The determinants of cardiac output

Best to use as flashcards with answer with definition.

2 circulatory concepts shown by Guyton et al

1. Heart is necessary to maintain cardiac output

2. Heart does not normally limit cardiac output

At what pressure do veins collapse?

1-2 mmHg below atmospheric pressure

At what point can the heart no longer increase the arteriovenous pressure gradient?

Venous pressure becomes negative

Central venous pressure in a healthy heart?

Almost zero

What limits cardiac output?

Venous return

What is the main determinant of cardiac output?

Mean systemic filling pressure

What determines the maximum flow rate for a given resistance?

The mean pressure

Mean systemic filling pressure

Mean pressure in the system - pressure that would eventually exist everywhere if the heart stopped.

How can mean systemic filling pressure be increased?

extra filling (blood transfusion); constricting filled volume

Unstressed volume

volume of blood that just fills the circulation without stretching the vessel walls

What is the maximum arteriovenous pressure gradient set by?

mean filling pressure

Can the heart change the mean pressure?

No

What determines the maximum cardiac output?

Mean pressure

CO

Cardiac output (l min^-1)

ABP

Arterial blood pressure (mmHg)

TPR

Total peripheral resistance (mmHg.min.l^-1)

MSFP

Mean systemic filling pressure (mmHg)

RAP

right atrial pressure (mmHg)

Darcy’s law

Flow = Pressure gradient / resistance

Why does increased afterload increase force of cardiac contraction?

Ventricles stretch

What does increased total peripheral resistance cause?

Increased arterial blood pressure

What is mean systemic filling pressure determined by?

Volume of blood

Mean tension in blood vessel walls

What is happening at point A? (solid lines = without stimulation; dashed lines = with sympathetic stimulation)

System is not stimulated (resting)

What is happening A-B? (solid lines = without stimulation; dashed lines = with sympathetic stimulation)

Sympathetic venoconstriction increases mean systemic filling pressure

Venous return curve shifted upwards

Venous return exceeds cardiac output

Right atrial pressure increases

cardiac output increases

system settles at point B

What is happening at B? (solid lines = without stimulation; dashed lines = with sympathetic stimulation)

Raised right atrial pressure and cardiac output

What is happening from B-C? (solid lines = without stimulation; dashed lines = with sympathetic stimulation)

Cardiac output curve shifts to higher outputs at right atrial pressure

Drives right atrial pressure negative

Increase in cardiac output is minimal

What is happening at point D? (solid lines = without stimulation; dashed lines = with sympathetic stimulation)

Higher cardiac output

right atrial pressure remains at zero

How are Guyton curves helpful?

Consider effect of various changes to the circulation

Circulatory shock

When cardiac output is inadequate to supply sufficient metabolic substrates for aerobic respiration to all tissues

Symptoms of circulatory shock

Hypotension, tachycardia, reduced organ perfusion (e.g. low urine output, loss of consciousness)

Hypovolaemic shock

Failure of cardiac output due to severe loss of circulating volume.

Cardiogenic shock

Shock caused by cardiac pathology

Distributive shock

Shock caused by a severe fall in vascular tone