Myocardial Oxygen — Tachycardia

Coronary Blood Flow

• 80 mL/min/100 g
• or 200-250 mL/min
• 5% of CO at rest
• Can increase by 3-4 times (up to 400mL/min/100g)

$$CoroBF \; = \; {{CPP} \over {CVR}}$$

Coronary Perfusion Pressure

$$CPP \; =$$

$$ADP \; – \; (larger \; of \; LVEDP \; or \; RAP)$$

• ADP = Ao Diastolic Pressure
• Varies throughout the cycle and b/w ventricles

Coronary Vascular Resistance

• Physical Factors
  • Extravascular compression (CPP factors)
  • LCA flow stops in systloe
  • RCA flow continuous throughout the cycle
• Neural and Neurohumoral Factors
  • ↑ SNS tone →
    • α receptor mediated vasoconstriction
    • β receptor mediated vasodialtion 
    • ↑ force and rate of contractions → ↑ vasodialtor metabolite release
    • Overall effect is dilation
  • ↑ PSNS tone → KACh stimulation → mild ↓ Coronary vascular resistance
• Metabolic Factors (main)
  • Vasodilatory (↑’d with ↑ HR)
    • ↑ Adenosine, H, K, CO2, Lactate
    • NO → GTP
    • ↑ O2 demand → ↓ ATP → ↑KATP channel activation → hyperpolarisation → vasodilation
• Myogenic autoregulation (keep CPP 60-180 mmHg)

Determinants of myocardial O₂ demand:

• MVO₂ (ie extraction)
• High at rest (55-65%) cf. body average of 25%
• Extraction ratio can only rise by factor of < 2 to 90%
• AV Δ O₂ = 11 mL/dL
• Coronary venous O₂ content = 5 mL/dL
• Normally consumption  21 to 27 mls of O2/min.
• Determined by
  • Wall tension.
    •  preload (EDV/EDP) and afterload (SVR)
  • Contractility.
  • Heart rate.
    • ↑HR → ↓ supply
    • ↑HR → ↑ demand (as ↑ MRO2)

CICMWrecks 2016