Unit 2: Overview of Cardiac Conduction System Flashcards
Automaticity
allows the cardiac cells to generate an impulse
Sinoatrial Node (SA)
pacemaker of the heart
-60 to 100 bpm
Atrioventricular Node (AV)
40 to 60 bpm
- the gatekeeper of the hearts electrical conduction system
- responsible for allowing impulses to travel between the atria and the ventricles
- if SA node fails, the AV node can generate impulses at a rate of 40 to 60 bpm
Ventricular Cells (Left and Right bundles of HIS, the Purkinje fibers, and Ventricles)
20-40 bpm
Excitability
the ability to respond to a stimulus and generate an impulse
Conductivity
allows cardiac tissue to transmit the impulses to neighboring connected cells
Cardiac Action Potential
a process in which the membrane potential, the difference in charge between the inferior and exterior of the cell, changes or goes up and down in a consistent pattern
-a stimulus begins the movement that produces the cardiac action potential
Depolarization
the movement of ions proceeding and facilitating cardiac mechanical contraction
Repolarization
the movement of ions back to the resting state
- the cardiac resting membrane potential of -90 mV, to allow for the initiation of another action potential
- 5 phases of the action potential
Absolute refractory (means resistant) period
cells are unresponsive (during and after depolarization)
Refractory Period
a greater than normal stimulus my initiate an impulse
Electrocardiogram (ECG)
graphic representation of the hearts electrical activity
- has 12 leads/views of the electrical activity in the heart
- each lead has a (+) and (-) electrode or pole; electrical impulse is measured as it moves between 2 poles/points
- electrical impulse moves towards a (+) pole, the waveform will be upright on the ECG
- if the electrical impulse is moving away from the (+) pole it will produce a downward or negative (-) deflection of the ECG
- best lead to identify the hearts rhythm is lead II (2)
- Lead II: mimics the hearts natural electrical direction in a healthy, normal positioned heart
What is the best lead to identify or interpret the hearts rhythm?
lead II
Waveforms on a ECK/EKG
- P wave
- QRS complex
- T wave
- U wave
P wave
first wave that is normally seen
- represents the SA node sending out an electrical impulse
- atrial depolarization
- should be upright (positive deflection) with a rounded top in lead II
- not be longer than 0.10 sec
- no higher than 2.5 mm
QRS Complex
2nd waveform
- ventricular depolarization
- normal: pointy and skinny in width
- wide QRS: (greater than 0.10 sec or two and a half small boxes wide) means there is something occurring in the ventricles causing the QRS to widen
ex: may indicate that the impulse originated in the ventricles or that there is a block in the ventricles delaying the impulse from traveling through the ventricles to allow for normal depolarization
T wave
after QRS complex
- ventricular repolarization
- wave should be upright and rounded in lead II
- helpful in monitoring electrolyte imbalances (hypo/hyperkalemia), disturbances with myocardial oxygen supply, or other cardiac disorders (pericarditis or ventricular aneurysm)
U wave
rarely seen
- represents purkinje fiber repolarization
- can be seen w/ certain medication toxicities (digoxin) an electrolyte imbalances (hypokalemia)
- small rounded wave in lead II
- can be confused with P wave
Intervals in a ECG/EKG
- PR Interval
- QRS Interval
- QT Interval
PR Interval
from the beginning of the P wave to the beginning of the QRS complex
- reflects time it takes an electrical impulse to depolarize the atria and travel to the ventricles
- 0.12 to 0.20 sec (5 small boxes on ECG)
QRS Interval
measure of time to depolarize the ventricles
- measured from where the QRS leaves the baseline to where it returns to baseline
- 0.06 to 0.12 sec
- greater than 0.12 (equal to or more than 3 small boxes) may be a sign of disturbance within the ventricle itself or blockage
- atrial repolarization is “buried” in the QRS complex
QT Interval
measure of time it takes the ventricle to depolarize and repolarize
- measured from where the QRS leaves the baseline and where the T wave returns to baseline
- less than or equal to 0.52 seconds (13 small boxes)
- never more than half the distance from one QRS to the next
Steps In ECG Interpretation
Ask the following
- is the rate fast, slow, or normal?
- is the rhythm regular; are complexes an equal distance from one wave to the next?
- distances from P wave to P wave equal throughout strip? (P-P interval)
- distances from QRS wave to QRS wave equal throughout strip? (R-R interval)
- P waves present?
- QRS complexes present?
- T waves present?
- are the intervals within normal limits? (e.g. PR interval (0.12 to .20 sec), QRS interval (0.6 to 0.12 sec))
- is there a P wave before every QRS?
- is there a QRS after every P wave?
What happens if SA node and AV nodes fail to generate an impulse to stimulate the ventricles?
Ventricular pacer cells can generate impulses at a rate of 40 bpm or less
-worst case scenario, Purkinje fibers can generate a ventricular rate of 20 bpm
Normal Sinus Rhythm (NSR)
when the impulse travels along the normal conduction pathway generated by the SA node
Normal Conductive Pathway
SA Node–> AV Node–> Bundle of HIS–> Purkinje fibers
-normal sinus rhythm (NSR)
ECG: how many seconds does 1 small box indicate?
0.04 seconds
ECG: How many seconds does 5 small boxes indicate?
0.2 seconds
ECG: How many seconds does 30 large boxes represent
6 seconds
-3 lines at the top of strip
Calculating HR on ECG
- mark off 6 seconds of the strip (30 large boxes)
- count the number of QRS complexes and multiply by 10
