Lecture 55 - The Cardiac Cycle Flashcards
What prevents the atria and the ventricles from contracting at the same time?
What is this made of?
The fibrous skeleton of the heart
It is made of fibrous connective tissue
Compare the pressure in the two sides of the heart
Right side: low pressure
Left side: high pressure
Compare the right and left ventricles’ structure
Right: thin muscular wall
Left: thick muscular wall
Compare the function of the right and left ventricles
Right: pumps deoxygenated blood to the lungs
Left: pumps oxygenated blood to the body
Which circulation has greater resistance?
Systemic has greater resistance than pulmonary
What generates the force in the heart?
The contraction of the muscular walls of the heart
What are the stages of ventricular systole?
Isovolumetric contraction
Ventricular ejection
What are the stages of ventricular diastole?
Isovolumetric relaxation
Ventricular filling
Describe the mechanical events of the cardiac cycle
- Late diastole: chambers filling passively
- Atrial systole: blood forced into ventricles
- Isovolumetric ventricular contraction
- Ventricular ejection
- Isovolumetric ventricular relaxation
Describe the electrical events of the cardiac cycle
- SA node depolarisation
- P wave: atrial depolarisation
- P-Q segment: conduction through AV node and bundle
- Q wave: depolarisation through bundles of His
- R wave: depolarisation at apex of heart
- S wave: depolarisation through purkinje fibres through ventricle
- S-T segment: ventricular contraction
- T wave: ventricular repolarisation
Describe what is happening at the SA node
Autorhythmic
70 bpm
What controlls the path of conduction through the heart?
Insulation
Fibrous skeleton
How is depolarisation conducted from the SA to the AV node?
Internodal pathways
Compare depolarisation rates throughout the heart
Quick:
• along internodal pathways
• bundles of His
Slow:
• through atria
Why would it be pointless to have sustained contraction of the heart?
How is this overcome?
This would not allow the heart to function properly, as the principal function of the heart is to pump blood. Sustained contraction would not allow the heart to refill
The refractory period ensures that the heart can refill between beats.
Describe how an ECG is recorded
3 leads placed on the body
Einhoven’s Triangle
Leads detect the direction of flow of electric current.
Describe what we see on an ECG recording.
Draw this
Slide 17
P wave
QRS complex
T wave
What does the P wave represent?
Atrial depolarisation
What does the Q wave represent?
Depolarisation of the Bundles of His
What does the R wave represent?
Spread of depolarisation around the apex of the heart
What does the S wave represent?
Spread of depolarisation up through the ventricles
What does the QRS complex represent as a whole?
Ventricular depolarisation
What does the T wave represent?
Ventricular repolarisation
What does a reading above the baseline on an ECG recording indicate?
Net movement of charge towards the positive electrode
What do we look for in a normal ECG?
- Is the rate regular?
- Is rhythm regular
- Are the waves regular?
- One QRS compex per P wave?
- Is P-R segment regular in length?
What is regular heart rate?
60-100 bpm
What is atrial fibrillation?
Erratic atrial activity; not contracting in a coordinated manner.
Depolarisation does not reach the ventricle each time
What is ventricular fibrillation?
Erratic electrical activity in the ventricle
Ventricle beating on its own, under the ventricular pacemakers
What causes ventricular fibrillation?
Damaged muscle of the heart
How can ventricular fibrillation be overcome?
To restore the normal rhythm, the heart must be in the refractory period. This doesn’t happen during ventricular fibrillation.
De-fibrillator give a high voltage shock to synchronise all the muscle
What causes the valves of the heart to open an close?
No muscle - it is a passive process.
The pressure changes cause the valves to open and close
When do the AV valves open?
When pressure in the atria is greater than in the ventricles
When do the AV valves close?
When pressure in the ventricles is greater than in the atria
When do the semilunar valves open?
When pressure in the ventricles is greater than in the vessel (aorta, pulmonary trunk)
When do the semilunar valves close?
When pressure in the ventricles is less than in the vessels (aorta, pulmonary trunk)
Which side of the heart has the tricupsid valve?
The right side of the heart
What is happening during isovolumetric ventricular contraction?
The muscle of the ventricles is contracting
However, the valves aren’t open.
The pressure in the ventricles is rising up to 120 mmHg
Why does blood flow into the ventricles?
Blood has been ejected from the ventricles, and the muscle of the ventricles in relaxting, so the pressure in the ventricles drops very low.
The pressure in the atria is higher than in the ventricles, so blood flows into the ventricles
What happens in rheumatic heart disease?
AV valves aren’t closing fully during ventricular contraction
Blood flowing back into the atria
Turbulent flow, engendering a heart murmur
What are heart sounds due to?
What are they?
Closure of valves
Sound 1: AV valves closing
Sound 2: Semilunar valves closing
Does blood flow cause heart sounds?
Laminar flow (ie normal blood flow) does not bring about heart sounds
Turbulent flow through a leaky valve will bring about a murmur
At which point in the ECG will the AV valves close?
QRS complex
ie ventricular contraction
At which point during the ECG will semilunar valves close?
T wave
What things does the Wigger’s diagram show us?
1/ ECG 2/ Pressure in: • aorta • left ventricle • left atrium 3/ Heart sounds 4/ Left ventricular volume
