ECG 1 Flashcards
Summarize the cardiac cycle
Action potential is generated in the sinus node (SA node) and spread to the atria producing atrial contraction —>
- Atrial systole
S1-mitral and tricuspid valve
- Isovolumetric contraction
- Rapid ventricular ejection
- Reduced ventricular ejection
S2- aortic & pulmonary valve closure
- Isovolumetric ventricular Relaxation
- Rapid ventricular filling
- Reduced ventricular filling
Explain the sequence of depolarization of the cardiac muscle
AP generation in the SA node
Right/left atrium depolarization —> atrial contraction
AV node depolarization (delay)
Bundle of His
Septum depolarization
Apex depolarization
Ventricle walls depolarization —> ventricular contraction
Atria repolarization (atria relaxation) occurs during ventricular depolarization
What is an electrocardiogram?
Is an amplified, timed recording of the electrical activity of the heart, as detected on the surface of the body.
The recording gives a plot of voltage as a function of time
Body serves as a volume conductor
The heart generates a collection of changing dipoles (vectors) during DEPOLARIZATION & REPOLARIZATION
What is the recording speed of calibratated paper ?
Recording speed:
25mm/s
1mm= 0.04s
Describe the horizontal axis of calibratated paper
Horizontal axis- time
1 small square= 0.04 seconds
1 big square = 5 small squares
1 big squares= 0.2 secs
300 big squares= 1 min
Describe the vertical axis kf the calibratated paper
1 big square 0.5 mV
1 small square= 0.1 mV
What does calibratated paper record?
Depolarization or repolarization—> voltage change(mV)—> recorded on moving calibrated paper against time
What are the components of ECG ?
- Waves are deflections above or below baseline
- P, QRS, T, U waves
Segments baseline between two waves
- PR segment
- ST segment
- TP segment
Intervals include wave(s) & segments
PR interval
QT interval
RR interval
What are the waves of the ECG?
- p wave
- QRS segment
- T wave
- U wave
What are the segments of the ECG?
PR segment
ST Segment
What is the point of ECG?
J point
What are the intervals of the ECG?
PR interval
ST interval
QT interval
What is a dipole?
The difference of polarity between two neighboring locations is called dipole
When a portion of myocardium becomes depolarized from an Action potential , it’s polarity is temporarily reversed, becoming positive on the inside and negative on the outside relative to the neighboring of opposite charge, or polarity, within the myocardium
When do you have a dipole?
When the myocardium it’s partially depolarized.
-Dipoles are present only when a portion of the myocardium is in the process of depolarization or repolarization while other portions are not.
They are not formed when the entire myocardium is depolarized or repolarized
What is a vector?
A vector is an object that has both magnitude and direction
Examples: force, velocity, dipoles generated in the heart during depolarization and repolarization
How to record the electrical activity of the heart?
Lead- electrode connection that records the potential difference between 2 electrodes and one of those electrodes is designated as the positive input
How to record the electrical activity of the HEEART?
All the cells are in phase 4 (resting membrane potential) isoelectric line IN ALL THE LEADS (ECG: TP segment)
All the cells are in phase 2 (plateau phase) isoelectric line IN ALL LEADS (ECG: ST segment)
Depolarization vector pointing towards the positive electrode
Positive deflection wave
Repolarization vector traveling away from a positive electrode produces positive deflection
Depolarization vector perpendicular to an ELECTRODE AXIS
Produces no net deflection
What are leads?
Refers to an imaginary line between two ECG electrodes
What is Einthoven’s triangle ?
3 leads
What are the 5 ECG Interpretation rules?
- A wave of depolarization traveling toward a a positive electrode results in a positive deflection in the ECG tracing
Corollary: a wave of depolarization traveling away from a positive electrode results in a negative deflection - A wave of repolarization traveling towards a positive electrode results in a negative deflection.
Corollary: a wave of repolarization traveling away from a positive electrode results in a positive deflection - A wave of depolarization or repolarization oriented perpendicular to an electrode axis has no net deflection
- The instantaneous amplitude if the measured potentials depends upon the orientation of the positive electrode relative to the mean electrical vector
- Voltage amplitude (positive or negative) is directly related to the mass of tissue undergoing depolarization or repolarization
Explain the origin of ECG wave forms
Moment to moment orientation and magnitude of net dipoles in the heart determine the formation of ECG wave forms
- Atrial depolarization: p wave
- AV node and bundle of His delay: PR segment
-Ventricular depolarization: QRS complex
Atrial repolarization is masked by the ventricular depolarization
- Ventricles are totally depolarized: ST segment
- Ventricles repolarization: T wave
- Arrium and ventricles at resting membrane potential (phase 4): TP segment
What is the origin of the P wave?
Wave of depolarization that spreads from SA node throughout the atria
Duration 0.08 seconds - 0.1 seconds (2-2.5mm)
Atrial repolarization is not visible in the ECG because it occurs during ventricular depolarization
What is the origin of the PR segment?
Brief isoelectric (zero voltage ) period after P wave
Represents the time in which the atrial cells are depolarized, and the impulse is traveling within the AV node (decreased conduction velocity)
What is the origin of the PR interval?
Period from the onset of P wave to the beginning of QRS complex
Duration: 0.12-0.20(3 mm- 5 mm)
Represents the time between the onset of atrial depolarization and the onset of ventricular depolarization
PR interval > 0.2s (5 mm): AV block
What is the QRS complex?
Represents ventricular depolarization
Duration: 0.06s - 0.1s (1.5 mm- 2.5 mm)
QRS complex duration> 0.12s: intraventricular block
Summarize ventricular depolarization sequence
- Septum depolarization
- Free wall depolarization
- Base of the LV depolarization
What is the function of ST segment?
Isoelectric period following QRS complex
Entire ventricle is depolarized
Roughly corresponds with the plateau phase of the ventricular myocyte action potential
ST depression/elevation: diagnosis of ischemia
Where is the J point?
Junction between end of QRS complex and start of ST segment
Whaat is the function of T wave?
Represents ventricular repolarization (phase 3 of the action potential)
Last longer than depolarization
T-wave inversion may indicate ischemia or recent myocardial infarction
Describe the ventricular repolarization
Depolarization of the ventricles occurs first in the subEndocardial region and then in the subepicardial region.
Duration of ventricular action potentials in the subEndocardial myocardium is longer than in the subepicardial myocardium, therefore, subendocardial myocardium is the first to depolarize and the last to repolarize
Describe atrial repolarization appearance
Atrial repolarization do not appear as a separate deflection of the ECG because it generates a low voltage and it is masked by much larger QRS complex which is present at the same time
First myocardial cells to to repolarize, therefore, it produces a negative deflection
What does the QT interval represent?
Represents ventricular depolarization-ventricular contraction-ventricular repolarization
Roughly estimates duration of ventrivular action potential
Duration: 0.2-0.4s (depending on heart rate)
In practice, QT interval is expressed as a corrected QT (Q-Tc) interval:
Q-Tc= QT interval/ (square root of RR interval). Normal Q-Tc interval < 0.44
> 0.44: prolonged QT interval, high risk of arrythmias
What is the function of TP segment?
End of T wave to beginning of the P wave
Phase 4 of ventricular action potential
Represents the electrical resting state
It is traditionally used as the baseline reference from which to assess PR and ST deviations
Explain RR (QRS-QRS interval) interval:
This equals 1 heart beat
It is measured from one point on a given QRS complex to the corresponding point on the next (usually the peak of R-wave, or nadir of an S or QS wave)
The instantaneous heart rate (beats per min) = 60/RR interval when the RR is measured in seconds (sec)
Describe regular HR
The “count off” method requires memorizing the sequence:
300- 150- 100- 75- 60-50
Then use this sequence to count the number of large boxes between two consecutive beats:
The second QRS falls between 75 and 60 bpm; therefore, the heart rate is approximately midway between them about 67 bpm. Knowing that the heart rate is approximately 60-70 bpm is certainly close enough
Describe irregular heart rate
ECG recording paper often indicates 3 sec time makers at the top of bottom of the tracing
To calculate the heart rate, count the number of QRS complexes between the 3 sec markers (= 6 beats in this example) and multiply by 20. Thus, the heart rate here is approximately 120 bpm
It’s even easier (and more accurate) to count the number of complexes between the first and third markers on the strip (representing 6 sec of the recordin) and then multiply by 10 to determine the heart rate.
Method 3 is particularly helpful for measuring irregular heart rates
What are the ECG manifestations during acute ischemia?
Within hours: peaked T-waves and ST-segment changes (either ST segment depression of elevation )
Within 24 hours: T-wave inversion and ST-segment resolution
Within a few days: pathologic Q waves
-Q wave duration 40 msec
- Q-wave more than one third(1/3) of the QRS amplitude
- Scar tissue, electrically dead
Contrast subEndocardial infarcts and transmural infarcts
Subendocardial infarcts- ST segment depression
Transmural infarction-ST segment elevation
Describe current of injury
Abnormal current flow caused by acute ischemia
Current of injury underlies the pathophysiology of both ST elevations and ST depressions caused by acute ischemia
- diastolic current theory
- systolic current theory
Contrast systolic current theory and diastolic current theory of current injury
Diastolic current theory- partial depolarization of injured myocardium in diastole before stimulation phase 4
Shifts ECG baseline downward/upward
Systolic current theory- reduced resting membrane potential and repolarize more rapidly phases 0- 3
Current of injury from normally depolarized cells to the ischemic cells
What is the origin of current of injury?
Cell ischemia—> less ATP —> decreased Na+/K+ ATPase pump—> ischemic cell action potential:
Reduction of the resting membrane potential (phase 4)
Slower and shorter action potential (decreased slope of and less amplitude of phase 0)
repolarization more rapidly
