2.06 - Cardiac Work

Question 1

What is the definition of work?

Answer 1

Energy (J) = Work (W) = Force (F) • Height (H)

Question 2

How is the formulat for Work adapted for the heart?

Answer 2

W = F•H; F= P•A

W = P • A • H; A•H=Volume (V)

W = deltaP•V

Question 3

Draw and label a left ventricular PV Loop. What are major points

Answer 3

A. Mitral valve close

Upstroke = isovolumetric contraction

B: Aortic valve opens

Ejection phase

C: Aortic valve closes

Downstroke: Isovolumetric relaxation

D: Mitral valve opens

Filling phase

Question 4

What are the forms of Work?

Answer 4

External work (physical work)

Internal work (largely heat)

Question 5

I what phase(s) of the cardiac cycle does the most work get done?

Answer 5

During systole. Particularly during the isovolumetric and fast ejection phase

Question 6

If the total energy consumption of the heart is 11.0W. How much is external and how much is internal?

Which do we want to keep at a minimum?

Answer 6

All external work: 1.5W

Internal Work = 9.5W

Want to keep internal work to a minimum

Question 7

Where is all the internal work of the heart spent?

Answer 7

Isovolumetric work

Ionic channels & pumps (sarcoplasmic and action potential)

Base metabolism

Sounds/murmurs

Question 8

What is the energy source for the work done by the heart?

Answer 8

All external and internal work relies on ATP

Most of the ATP comes from oxidative phosphorylation (using fatty acids and carbohydrates)

Question 9

What is the lmiting factor in energy produciton in the heart?

Answer 9

All the O₂ needs to be supplied via the coronary arteries. Any problems with the coronary arteries can lead to failure of the heart in some way and is thus a limiting factor.

Question 10

How does increasing HR affect cardiac output and energy consumption

Answer 10

If CO in increased via an increased in HR, systole becomes more dominany (because diastole shortens), this leads to an increase in internal work, as this required much more energy

Question 11

How does increasing SV affect CO and energy consumption?

Answer 11

If CO is increased by an increase in SV, a much smaller increase in internal work is demanded, because largely external work is increased

Question 12

What is the effect of Afterload on work?

Answer 12

Afterload increases internal work and is therefore energetically costly (one of the problems with hypertension)

Question 13

Describe the association between CO and venous return

Answer 13

Homeostatically, blood volume ejected by the left ventricle (CO) has to be matched by blood volume returned to the right atrium (VR)

Over time CO=VR

Question 14

What effect does sympathetic stimulation of the venous system have?

Answer 14

Sypathetic stimulation (via alpha receptors) increases wall tension

Question 15

What are the determinants of Venous Return?

Answer 15

Mean systemic filling pressure

Muscle pump

Abdominal pressure

Intrathoracic pressure (effect of breathing, negative during inspiration)

Valvular plane displacement

Question 16

What is mean systemic filling pressure?

Answer 16

The pressure left in the vessels after it has gone through the capillary bed.

~7mmHg

This pressure is required to return the blood to the heart

Determined by peripheral resistance and compliance (extent of blood pooling)

Question 17

How can mean systemic filling pressure be modulated?

Answer 17

Sympathetic stimulation (increased MSFP)

NO donors –> decreased MSFP

Increased by infusion or decreased by bleeding

If MSFP is low, venous pooling until it rises again

Orthostatic manoeuvre (standing up decreases MSFP)

Question 18

At what ‘right atrial pressure’ are VR and CO matched?

Answer 18

0mmHg

An increase in P_RAincreases CO but decreases VR

And vice versa

Question 19

How are CO and VR affected by Intrapleural pressure?

Answer 19

The heart is exposed to the intrapleural space and therefore vary with intrapleural pressure (most effect on RA)

Atrial filling increases when intrapleural pressure (lung) pressure drops during inspiration (suction)

This lead to an increase venous return. Initally there is then a decrease in CO but after a delay an increase in CO