Lecture 13 Flashcards
What is an ELECTROCARDIOGRAM
(ECG)?
An ELECTROCARDIOGRAM (ECG) is an amplified, timed recording of the electrical activity of the heart, as detected on the surface of the body.
-Plot of voltage as a function of time.
Describe the lines of an ECG
-3 lines (Leads) w 4 parts
Parts:
1) Standard bipolar leads
2) Augmented limb leads
3) Chest/precordial leads
Parts 1 and 2:
-Record electrical activity of the heart in frontal (Coronal) plane
Parts 3 and 4:
-Record electrical activity of the heart in the horizontal plane (Chest)
Whats the recording speed of the Calibrated paper?
-Measures the horizontal axis-time (milliseconds)
1 Small square:
25 mm/s = 0.04 seconds
1 Big square= 5 small squares
0.20 seconds
300 big squares
1 min
Whats the recording voltage of the Calibrated paper?
-Measures the vertical amplitude axis-millivolts
1 big square = 0.5 mV
1 small square = 0.1 mV
What are the components of ECG tracing?
1) Waves –> deflections/baseline
- P, QRS complex, T, U wave
2) Segments –> baseline between 2 waves
- PR, ST, TP segments
3) Intervals –> Includes waves and segments
- PR, QT, RR interval
What’s the significance of lead (line 2)?
-Reference point for many factors (Ex: HR) bc has main vector of heart
Cycle:
-P wave =
- QRS = reference –> ventricular depolarizaion
- T wave =
Origin of ECG waveforms
Dipoles
Depolarization/Repolarization –> Dipole vectors –> Leads record ECG
Dipoles:
-Difference of polarity between two neighboring locations (One pos, one neg)
-Occur When the myocardium is partially depolarized
- = repolarization
+= depolarization
(Follows charge direction)
What’s a Vector
-an object that has both magnitude and direction
(Ex: Force, velocity, dipole, depolarization, repolarization)
-Can combine vectors that go into the same direction (Ex: SA Node vectors)
How do we record electrical activity of the heart?
1) Lead –> Electrode connection
- Records the potential difference between 2 electrodes
- (One positive one negative electrode)
2) Electroaxis –> Connects positive and negative electrode (Imaginary)
3) Isoelectric line
- All cells are in phase 2 (plateau phase) or Phase 4 (resting)
- In all leads (Flat line)
Origins of the ECG Waveforms
P wave
Pr Segment
QRS Complex
ST Segment
T Wave
TP Segment
P wave
-Atrial depolarization
PR Segment
-A-v node and bundle of HIS delay
QRS Complex
-Ventricular depolarization
ST Segment
-Ventricles totally depolarized
T Wave
-Ventricular repolarization
TP Segment
-Atrium and ventricules at resting membrane portential (phase 4)
ECG INTERPRETATION
Rules
1) A wave of depolarization traveling toward a
positive electrode results in a positive deflection
in the ECG tracing
2) A wave of repolarization traveling towards a positive electrode results in a negative deflection
3) A wave of depolarization or repolarization oriented perpendicular to an electrode axis has no net deflection.
4) The instantaneous amplitude of the measured
potentials depends upon the orientation of the positive electrode relative to the mean electrical vector
5) Voltage amplitude (positive or negative) is directly
related to the mass of tissue undergoing depolarization or repolarization
P Waves:
- Wave of depolarization that spreads from SA node throughout the atria
- Duration 0.08 – 0.10 seconds (2 – 2,5 mm)
- Atrial repolarization is not visible in the ECG because it occurs during ventricular depolarization
- Positive deflections
P Waves:
- Wave of depolarization that spreads from SA node throughout the atria
- Duration 0.08 – 0.10 seconds (2 – 2,5 mm)
- Atrial repolarization is not visible in the ECG because it occurs during ventricular depolarization
- Positive deflections in triangle
- Highest voltage P wave leads will be the one that goes in the same direction (Parallel) as the projection vector
PR Interval:
Period from the onset of P wave to the beginning of QRS complex
Duration: 0.12 – 0.20 (3mm – 5mm)
Represents the time between the onset of atrial depolarization and the onset of ventricular depolarization
PR interval > 0.20 s (5mm): AV block
QRS Complex:
Represents ventricular depolarization
Duration: 0.06 s – 0.10 s (1.5mm – 2.5 mm)
QRS complex duration > 0.12 s: Intraventricular block
ST Segment
Isoelectric period following QRS complex
Entire ventricle is depolarized
Roughly correspond with the plateau phase of the ventricular myocyte action potential
ST depression/elevation: diagnosis of ischemia
J Point
Junction between end of QRS complex and start of ST segment
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
Ventricular Depolarization sequence
1) Septum depolarization
2) Free wall depolarization
3) Base of the LV depolarization
-These make up QRS Complex
Ventricular Repolarization
-Depolarization of the ventricles occurs first in the subendocardial region and then in the subepicardial region
Atrial repolarization
-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.
Qt Interval
Represents ventricular depolarization-ventricular contraction- ventricular repolarization
Measures the avg of the Duration of the ventricular action potential
Roughly estimates duration of ventricular action potential
Duration: 0.2 – 0.4 s (depending on heart rate)
In practice, QT interval is expressed as a corrected QT (Q-Tc) interval:
Q-Tc =
𝑸𝑻 𝒊𝒏𝒕𝒆𝒓𝒗𝒂𝒍/√𝑹𝑹 𝒊𝒏𝒕𝒆𝒓𝒗𝒂𝒍
Normal Q-Tc interval < 0.44
> 0.44: prolonged QT interval, high risk of arrythmias
TP Segments
End of the 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
The RR (QRS–QRS) interval: = 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).
Normal HR
Fast HR
Irregular HR
Normal HR
-Second QRS falls between 75 and 60bpm = HR halfway between them
(Must memorize sequence: 300, 150, 100, 75, 60, 50) = big squares
Fast HR
>100bpm
Irregular HR
-Use 3 second markers and count QRS complexes between them, then multiple them by 20 = HR
Ventricular Action Potential and ECG
Phases:
0 = Depolarization
-QRS
1 =
-ST segment
2 =
-ST Segment –> all cells depolarized
3 = Repolarization
-T
4 = Resting
-QT represented by phases 1-3
Myocardial Ischemia/Infarction
ECG manifestations during ACUTE ISCHEMIA
Within Hours:
-Peaked T-waves
-ST-segment changes (either
ST-segment depression or elevation)
Withing 24hrs:
- T-wave inversion
- ST-Segment resolution
Within a few days:
- Pathologic Q wave
- Q-wave duration 40msec
- Q-wave more than one third (1/3) of the QRS amplitude
- Scar tissue, electrically dead
Subendocardial Infarcts
Transmural Infarcts
1) Subendocardial Infarcts
- ST segment depression
2) Transmural Infarcts (Whole wall of ventricule)
- ST segment elevation
Origin of ST segment
deviation
Diastolic Current Theory
Systolic Current Theory
Current of injury = Abnormal current flow caused by acute ischemia
-Pathologies caused by ST elevation/ depression
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 happens in result of cell ischemia?
-Less ATP produced –> Decrease Na+/K+ ATPase PUMP –> Resting membrane potential decreases
Systolic Current Theory
phase 2 increases (Shifting ST directly)
Diastolic Current Theory
-phase 4 decreases (Shifting resting baseline)
St Segment Elevation Myocardial Infarction Localization
Infarct Location, Artery, Leads with ST elevation or Q waves:
1)Anteroseptal
-Left Anterior, Descending Artery (LAD)
V1-V2
2) Anteroapical
- distal LAD
- V3-V4
3) Anterolateral
LAD or Left Circumflex, Artery (LCX)
-V5-V6
4) Lateral
- LCX I, aVL
5) Inferior
- Right Coronary Artery
- II, III, aVF
6) Posterior
-Posterior Descending Artery
-V7-V9, ST depression in V1-V3 with
tall R waves
