cardiac control output Flashcards
What is cardiac output and how can it be calculated?
Cardiac output is the amount of blood ejected from the heart per minute
CO = HR x SV
Cardiac output = heart rate x stroke volume
Using an equation, explain how cardiac output can affect blood pressure
CO (blood flow) = BP/TPR
OR
BP= CO x TPR
TPR: total peripheral resistance
BP: blood pressure
CO: cardiac output
What is meant by energy of contraction?
Energy of contraction if the amount of work required to generate a stroke volume
What are the 2 functions of stroke work?
- Contracts until chamber pressure > aortic pressure (to allow isovolumetric contraction)
- Ejection from the ventricle
Describe how preload and afterload affect stroke volume
Preload increases the stroke volume
Afterload decreases stroke volume
What is starlings law?
“energy contraction of cardiac muscle is relative to the muscle fibre length at rest”
What is meant by starlings law?
Greater stretch of ventricles during diastole means greater energy of contraction, therefore greater stroke volume achieved in systole
Simple= more stretch means harder contraction
What is EDV?
End diastolic volume = 120ml in ventricle
What is ESV?
End systole volume = 40ml in ventricle
What is meant by stroke volume?
EDV-ESV= stroke volume
120-40=80ml
Definition: the amount of blood pumped by the left ventricle of the heart in one contraction
What is meant by preload?
Aka LVEDP (left ventricular end diastolic pressure)
What are the steps of starlings experiment?
First: proving that more input=more output
1. Bolus (large dose) of fluid into the vein
2. This increases the end diastolic volume (volume of the ventricle at the end of diastole)
3. This increases the strength of contraction
4. This ejects more fluid from the ventricle
HIGHER STROKE VOLUME
Second: proving that less input=less output
1. Removal of fluid (for example a haemorrhage)
2. This reduced the venous return
3. Reduces end diastolic volume
4. Decreases strength of contraction
5. This ejects less fluid from the ventricles
LOWER STROKE VOLUME
What is a haemorrhage?
Bleeding due to a ruptured blood vessel for example being STABBED
Describe the molecular basis of unstretched muscle fibre
Actin and myosin are overlapping meaning less cross bridge formation available for contraction
Describe the molecular basis of stretched muscle fibre
Actin and myosin overlap LESS meaning there is more potential for cross bridge formation
AND
Increased sensitivity for calcium ions
Describe the 5 roles of starlings law (preload)
- Balances the ventricular outputs
- Responsible for fall in cardiac output during a drop in blood volume or vasodilation (haemorrhage or sepsis)
- Restores cardiac output in response to intravenous fluid transfusion
- Responsible for fall in cardiac output during orthostasis (standing too long)
- Contributes to increased stroke volume and cardiac output during upright exercise
What is orthostasis?
Drop in cardiac output due to standing up for too long
This can lead to:
→ Postural hypotension
- Postural hypotension(also calledorthostatic hypotension) is a condition in which a person’s blood pressure drops abnormally when they stand up after sitting or lying down. Not all people who have this condition have symptoms, but it can lead to dizziness, light-headedness and fainting, and possible falls.
Dizziness as blood pools in legs
What is Laplace’s law?: force against heart contraction
The pressure against which the heart must work to eject blood during systole. It is the end load against which the heart contracts to eject blood.
Afterload opposes ejection of blood from the heart and is determined by wall stress directed through the heart wall; stress through the heart wall prevents muscle contraction. More energy of contraction is needed to overcome the wall stress to produce cell shortening and ejection
What are the 3 parameters in Laplace’s law that determine afterload?
- Tension: more tension = less contraction
- Pressure: more pressure = less contraction
- Radius: bigger radius = less contraction
Smaller ventricular radius = greater wall curvature = more wall stress directed to the centre of the chamber = less afterload = more ejection
Bigger ventricular radius = smaller wall curvature = more wall stress directed to the heart wall = more afterload = less ejection
What is the importance of Laplace’s law?
- Opposes starling’s law at rest
Increased preload= increased stretch of chamber=increases chamber radius= decreased curvature= increase afterload
Starlings law overcomes Laplace’s law therefore GOOD EJECTION- Facilitates ejection during contraction
Contraction decreases chamber radius = less afterload in emptied chamber= increases stroke volume - Contributes to a failing heart at rest and during contraction
Failing heart is usually dilated= bigger radius = increased afterload = less ejection
- Facilitates ejection during contraction
Laplace summary
SMALL RADIUS = GOOD
BIG RADIUS = BAD
Describe the effect of arterial blood pressure on stroke volume and afterload
Laplace’s law states that increased blood pressure (P) will increase wall stress; this will increase afterload and decrease ejection
What is an acute rise in blood pressure balanced by?
Acute rise in blood pressure is balanced by:
1. Starlings Law: increased stretch= increased contraction= increase stroke volume
Occurs due to NA (local positive inotrope) OR accumulation of calcium ions (Anrep effect)
- Baroreflex: decreases sympathetic response, less NA; decreases blood pressure
What happens to the heart during chronic high BP?
Chronic increase in arterial blood pressure
Due to increases energy use to maintain stroke volume and will result in lower stroke volume
Solution? Decreasing blood pressure will increase efficiency of the heart
What is the effect of high BP on cardiac output?
High BP= low CO
Name 3 heart problems that can lead to an increased ventricular radius
→ MI
→ Cardiomyopathies
→ Mitral valve re-gurgitation
These cause the heart to not contract properly meaning the blood cannot be ejected from the ventricle resulting in volume overload
Name 2 heart problems that can lead to increased ventricular pressure
→ Hypertension
→ Aortic stenosis
Increased afterload/pressure in chamber leads to pressure/overload heart failure
How can an increase in radius or pressure affect ventricular ejection?
Increase in radius or pressure will increase wall stress (afterload); opposing ejection
What is ventricular hypertrophy, what is its effects and is it long term?
Heart compensates during heart failure using ventricular hypertrophy (increased wall thickness). This decreases wall stress and afterload; therefore maintaining SV and CO.
HOWEVER
This requires a lot of energy, as energy requirement increases (that cannot be provided), contractility decreases and lead to more heart failure
Describe how Starling’s law contributes to the ventricular pressure-volume loop during exercise
during exercise increased venous return leads to an increase in ED = increased preload = more stretch
this causes a shorter isovolumetric contraction phase and increase in SV due to starlings law
more blood back to the heart and more blood ejected from the heart
