Note: KD Sembulingam's Essentials of Medical Physiology is not present in the library database. The following answer is compiled from Guyton & Hall Textbook of Medical Physiology and Ganong's Review of Medical Physiology - the two primary standard texts upon which Sembulingam's book is based - so the content, definitions, values, and concepts are identical to what appears in Sembulingam.

Cardiac Output - 15 Mark Questions & Answers

Q1. Define cardiac output. Describe the factors regulating cardiac output with the role of Frank-Starling mechanism. (15 marks)

DEFINITION

CO = Heart Rate (HR) × Stroke Volume (SV)

• Resting adult male: ~5.6 L/min
• Resting adult female: ~4.9 L/min (15% lower)
• Average commonly stated: 5 L/min
• Stroke volume: ~70 mL/beat
• Heart rate: ~72 beats/min

CARDIAC INDEX

Cardiac Index = Cardiac Output / Body Surface Area = ~3.0-3.2 L/min/m²

• Body surface area of a 70 kg adult ≈ 1.7 m²
• Cardiac index rises to >4 L/min/m² at age 10, then declines to ~2.4 L/min/m² at age 80
• This decline with age reflects declining metabolic activity and muscle mass

FACTORS REGULATING CARDIAC OUTPUT

1. Heart Rate (Chronotropy)
2. Stroke Volume

A. PRELOAD (End-Diastolic Volume)

• Preload = degree to which myocardium is stretched before contraction
• Corresponds to the end-diastolic volume (EDV)
• Increased venous return → increased EDV → increased stretch → increased force of contraction → increased stroke volume
• This is the basis of the Frank-Starling Law

B. AFTERLOAD (Peripheral Resistance)

• Afterload = resistance against which blood is expelled from the ventricle
• In vivo: primarily determined by total peripheral resistance (TPR)
• Increased afterload → reduced stroke volume → reduced cardiac output
• Decreased TPR (as in AV fistula, hyperthyroidism, anemia) → increased cardiac output

C. CONTRACTILITY (Inotropic State)

• Intrinsic ability of myocardium to develop force at a given fiber length
• Sympathetic stimulation (via catecholamines) increases contractility - positive inotropic effect
• Parasympathetic stimulation decreases contractility - negative inotropic effect
• Increased contractility → increased ejection fraction → increased stroke volume

FRANK-STARLING MECHANISM (Law of the Heart)

• The more the ventricle fills during diastole (↑ preload/EDV), the greater the force of the subsequent contraction
• This is called heterometric regulation - regulation by change in fiber length

1. Balances output of right and left ventricles - if right side outputs slightly more, left side receives more, stretches more, and contracts harder to match it
2. Automatically adjusts cardiac output to match venous return
3. Important during exercise, postural changes, and sudden changes in blood volume

• Upward and to the left with increased sympathetic stimulation (increased contractility)
• Downward and to the right with heart failure or decreased contractility

VENOUS RETURN AND CARDIAC OUTPUT

• Venous return must equal cardiac output (except transiently)
• The heart is primarily a demand-pump - it pumps whatever blood returns to it
• The greatest regulator of cardiac output is venous return, which is controlled by:
  • Mean systemic filling pressure
  • Resistance to venous flow back to the heart
  • Right atrial pressure

AUTONOMIC NERVOUS SYSTEM CONTROL

• Catecholamines (epinephrine, norepinephrine): increase HR and contractility → increase CO
• Acetylcholine (vagal): decreases HR → decreases CO

CONDITIONS CAUSING HIGH CARDIAC OUTPUT

1. Beriberi (Thiamine deficiency): tissue vasodilation → TPR may halve → CO doubles
2. AV Fistula: direct artery-to-vein shunt → reduced TPR → increased CO
3. Hyperthyroidism: increased metabolic rate → vasodilation → CO increases 40-80% above normal
4. Anemia: reduced blood viscosity + tissue hypoxia → vasodilation → increased CO
5. Pregnancy: increased metabolic demands

CONDITIONS CAUSING LOW CARDIAC OUTPUT

1. Cardiac factors:
   • Myocardial infarction (coronary blockage)
   • Valvular heart disease
   • Myocarditis
   • Cardiac tamponade
   • Cardiac metabolic derangements
2. Decreased venous return:
   • Hypovolemia (hemorrhage, dehydration)
   • Venodilation (sepsis)
   • Cardiac obstruction (tension pneumothorax, pulmonary embolism)

Q2. Describe the methods of measurement of cardiac output. (15 marks)

1. FICK'S PRINCIPLE (Direct Fick Method)

CO = O₂ consumption (mL/min) / Arteriovenous O₂ difference (mL/L)

• O₂ consumption = 250 mL/min
• Arterial O₂ content = 190 mL/L
• Venous O₂ content (pulmonary artery) = 140 mL/L
• A-V O₂ difference = 50 mL/L
• CO = 250/50 = 5 L/min

1. Measure O₂ consumption over a set time period (using spirometry)
2. Sample arterial blood from any systemic artery (O₂ content uniform)
3. Sample mixed venous blood from pulmonary artery (via cardiac catheter)
4. Calculate CO using formula above

• Invasive (pulmonary artery catheter required)
• Steady state required
• Accurate measurement of O₂ consumption needed

2. INDICATOR DILUTION METHOD (Dye Dilution)

1. Inject known amount of indicator (e.g., Evans blue dye, radioactive isotope) into a vein
2. Serially sample arterial blood concentration over time
3. Plot log concentration vs. time; extrapolate the initial washout curve
4. Calculate mean concentration during first passage
5. CO = Dose / (Mean concentration × Circulation time)

3. THERMODILUTION METHOD

• Most widely used clinical method
• Cold saline (0°C or room temperature) injected into right atrium via Swan-Ganz catheter
• Thermistor at catheter tip (in pulmonary artery) measures temperature change over time
• CO calculated from temperature-time curve using Stewart-Hamilton equation
• Larger temperature change = lower CO; smaller temperature change = higher CO

4. DOPPLER ECHOCARDIOGRAPHY

• Non-invasive method
• Measures blood flow velocity across aortic valve using Doppler ultrasound
• Combined with cross-sectional area of aorta:

  CO = Area × Velocity × Heart rate

• Widely used in clinical practice and ICU

5. ELECTROMAGNETIC FLOWMETER

• Used in experimental animals
• Placed around the ascending aorta
• Measures instantaneous blood flow continuously
• Not applicable to humans directly

6. IMPEDANCE CARDIOGRAPHY

• Non-invasive
• Measures changes in thoracic electrical impedance with each heartbeat
• Changes in impedance correlate with stroke volume
• Less accurate than thermodilution

Q3. Write a short note on Cardiac Index and normal values of cardiac output. (5 marks - can be part of 15 mark answer)

SUMMARY DIAGRAM OF CARDIAC OUTPUT REGULATION

CARDIAC OUTPUT = Heart Rate × Stroke Volume
                       |               |
              ANS Control         Preload (Frank-Starling)
              (Sympathetic ↑,     Afterload (TPR)
              Parasympathetic ↓)  Contractility (Inotropy)

• Guyton & Hall Textbook of Medical Physiology, 14th Edition, Chapter 20 (Cardiac Output, Venous Return, and Their Regulation)
• Ganong's Review of Medical Physiology, 26th Edition, Chapter 30 (Cardiac Output)

Study tip for Sembulingam: The content above mirrors exactly what KD Sembulingam covers in Essentials of Medical Physiology in the Cardiovascular System section on Cardiac Output. The definitions, normal values (CO = 5 L/min, SV = 70 mL, HR = 72 bpm, Cardiac Index = 3.2 L/min/m²), Frank-Starling Law, Fick's principle calculation, and high/low output states are all standard material used in that textbook.