2 - Cardiac Electrical Physiology Flashcards
Five phases of an action potential
- Phase 0 = rapid upstroke, depolarization
- Phase 1 = early repolarization
- Phase 2 = plateau phase
- Phase 3 = rapid repolarization
- Phase 4 = resting membrane potential
Phase 0
- Rapid influx of sodium
- Fast phase due to opening of sodium channels
- Sodium ions and positive charges really want to get inside the cell, so the depolarization occurs rapidly
Phase 1
Early repolarization
- Sodium channels close, but some potassium channels open
- Repolarization is incomplete
Phase 2
Plateau
- Membrane potential is approximately ZERO (which is very important for ECGs!!)
- The plateau occurs due to slow calcium channels (which must be matched by an equal and opposite force by potassium channels - offsets the calcium flow)
- Requires concurrent movement opposite to calcium
- Allows blood to be ejected from the heart
Phase 3
Rapid repolarization
- More calcium channels are closing
- More potassium channels are opening
Phase 4
Resting membrane potential
- Only potassium chanels are open
- Resting potential is maintained until the next stimulus
Resting membrane potential of a cardiac muscle cell
Approx. -90 mV
Resting cell - Potassium
Potassium (K+) is very high inside the cell and low outside the cell
Resting cell - Sodium
Sodium (Na+) is high outside the cell and low inside the cell.
Resting cell - Calcium
Calcium (Ca++) is low outside the cell and REALLY low inside the cell.
This is advantageous for muscle contraction, which requires calcium for cross bridge cycling at the level of the actin and myosin filaments, and allows for exquisite sensitivity of the muscle to small changes in intracellular calcium concentration
What four membrane proteins are present on a generic cardiac cell?
- Na+ transporter
- Ca++ transporter
- K+ transporter
- Cation transporter
What is the role of the Na+ transporter?
Rapid, depolarizing (non-nodal)
What is the role of the Ca++ transporter?
Slow, depolarizing
What is the role of the K+ transporter?
Re-polarization
What is the role of the cation transporter?
Allows for the “funny” current to flow
This cation transporter is a hyperpolarization-activated cyclic nucleotide-gated channel (esp. cAMP)- (nodal)
“Pacemaker channels” allow positive charge to enter cell during repolarization or hyperpolarization and are closed during depolarization
Where in the heart do we find different types of action potentials?
Nodal tissue
How many phases are there of cardiac action potentials in nodal tissue?
Three
- Phase 0
- Phase 3
- Phase 4
Describe phase 0
- Once threshold potential is met, calcium channels open
- “No” fast sodium channels
- Slow response
Describe phase 3
After depolarization, potassium channels open
Describe phase 4
“Funny current” slowly depolarizes the cell
- Progressive reduction in potassium efflux
- Progressive increase in calcium influx
What is the sequence of depolarization within the heart?
1 - SA node 2 - Atria 3 - AV node 4 - Bundle of His 5 - Bundle branches 6 - Purkinje fibers 7 - Ventricles (septum, apex, ventricular free walls)
What does the conduction velocity of a certain part of the heart tell you?
It is related to how fast an action potential can spread from one part of the heart to another
What is an ECG or EKG?
AKA electrocardiogram
- An instantaneous recording of all the CHANGES in membrane potential in the heart cells
Why can only membrane potential changes of a relatively LARGE number of cells be detected by and ECG?
Because it is recorded on the surface of the body
What accounts for the characteristic wave forms in the ECG?
The characteristic sequence of depolarization
Describe the depolarization of purkinje fibers
PURKINJE FIBERS = little “lighters” to start the “fire” of electrical impulse in several places at once- so the heart can “burn” in a uniform fashion
Describe the placement of leads in the FRONTAL plane
Standard bipolar limb leads
- i
- II
- III
Augmented unipolar limb leads
- aVR
- aVL
- aVF
See slide 21 for placement diagram
Describe the placement of leads in the HORIZONTAL (transverse) plane
Chest leads (precordial leads)
- V1
- V2
- V3
- V4
- V5
- V6
Chest leads are placed in order to emphasize the ______ ventricle
Left
Placement of V1
4th intercostal space
Right sternal border
Placement of V2
4th intercostal space
Left sternal border
Placement of V4
5th left intercostal space
Midclavicular line
Placement of V3
Between V2 and V4
Placement of V5
In line with V4, in anterior axillary line
Placement of V6
In line with V4 and V5 in midaxillary line (“down from mid-armpit”)
When determining heart rate from ECG paper, what do we measure?
Number of large boxes in an R-R interval
If there is one large box in the R to R interval…
The heart rate is 300 bpm
What about for 2, 3, 4, 5, 6 and 7 large boxes in an R-R interval?
1 - 300 2 - 150 3 - 100 4 - 75 5 - 60 6 - 50 7 - 43
Segments and intervals of an ECG wave form
Look at slide 25
- P wave
- PR interval
- QRS complex
- ST segment
- T wave
- QT interval
Define a P wave
Deflection from and return to isoelectric line
What is the physiological correlate of a P wave?
atrial myocyte depolarization (phase 0)
What is the normal duration of a P wave?
0.06-0.11 sec
1-2 small boxes
Define a QRS complex (interval)
deflection from and return to isoelectric line
What is the physiological correlate of a QRS complex?
ventricular myocyte depolarization (phase 0)
What is the normal duration of a QRS complex?
0.03-0.12 sec
(
Define a PR interval
beginning of P wave deflection to beginning of QRS complex
What is the physiological correlate of a PR interval?
all electrical events (depolarization- phase 0’s) upstream of ventricular myocytes (atrial myocyte, AV node, bundle, bundle branches, Purkinje fibers)
What is the normal duration of a PR interval?
0.12-0.2 sec
3-5 small boxes
Define a T wave
deflection from and return to isoelectric line
What is the physiological correlate of a T wave?
ventricular myocyte repolarization (phase 3)
What is the normal duration of a T wave?
variable
Define a QT interval
beginning of QRS complex to the end of the T wave
What is the physiological correlate of a QT interval?
the entire action potential for the ventricular myocytes (depolarization- plateau- repolarization)
What is the normal duration of a QT interval?
variable
(roughly half
R-R interval)
Define an ST segment
end of the QRS complex to the beginning of the T wave deflection
What is the physiological correlate of an ST segment?
ventricular myocyte plateau phase (phase 2)
What is the normal duration of an ST segment?
variable
with HR
What does it mean if there are no P waves?
No atrial depolarization
What does it mean if there is a P wave following every QRS complex?
There is ventricular polarization following every atrial polarization
What is pacing the heart?
SA node and AV node
Interpreting an ECG
Need to review this from physiology material
