2. Cardiac Cycle And Output Flashcards
As heart rate increases, what happens to the length of systole and diastole?
Systole - stays same length
Diastole - shortens
What are the seven phases of the cardiac cycle?
Atrial contraction Isovolumetric contraction Rapid ejection Reduced ejection Isovolumetric relaxation Rapid filling Reduced filling
What happens in phase 1 of the cardiac cycle, atrial contraction, including what would be seen on a Wiggers diagram?
End diastole Atrial pressure rises (A wave on left atrial pressure) Accounts for 10% of ventricular filling P wave on ECG Reaches end-diastolic volume
What happens in phase 2, isovolumetric contraction?
Start systole
Mitral value closes (C wave on left atrial pressure curve)
QRS complex on ECG signifies onset of ventricular depolarisation
Isovolumetric
Rapid rise in ventricular pressure
Closure of mitral and tricuspid valves create first heart sound (S1)
What happens in phase 3 of the cardiac cycle, rapid ejection?
Aortic valve opens
Atrial pressure decreases as base is pulled down (X-descent on left atrial pressure curve)
Decrease in ventricular volume
What happens in phase 4, reduced ejection?
End systole
Repolarisation of ventricle - pressure and output decreases, T wave on ECG
Atrial pressure rises (V wave on left atrial pressure curve)
Aortic valve still open
What happens in phase 5 of the cardiac cycle, isovolumetric relaxation?
Start diastole
Aortic valve closes as intraventricular pressure falls, gives dicrotic notch in aortic pressure curve
Ventricular volume remains constant, as all valves closed, reaches end systolic volume
Closure of aortic and pulmonary values gives second heart sound (S2)
What happens in phase 6, rapid filling?
Mitral valve opens
Atrial pressure falls (Y-descent on left atrial pressure curve)
Ventricular filling is normally silent, but third heart sound sometimes present, is normal in children but can be a sign of pathology in adults
What happens in phase 7 of the cardiac cycle, reduced filling?
Ventricle begins to reach its inherent relaxed volume, so rate of filling slows, but further filling is still driven by venous pressure.
90% full by end of phase
What are the causes of aortic valve stenosis?
Degenerative (senile calcification/fibrosis)
Congenital (bicuspid)
Chronic rheumatic fever, leading to inflammation and commissural fusion
What heart sound does aortic valve stenosis cause?
Crescent-decrescendo murmur
What can aortic valve stenosis lead to?
Increased LV pressure - LV hypertrophy
Left sided heart failure - Syncope and Angina
What causes aortic valve regurgitation?
Aortic root dilation
Valvular damage caused by endocarditis rheumatic fever (group A streptococcus)
What heart sound does aortic valve regurgitation make?
Early decrescendo diastolic murmur
What does aortic valve regurgitation lead to?
Increased stroke volume (more blood in ventricle)
Increased systolic pressure and decreased diastolic pressure, thus increasing pulse pressure
Bounding pulse due to increased pulse pressure. Gives head bobbing and Quinke’s sign (nail beds changing colour with heart beat)
LV hypertrophy
What causes mitral valvular regurgitation?
Myxomatosis degeneration, weakening the tissue, leading to prolapse
Damage to papillary muscle after heart attack
Left sided heart failure leading to LV dilation
Rheumatic fever leading to leaflet fibrosis
What does mitral valve regurgitation lead to?
Increased LA preload, more blood enters LV in subsequent cycles, and LV hypertrophy occurs
What is the heart sound of mitral valve regurgitation?
Holosystolic murmur
What causes mitral valve stenosis?
Rheumatic fever, leading to commissural fusion of valve leaflets
What heart sound occurs in mitral valve stenosis?
Snap as mitral valve opens and then diastolic rumble
What does mitral valve stenosis lead to?
Increased LA pressure leads to pulmonary oedema, dyspnea, pulmonary hypertension and therefore RV hypertrophy as blood backs up
Also leads to LA dilation, and therefore atrial fibrillation then thrombus formation, and oesophagus compression then dysphasia
Define afterload
The load the heart must eject blood against (approx aortic pressure)
Define preload
Amount the ventricles are stretched in diastole, related to end diastolic volume or central venous pressure)
Total peripheral resistance
Resistance to blood flow offered by all the systemic vasculature, sometimes called systematic vascular resistance
Constriction of arterioles increases the resistance, so pressure on the arterial side rises. What happens to pressure in the capillaries and venous side?
Pressure falls
What happens if total peripheral resistance falls and cardiac output stays the same?
Arterial pressure falls
Venous pressure increases
What happens in total peripheral resistance increases and cardiac output stays the same?
Arterial pressure increases
Venous pressure falls
What happens if cardiac output increases and total peripheral resistance stays the same
Arterial pressure increases
Venous pressure decreases
What happens if cardiac output decreases and total peripheral resistance stays the same?
Arterial pressure falls
Venous pressure rises
What dilates and contracts to increases or decrease peripheral resistance?
Arterioles and precapillary sphincters
By what mechanisms does the heart respond to changes in central venous pressure and arterial blood pressure?
Intrinsic - increased stroke volume and increased filling of the heart, ensuring both sides of the heart pump the same amount
Extrinsic - sympathetic stimulation and circulating adrenaline, increasing contractility of the heart (and the opposite to decrease)
How do you calculate cardiac output?
Stroke volume * heart rate
How do you calculate stroke volume?
End diastolic volume - end diastolic volume
What happens to heart filling is venous pressure increases?
It fills more
What does the ventricle fill until?
Intraventricular pressure equals venous pressure
When does ventricular compliance increase or decrease?
In diseased states
What does the Frank-Starling law of the hear say?
The more the heart fills the harder it contracts.
An increase in venous pressure, heart fills more, contracts harder, and stroke volume increases
What happens if sarcomere length is too short in cardiac muscle fibres?
Filament overlap interferes with contraction
What happens in cardiac muscle as the muscle fibres are overstretched?
An increase in calcium sensitivity
What is contractility?
Force of contraction for a given fibre length. An increase in contractility, increases the stroke volume for a given left ventricular end-diastolic pressure
What is the effect of increasing arterial pressure on stroke volume?
Afterload is increased when peripheral resistance is increased, reducing venous pressure, and reducing filling of heart, thus reducing stroke volume
What factors determine cardiac output?
How hard the heart contracts - depends on end diastolic volume and contractility
How hard it is to eject blood - aortic impedance (aortic pressure)
What response does a decrease in arterial BP cause in the nervous system?
Reduces parasympathetic nervous system activity, stimulates the sympathetic nervous system, to increase heart rate and contractility
What is the effect on total peripheral resistance if the metabolism of the body increases?
Total peripheral resistance falls to supply more blood, leading to a fall in arterial pressure and increase in venous pressure. Cardiac output increases
How does the CVS respond to eating a meal?
There is local vasodilation in the gut. Total peripheral resistance falls, so arterial pressure falls and venous pressure rises. This increases the stoke volume of the heart, and the fall in arterial pressure increases heart rate. Cardiac output therefore increases and the arterial pressure increases, and venous pressure falls
What happens to venous pressure, cardiac output and arterial pressure on standing?
They all fall. They then cannot be adjusted by intrinsic mechanisms, so baroreceptor reflex and ANS increase HR and total peripheral resistance. If reflexes don’t work then leads to postural hypotension
What happens to venous pressure, heart rate, contractility and cardiac output in exercise?
They all increase.
