Lecture 20 : Cardiac Cycle & Control Flashcards
What is the difference between a normal ECG and an ECG of a 2nd degree AV block?
Normal ECG: Nice spaced out wavs
AV block:
* Missing a QRS complex - signal is getting blocked at AV node
* Atria depolarizing normally
What are arrythmias?
The uncoordinated atrial and ventricular contractions caused by a defect in the conduction system
What is a fibrillation?
A rapid and irregular (usually out of phase) contraction where the SA node is no longer controlling heart rate
What does atrial fibrillation cause and how is it prevented?
Can cause clotting (stroke) and inefficient filling of the ventricles
- Warfarin used to prevent stroke
What does an ECG of atrial fibrillation look like?
Tracing shows tiny, irregular “fibrillation” waves between heartbeats - rhythm is irregular and erratic
What is ventricular fibrillation?
The ventricles pump without filling and if the rhythm is not rapidly reestablished then circulation stops and brain death occurs
- More life threatening
What is the treatment for ventricular fibrillation?
Defibrillation - the application of an electrical stimulus to shock the heart back into a normal SA rhythm
What is the cardiac cycle?
All the events involved with the blood flow through the heart during one heart beat
What is systole?
Contraction phase
- Begins with ventricular contraction
- Ends when ejection finishes
What is diastole?
Relaxation phase
- Begins when ejection ceases as ventricles relax
- Ventricular filling begins after sufficient relaxtion occurs
What type of events come together to form the cardiac cycle?
- Electrical events - spontaneous - ordered sequence
- Followed by mechanical events - coordinated - contraction and relaxation
- Both sides of the heart contract at the same time
- Pressure changes close valves at appropriate times
How long is one cardiac cycle?
For a heart rate of 72 beats/minute, each cardiac cycle = 0.8sec
* Systole – 0.3 sec
* Diastole – 0.5 sec
What are the 5 phases of the cardiac cycle?
- Atrial systole
- Isovolumetric contraction - ventricular systole
- Ventricular ejection - ventricular systole
- Isovolumetric relaxtion - ventricular diastole
- Ventricular filling - ventricular diastole
Explain the process of atrial systole:
- Late diastole, atrial depolarisation (P wave on ECG)
- Atrial contraction
- Atrial pressure rises
- Last 10-20% blood ejected from atrium into ventricle ”top up”
Explain the process of isovolumetric contraction:
- Ventricles are filled
- Ventricles depolarise (QRS complex on ECG)
- Ventricles contract
- Ventricular pressure rises
- AV valve closes (first heart sound,S1)
- All valves are shut
- No blood flow
- No blood volume change
- Isovolumic contraction
Explain the process of ventricular ejection and atrial refill:
- Ventricular pressure exceeds aortic pressure aortic valve opens
- Blood ejected into aorta
- Arterial blood volume and pressure increase
- LVP and AP rise in parallel
- 2/3 of blood ejection occurs in first 1/3 of ejection time - rapid ejection
- Late systole
- Both LVP and AP fall
- Slow ejection
- Repolarisation of ventricles (T wave in the ECG)
- LVP falls below AP
- Semilunar valves close (2nd heart sound, S2)
Explain the process of isovolumetric relaxation:
- All valves are shut
- No change in blood volume in ventricle
- Minimal Ventricular volume
(end-systolic volume (ESV)) - Ventricles are relaxing
- Atrial pressure lower than ventricular
- Atrium fills with blood returning to heart
- Atrial pressure rises above ventricular pressure
Explain the process of ventricular filling:
- AV valve opens passively
- Ventricular filling begins
- 80-90% ventricular filling occurs passively down pressure gradient
How long is each phase of the cardiac cycle?
- Systole 1: 0.05s
- All valves shut
- Isovolumetric Contraction
- Systole 2: 0.3s
- SL valves open
- Ventricular Ejection
- Diastole 1: 0.05s
- All valves shut
- Isovolumetric Relaxation
- Diastole 2: 0.6s
- AV valves open
- Ventricular Filling, Atrial “top-up”
Describe the ventricular blood volumes in each phase of the cardiac cycle:
Diastole:
- Constant during isovolumic relaxation
- Early Rapid Ventricular Filling
- Late Slow Ventricular Filling
- Top up after Atrial Contraction
- Maximum at end of diastole (EDV) after atrial systole
Systole:
- Constant during isometric contraction
- Rapid fall during early ejection
- Slow fall during late ejection
- Minimum at end-systole (ESV)
Describe the ventricular pressure at each phase of the cardiac cycle:
Diastole:
- Drops to minimum at beginning of diastole after isovolumetric relaxation
- Rises a little during ventricular filling and
- Atrial Contraction
Systole:
- Rises dramatically during isometric contraction
- Continues to rise to a maximum during ejection phase
- Falls during late-systole
Describe the arterial/aortic pressure at each phase of the cardiac cycle:
Diastole:
- Continues to fall during diastole to minimum
- Minimum at end of diastole/start of systole (Diastolic blood pressure)
Systole:
- LVP > AP valves open
- Rises to a maximum (Systolic blood pressure)
At what phases of the cardiac cycles do the heart sounds occur?
- First heart sound (S1)
- Closure of AV valves
- Low, long sound (lub)
- Second heart sound (S2)
- Closure of semilunar valves
- Short, high pitch (dub)
When are the AV valves open/closed?
AV open:
- Mid diastole
- Late diastole
- End diastole
AV closed:
- Early systole
- Mid systole
- End systole
- Early diastole
When are the semilunar valves open/closed?
Semilunar open:
- Mid systole
- End systole
Semilunar closed:
- Early diastole
- Mid diastole
- Late diastole
- End diastole
- Early systole
How can the cardiac cycle change?
Heart does not function under constant conditions
e.g posture, exercise, volume depletion
-> Cardiac function needs to be adaptable
CO = HR x SV (ml/min)
-> Control / alter HR or SV or both
