Myocardial oxygen consumption

Myocardial oxygen consumption = MVO2

Directly proportional to work performed

Determinants of MVO2

1. Myocardial wall tension
2. Heart rate
3. Contractility
4. Others
   * Basal energy metabolism (25% of MVO2)
   * External work performed
   * Energy for electrical activation (0.5-1% of MVO2)

NB:

• Pressure work requires more myocardial oxygen consumption than volume work.
• Work of RV is 1/7 that of LV
• Net efficiency = 18%
  [BL8:p235]
• Heart cannot develop oxygen debt
  * i.e. All metabolism are aerobic.
• Oxygen extraction ratio is already high
  --> Any increase in MVO2 must be met with increase in perfusion

Calculation of MVO2

MVO2
= 10% of total basal oxygen consumption

Because

• Basal oxygen consumption = 250mLs O2/min
• Heart weights ~ 300g

Thus,

MVO2
= 25 mLs O2 /min
= 8 mLs O2/min/100g

NB:

[KB2:p52]

• MVO2
  = 21-27 mLs O2/min
  = 7-9 mLs O2/min/100g
• Oxygen extraction ratio = 55-65%
  * ?????? Not sure why it's 5-15% higher than the 50% that can be calculated assuming 10% basal oxygen consumption.

[BL8:p234]

• MVO2
  = 8-10 mLs O2/min/100g
• Cardiac venous O2 content
  = 5mL/dl
  * Which implies extraction of 75% (CaO2=20mL/dl)

[WG21:p615, p625]

• MVO2
  = 29 mLs O2/min
  = 9.7 mLs O2/min/100g
• Oxygen extraction ratio
  = 70-80%