67 Metabolism
67.1 What this covers
- 30 - 60 mg of fixed acid is removed in kidney
- 14,000 mg of volatile acid is removed by the lungs
- Convective / Bulk-flow trasport: tidal movement of fluid (gas) through the airways and circular flow of fluid from the heart to the peripheral and back again.
67.2 Learning objectives
- 30 - 60 mg of fixed acid is removed in kidney
- 14,000 mg of volatile acid is removed by the lungs
- Convective / Bulk-flow trasport: tidal movement of fluid (gas) through the airways and circular flow of fluid from the heart to the peripheral and back again.
- Diffusive transport: from alveoli in to blood, and from capillaries into tissues.
- Energy expenditure (kcal/d) [(VO2 3.941) + (VCO2 1.11) + (uN2 217) 1440.
67.3 Bottom line / summary
- VO2 (aka metabolic rate) Qt (aka CO by fick) C(a-v)O2 (aka o2 extraction) S(a-v)O2 10 Hgb)
- Schematically: 2 types of transport enable respiration, which can be conceptualized as movement of O2 from ambient pO2 to low pO2 mitochondria (mitochondrial pO2 0.1-2 mmHg, but can’t be clinically assessed)
- Director Calorimetry measure heat produced (via thermally sealed chamber)
- Indirect Calorimetry estimate heat produced by measuring O2 consumed and CO2 produced (because C6H12O6 + 6O2 -> 6CO2 + 6H2O + heat)
- UN2 urinary nitrogen component, often ignored b/c less than 4% of energy generally.
67.4 Approach
- 30 - 60 mg of fixed acid is removed in kidney
- 14,000 mg of volatile acid is removed by the lungs
- Convective / Bulk-flow trasport: tidal movement of fluid (gas) through the airways and circular flow of fluid from the heart to the peripheral and back again.
- Diffusive transport: from alveoli in to blood, and from capillaries into tissues.
- Energy expenditure (kcal/d) [(VO2 3.941) + (VCO2 1.11) + (uN2 217) 1440.
67.5 Red flags / when to escalate
- TODO: List red flags that require urgent escalation.
67.6 Common pitfalls
- Note: practical limitation - VO2 Vi(FiO2) - Ve(FeO2), but practically measuring FiO2 when over 60% predisposes to moer error so generally indirect calorimetry will be less acurate for Fio2 over 60% (or, of course, if FiO2 is changing)
67.7 References
TODO: Add landmark references or guideline citations.
67.8 Source notes
67.8.1 Metabolism
68 Metabolism
Acid removal:
- 30 - 60 mg of fixed acid is removed in kidney
- 14,000 mg of volatile acid is removed by the lungs
VO2 (aka metabolic rate)= Qt (aka CO by fick) * C(a-v)O2 (aka o2 extraction) = S(a-v)O2 * 10 Hgb)
Schematically: 2 types of transport enable respiration, which can be conceptualized as movement of O2 from ambient pO2 to low pO2 mitochondria (mitochondrial pO2 = 0.1-2 mmHg, but can’t be clinically assessed)
- Convective / Bulk-flow trasport: tidal movement of fluid (gas) through the airways and circular flow of fluid from the heart to the peripheral and back again.
- Diffusive transport: from alveoli in to blood, and from capillaries into tissues.
Director Calorimetry = measure heat produced (via thermally sealed chamber)
Indirect Calorimetry = estimate heat produced by measuring O2 consumed and CO2 produced (because C6H12O6 + 6O2 -> 6CO2 + 6H2O + heat)
- Energy expenditure (kcal/d) = [(VO2 * 3.941) + (VCO2 * 1.11) + (uN2 * 217) * 1440.
UN2 = urinary nitrogen component, often ignored b/c less than 4% of energy generally.
TEE (total energy expenditure) = BEE (basal) + DIT (diet-induced thermogenesis) + AEE (activity energy expenditure).
Basal = 5 hours of fasting, no physical activity, abstinence from all stimulants (ceffeine, nicotine). Rare, thus resting energy expenditure (just no current activity) used as surrogate in hospitalized patients.
RER = ventilatory estimate of RQ 0.67 - 1.2 is physiologic range for RQ (amount of O2 metabolized per CO2 - over 1 means some anaerobic)
Harris-Benedict equation (or, in case of burns, Ireton-Jones) often used to estimate.. but vary widely.
[ ] data on benefit of measuring energy expenditure? vs dosing feeds off of weight etc.
Note: practical limitation - VO2 = Vi(FiO2) - Ve(FeO2), but practically measuring FiO2 when over 60% predisposes to moer error so generally indirect calorimetry will be less acurate for Fio2 over 60% (or, of course, if FiO2 is changing)
Limitation 2: hemodialysis removes CO2 gas from the venous bed, thus will lead to a underestimate estimate of VCO2. this is true of iHD, CRRT. There is not much loss of O2.