Cardio Block 4: Excitation + ECG

Give the name of the specific pore which essentially links two cells of the myocardium together?

Connexin 45 or CX45 pore

Why can arrhythmias occur in myocardium

• One cardiac myocyte might have many different connections to allow it to transmit that signal throughout the myocardium. 

• a single cell could potentially fire a spontaneous action potential that could spread throughout the myocardium = this is the basis of many arrhythmias 

What mechanism stops arrhythmias from happening?

• cells of the myocardium have a refractory period where it is much more difficult to restimulate a cell once the depolarisation spreads through them

• therefore, refractoriness makes the spread of excitation UNIdirectional in normal physiological conditions

What channel ensures the refractory peroid occurs?

• voltage-gated sodium channels

• these channels are basically inactive within 2 milliseconds of the action potential initiating and they won’t recover again until the end of the action potential

Name the 2 refractory periods of myocardial cells:

1. Absolute Refractory Period

2. Relative Refractory Period

What is the difference between the ARP and the RRP?

• the ARP is where another action potential CANNOT be stimulated because ALL of the sodium channels are inactive

• the RRP is where another action potential could potentially be stimulated because SOME of the sodium channels have recovered from inactivation

What happens to the sodium channels to inactive them?

• a ball peptide on the end terminus of the channel moves into the pore and physically blocks it

• Once that’s plugged in place, it is not coming out again until the membrane potential repolarises in phase 3. 

What does the Electrocardiogram plot?

• voltage changes of the heart tissue

• against time

What heart problems can an ECG detect?

1. arrhythmias; tachycardia + bradycardia

2. atrial fibrillation

3. rate changes

4. rhythm disturbances

5. ischaemia (reduced blood flow to heart)

Draw and label a typical ECG graph:

What does the P wave represent?

• atrial depolarisation

What does the QRS complex represent?

• ventricular depolarisation

What does the R wave represent?

• atrial repolarisation occurs here but is hidden by ventricular depolarisation

What does the T wave represent?

• ventricular repolarisation

What is the ST segment?

• represents the period when the ventricles are fully depolarised and contracting, just before repolarisation begins

What deflection do you see on an ECG when a wave is moving towards an electrode?

• positive deflection (up stroke)

What deflection do you see on an ECG when a wave is moving away from an electrode?

• small negative deflection (down stroke_

When there is no NET movement of charge, what can be seen?

• an isoelectric flat line

How many ECG leads are there?

12

How many electrodes are there?

• 10

What are the two lead types for an ECG?

• limb leads

• chest leads

Where do the 6 chest leads lie for a 12 lead/unipolar ECG?

• on the perpendicular transverse plane

• from fourth intercostal space to sixth intercostal space

Where are the standard limb leads placed on bipolar ECG?

• Lead I = right arm → left arm

• Lead II = right arm → left leg

• Lead III = left arm → left leg

How does Bipolar lead arrangement measure heart activity?

• the leads record the electrical potential difference between 2 active electrodes

• forms Einthoven triangle

What is Wilson’s central terminal?

• a common virtual electrode

• an average of the measurements from RA, LA and LL to give an ‘average potential of the body’

How does unipolar lead arrangement measure the activity of the heart?

• uses on positive (exploring) electrode

• uses one indifferent/reference electrode

List the Leads in a unipolar ECG arrangement:

1. chest precordial leads V1-V6

2. limb leads: RA, LA, LL, RL

3. Augmented limb leads aVR, aVL, aVF

What are augmented/virtual limb leads?

•  virtual leads use a reference created by combining signals from multiple electrodes to estimate the electrical activity from a specific direction.

What does the aVF measure?

• the difference between positive LL and the mean of the negative RA and LA

What does the aVL measure?

• the difference between positive LA and the mean of the negative RA and LL

What does the aVL measure?

• the difference between the positive RA and the mean of the negative LA and LL

How can limb leads also be placed on the chest?

• the leads are arranged in a wheel

Draw a diagram of the 3 augmented unipolar leads, coupled with the 3 standard bipolar leads:

Which segment of the ECG normally shows a myocardial infarction?

• ST segment

• instead of flat it will have elevation

What view of the heart do Leads II, III and aVF give?

• inferior diaphragmatic surface of the heart

What view does LI. aVL, V5 and V6 give?

• lateral wall of left ventricle

What view of the heart do leads V1 and V2 give?

• septal surface (interventricular septum)

What view of the heart do leads V3 and V4 give?

• anterior wall of right and left ventricles - sternocostal surface of the heart

How do you calculate the heart rate from an ECG?

• count the number of large squares between an R and an R

• do 300 divided by that amount to get rate

What is ventricular APD?

• ventricular action potential duration

Faster heart rate, requires a ___ APD?

• shorter

longer QT period =

slower heart rate

shorter QT period =

faster heart rate

How is the QT period measured?

• intersection of T wave maximum slope with the isoelectric line