg +
left arm - to left leg +
right arm - to left arm +
The magnitude of the dipole is determined by what two things?
- how many cells are depolarizing
- consistency of individual dipole orientation (same or opposite)
Direction of the deflection on the KEG tracing tells us what?
the path the depolarizing wave front is taking at that instnat, so that an upward deflection occurs when the positive wave front heads toward the positive electrode on the foot and downward deflection occurs when the positive wave is moving away from the positive electrode.
Charge to like electrode = positive deflection
Based on this, why is the QRS so deflection so positive?
it’s ventricular depolarizaiton moving towards the positive node on the left leg.
Why is ventricular repolarization represented as a positive deflectoin T wave? Shouldn’t it be negative?
You would think it would be negative as a negative repolarization travels down the ventricles, but that’s actually not what happens. The last ventricular cells to depolarize are the FIRST to repolarize, so it’s a negative wave traveling towards the negative node on the arm, hence a positive deflection. we don’t know why
What is the mean electrical axis? What do we use it for?
the orientation of the cardiac dipole during the most intense phase of ventricular depolarization (during the R wave)
We use it as a clinical indicator of whether depolarization is proceeding over normal pathways
How is mean electrical axis reported?
in degrees according to the axial reference system convention
Describe the axial reference system convention.
If you take the leads in the triangle, superimpose them on a wheel with three spokes, lead I becomes O degrees, lead II is 60 degrees and lead III is 120 degrees (in one version - it depends on which one you use)
What is considered normal deviation? Left axis deviation?
Right axis deviation?
normal = 0 to 90 degrees (although some say -30 to 90 degrees)
left axis deviation = less than 0 degrees
right axis deviation = more than 90 degrees
What can cause a left axis deviation?
physical displacement of the heart to the left
left ventricular hypertrophy
loss of electrical activity to the right ventricle.
What can cause a right axis deviation?
physical displacement of the heart to the right
right ventricular hypertrophy
loss of electrical activity to the left ventricle
What is the first way you can calculate mean electrical axis?
- find the lead with the largest R wave
- on the axial reference system, find that lead’s dipole and follow it
- If the R wave is positive (like usual), the mean electrical axis IS that dipole. If it’s negative, then the dipole is in the opposite direction
How do you calculate mean electrical axis with vectorcardiography?
you graphically represent all the net dipolar amplitudes thorughout the entire length of the cardiac cycle, tracing it around the axial reference system
this ends up looking like a series of loops resembling a heart!
You take the longest loop and draw in the dipole. this is the mean electrical axis.
What are augmented leads?
they’re unipolar limb leads - one positive electrode is references against a combination of other electrodes - they are aVR (right arm), aVL (left arm) and aVF (left foot)
How are the augmented leads added to the axial reference system?
Stick the center of the reference system in the middle of the patient’s chest
aVR is at -150 degrees
aVL is at -30 degrees
aVF is at 90 degrees
How do you calculate the mean electrical axis with the equiphasic approach?
- determine which lead contains the most equiphasic QRS complex (same amount positive as negative deflection)
- determine which lead lies 90 degrees away from the most equiphasic lead and look at the tracin
- If the QRS complex is predominantly positive, then the direciton of this lead is approximately the QRS axis
If the QRS complex is preodminantly negative, then the QRS axis is 180 degrees away from the direction of this lead
What does a 12-lead ECG allow for?
the standard 6 lead allows you to look at conduction in a single plane
by adding 6 more chest unipolar leads, you can also look at activity in the transverse plane
What are the 6 unipolar chest peads designated as?
V1 (most midline) to V6 (most lateral)
