Flashcards in Lecture 11: The Cardiac Cycle, Overview and Events Deck (73):
What is the cardiac cycle?
A highly coordinated stereotyped series of events that occur repetitively with each heart beat
These events are responsible for the heart sounds, murmurs and pulsations observed on the physical
Governed by biophysics, specifically fluid statics and fluid dynamics
What is the purpose of the cardiac cycle?
1. To fill the ventricles to an optimal volume during diastole
2. execute a coordinated contraction of ventricular myocardium to eject blood into the great vessels during systole
What are the requirements for satisfactory circulatory performance?
1. Flow rate commensurate with tissue requirements
2. Appropriate pressure within a vessel or cardiac chamber to permit adequate fluid/solute exchange
Summarized as appropriate blood flow at a proper pressure
What is the difference between pressure and flow?
Pressure = normal force exerted by fluid on wall of vessel
-measured as difference between two pressures
-seen in Dynes/cm^2 and mmHg
Flow = ways to characterize te movement of a fluid or gas from one place to another
-determined by flow RATE (volume/time or L/min)
-determined by flow VELOCITY (distance/time or cm/s)
What are the two paradigms for understanding CV performance?
1. pressure-flow paradigm
2. velocity-displacement paradigm
What is the difference between flow RATE and flow VELOCITY?
Flow rate – VOLUME/time
Flow velocity –DISTANCE/time
What is the pressure-flow paradigm?
The pressure available to drive flow
Physicochemical forces determine amount of fluid exchange across capillary membranes
What is the velocity-(volume) displacement paradigm?
Relationship between velocity of the flow of blood to the VOLUME displacement of the blood itself
What is the significance of comparing the pressure-flow and the volume displacement-velocity paradigms?
As you increase pressure, you open valves and you have a large velocity (of volume of blood displaced)
Once there is no more pressure gradient, no blood transfer
What is Bernoulli’s paradigm?
The relationship between pressure and velocity
Inversely proportional (higher velocity^2 = less pressure)
-deltaP = deltaV^2
What is LaPlace’s paradigm?
Determines the wall forces required to pump blood and forces distending chambers and vessels
Tension and force associated with chamber cavity pressure and wall thickness
Force (tension) is directly correlated with pressure and radius in a vessel
Force (tension) is inversely correlated with thickness of the wall of the vessel
What is the LaPlace equation?
Wall tension = pressure x radius/wall thickness
What does inveigled mean?
To persuade someone to do something by means of deception or flattery
What are the two functional states of the myocardium?
Systole and diastole occur at different times in the atria/ventricles
What are the key characteristics of systole?
1. chamber muscle is in its active (contracting) state
2. chamber pressure increases, muscle shortens, and volume decreases
What are the key characteristics of diastole?
1. chamber muscle is in its relaxed state but diastole is an ACTIVE process so requires metabolic activity too
2. chamber pressure decreases (muscles lengthen, volume increases)
What are the ventricular phases for the cardiac cycle?
1. Isovolumetric relaxation
2. Diastolic filling
3. Isovolumetric contraction
4. Systolic ejection
What are the key characteristics of isovolumetric relaxation?
12-16 ms, semilunar valve closure to AV opening, rapid ventricular pressure decline due to the ventricular myocardial active state terminaton
Ends when atrial pressure higher than ventricular pressure (which precipitates the opening of the AV valves)
Volume does not change
What are the key characteristics of diastolic filling?
150-800 ms, aV valve opening to AV valve closing, progressive filling of ventricles from atria finished by contribution from atrial contraction
Majority of filling occurs in EARLY diastole
Only a little bit of filling at late diastole (due to LaPlace relationship that is overcome by atrial contraction then)
Why is filling rate not uniform?
1. myocardial diastolic compliance is length dependent. As ventricles fill, the pressure increment required to achieve a volume increment increases
2. Atrial systole occurs at the end of the diastole delivering a final increment of blood into the ventricle just before the end of diastole
What are the key characteristics of isovolumetric contraction?
10 ms, AV valve closure to semilunar valve opening
Rapid ventricular pressure increase due to development of ventricular myocardial active state
What are the key characteristics of systolic ejection?
250-280 ms, semilunar valve opening to semilunar valve closure ejection due to shortening of ventricular muscle
When does atrial systole and ventricular systole overlap?
During isovolumetric contraction
Ventricular pressure is much higher so even with atrial contraction, the AV nodes are closed
When does atrial systole end/diastole starts?
At the beginning of systolic ejection (right after isovolumetric contraction)
What is the timing relationship between atrial and ventricular events? (
Atrial systole occurs 120-160ms before ventricular systole
Achieved by delay of passage of activation wavefront through AV node
Atrial systole is shorter than ventricular systole but they OVERLAP
Last bit of atrial systole occurs after AV valve closes
How do the waveforms of atria and ventricle compare?
Ventricle has longer AP than atria
Why is diastole an active process?
Because ATP is required for reuptake of calcium ions in order for muscles to relax
Also ATP is required to terminate myosin/tropomyosin binding
What happens if ventricular filling is disrupted?
Not all of atrial blood goes to ventricle
Therefore backs up the pressure behind the left atria
Can lead to diastolic heart failure
What is diastolic heart failure?
When there is a failure to relax (so that the cardiac chambers are less compliant and less blood comes in)
Less compliant ventricle = more blood left in the LA = elevated systemic and pulmonary venous pressure
Can be caused by an inability of heart muscles to adequately take up Ca into the SR (diastolic dysfunction of ventricle)
Less blood perfusion as well
What is the function of cardiac valves?
Directs flow of blood through the heart
Helps ventricles cycle between equilibrium pressure with atria and great vessels
Open to permit unobstructed flow from upstream to downstream chamber
Also prevents backward flow
PASSIVE structures that respond to hemodynamic forces
What are the components of the AV valves?
1. valve annulus
2. valve leaflets
3. Chordae tendinae (attached to papillary muscles)
4. Papillary muscles (attached to ventricles)
What are the requirements for satisfactory closure of AV valves?
1. passive movement of leaflets toward atria driven by ventricular pressure
2. contraction of circumference of the valve annulus
3. contraction of the papillary muscles to control position of the valve leaflets
4. appropriate contraction of the ventricle to control the position of the papillary muscles
Disruption of any of these 4 steps = regurgitation
What are the key characteristics of the semilunar valves?
Less complicated than AV valves
Valve opening is passive
Requirements for competence
i. appropriate valve annulus diameter
ii. appropriate leaflet geometry
What are the consequences of valve dysfunction?
1. regurgitation: failure to close properly
2. stenosis: failure to open properly
What are the functions of the aortic valve?
1. Allows equal LV and Ao pressure during systole
2. maintains Ao pressure > LV pressure during diastole
What are the functions of the mitral valve?
1. Allows equal LV and LA pressure during diastole
2. Maintains LV pressure > LA during systole
1. Allows equal LV and LA pressure during diastole
2. Maintains LV pressure > LA during systole
1. rapid upstroke at the onset of ventricular systole
2. rounded peak in mid and late systole
3. gradual decay of pressure during diastole
What is the dicrotic notch?
What is seen in the “rounded peak” phase of the arterial pressure waveform
Indicative of semilunar valve closure (old thinking)
New thinking: caused by pressure wave reflections
Comes back and hits the aorta right after ventricular systole ends (nice timing)
Augments ascending aortic pressure during diastole
Facilitates coronary artery flow
What are the determinants of the arterial pressure waveform?
1. ejection into the great vessel
2. Great vessel compliance (energy of ventricular ejection is stored by distension)
3. Reflected pressure waves
What are the four parameters of ARTERIAL pressure waveform?
1. systolic pressure
2. diastolic pressure
3. pulse pressure
4. mean pressure
What is pulse pressure?
The arithmetic difference between systolic and diastolic pressure
Systolic – Diastolic = pulse pressure
What is mean pressure?
The time-averaged pressure over multiple cardiac cycles
What are the components of the ATRIAL pressure waveform?
1. A wave
-positive deflection due to atrial contraction
-occurs just before the end of ventricular diastole
2. X descent
-negative deflection due to atrial diastolic relaxation
3. V wave
-positive deflection due to atrial filling
4. Y descent
-negative deflection due to rapid atrial emptying
-occurs at the start of ventricular diastole
**C wave = Closure of AV valves during atrial systole but not mentioned here**
What is the A wave?
Increase in atrial pressure due to atrial kick
What is the C wave?
Increase in atrial pressure due to closure of AV valve during atrial systole (atrial kick)
He says it is a figment of people’s imagination…doesn’t exist lol
What is the V wave?
Increase in atrial pressure when blood is filling the atrium during diastole
What is the X descent?
Negative deflection after A/C wave that demonstrates atrial diastolic relaxation
What is the Y descent?
Negative deflection due to rapid atrial emptying (so less blood volume in the atrium)
Occurs at start of ventricular diastole
When does the majority of atrial filling occur?
During ventricular systole
What are the characteristics of the VENTRICULAR pressure waveform?
Cycles between the shape/magnitude of the arterial wave during systole (when semilunar valves are open)
The atrial wave form during diastole (when the AV valves are open)
What is LVEDP?
Left ventricular end diastolic pressure
Pressure in the left ventricle at the end of ventricular diastole
Principal determinant of Preload
Also indirect measure of LV myocardial fiber length at point of ventricular systole
What is the relationship between the LV and systemic arterial system?
Known as Ventriculo-vascular coupling
Systemic arteries deliver blood from LV to capillaries
Systemic arteries also cushion the cardiac pulsations so capillary blood flow is
What is the importance of having elastic tissue in large arteries?
1. It dampens the pressure oscillations that accompany ventricular ejection
2. It shows the transmission (and reflection) of the arterial pressure pulse, which is a major determinant of the wave form of arterial pressure pulses
3. compliance of elastic artery stores blood to maintain diastolic pressure
What happens to large arteries as people age?
Elastic tissue is replaced by fibrous tissue
Therefore arterial systolic and pulse pressure increases
HTN that requires LV to generate higher systolic pressure
This then increases the stress on the large artery walls, leading to more fibrosis
How does the pressure pulse wave reflect?
It gets reflected from sites in the distal arterial system
Reflected wave propagates retrograde back to aortic valve
Takes 160ms to get to distal arterial system and 160ms to get back to aortic valve
This reflection increases the arterial pressure felt during ventricular diastole, thereby dampening pulse pressure (since diastolic pressure increases)
How does blood pressure go up as you get farther from the heart?
Pressure wave reflections
Arteries farther away from heart are CLOSER to the point of wave reflections
Therefore, they get the bump up in pressure sooner (80ms) vs the aorta which is farther away from site of pressure wave reflection (160ms)
How does wave reflections affect a young individual?
Early arrival of reflected wave augments SYSTOLIC pressure
-as can be seen in the higher pressures in the iliac arteries
Late arrival of reflected wave augments DIASTOLIC pressure
-as can be seen by dichrotic notch of aorta
What are the consequences of the increase of pulse wave velocity as people age?
The reflected wave form will get to aortic valve early enough to add to systolic pressure
Therefore, the faster the reflected wave travels, the greater the systolic pressure becomes
This can lead to HTN
What happens to pulse wave velocity as people age?
Normal = 5 M/s
Older people = 12 M/s
Why do older individuals have BPs that are 140/50?
High systolic due to the fact that the pressure waveforms get back to aorta faster (133ms vs. 320 ms in young adults)
This means that your “dichrotic notch” occurs during ventricular systole rather than after it
Low diastolic due to the decreased compliance of vessels
Less compliance = less stored blood by elastic large vessels = less blood to flow during diastole
What is diastole?
The pressure that is exerted on the walls of the various arteries when the body is relaxed
Maintained by the blood stored by the elastic arteries (or else flow would turn off like a faucet)
How does the compliance of elastic vessels maintain diastolic pressure?
When blood is pumped to elastic arteries, those arteries expand and “store” some blood that is then released as soon as ventricular systole is over
Thus, vessel compliance keeps BP within a relatively narrow range
Why can large elastic arteries be thought of as capacitors?
Because they store blood during systole and release it during diastole to maintain diastolic pressure
Diagram below shows how the loss of vascular compliance and increased pulse wave velocity leads to HTN in older individuals
Why do older individuals have greater pulse pressures?
Because systolic pressure is getting higher due to faster reflected wave lengths (due to fibrotic vessels and the continuity equation, since smaller area = higher velocity)
And diastolic pressure is getting lower due to decreased compliance/ability to store blood in elastic arteries
Pulse pressure = systolic – diastolic pressure
Why are reflected pulse waveforms faster in older individuals?
Older people have fibrotic vessels that are less compliant
Less compliance = small cross sectional area
As per the continuity equation, the velocity in a small area chamber is higher (so it travels and reflects faster)
What is the difference between relative pressure values on the left and right sides of the heart?
1. Right side systolic pressures are substantially lower than left side
2. right sided diastolic pressures are lower than left side
What is the difference in systolic pressure for right and left side?
Right side: Peak pulmonary artery systolic pressure = 25mmHg
Left side: peak aortic systolic pressure = 120mmHg
Difference is due to the difference in level between heart and lungs and heart and head
What is the difference in diastolic pressure for right and left side?
Mean RA pressure = 5mmHg
Mean LA pressure = 12mmHg
Difference due to variation in diastolic compliance between LV and RV
RV is MORE compliant than LV, therefore the RA needs less pressure to dump its blood reservoir off to the RV
What are the clinically important intracardiac pressure variables?
1. aortic pressure
2. Left ventricular end diastolic pressure (LVEDP)
3. Mean left atrial pressure
4. Right atrial pressure
What is the significance of aortic pressure?
1. The perfusion pressure that drives the systemic circulation
2. The load that the left ventricle has to support
3. The load that distends the arterial system
What is the significance of the LVEDP?
The pressure that achieves the LV dimension at end diastole
Determinant of myocardial fiber length at systole (preload)
What is the significance of the mean left atrial pressure?
The pressure that generates left ventricular end diastolic pressure
The pressure that pulmonary capillaries are exposed to