Unit 2 Lecture 4: Electrocardiogram & Cardiac Cycle Flashcards
What is an ECG defined as?
The sum of all cardiac action potentials (multiple cardiac cells not just one)
- Electrical activity in the heart comes from cardiac muscle
- Around the membrane of cardiac muscle cell
Explain the P-wave, QRS complex, and T-wave?
- P-wave is atrial depolarization which is also ventricular diastole
- QRS complex is ventricular depolarization and large amplitude indicates large electrical activity
- T-wave is ventricular repolarization
Flat line after T-wave is the ventricle filling (ventricular diastole)
Why does the QRS complex have such a large amplitude compared to the P-wave?
QRS complex is comprised of ventricles which the heart contains more cells of cardiac muscle cells meaning more electrical activity is occuring
Why is the QRS complex delayed after the P-wave?
AV nodal delay (P-R segment)
P-R segment is also where atrial contraction occurs
What is the ST Segment?
Plateau phase of the action potential which is contraction of ventricles (accumulation of Ca2+)
Why is the QRS complex so fast?
Due to the purkinje fibres spreading all around the ventricles so it passes the signal through ventricle muscle cells much more quickly
T-wave is slow because it depends on cell-to-cell communication unlike QRS which relies on purkinje fibres to speed up communication and passing signal
Explain what the underlying problem is for each case
- Extrasystole - Large disturbance of electrical signal in QRS complex so there is a disturbance in the ventricle
- Ventricular Fibrilation - Not fully relaxing and fully contracting. Ventricles are contracting in a rapid and uncoordinated manner
- Complete Heart Block - Multiple P-waves before the QRS complex is reached so there is a problem between the SA and AV node where they are not coordinated. This is also why the QRS has a high amplitude due to AV node pacemaker potential running
- Myocardial Infarction - Blood supply blockage
What is the main idea of the cardiac cycle?
Blood flows via bulk flow: F = ΔP/R
Pressure differences are generated by muscle contraction
What is systole known as?
Ventricular contraction and ejection
Explain how Systole works?
- Contains Isovolumetric Ventricular Contraction which is no volume change but contraction still occurs due to pressure changes
- Ventricular ejection is just blood moving out of the ventricles
In systole the AV, Pulmonary and Aortic valves are closed. What is the reason for that?
Isovolumetric ventricular contraction shows that the pressure is greater in the ventricles than the atria so the AV valves are closed but the pulmonary and aortic valves are closed too because not enough pressure from the ventricles is being generated as contraction occurs
Pulmonary and Aortic open up at the end of ventricular contraction when pressure overwhelms both - ventricular ejection (blood flows out through the pulmonary and aortic valve)
What is Diastole known as?
Ventricular relaxation and filling
Explain how Diastole works?
- Isovolumetric ventricular relaxation there is no volume change and all the valves are closed
- Ventricular filling blood is passively flowing into the ventricles during diastole (AV valves open)
- Ventricle filling also involves the atria contracting so it has to have more pressure in order for the ventricles to fully fill
NOTE: Most of the filling of the ventricles is due to passive filling while the remainder is from atrial contraction which we can see on Wigger’s diagram where the atrial pressure drops at first during ventricular filling and then starts to increase halfway indicating that the atria are contracting to complete ventricular filling
What indicates your diastolic and systolic pressure?
Aortic pressure
- End of diastolic volume = diastolic pressure
- End systolic volume = systolic pressure
What is a dicrotic notch? Where does it occur?
Dicrotic notch is when aortic pressure suddenly jumps up slightly due to the blood being pumped through the ventricles into the aorta (right before isovolumetric relaxation.) This occurs in the ventricles to aorta, and the aorta due to it being elastic creates a pressure against itself which causes this dicrotic notch to exist.
REMEMBER: Dicrotic notch is due to aorta being elastic and happens during isovolumetric relaxation
Explain Wiggers Diagram with respect to how it correlates to Left Ventricular Volume
- End of diastole, blood fills into ventricles & atrial pressure must be greater than ventricles to keep AV valves open for filling
- Atrial depolarization occurs where we see atria contract right after to push the final bit of blood into the ventricles
- End-diastolic volume is reached where ventricles are fully filled and are relaxed
- Ventricular depolarization ==> Ventricles contract to generate pressure greater than atria (AV closes to make first heart sound)
- Aortic Valve continues to stay closed during ventricles contracting until the pressure is overcome by ventricular pressure (during isovolumetric contraction)
- Aortic valve opens and aortic and ventricular pressure increase to around 120 (systolic pressure) but ventricular volume decreases
- End-systolic volume is reached and the Aortic Valve closes (2nd heart sound is heard)
- Ventricular pressure is greater than atrial but aortic is greater than ventricular so all valves stay closed; dicrotic notch happens
- Ventricular repolarization occurs during isovolumetric relaxation and atrial pressure becomes slightly greater than ventricular pressure which allows for the filling of the ventricles and the AV valves to open to do said filling
- While atrial pressure is greater we then see rapid filling (passive filling) & reduced filling due to atria contracting
Note: At step 8. dicrotic notch comes into play which allows aortic pressure to stay at 80mmHg during isovolumetric filling which allows for all valves to be closed until that pressure is overcome
