Electrical function and ECG Flashcards
Where do action potentials in the heart originate from?
Nodes
How is the action potential in cardiac muscle is propagated from cell to cell?
- branching of the cells
- intercalated discs which contain gap junctions
What is automaticity?
Spontaneous depolarisation generating an action potential
Where are the fastest pacemaker cells located?
sinoatrial node
What are ectopic pacemakers and what is their function?
Other cells in the myocardium that develop automaticity after injury
– can be responsible for some clinically serious abnormal rhythms
How do action potentials occur in the skeletal muscle surrounding the heart?
- the action potential is initiated by somatic motor neurones
- the skeletal muscle cells are insulated from each other
- each needs to receive neurological stimulation
How does the ANS innervate the heart/for what reasons?
Affects the frequency and speed of action potentials to modulate the heartbeat
Which type of muscle cell had a longer action potential duration?
Cardiac muscle cells
Which ion channel in cardiac muscle causes the prolonged plateau phase?
Calcium ion channels
What is the purpose of the refractory period?
Ensures a new action potential cannot be initiated before the previous one is completed.
Makes sure each contraction of the heart is distinct
The time during which a new AP cannot be generated is called the … ?
Absolute refractory period
What is temporal summation and why does it not occur in cardiac muscle?
- multiple action potentials within the space of one muscle contraction
- cardiac muscle needs to relax to allow for diastolic filling
Why are atrial action potentials are shorter than ventricular action potentials?
- The slow Ca++ channels stay open for less time
- K+ channels close for less time
- shorter refractory period
How is a pacemaker potential different to an action potential?
- slow rise to threshold level following an action potential
- no fast Na+ channels, so no sudden influx of sodium, have pacemaker Na channels instead
- K+ channels close
- Ca++ channels open
- slower depolarisation
Describe the calcium excitation-contraction coupling that occurs in cardiac muscle
- extracellular Ca++ enters through the voltage-gated Ca++ channels of the T-tubule
- This stimulates calcium induced calcium release from the sarcoplasmic reticulum
What does the ECG record and measure?
- Records the electrical activity of the heart by measuring the voltage
What does the ECG plot a graph of?
Voltage against time
If an action potential is travelling towards the +ve electrode what deflection will be shown on the graph?
An upwards deflection
What do the height and width of a deflection reflect?
Height = the size of the electrical potential difference perceived by the electrodes Width = the time taken for the potential difference to return to zero
What do the following waves represent: P wave? Q wave? R wave? S wave? T wave?
P = atrial depolarisation Q = early ventricular depolarisation R = Full ventricular depolarisation S = late ventricular depolarisation T = ventricular repolarisation
What is occuring due to the P-R interval when the ECG graph returns to baseline?
Represents the delay in conduction which is due to the action potential travelling through the AV node
What should the ratio of P:QRS be?
1:1
What do leads 1-3 compare?
Lead I - compares the voltage at the left forelimb to the right forelimb
Lead II - compares the left hindlimb to the right forelimb
Lead III - compares the left hindlimb with the left forelimb
What is the function of the augmented unipolar leads – AVR, AVL and AVF?
These measure the electrical potential between a positive electrode at one limb with the average voltage from the other two
What does each augmented unipolar leads – AVR, AVL and AVF compare?
AVR = right forelimb to the average of the left forelimb and left hindlimb
AVL = left forelimb to the average of the right forelimb and left hindlimb
AVF (foot) = left hindlimb to the average of the two forelimbs
How can heart rate be measured from an ECG?
- Measuring the R-R interval and dividing 60 (one minute) by this number
- Counting the number of PQRST complexes in 3 or 6 seconds and multiplying that by 20 or 10
What is the normal heart rate for
- dogs?
- cats?
Dog = between 70bpm-160bpm Cat = between 120-220bpm
What abnormality does a tall p wave indicate?
Right atrial enlargement
What abnormality does a wide P wave indicate?
Left atrial enlargement
What is myocardial ischaemia?
Blood supply to the heart is inturrupted
Define the following:
- bradycardia
2. tachycardia
- abnormally slow heart rate
2. abnormally fast heart rate
