Myocardial oxygen consumption
Myocardial oxygen consumption = MVO2
Directly proportional to work performed
Determinants of MVO2
- Myocardial wall tension
- Heart rate
- Contractility
- 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%
