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