Brian Locke

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)

  1. 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.
  2. 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.