Week 2 Flashcards
P wave characteristics
Location: precedes QRS complex
Amplitude: 2-3 mV
Duration: 1.5 - 3 boxes (0.6-0.12 seconds)
Configuration: upright and rounded - except aVR
PR interval characteristics
Location: beginning P wave to beginning of QRS complex
Amplitude: isoelectric
Duration: 3 - 5 boxes (0.12 - 0.2 seconds)
Represents: time when atria are contracting
What is the function of the AV node?
It delays conduction through by 0.04 seconds to allow optimal ventricular filling. It is also acts as a safety mechanisms (prevents rapid atrial impulses from spreading to the ventricles)
What is associated with shortened PR intervals?
Pre-excitation syndromes (WPW) and junctional rhythms
What is associated with prolonged PR intervals?
Bundle branch block or impaired conduction from atria to ventricles, e.g. digoxin toxicity
Which direction does electrical activity in the septum flow?
Between bundle branches from left to right
What does the Q wave represent?
Septal depolarisation/repolarisation of the atria
What direction is septal depolarisation
Depends whether the lead is looking at the heart from the left or right (left = downward, right = upward)
What is the first downward deflection called?
Q wave
What is the first upward deflection called?
R wave
Downward deflection after R wave?
S wave
QRS deflection depends on what?
Whether the R or S wave is bigger
What dominates the deflection of the QRS complex?
The left ventricle voltage as it contains more muscle mass
How does the QRS complex change across the chest leads?
Transition from predominately negative to positive wave
What do deep/wide Q wave represent?
Previous MI
What is the characteristics of a pathological Q wave?
(1) More than 1/4 height of R-wave (2) duration longer than 0.04 seconds
What is a Q-wave MI called and what does it signify?
Transmural myocardial infarction - damage of all layers of the heart.
Why does infarcted tissue result in a Q wave?
The lead ‘looks’ through the infarcted tissue as it is no longer conductive. Since electrical activity moves from the inside to the outside of the heart, it is seen as a downward deflection on the ECG.
What is a subendocardial MI?
MI when not all layers of the heart are damaged and pathological Q waves are not evident
Characteristics of a normal QRS?
- Location: follows PR interval
- Amplitude: 5 to 30 mm high (1 large to 6 large boxes)
- Duration: 0.06 - 0.12 seconds (1.2 - 2.5 square)
What does the ST segment represent?
The end of ventricular depolarisation and the beginning of ventricular repolarisation
What does the J-point mark?
The end of the QRS and beginning of ST segment
During the ST segment, is the intracellular membrane potential positive or negative?
Positive
ST segment depression can be seen when?
During ischemia and angina (which is a symptom of CVD).
