Topic 26 - Parameters of the cardiac cycle; volume fractions; factors influencing cardiac output

Question 1

Words to include

Answer 1

• Volume fractions
  
  • End diastolic volume (EDV)
    
    • Maxilmally filled ventricles
  • End systolic volume (ESV)
    
    • Maxilmally emptied ventricles
  • Stroke volume (SV)
    
    • Aorta
    • Cycle
    • EDV ÷ ESV
  • Cardiac output (CO)
    
    • Circulation
    • CO = (EDV - ESV) x frequency = CO = SV x Frequency
• Measuring cardiac output
  
  • Aorta
  • Left ventricle
  • Flick’s principle
    
    • Oxygen uptake
    • Arterio-venous O₂ difference
    • Lung
    • Tissues
  • Stewart’s principle
    
    • Evans-blue
• Ventricular compliance
  
  • Compliance
    
    • Dilating capacity
    • Elastic fibers
• Work of the heart
  
  • Total work of the heart
    
    • Outer work
      
      • Mechanical work
      • Volume work
      • Stroke volume
      • Pressure difference
      • ΔP
        
        • Aorta
        • Vena cava
    • Inner work
      
      • Heat production
    • W_t = W_outer + W_inner
  • Kinetic component
  • Oxygen consumption
  • Tissue
  • W_t = oxygen consumption x energy equivalent of O₂
  • Efficiency of the heart (E)
    
    • E = W₀ / W_t
• Performance of the heart
  
  • Performance = work / time
    
    • W/t = P x V /t
  • Constant peripheral pressure (constant P)
  • Volume flow
  • Cardiac output
• Rushmer diagram
  
  • Cardiac work (Wo)
  • Volume
  • Pressure
  • Left ventricle
  • Cardiac cycle
  • Systole beginning
    
    • Mitral valve (close)
    • Isovolumetric contraction
  • Aortic pressure
    
    • Semilunar valves (open)
    • Ejection phase
  • Systole end
    
    • Semilunar valves (close)
    • Isovolumetric relaxation
• Law of Laplace
  
  • O₂ consumption
  • Ventricular volume
  • Constant pressure
  • Wall tension
• Factors influencing cardiac output
  
  • CO = (EDV-ESV) x frequency
  • EDV
    
    • Compliance
    • Ventricular filling time
    • Central venous pressure
      
      • Preload
  • ESV
    
    • Arterial pressure
      
      • Afterload
    • Contractility
      
      • Sympathetic stimulation ↑
      • Parasympathetic stimulation ↓
  • Frequency
    
    • Sympathetic effect
      
      • Artificial increase
        
        • Pacemakers
      • Natural increase
    • Parasympatheric effect
• Autonomic nervous system
  
  • Chronotropic
    
    • Frequency
    • Contraction time
  • Dromotropic
    
    • Conduction speed
  • Bathmotropic
    
    • Threshold
  • Ionotropic
    
    • Contraction force
• Sympathetic
• Parasympathetic

Question 2

Topics to include in the essay

Answer 2

1. Volume fractions
2. Measuring of cardiac output
3. Ventricular compliance
4. Work of the heart
5. Performance of the heart
6. Rushmer diagram
7. Law of Laplace
8. Factors influencing cardiac output
   
   • EDV
   • ESV
   • Frequency
   • Effects of ANS on the heart frequency

Question 3

Volume fractions

Give the volume fractions

Answer 3

1. End diastolic volume
   
   • The amount of blood found in the heart by the end of diastole
   • The ventricles are maximally filled
2. End systolic volume
   
   • The amount of blood remaining in the heart by the end of systole
   • When the ventricles are maximally emptied
3. Stroke volume
   
   1. The volume fraction passes into the aorta at each cycle
   2. EDV - ESV = stroke volume
4. Cardiac output
   
   • The volume of blood pumped into the circulation by the heart in one minute
   • CO = (EDV - ESV) x frequency
     
     • ​CO: cardiac output
     • (EDV - ESV): stroke volume
     • Frequency: heart beat, or beats per minute
   • Measuring CO:
     
     • Fick’s principle: CO equals total oxygen consumption divided by the arterio-venous oxygen concentration difference
     • Stewart’s principle​

Question 4

Volume fractions

Measuring of cardiac output

Answer 4

• Cardiac output is one of the most important physiological and clinical parameters describing the performance of the heart
  
  • CO equals the amount of blood forwarded into the aorta from the left ventricle per unit time
• Stewart’s principle
  
  • ​Evans-blue
• Fick’s principle
  
  • ​CO is based on the fact that the amount of oxygen that is taken up by the lung per unit time should equal the amount taken up by the tissues
  • CO = total O₂ uptake (l/min) / arterio-venous O₂ difference (l/l)

Figure: Fick’s principle

Question 5

Parameters of the cardiac cycle

Give the parameters of the cardiac cycle

Answer 5

1. Ventricular compliance
2. Work of the heart
3. Performance of the heart
4. Rushmer diagram
5. Law of Laplace

Question 6

Parameters of the cardiac cycle

Ventricular compliance

Answer 6

• Compliance: dilating capacity
• Important parameter of the adaptability of the heart
• Decreased compliance in old animals
  
  • Due to increased rigidity of elastic fibers and the aging of muscle cells

Question 7

Parameters of the cardiac cycle

The work of the heart

Answer 7

• The total work of the heart is composed of:
  
  1. Outer work
     
     • ​Mechanical work
     • Volume work (major component of outer work)
     • Kinetic component (minor component of outer work)
     • Outer work is the product of the stroke volume times the pressure difference between the aorta and the vena cava (∆P)
  2. Inner work
     
     • ​Heat production
• W_t = W_outer + W_inner
• Since the heart gains its energy purely from the oxidative processes, the total work of the heart can be determined by measuring the total oxygen consumption of the tissue
  
  • ​W_t = oxygen consumption x energy equivalent of O₂

Question 8

Parameters of the cardiac cycle

The performance of the heart

Answer 8

• Performance = work / time
• W/t = PxV/t
• Since the heart maintains a close to constant peripheral pressure (constant P), the performance (W/t) is mainly determined by the volume flow
• Performance is proportional to the cardiac output

Question 9

Parameters of the cardiac cycle

Rushmer diagram

Answer 9

• Analyses the cardiac work (Wo) as a function of volume and pressure in the left ventricle during cardiac cycle

1. At the beginning of systole:
   
   • Mitral valve close
   • Isovolumetric contraction
2. Aortic pressure is reached:
   
   • Semilunar valves open
   • Ejection phase
3. At the end of systole:
   
   • Semilunar valves close
   • Isovolumetric relaxation
4. Mitral valves open, filling

Question 10

Parameters of the cardiac cycle

Law of Laplace

Answer 10

• ↑ ventricular volume ↑ O₂ consumption
  
  • Constant pressure of an organ of increasing radius can only be maintained by increased wall tension

Question 11

Factors influencing cardiac output

How to calculate cardiac output?

Answer 11

CO = (EDV-ESV) x frequency

• CO: Cardiac output
• (EDV-ESV): Stroke volume
• Frequency: Heart beat, or beats per minute

Question 12

Factors influencing cardiac output

Give the parameters and their influencing factor

Answer 12

• EDV
  
  • ​Compliance of ventricles
    
    • Decreases with age
  • Ventricular filling time in diastole
  • Central venous pressure (preload)
• ESV
  
  • ​Arterial pressure (afrerload)
    
    • ​When the semilunar valves are still open
  • Contractillity
    
    • Sympathetic stimuli ↑
    • Parasympathetic stimuli ↓
• Frequency
  
  • _​_Sympathetic effect
    
    • ​Artificial increase (pacemaker)
    • Natural increase
  • Parasympathetic effect
    
    • ​Normal control of the heart

Question 13

Factors influencing cardiac output

Effects of autonomic nervous system (ANS) on the heart frequency

Answer 13

• Heart rate is a determinant of the cardiac output
  
  • Heart rate ↑ will not guarantee cardiac output ↑
  • Reason: Heart rate ↑ does not mean contractility ↑