Cardio: Cardiac Output Variables

Question 1

Stroke Volume

Answer 1

• Stroke Volume (SV) affected by:
  
  • Contractility ​
  • Preload
  • Afterload
• *↑ *Contractility –> ↑ SV
• ↑ Preload –> ↑ SV
• ↓ Afterload –> ↑ SV

Question 2

Contractility ↑ w/:

Answer 2

• Catecholamines (↑ activity of Ca²⁺ pump in SR)
  
  • ​↑ Intracellular Ca²⁺
  • ↓ Extracellular Na⁺ (↓ activity of Na⁺/Ca²⁺ exchanger)
• Digoxin
• Digitalis (blocks Na⁺/K⁺ pump)
  
  • **​↑ Intracellular Na⁺ –> ↓Na⁺/Ca²⁺ exchanger activity –> ↑ intracellular Ca²⁺ **

Question 3

Contractility (and SV) ↓ w/:

Answer 3

• β1-blockade (↓ cAMP)
• Heart Failure w/ systolic dysfunction (loss of myocardium)
• Acidosis
• Hypoxia / Hypercapnea (↓ PO₂ / ↑ PCO₂)
• Non-dihydrophyridine Ca²⁺ channel blockers
• Dilated cardiomyopathy

Question 4

Preload

Answer 4

• Preload approximated by ventricular End Diastolic Volume (EDV); depends on venous tone and circulating blood volume
• Force of Contraction is proportional to End-Diastolic length of cardiac muscle fiber (preload)
• Venodilators (nitroglycerin) –> ↓ Preload –> ↓ SV
• ACE inhibitors and ARBs ↓ both Preload and Afterload

Question 5

Afterload

Answer 5

• Afterload is approximated by Mean Atrial Pressure (MAP)
• MAP = Cardiac Output x Total Peripheral Resistance
  
  • MAP = CO x TPR
• MAP = 2/3 Diastolic mmHg + 1/3 Systolic mmHg
• Wall Tension = (pressure x radius) / (2x wall thickness)
• LV compensates for ↑ afterload by hypertrophy
• Vasodilators (Hydralazine) ↓ Afterload
• ACE inhibitors and ARBs ↓ both Preload and Afterload

Question 6

Ejection Fraction

Answer 6

EF = SV / EDV

EF = (EDV - ESV) / EDV

• Left ventricular EF is an index of ventricular contractility
• Normal EF is > 55%
• EF ↓ in systolic heart failure
  
  • EF is normal in diastolic heart failure

Question 7

Reistance

Answer 7

• R = Change in Pressure / Volumetric flow Rate
• R = ΔP/V
• R = 8 η x length / (π r⁴)
• η = viscosity
  
  • Increases with Polycythemia (Bonemarrow disorder), Hyperproteinemic states (multiple myeloma), and Hereditary spherocytosis ( familial anemia)
  • Decreases with anemia