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Cardiac output sturctured answer

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Here is your complete structured answer on Cardiac Output, compiled from all four pages of your notes:

CARDIAC OUTPUT - Structured Answer

1. DEFINITION

Cardiac Output (CO) = Heart Rate (HR) x Stroke Volume (SV)
  • Normal CO = 5 L/min
  • Cardiac Index = CO per m² of body surface area; Normal = 3.2 L/min/m²

Autonomic Regulation:

StimulationEffect on HREffect on CO
Sympathetic↑ HR↑ CO
Parasympathetic↓ HR↓ CO

2. DETERMINANTS OF STROKE VOLUME

Mnemonic: CAP - Contractility, Afterload, Preload

A. PRELOAD

  • Determined by: End Diastolic Volume (EDV) / Venous Return (VR)
  • Best marker: EDV
  • Relation to CO: Preload ∝ CO
  • Factors that ↑ Venous Return:
    1. Calf muscle contraction
    2. One-way valves
    3. Deep fascia (prevents collapse of veins)

B. CONTRACTILITY (Inotropy)

  • Determined by / Best marker: Ejection Fraction (EF)
  • Relation to CO: Contractility ∝ CO
  • Frank-Starling Law (Length-Tension Relationship):
    • ↑ EDV → ↑↑ Force of contraction (within physiological limits), then plateau
  • Applied - Digoxin:
    • Positive inotropic agent
    • ↑ Contractility → ↑ EF
    • Used in treatment of heart failure

C. AFTERLOAD

  • Determined by: Total Peripheral Resistance (TPR) + Aortic pressure
  • Best marker: TPR
  • Relation to CO: Afterload ∝ 1/CO (inversely related)
  • Applied - Heart & Kidney parallel circuit:
    • Removal of a kidney → ↑ TPR → ↑ Afterload → ↓ CO

3. FACTORS AFFECTING FORCE OF CONTRACTION

FactorMechanism
Bowditch Effect (Force-frequency relationship / Treppe)↑ HR → ↑ Force of contraction
Anrep EffectAbrupt ↑ in afterload → ↑ Force of contraction

4. MEASUREMENT OF CARDIAC OUTPUT

  1. Fick's Principle:
    • CO = O₂ Consumption / Arteriovenous O₂ difference = 5 L/min
    • Heart has the highest A-V O₂ difference = 114 mL/L
  2. Echocardiography (m/c): Clinically used
  3. Thermodilution Method } → Theoretical
  4. Dye Dilution Method }

5. DISTRIBUTION OF CARDIAC OUTPUT

OrganBlood FlowO₂ Consumption
Liver1500 mL/min (Highest total)51 mL/min (Highest total)
Brain750 mL/min46 mL/min
Kidney1250 mL/min; 440 mL/100g/min (Highest per gram)-
Resting Skeletal Muscle840 mL/min; 3 mL/100g/min-
Heart250 mL/min (5% of CO)10 mL/100g/min (Highest per gram)

6. CORONARY CIRCULATION

  • Heart receives majority of blood during diastole
  • Subendocardial part: more prone to ischemia
  • Myocardial O₂ demand ∝ HR, duration of systole, systolic BP

Regulation of Coronary Blood Flow:

  1. Nervous control: minimal role
  2. Local factors: Accumulation of adenosine, NO, H⁺, metabolic end products → ↑ coronary blood flow
  3. Postural change (Supine → Immediate standing):
    • Transient fall in BP (BP = CO × TPR)
    • Parameters affected: VR↓, CO↓, TPR↑, HR↑, CVP↓
    • Mechanisms maintaining BP:
      • Baroreceptor reflex
      • ↑ TPR (to counter ↓ CO)

7. SHOCK

Types of Shock and Effect on Cardiac Output:

TypeSeen InCOMechanism
Hypovolemic (m/c)RTA↓Volume loss
NeurogenicGeneral/spinal anesthesia↓Sympathetic inhibition → extensive vasodilation → ↓ TPR
CardiogenicCardiac disorders, cardiac tamponade↓Pump failure
AnaphylacticBee sting↓Histamine release → ↑ capillary permeability → extensive vasodilation
SepticBacterial infection↑Septicemia → ↑ body temperature → Activation of SA node

8. HYPOVOLEMIC SHOCK - STAGES

FeatureStage 1: Non-ProgressiveStage 2: ProgressiveStage 3: Irreversible
MechanismCompensatory mechanisms presentWithout Rx → DeathDeath inevitable
Key Events- Sympathetic activation → Norepinephrine release - Heart: ↑ HR, ↑ contractility, ↑ CO, ↑ BP - Kidney: vasoconstriction of afferent arteriole → diverts blood to heart & brain - ADH release → ↑ water reabsorption → ↑ blood volume- ↓ Coronary blood flow → Cardiac deterioration ↔ Positive feedback loop - Clot formation in small blood vessels → ↓ blood flow to heart- Loss of energy sources: ATP, Adenosine regeneration (slow process), Creatinine phosphate
High-yield points to remember:
  • Septic shock is the ONLY type with ↑ CO
  • Kidney has the highest blood flow per gram of tissue
  • Heart has the highest O₂ consumption per gram
  • Liver has the highest total blood flow and total O₂ consumption
  • Frank-Starling law = Length-tension relationship (EDV → Force of contraction)
  • Digoxin = positive inotrope used in heart failure
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