Physio - Cardio Cycle

Question 1

L.V. Performance Pressure Volume Loops

Answer 1

1: Mitral Valve Closing (Bottom right)

1 –> 2: Isovolumetric contraction

2: Aortic Valve Opening (Top right)

2 –> 3: Ventricular ejection (systole)

3: Aortic Valve Closing (Top left)

3 –> 4: Isovolumetric Relaxation

4: Mitral Valve Opening (Bottom left)

4 –> 1: Ventricular Filling (diastole)

Question 2

What effects happen if we have increased preload, afterload, contractility?

Answer 2

Increased Preload

• Increased volume = Increased Preload
• ESV = same
• EDV = increased
• Volume = increased

Increased Afterload

• Increased Aortic Pressure = Increased Afterload
• Ventricle does not empty as well
  
  • Increased ESV & EDV (preload) & Pressure
  • Decreased Volume

Increased Force of Contraction

• Increased Contractility = more blood to be ejected
• EDV = same
• ESV = decreased
• Volume = increases

Question 3

What is Cardiac Work?

Answer 3

Basics:

• Work = F x d
• Stroke Work = Aortic Pressure x SV

Area under the curve:

• A = estimates stroke work (for healthy people)
• B = estimate stroke work (double work)
  
  • Increased preload

Question 4

Pressure work vs. Volume work

Answer 4

Cardiac Minute Work:

• Cardiac Minute Work = Aortic Pressure x SV x HR

Pressure work (aka internal work)

• ~aortic pressure
  
  • cost much more energy than volume work
  • changes in afterload greatly affect O2 consumption

Volume work (aka external work)

• ~ SV x HR
• cost less energy
• changes in preload do NOT affect O2 as much

Notes:

• Aortic stenosis (loop C)
  
  • work = significantly greater compared to loop B
    
    • MAP = increases substantially
  • despite both loops being double loop A

Question 5

What is a Rate Pressure Product?

Answer 5

Basics:

• Rate Pressure Product = Aortic Pressure x HR
  
  • assumes SV = constant

Utility:

• Useful since it can be acquired non-invasively
• Usedin activity trackers/smartphones

Question 6

How is Law of Laplace related to myocardial O2 consumption (MVO₂) ?

Answer 6

Basics:

• HR & Increased Preload affect MVO₂
  
  • _​Wall tension = closely related to MVO₂
• Pressure = DIRECTLY proportional to wall tension
  
  • INVERSELY proportional to radius

Equation:

• P = 2HT/r
  
  • P = Pressure
  • HT = Wall tension
    
    • H = wall thickness
    • T = stress
  • r = radius

Example: Physiological Hypertrophy

• 2HT = P x r
• Left ventricle = thicker wall than R.V.
  
  • pressure work of L.V. >>> R.V. although same volume passes thru
  • thickens as compensation for increased workload

Question 7

What is the relationship between Fick Principle and CO?

Answer 7

Basics:

• Body O₂ consumption = O₂ leaving the lung in PV – O₂ returning to lungs in PA

Equation:

• CO = (O₂ consumption) / ([O₂]_pv - O₂]_pa)
  
  • O₂ consumption = Spirometry
  • O_2pv = Peripheral artery (arterial blood)
  • O2_pa = Pulmonary artery (mixed venous blood)

Question 8

Events of Cardiac Cycle & Heart Sounds

Answer 8

• First Heart Sound
  
  • Mitral Valve closes
  • Isovolumetric Ventricular Contraction (QRS complex)
• Second Heart Sound
  
  • Aortic Valve closes
  • Isovolumetric Ventricular Relaxation
• Third Heart Sound
  
  • Mitral Valve opens
  • Rapid Ventricular Filling
• Fourth Heart Sound
  
  • P wave/ PR interval
  • Atrial Systole
    
    • artia contracts