7.7 EKG Flashcards
Firing Rates:
SA
AV
Bundle of His
SA: 60-100 action potentials per minute
AV: 40-60 beats per minute
20-40 per minute
Where do you see the SA node electrical activity on EKG?
You don’t! It’s very small.
P Wave
Depolarization, contraction of both atria
Ventricular Depolarization
QRS Complex
Ventricular Contraction on an EKG
Begins with the QRS, ends with the T
Spans depolarization and repolarization
What does a long QT interval indicate?
Warns that the patient is vulnerable to irregular rhythms
Normal intervals of:
- PR
- QRS
- QT
PR: <0.4 seconds
What is the purpose of limb leads?
Record the activity in the coronal or frontal plane of the body
How do limb leads work? What type of configuration?
- Bipolar configurations
- One positive pole electrode and one negative pole electrode such as leads I, II and III.
- Energy moves from the negative electrode toward the positive electrode.
What is the purpose of chest leads?
Record activity in the axial (horizontal) plane of the body
Where are chest leads placed?
V1&2: 4th ICS, either side of sternum V4: 5th ICS MCL V3: Between V2 and V4 V6: 5th ICS: Mitral area V5: 5th ICS between V4 and V6
Meaning of positively deflected v negatively deflected
- Electrical activity moving from negative electrode to positive electrode = positively deflected
- Electrical activity moving away from positive electrode = negatively deflected
ECG paper: How fast does it roll out?
25mm or 1” per second
How large is each small square on ECG graph paper? How much time does this represent?
1mm, 0.04 seconds
On ECG paper, each large box is made up of how many small boxes? Each large box represents how much time?
five small boxes, representing 0.20 seconds.
Regular heart rate
Bradycardia
Tachycardia
Regular: 60-100
Bradycardia 100
How do you calculate rate?
What numbers do you need to know to do this (6)?
Count R-R interval and large boxes.
300 - 150 - 100 - 75 - 60 - 50
What is heart rhythm?
- The quality of timing as one heart beat is compared to the next, regardless of rate
- Determined by comparing the length of several adjacent R-R intervals
If all R-R intervals are of equal length, you would say that the rhythm is _____
REGULAR
If R-R intervals are of different lengths but there’s an overall pattern pressent
Regularly irregular
If there is no overall R-R pattern
Irregularly Irregular
What causes a premature atrial beat?
Atrial focci becomes irritable.
- Due to sympathetic stimulation (adrenergic substances like adrenaline)
- Beta 1 receptor stimulants (Caffeine, cocaine, amphetamines, excess digitalis, hyperthyroidism
Two things to note about the ECG when observing a premature atrial beat.
- The pause following the PAC is longer than the normal P-P interval, but shorter than twice the normal P-P interval.
- Note the different shape of the P-P wave.
What happens during atrial flutter? (2)
In atrial flutter, a single strong ectopic focus in an atria start to beat fast (240-360 beats per minute)
**The AV node acts as the gatekeeper, blocking some of the impulses to the ventricles.
What happens during atrial fibrillation? (3)
- Many weak ectopic foci in the atria beat in an uncoordinated pattern, resulting in an uneven baseline of many tiny P waves.
- Eventually, the ventricles recieve enough electrical stimulation to contract, or contract on their own.
- IRREGULARLY IRREGULAR.
Supraventricular Tachycardia
- What happens?
- What’s the HR?
- Occurs when there is a fast, strong stimulus from an ectopic focus above the ventricles but below the SA node, and the heart conducts every beat.
- HR is 150-250 per minute
Premature ventricular contractions
- Ventricular focus can be made irritable by low O2 and low potassium
- Irritable ventricular focus causes a premature ventricular contraction (PVC), which originates suddenly and produces a giant ventricular complex on the EKG.
What are some things that can cause low O2? (6)
Airway obstruction, absence of air, low O2 content, reduced cardiac output, poor to absent coronary blood supply due to infarction or insufficiency.
What causes ventricular tachycardia?
One strong ventricular ectopic focus that hijacks the conduction system of the heart (not sustainable)
What is ventricular fibrilation?
The beating of many weak ectopic foci in the ventricles, resulting in uncoordinated contractions.
(Cannot circulate blood, not compatible with life.)
Four Conduction Abnormalitites
1) First-degree Atrioventricular Block
2) Second-degree Atrioventricular Block, Type I
3) Second-degree Atrioventricular Block, Type II
4) Third-degree Atrioventricular Block
A First-degree Atrioventricular Block does what to the ECG reading?
Makes the PR interval >0.20
What happens with a First degree AV Block?
- The impulse within the AV node is delayed, making a longer-than normal pause before ventricular stimulation (PR interval > 0.20)
- Prolongation consistent with each cycle
What happens with a second degree AV block (Mobitz Type I Wenckebach)
Progressively longer PR duration until non-conducted PR
going, going, gone
What happens with a second degree AV block (Mobitz Type II Wenckebach)
- Consistently normal PR interval, but then a normal punctual P wave with no QRS response
- Can be a 2:1 or 3:1 mobitz block (happens every 2 or 3 P Waves)
What happens with a complete / 3rd degree block
No relationship between the P Waves and the QRS complexes (AV node is completely blocked)
Sympathetic / Norepinephrine:
- Activate what kind of receptors
- What response occurs (3?)
B1 adrenergic receptors in the heart
Elicit an excitatory response:
- Increase SA node-pacing
- Increase force of myocardial contraction
- Constricts arteries – increases BP
Parasympathetic / acetylcholine:
- Activate what kind of receptors
- What response occurs (3)?
Cholinergic receptors
Inhibitory effect:
- Decrease SA node pacing
- Decrease force of contraction
- Dilates arteries – decreases BP 35
