Chapter 4 : Vectors and the Basic Beat Flashcards
What is the sum of all ventricular vectors?
Electrical Axis
What do vectors represent?
Energy and direction
What are electrodes?
Sensing devices that pick up the electrical activity occurring beneath them
When a positive electrical impulse is moving away from the electrode the ECG machine converts it into a what?
Negative Wave
When a positive electrical impulse is towards from the electrode the ECG machine converts it into a what?
Positive Wave
What is the position for leads V1 and V2?
Each side of the sternum at fourth intercostal space
What is the position for lead V4?
Fifth intercostal space in the midclavicular line
What is the six limb leads? (Hexaxial system)
I, II, III, aVR, aVF, aVL
What is the lead aVR?
The augmented unipolar right arm lead faces the heart from the right side and is usually orientated to the cavity of the heart. Therefore, all the deflections P, QRS and T are normally negative in this lead
What is the lead aVF?
The augmented unipolar left leg lead, orientated to the inferior surface of the heart Precordial Chest Leads
What is the lead aVL?
The augmented unipolar left arm lead faces the heart from the left side and is orientated to the anterolateral surface of the left ventricle
How many degrees apart are all the leads using the haxaxial system?
30 degrees
The six limb leads view the heart at what angle?
Coronal
The six precordial leads view the heart at what angle?
Transverse
The Hexaxial System
What is the normal duration of the P wave?
0.08 to 0.11 seconds
What does the P wave represent?
Electrical depolarization of the atria
The axis of the P wave is usually directed in what direction?
Downward and to the left
What does the TP wave represent?
Repolarization of the atria
It deflects in the opposite direction of the P wave but is usually not seen because it occurs at the same time as the QRS wave.
If the PR segment is depressed more than 0.8mm it could be a sign of what?
Pericarditis or atrial infarct
What happens in the PR segment?
The initiation of electrical impulse in the SA node up to the moment of ventricular depolarization
What is the normal duration of the PR segment?
0.12 - 0.20 seconds (3 to 5 little boxes)
What does the QRS complex represent?
Ventricular depolarization
The Q wave can be present or absent
Q wave significance
Can be benign, or it can be a sign of dead myocardial tissue. The Q wave is considered significant if it is 0.03 seconds or wider, or its height is equal or greater than 1/3 the height of the R wave.
If it meets that criteria it indicates a Myocardial Infarction (MI)
What happens during the R wave peak time?
The amount of time it takes the electrical impulse to travel from the purkinje system in the endocardium to the surface of the epicardium immediately under an electrode.
How long is the R wave peak time?
Up to 0.035 seconds in the right precordial leads (v1 and v2) because the right ventricle is thin compared to the left.
0.045 seconds in the left precordial leads (v5 and v6) because of the left ventricles thickness
ST elevations?
It can vary up to 1mm from baseline in limb leads and 3mm in precordials.
Any ST elevation in a symptomatic patient should be considered significant and representative of myocardial injury or infarction.
What does the T wave represent?
Ventricular repolarization
Should the T wave be asymmetric or symmetric?
Asymmetric
Why is the T wave in the same direction as the QRS if it represents repolarization?
The increased pressure on the endocardium during contraction, the repolarization wave travels in the opposite direction from the epicardium back to the endocardium.
What does QTc stand for? And what is its purpose?
QT corrected interval
It is corrected for heart rate. As the heart rate decreases, the QT interval lengthens. And as the heart rate increases the QT interval shortens.
The QTc is less than 0.45 in males and 0.46 in females. Anything above this is considered prolonged. Considered at risk of lethal arrhythmias.
