EKG-Session 1 Flashcards
Contractility
ability of heart muscle fibers to shorten, generate force, and pump blood
Automaticity
ability of heart tissue to independently and rhythmically generate electrical impulses
Conductivity
distribution of electrical impulses throughout the heart via a conduction system
Excitability (irritability)
ability of cardiac cells to respond to external stimuli: chemical, mechanical, or electrical
Polarized (resting) state
Interior of the myocyte is negatively charged
Depolarization
Interior of the myocyte becomes positive
Influx of Na+ ions
Results in myocardial contraction
Repolarization
Recovery phase
Interior of the myocyte returns to negative charge
Accomplished by transfer of potassium (K+) ions out of the myocytes
What is an absolute refractory period?
- QRS to peak of T wave
- Myocardial cell will NOT respond to further stimulation
What is relative refractory period?
- Downslope of T wave
- Some cardiac cells depolarized; can be stimulated to depolarize with a stronger-than-normal stimulus
What is a supernormal period?
Weaker-than-normal stimulus can depolarize
Explain the principle of automaticity.
Each structure within the conduction system:
- Conducts electrical activity at a specific velocity
- Can initiate electrical activity independently at a characteristic discharge rate
- Under appropriate conditions, each structure could act as a pacemaker
Explain the concept of Overdrive suppression
Suppression of the automaticity and independent depolarization of cells with pacemaker potential by tissues firing at a higher intrinsic rate of automaticity.
Overdrive suppression explains why the SA node normally functions as the pacemaker of the heart, despite the presence of other tissues with automaticity
What is the inherent rate of spontaneous depolarization of the Sino-Atrial (SA) node?
60-100min
What is the inherent rate of spontaneous depolarization of the Atrial Cells?
60-75/min
What is the inherent rate of spontaneous depolarization of the Atrioventricular node?
40-60/min
What is the inherent rate of spontaneous depolarization of the Ventricular cells?
30-45/min
What does the horizontal axis of an EKG record?
time
What does the vertical axis of an EKG record?
voltage/velocity
Fast response cardiac action potentials work through what type of channels?
Sodium
Slow response cardiac action potentials work via what types of channels?
Calcium
Where in the heart do slow response cardiac action potentials occur?
SA and AV nodes
Where in the heart do fast response cardiac action potentials occur?
normal myocardial fibers in atria, ventricles, and Purkinje fibers
Describe ‘positive deflections’ with regards to measuring velocity on an EKG.
Record impulse moving toward positive electrode
Deflections above the baseline
Describe ‘negative deflections’ with regards to measuring velocity on an EKG.
Record impulse moving away from positive electrode
Deflections below the baseline
How many limb leads are there?
6
List the 3 bipolar leads
Lead I, II, III
List the 3 Augmented limb leads
Lead aVR-Augmented voltage (right)
Lead aVL-Augmented voltage (left)
Lead aVF-Augmented voltage (foot)
Describe chest lead placement for V1
4th intercostal space, right sternal border
Describe chest lead placement for V2
4th intercostal space, left sternal border
Describe chest lead placement for V3
Midway between V2 and V4
Describe chest lead placement for V4
5th left intercostal space, mid-clavicular line
Describe chest lead placement for V5
Midway between V4 and V6
Describe chest lead placement for V6
5th left intercostal space, mid-axillary line
Atrial depolarization
Represented by P wave on EKG strip
Results in atrial contraction
(SA node firing, wave spreading through atrium, results in atrial contraction)
Atrial repolarization
Lost in QRS complex on EKG strip
QRS Complex
Ventricular depolarization
(Impulse travels from AV node through bundle branches and Purkinje fibers, Results in ventricular contraction
Q Wave
First negative deflection after the P wave
R Wave
First positive deflection after the P wave
S Wave
Negative deflection after an R wave
T Wave
Electrical deflection following QRS complex Normally positive (upward) -Represents the rapid phase of ventricular depolarization
U Wave
Follows T wave
-Perkinje repolarization
-Usually NOT present
May indicate clinically relevant pathology (HYPOKALEMIA)
What does the PR Segment represent?
Isoelectric (at the baseline)
End of P wave to beginning of QRS complex
Represents delay of electrical impulse through the AV node (allows time for ventricles to fill with blood)
Where does the PR Interval occur on an EKG strip and what does it represent?
located from the beginning of P wave to beginning of QRS complex
Represents atrial contraction and AV node delay
Intervals are horizontal lines and therefore measurements of TIME or DURATION
What is the normal duration of a PR Interval?
0.12-0.20 seconds
Less than 5 small boxes/less than 1 big box
Varies with heart rate
Where does the QRS interval occur on an EKG strip and what does it represent?
Beginning of Q to end of S
Represents ventricular depolarization
Very Rapid
What is the normal duration of a QRS interval?
Less than or equal to 0.12 seconds (3 small boxes)
Where does the ST segment occur on an EKG strip and what does it represent?
From the end of the QRS to the start of the T wave
Represents initial phase of ventricular repolarization
Normally isoelectric
Elevation or depression often clinically significant
Where does the QT Interval occur on an EKG strip and what does it represent?
From the start of the QRS complex to the end of the T wave
Represents ventricular depolarization/repolarization
What is the normal duration of a QT interval?
Should be 0.30-0.44 seconds
Should be less than half of the R-R interval
Corrected for rate (QTc)
Where does the R-R interval occur on an EKG strip and what is its significance?
Distance from one QRS complex to next
Used to calculate rate
