Lecture 7 - Exam #2 material - ECG Flashcards
In regard to a reference lead, when will a vector have the highest amplitude?
When the vector is very close to the direction of the reference lead.
When will a vector, in regard to a reference lead, have the smallest amplitude?
When the vector is perpendicular to the reference lead.
The sum of all vectors at a given instant
Instantaneous mean vector
Mean electrical axis of the heart
59*
Repolarization of the heart is in the direction of the (base/apex)
Apex
Repolarization of the ventricles begins on what region of the heart (endocardium or heart surface)?
heart surface
What region of the heart begins to repolarize first during the atrial Twave?
area around the sinus node (repolarization essentially follows depolarization)
Explain the pathophysiology of axis deviation in ventricular hypertrophy.
Larger muscle mass causes increased electrical activity. Depending on the area of hypertrophy, the mean electrical axis may shift.
Why does fluid in the pericardium cause an shift in electrical axis?
Fluid acts as an insulator. As a result, only certain currents may get through, which will shift the axis.
What is the criteria for determining a “High voltage ECG?”
When the sum of the QRS complexes in Leads I, II, and III are > 4 mV
Explain why conditions around the heart are responsible for low-voltage ECGs
Conditions (such as pleural effusion or tamponade) act to insulate the heart’s electrical activity from conduction to the skin. This results in less electron flow, and a lower ECG recording.
Normal QRS duration?
0.06 sec to 0.12 sec
Currents that flow between normal areas and damaged areas of conduction
Current of injury
The reference point in determining cardiac damage
J point
The voltage of the J-point
0 mV
A line with both magnitude and direction
vector
The J-point is an indicator of what “polarization state” of the ventricles?
Completely depolarized or completely repolarized (J-point=0mV)
Tachycardia that begins and ends suddenly
paroxysmal tachycardia
What three physiologic conditions cause “circus movements” in cardiac tissue?
1) A conduction pathway is too long
2) A conduction velocity has been lengthened
3) A refractory period has shortened
Explain why a lengthened conduction pathway may result in fibrillation.
If a conduction pathway has lengthened, the tissue where the AP originates has entered either relative refractory period or depolarization. Therefore, another AP can be fired.
Explain why a slowed conduction velocity may result in fibrillation.
If an AP has slowed, by the time it arrives back at the originating tissue, that tissue is either in a relative refractory period or a repolarized state.
Explain why a shortened refractory period may result in fibrillation.
If the refractory period is shortened the tissue is repolarized more rapidly. An arriving AP at the point of origin will trigger another action potential.
Explain by what 2 mechanisms a refractory period may be shortened.
1) Increased Na/K/ATPase activity
2) Increased K efflux
