Lab 1 - ECG Flashcards
What is an ECG?
Electrocardiogram - measures the electrical activity of the heart
What event in the cardiac cycle does the T wave represent?
Ventricular repolarisation
What event in the cardiac cycle does the P wave represent?
Depolarisation of the atria
What event in the cardiac cycle does the QRS complex represent?
Depolarisation of the ventricles
Q wave: The initial negative deflection following the P wave.
R wave: The subsequent large positive deflection.
S wave: The negative deflection following the R wave.
Why is ventricular repolarisation a positive deflection?
Because repolarisation is when it gets more negative and when going towards the negative lead (negative + negative = positive).
Explain why the amplitude and polarity of the P, QRS and T waves vary in recordings of 6 different limb leads?
Polarity is to do with the nature in change in membrane potentials but also to do with the orientation and direction - therefore when we change the leads and this the orientation/direction the recordings will vary.
Amplitude also depends on orientation - how parallel it is to the electrode - the more parallel the larger the amplitude.
If the amplitude is towards positive lead is it depolarisation or repolarisation?
Depolarisation - it is depolarisation when they match e.g., when towards positive lead or away from the negative lead.
It is repolarisation when they don’t match e.g., away from positive and towards negative.
What deflection its shown when depolarisation is towards a positive electrode?
Positive deflection
What deflection is shown when repolarisation is away from a positive electrode?
Positive deflection
If there is poor alignment of lead and electrical activity will the amplitude be large or small?
Small
Why does the recording of the aVR lead appear to be an inversion of the recording of standard limb lead 2?
Because the electrodes are reversed for lead 2 and aVR which means they look at the heart from the opposite direction thus the ECG is inverted
What prevents depolarisation of skeletal muscle every time the heart beats?
Isolation of the hearts electrical activity from the body by pericardium and cartilage around the valve. This prevents depolarisation of skeletal muscle very time the heart beats.
What is Mean Electrical Axis?
MEA is the average direction of all the electrical activity occurring during depolarisation of the ventricles.
What ECG lead will have the largest R wave?
The one that is more aligned with the direction of ventricular depolarisation - which is typically lead 2 (right arm and left leg)
What is the normal range of MEA?
-30 to +120 degrees
The MEA of individuals with a shorter and broader torso will likely be:
More horizontal
The MEA of individuals with a tall and thin torso will likely be:
More vertical
Why is the MEA effect by height?
Because it changes the thorax space for the heart e.g., shorter people have less thorax space and therefore heart sits in a more horizontal position
How does fitness effect MEA?
Left Ventricular hypertrophy - muscle growth will mean that it sits more horizontally
Is the MEA of heart more horizontal during inspiration or expiration?
Expiration (heart rises in chest due to the diaphragm relaxing and expanding)
What happens to the diaphragm as we inspire?
it contracts and flattens
What factors effect MEA?
Height
LV hypertrophy
gastric distension
obesity
pregnancy
myocardial infarction on right side
What three things cause LV hypertrophy?
Fitness
Aortic stenosis (increases aortic pressure = increase afterload)
Hypertension (age)
What causes RV hypertrophy?
Pulmonary hypertension
High altitude
What direction does the heart move for LV and RV hypertrophy respectively?
LV = moves horizontally (anti clockwise)
RV = move vertically (clockwise)
What is the consequence of a missed QRS complex?
NO QRS = no ventricular depolarisation or contraction which means a missed heart beat
It means that no blood have been pumped into the body and lungs for that cardiac cycle.
Is every P wave followed by a QRS complex in 2nd degree AV block?
No - there is no R wave after the 5th P wave.
IS the PR interval consistent during 2nd degree AV block?
No there PR interval gets progressively longer
