ECG pt.1 Flashcards
Importance of ECG
- Most important test for interpretation of the cardiac rhythm, conduction system abnormalities, and the detection of myocardial ischemia
- Great value in the evaluation of other types of cardiac abnormalities including valvular heart disease, cardiomyopathy, pericarditis, and hypertensive disease
- Can be used to monitor drug treatment (specifically antiarrhythmic therapy) and to detect metabolic disturbances
What is an ECG?
- A plot of voltage on the vertical axis against time on the horizontal axis
- Reflects the net electrical activity at a given moment
How does an ECG work?
- Electrodes are connected to a galvanometer that records a potential difference
- The needle (or pen) of the ECG is deflected a given distance depending upon the voltage measured
- At every beat, the heart is depolarized to trigger its contraction. This electrical activity is transmitted throughout the body and can be picked up on the skin
- This is the principle behind the ECG
What is an ECG recorded on? Characteristics of it
- Recorded on special graph paper that is divided into 1 mm2 grid-like boxes
- Has large boxes (heavy lines) of 0.5 cm sides with each large box having smaller boxes on the horizontal axis and 5 smaller boxes on the vertical axis
What does each large box represent on the horizontal axis?
Represents 0.2 seconds at a typical paper speed of 25mm/sec
ECG paper speed
- Ordinarily 25 mm/sec
- On occasion, the paper speed is increased to 50 mm/sec to better define waveforms
What does each small box represent on the horizontal axis?
0.04 seconds at a typical paper speed of 25mm sec
What does each box represent on the vertical axis?
- Each small box (1mm) represents 0.1mv, large box (5mm) 0.5mv, two large boxes (10mm) 1 mv
- On occasion, particularly when the waveforms are small, double standard is used (20 mm equals 1 mv). When the waveforms are very large, half standard may be used (5 mm equals 1 mv)
/
/
/
/
What does the ECG measure on the X axis?
Measures the height (amplitude) of a given wave or deflection
What would each box represent on y axis if the paper speed is increased to 50mm/sec?
Each large box is only 0.10 sec and each small box is only 0.02 sec
How would the heart rate appear on ECG with a speed of 50mm/sec?
The heart rate appears to be one-half of what is recorded at 25 mm/sec
ECG intervals in 50 mm/sec vs 25 mm/sec
All of the ECG intervals are twice as long as normal on a 50 mm/sec paper speed
Where can we find the paper speed and voltage used in an ECG?
Paper speed and voltage are usually printed on the bottom of the ECG for reference
What is a normal ECG composed of?
Several different waveforms that represent electrical events during each cardiac cycle in various parts of the heart
What waveforms do we find on an ECG?
- ECG waves are labeled alphabetically starting with the P wave, followed by the QRS complex and the ST-T-U complex (ST segment, T wave, and U wave)
- The J (junction) point is the junction between the end of the QRS and the beginning of the ST segment, the point where the QRS complex joins the ST segment
What does the P wave represent?
Atrial depolarization
What is is the P wave and how is the P wave seen on a normal ECG?at
Represents atrial depolarization and is seen as an initial low amplitude deflection preceding the QRS complex that is positive in most leads
Duration and amplitude of P wave?
Generally <0.12 sec (three small boxes) and the amplitude <0.25 mv (2.5 small boxes)
What represents atrial repolarization and when does it occur?
- Atrial repolarization sequence (Atrial ST and T wave phases) occurs just before, simultaneously, and just after depolarization of the ventricular myocardium
Atrial T wave presence on ECG
The atrial “T wave” itself is usually hidden by the QRS complex and not observed on the routine ECG
What is the PR interval and what does it represent?
- Time between the first deflection of the P wave and the first deflection of the QRS complex
- It includes time for atrial depolarization (the P wave) and conduction through the AV node and the His-Purkinje system (which constitute the PR segment)
What does QRS wave complex represent?
Ventricular depolarization
/
/
How to read QRS complex?
The rule is: if the wave immediately after the P wave is an upward deflection, it is an R wave; if it is a downward deflection, it is a Q wave
What does the Q wave represent?
Correspond to depolarization of the interventricular septum
What does the R wave represent?
Depolarization of the main mass of the ventricles –hence it is the largest wave
What does the S wave represent?
Signifies the final depolarization of the ventricles
What is the ST segment and what does it represent?
- The time between the end of the QRS complex and the start of the T wave
- Occurs after ventricular depolarization has ended and before repolarization has begun
- It is a time of electrocardiographic silence reflecting the period of zero potential between ventricular depolarization and repolarization
What does the T wave represent?
T waves represent ventricular repolarization (atrial repolarization is obscured by the large QRS complex)
What should every ECG reading start with?
Every ECG reading should start with an assessment of the rate, rhythm, and axis
What is the PR segment also known as?
PR interval
