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Cardiac Output
Cardiac output
The volume of blood pumped by either ventricle in one minute
The output of the two ventricles are equal over a period of time (not necessarily during each minute)
Cardiac output
Calculation
Cardiac output = stroke volume x heart rate
Cardiac output of a healthy adult at rest is approximately
5 L/min
It does not remain constant in any individual but varies from time to time depending on the needs of the body
Cardiac output
Significance –1. It is the total quantity of blood available for providing
adequate tissue perfusionThus CO must be kept at a level which is the same as the requirements of the body
Lower than requirement – CVS needs to make adjustments in the distribution of the available output
Greater than required – unnecessary workload on heart and increases the pressure within the arteries (leads to damage in the long term
Cardiac output
Significance –
2. It is a determinant of the pressure in the arteries
Low cardiac output – low pressure and reduced force available to drive the blood through the body
High cardiac output – high pressure damages blood vessels
Measurement of cardiac output
Classical methods –
Fick principle
Dye dilution method
Measurement of cardiac output
Classical methods – Fick principle
If one knows 1. How many boxes have been removed
If one also knows 2. How many boxes each truck can carry
The number of trucks that arrived can be calculated
Measurement of cardiac output
LUNGS
Venous bloodArterial blood
O2
Cardiac output = Total oxygen uptake/ amount of O2 taken by each ml of blood
Measurement of cardiac output
Classical methods – Dye dilution method
Inject a substance (not normally found in blood) into a vein and measure the concentration of the substance over a period of time.
Plotting the concentration of the substance over time enables one to calculate cardiac output
Time
Concn.
Measurement of cardiac output
2. Echocardiography
Ultrasound waves are used to measure blood flow in the aorta – cardiac output is calculated
Cardiac output is not a parameter usually measured in the evaluation of the function of the cardiovascular system in clinical practice. It is more relevant in research
Determinants of cardiac output
CO = SV x HR
= (EDV – ESV) x HR
Factors that determine cardiac output are those that influence:
EDV ESV HR
Determinants of cardiac output
Heart
Inotropy Chronotropy
1 2
3 4
1
2 4
3Preload
Afterload
Contractility
Heart rate
Determinants of cardiac output
1. Preload - Preload can be defined as the initial stretching of the cardiac myocytes prior to contraction and is related to the sarcomere length
In the normal heart, ventricular preload is determined by the volume of blood that fills the ventricle at the end of passive filling and atrial contraction
- the end-diastolic volume
Determinants of cardiac output
The effect of preload –
Frank-Starling Law
“ force of contraction increases when initial length of muscle increases”
“Heterometric autoreguation”
Force of contraction
Initial length of muscle fibre
Determinants of cardiac output
Preload –
A major determinant of preload is the amount of blood returning to the heart through the veins
“Venous return”
Venous return to the heart is the sum of all the local blood flows from individual segments of peripheral circulation
Usually applied to the return to the right heart A concept and not a measurement
Determinants of cardiac output
Factors which determine ventricular preload by influencing venous return:
venous blood pressure venous compliance and tone blood volume gravity muscle pump respiratory pump
The venous return will in turn determine the “filling pressure” - the pressure in the atria at the onset of ventricular filling
Clinically “central venous pressure” is measured as an indicator of adequacy of venous return
Determinants of cardiac output
Muscle pump Respiratory pump
Determinants of cardiac output
Factors other than venous return that affect ventricular preload
1. Ventricular compliance – distensibility of the ventricle,
2. Filling time – duration of ventricular diastole
3. Atrial contraction
Determinants of cardiac output
2. Afterload
Afterload can be viewed as the "load" that the heart must eject blood against.
Higher the afterload, higher the ventricular work.
Normally afterload is closely related to the aortic pressure
Determinants of cardiac output
Afterload
Afterload (on the left ventricle) is increased by an increase in
Aortic pressure Systemic vascular resistance Aortic valve stenosis
Determinants of cardiac output
3. Inotropy
Changes in ventricular inotropy (contractility) causes changes in the amount of blood pumped out in one beat (stroke volume)
If contractility increases, the stroke volume increases
If SV increases, end systolic volume decreases
Determinants of cardiac output
Inotropy Changes in inotropy produce significant changes in
ejection fraction (EF) [EF = SV/EDV %] Increasing inotropy leads to an increase in EF, while
decreasing inotropy decreases EF Therefore, EF is often used as a clinical index for
evaluating the inotropic state of the heart EF may be determined by echocardiography
Determinants of cardiac output
Contractility of the myocardium (Inotropy) is influenced by
Autonomic nervous activity Circulating catecholamines Oxygen and CO2 levels
H+ ion concentration Plasma electrolyte levels Loss of myocardium Drugs
Determinants of cardiac output
4. Heart rate
The number of times the heart beats per minute influences the amount of blood pumped out per minute
When heart rate increases the cardiac output is expected to increase
Determinants of cardiac output
4. Heart rate is influenced by Autonomic nervous activity Circulating catecholamines Oxygen and CO2 levels H+ ion concentration Plasma electrolyte levels Drugs
Determinants of cardiac output
Effect of increased heart rate on cardiac output –
Direct effect – increase in CO due to increased number of times the heart pumps
Indirect effect – shortening of ventricular diastole causing reduced ventricular filling – tends to reduce EDV and therefore CO
The net effect on CO depends on which of the above predominates
Determinants of cardiac output
Cardiac output increases with increasing heart rate until a limiting rate which is around 200 for adults
Beyond this the CO falls
CO
HR 200
Determinants of cardiac output
PreloadVenous return – filling pressure (affected by many factors)ventricular distensibilityfilling timeatrial contraction
Afterloadaortic pressuresystemic arteriolar resistance
Inotropyautonomic nervous systemcirculating catecholaminesfunctioning myocardiumoxygen supply to the heart
Heart rateautonomic nervous system
Regulation of cardiac output
PreloadVenous return – filling pressure (affected by many factors)ventricular distensibilityfilling timeatrial contraction
Afterloadaortic pressuresystemic arteriolar resistance
Inotropyautonomic nervous systemcirculating catecholaminesfunctioning myocardiumoxygen supply
Heart rateautonomic nervous system
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