Heart as a Pump

Question 1

Preload

Answer 1

The initial degree of stretch in the muscle when it begins to contract.

For cardiac contractility, this depends on end-diastolic blood volume: the amt of blood in the left ventricle just before contraction

Question 2

Recall the relationship between preload and force of contraction.

What is the graphic relationship between myocardial resting fiber length/end-diastolic volume and peak systolic ventricular pressure?

Answer 2

The top line (systole)

Question 3

Examples of factors that increase vs decrease preload

Answer 3

• Increase preload:
  
  • Large volume infusion of saline
  • Large volume blood transfusion
  • Exercise that increases venous return due to peripheral vasoconstriction
• Decreases preload:
  
  • Acute hemorrhage (blood loss decreases venous return)
  • Venous dilation (e.g. nitroglycerine)

Question 4

Afterload (the load the heart must overcome during contraction)

Answer 4

Aortic blood pressure when the ventricle ejects blood into the aorta.

• Increased by* aortic constriction, increased sympathetic activity, hypertension
• Decreased by arterial* vasodilation

Question 5

Equation for stroke volume

Equation for cardiac output

Equation for ejection fraction

Answer 5

SV = End-diastolic volume (EDV) - End-systolic volume (ESV)

CO = SV x HR

EF = SV/EDV

Question 6

Cardiac work

Answer 6

Cardiac work = Ejection pressure x Stroke volume

W = P x V

Question 7

Cardiac efficiency

Answer 7

Ratio of work done by the heart to the energy used to perform the work.

Chemical E of fatty acids are converted into heat and work. Normally 10-15%. Increases during exercise.

Question 8

Frank-Starling Relationship

Answer 8

Ventricular contraction increases in proportion to increased preload (i.e., venous return to the heart).

AKA, SV is directly proportional to End-diastolic Volume

However, if optimal lengthening of sarcomere is exceeded, contraction becomes less powerful.

Question 9

Starling’s law says that any chang ein the indices of the myocardial fiber length will result in a linear change in all of the indices of ventricular peformance.

What are the indices of each?

Answer 9

Question 10

What factors determine stroke volume?

Answer 10

• Preload (Starling’s law)
• Contractility: strength of contraction independent of preload
  
  • Chemical or hormonal influences
• Pressures in the aorta or pulmonary artery / Afterload
  
  • Impacted by narrowing of arteries or valves, hypertension, etc

Question 11

Echocardiography to find CO

Answer 11

Ultrasound the heart to produce 2D & 3D images of the heart -> estimate real time changes in ventricular dimensions to compute SV

-> use CO = SV X HR

Question 12

Thermodilution technique

Answer 12

Insert a catheter from a peripheral vein into the pulmonary artery and inject a cold saline solution to mix with blood as it passes through teh ventricle into the pulmonary artery.

This cools the blood and the computer can quantify this temp change and calculate flow out of the right ventricle (CO) based on blood temp information and the known temperature & volume of the injected solution

Question 13

Fick principle

Answer 13

O₂ in pulmonary vein =

O₂ in pulmonary artery + O₂ added in the lungs

(PV blood flow)(O₂ content) = (PA blood flow)(O₂ content) + quantity added in lungs

CO = O₂ consumption rate / (O₂ content in PV blood - O₂ content in PA blood)

Question 14

O₂ in any peripheral arterial blood sample is equal to ___

O₂ content in pulmonary artery blood is equal to ____

Answer 14

O₂ content in pulmonary venous blood

O₂ content in blood from right atrium (can collect by catheter)