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
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?
What is the first upward deflection called?
Downward deflection after R 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?
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?
ST segment depression can be seen when?
During ischemia and angina (which is a symptom of CVD).
Characteristics of normal ST segment
Deflection: isoelectric - may vary from 0.5 (DEPRESSION) to 1.0mm (ELEVATION) in some precordial leads
T wave characteristics
Location: following ST segment
Amplitude: 0.5 in leads I, II and III and up to 10mm in precordial leads
Configuration: typically rounded and smooth
Deflection: usually upright in leads I, II and V3 to V6
Tall and peaked T waves may indicate what?
myocardial injury, hyperkalemia.
Notched or pointed T waves may indicate what?
What can hyperkalaemia lead to?
How do R and T waves of opposite polarity occur?
Results of slowing of spread of the wave of excitation through the myocardium
What is the QT interval?
The entire duration of the depolarisation-repolarisation cycle through the ventricles
What influences the QT interval?
heart rate dependent - increased HR shortens the QT interval
Characteristics of normal QT?
Location: extends from the beginning of the QRS complex to the end of the T wave
Duration: 0.36 - 0.44 seconds (9-11 small boxes)
Shouldn’t be greater than half the distance between consecutive R waves
QT interval duration is influenced by what?
Age, sex and heart rate
What dose a prolonged QT interval indicate?
A longer relative refractory period
A long QT interval increases the risk of what ventricular tachycardia?
Torsades de points - can deteriorate in v.fib
What is the U wave?
Repolarisation of the interventricular septum or the purkinji fibres
What leads is the U wave most clearly seen?
V2 - V4
Reasons for prominent U wave?
Hypercalcemia, hypokalemia, digoxin toxicity
What is a lead?
View of the heart’s electrical activity between a negative and positive pole
The imaginary line between a negative and positive pole is what?
What is a plane?
Cross-sectional perspective of the heart’s electrical activity
What type of perspective does the frontal plane of the heart give?
What type of persecptive does the horiztonal plane of the heart give?
How many views does a 12-lead ECG give of the heart
12 views using 10 electrodes
What leads give information in the frontal plane?
the 6 limb leads - I, II, III, aVL, aVR, aVF
What are the bipolar leads?
I, II, III - have both a positive and negative electrode
Left axis deviation leads to an abnormally large R wave in which lead?