Week 3 Flashcards
The cardiac cycle
The movement of blood through the heart due to pressure changes generated by mechanical activity
Electrical activity-> mechanical activity-> pressure changes-> volume changes
ABP= SV * HR * TPR
Echocardiography: used to assess volume and function of heart
Cardiac catheter- the pressure and volume changes in the aorta and left ventricle can be measure by introducing catheters into these structures in cath lab
Basic principles of cardiac cycle
- Pressure (P) will increase in a chamber when muscle around it contracts. Amount of pressure generated is dependent on thickness of muscle around chamber so LV pressure is higher than RV pressure
- Valves will open when there is a pressure/energy gradient across them. Energy of blood= pressure* momentum
- Blood will flow down a pressure/energy gradient
- When valves are open, pressures in neighbouring chambers change together
- When valves are closed pressures in neighbouring chambers can be different
Partial> P ventricular AV valve opens
Heart rate=70bpm. Cardiac cycle= 850ms
If heart rate increases cardiac cycle duration decreases
Diastole 2/3rd- 600ms systole 1/3-250ms
If Heart rate increases its the length of diastole that shortens significantly
Ventricular diastole
- AV (mitral) valve open Patrial>P ventricular. Diastole ventricles filling with blood both atria and ventricle relaxed
- Aortic valve closed Pventricular< P aortic
- Blood flows into ventricle Ppulm vein> Patrial> P ventricular
Ventricular diastole, atrial systole, ventricular filling
- Atrial contraction increases Patrial
- Contributes ~5ml to ventricular filling at rest. Most filling done in diastole
Atrial wall is thin so contraction only gives small increase in pressure
In situations where a greater cardiac output is required and heart rate is faster the atrium can be stimulated to contact with greater force so more blood flows into ventricle. This offsets reduction in passive filling time as diastole shortens
Ventricular systole
Isovolumetric contraction: both valves closed volumes remain same. Main function to increase pressure in ventricle very rapidly so it > pressure in aorta
1. Ventricular contraction increases Pventricular large and rapid increase in pressure, wall is thick
2. A-V valve closes Pventricular>Patrial
3. Aortic valve is closed Pventricular<Paortic
Both valves closed, ventricle is a closed system
What keeps aortic valve open during ejection when Paortic> P ventricular
- Aortic valve opens Pventricular>Paortic
- Ventricular ejection into aorta. High velocity lots momentum
Halfway through ejection Paortic>Pventricular but momentum of blood leaving is large enough that energy of blood being ejected is greater than that in aorta
Despite reverse pressure gradient aortic pressure> ventricular pressure, ejection still continues
Second half of ejection phase the force of contraction decreases and pressure in ventricle and aorta begins to decrease
Ventricular diastole isovolumetric relaxation
Ventricular repolarisation will lead to ventricular relaxation decrease ventricular pressure. T wave
1. Ventricular relaxation decreases Pventricular
2. Aortic valve closes energy of blood leaving ventricle is reduced
3. AV valve closed Pventricular> Patrial
Isovolumetric relaxation- ventricle closed system both valves closed volume remains same
Pressure decreases readily important for Pv<Patrial> P ventricular
- as blood has been moving into atria while valve closed blood can enter ventricle very rapidly once open assisted by relaxation ventricle drawing blood in</Patrial>
Order of cardiac cycle
Ventricular diastole: rapid/passive filling
Atrial systole
Ventricular systole: Isovolumetric contraction
Ventricular systole: ejection
Ventricular diastole: isovolumetric relaxation
Ventricular diastole- filling phase Wiggers diagram
During the filling phase atrial pressure is slightly greater than ventricular pressure
This is enough of pressure gradient to allow blood to flow for the atria to ventricles, ventricular volume increases
There’s only a small amount ventricular filling during second half of diastole:
-in this stage the aortic pressure is much greater than ventricular pressure. Aortic pressure is also falling as blood flows off into peripheral circulation
Towards end of diastole the start of the p wave indicated atrial depolarisation this leads to atrial contraction
Contraction of the muscles leads to increase in pressure in that chamber. This corresponds to increase in atrial pressure however only small pressure change due to the atriums thin muscular wall
Pressure in the neighbouring chambers will change simultaneously if valves between them are open
Therefore as the AV valve is open the ventricular pressure is also changing along with the change in atrial pressure
Atrial contraction leads to small amount of ventricular filling
Ventricular systole- Isovolumetric contraction phase
The QRS complex of ECG represents ventricular depolarisation which in turn triggers ventricular contraction:
-contraction of the ventricular muscles causes ventricular pressure to rise >atrial pressure causing mitral valve to close-> ‘lub’ sound
-at this time aortic valve closed since ventricular pressure< aortic pressure
Therefore ventricle is closed system no blood being ejected no change in volume. ISOVOLUMETRIC CONTRACTION PHASE
Large pressure being generated because of thick muscle wall. Small peak of atrial pressure due to AV valve pressing back into atria
The main function of the Isovolumetric contraction is to raise ventricular pressure from low levels required for filling to above diastolic pressure in aorta so aortic valve can open and ejection can occur
What is diastolic pressure
The pressure in the aorta immediately before aortic valve opens
Must be the lowest pressure in aorta in cardiac cycle as pressure starts to rise as soon as valve opens and blood is ejected
Ventricular systole- ejection
As soon as pressure in the ventricle exceeds aortic pressure the aortic valve will open
Because the valve is open any pressure changes in ventricle are reflected in aorta:
-so both aortic and ventricular pressure increases and blood is ejected from ventricle into aorta due to slight pressure gradient
-ventricular volume falls rapidly during this phase
-about half way through ejection, aortic pressure exceeds ventricular pressure however ejection continues due to momentum of blood creating energy gradient between ventricles and aorta keeping aortic valve open despite reverse pressure gradient
Towards end of ejection phase; the momentum of blood and the aortic and ventricular pressure start to fall and the speed of ejection slows as ventricles start to relax
Ventricular diastole- Isovolumetric relaxation phase
The T wave on the ECG represents ventricular repolarisation leads to ventricular relaxation
As ventricle starts to relax the slowing of ejection means that the momentum of the blood will no longer keep the aortic valve open due to the aortic-ventricular pressure gradient and so the aortic valve will close - gives ‘dub’ sound
This means the ventricle is closed system as the AV valve and aortic valve closed
As ventricle continues to relax the ventricular pressure continues to fall rapidly Isovolumetric relaxation volume in chamber stays same pressure decreases
The main function of isovolumetric relaxation is to reduce ventricular pressure below atrial pressure so the AV valve can open and ventricular filling can start
Atrial pressure gradually increases through ejection and isovolumetric relaxation phased of cardiac cycle as atria continue to fill from pulmonary circulation whilst AV valves closed
Ventricular diastole- filling
The Isovolumetric relaxation phase ends when ventricular pressure falls below atrial pressure:
-this change in the pressure gradient allows the AV valve to open
-rapid filling of heart commences as blood is held in the atrium during previous phases can now rapidly move into the ventricle as soon as the AV valve opens. Steep gradient on ventricular volume trace
All phases repeat for each cardiac cycle
What happens if theres an increase in heart rate
The 2 phases that make up systole:
-isovolumetric contraction and ejection
Systole- 250ms Isovolumetric contraction= 50ms, ejection =200ms. Timing of these phases can’t change significantly
Diastole=600ms Isovolumetric relaxation=80ms filling=520ms
Timing of Isovolumetric relaxation cant change significantly so only passive filling phase allows us to change duration cardiac cycle
Length passive filling decreases with increasing heart rate
P wave can move a long way to the left before ventricular filling is compromised, occurs so atrial depolarisation occurs earlier
A greater force of atrial contraction can then compensate for the decrease in passive filling
The increase in sympathetic activity to the atria will increase force of atrial contraction and top up volume of blood in heart to greater extent than 5ml at rest
Heart rate would need to reach 180bpm before ventricular filling is compromised
