Electrical activity of the heart Flashcards
How do both sides of the heart beat in synchrony?
Electrically connected via gap junctions which go on to form intercalated discs. Physically connected via desmosomes.
What is tetanic contraction?
Sustained contraction of a skeletal muscle evoked when the motor nerve innervating the muscle produces APs at a fast rate.
What prevents cardiac muscle from exhibiting titanic contraction?
Cardiac muscle has a long AP (250ms) and therefore a long refractory period.
How can calcium regulate heart contractions?
Ca2+ release does not saturate the troponin so can be used to regulate the strength of contraction?
How does temperature modulate electrical activity of the heart?
Increases rhythm by around 10 beats per min per degree Celsius.
How does Hyperkalemia modulate the activity of the heart?
High plasma K+ causes heart fibrillation and block.
How does Hypokalemia modulate the activity of the heart?
Low plasma K+ causes heart fibrillation and block.
How does Hypercalcemia modulate the electrical activity of the heart?
High plasma Ca2+ increases the force of contraction and heart rate.
How does Hypocalcemia modulate the electrical activity of the heart?
Low plasma Ca2+ decreases heart rate and the force of contraction.
Give some examples of drugs that affect the electrical activity of the heart?
Ca2+ channel blockers decrease the force of contraction.
Cardiac glycocides – increase force of contraction
On an electrocardiogram what does the P wave correspond to?
Atrial depolarisation.
On an ECG what does the QRS complex correspond to?
Ventricular depolarisation.
On an ECG what does the T wave correspond to?
Ventricular repolarisation.
What is the PR interval?
The time between atrial depolarisation and ventricular depolarisation through the AV node. Usually around 0.12-0.2secs.
What is QRS?
The time taken for the whole of the ventricle to depolarise. Usually about 0.08 secs.
Why isn’t atrial depolarisation represented on an ECG?
Because it coincides with ventricular depolarisation which involves much more tissue depolarising much faster, masking the signal from atrium.
In what direction does the interventricular septum depolarise?
Left to right.
It is the first part of ventricle to depolarise.
In what tissue order does the bulk of the ventricle depolarise?
From endocardial to epicardial surface.
Why is the T wave positive going?
AP is longer in endocardial cells than epicardial so repolarisation wave runs in opposite direction to depolarisation. (moves away from electrode producing a positive blip.)
What extra info do the augmented limb leads give you?
By recording from one limb lead with respect to the other two combined, it gives you 3 other perspectives on events in the heart.
What extra info do the precordial leads give you?
Placed around the heart on the chest look at the same information but in the horizontal plane.
At what speed should the paper in an ECG run?
25mm per second.
How do you work out heart rate from an ECG?
Measure the R-R interval on the rhythm strip and then count how many occur in 60 secs.
Or count the R waves in 30 large squares of paper (=6secs) and then X by 10.
60-100 beats/min is normal
What does elevation of the ST segment tell us?
Elevation of the ST segment indicates something serious has gone wrong. Uses to determine the severity of a myocardial infarction. STEMI.
