Energy Expenditure, Metabolism, Requirements Flashcards
Describe direct calorimetry and how it is performed
Measures heat and chemical energy released from the body. Heat is considered a direct measurement of metabolic rate because the fuel energy used by the body is ultimately transformed to heat. Urine also is collected to account for the small amount of chemical energy lost in urea excretion in urine. These machines consist of a living space or chamber thermally isolated from the outside, with equipment to extract and measure the heat released into the air of the chamber from the subject residing within. Most appropriate in the study of healthy subjects in research/academic environments due to the individual needing to remain alone in the chamber without caregivers or medical equipment. Can last for days.
Describe indirect calorimetry and how it is performed
Gas exchange and heat measurements are obtained simultaneously and are used to deduce the ratios of gas exchange to head production and prove the equivalence pf the 2 processes. The measurement of gas exchange is shown to be an indirect but accurate reflection of energy expenditure. Subjects do not need to be isolated as long as respiratory gases can be completely captured for approximately 30 minutes. Most IC devices consist of sensors for measuring oxygen and CO2 concentrations in inspired and expired air, a device for measuring exhaled minute volume, and a computer system to calculate parameters and manage data.
What is the Reverse Fick Equation?
An estimation of oxygen consumption, derived from oxygen content differences in arterial and mixed venous blood multiplied by cardiac output measurements from pulmonary artery catheters. Not useful for clinical assessment of energy expenditure
What is the Doubly Labeled Water technique?
A method for assessing energy expenditure that involves administering a stable isotope of water (2H2O) to the patient and measuring its disappearance rate over time (days). The disappearance of 2H is proportional to water turnover, whereas the disappearance of 18O is proportional to water turnover plus VCO2 production. The difference between the 2 disappearance rates is the VCO2. VO2 is computed from the food quotient (FQ - the sum of the CO2 produced from all food eaten divided by the sum of the oxygen consumed during oxidation of all food eaten). Useful for research, not used in clinical care.
Name the relative indications for Indirect Calorimetry
BMI <20.5
BMI >80
Concern about overfeeding in a patient with unexplained high ventilator requirement or who for an unknown reason cannot be liberated from the ventilator
Unwanted weight loss over time not explained by volume status in a patient who regularly receives nearly 100% of target feeding
Massive tissue loss from amputation
Preadmission fluid overload (ascites, volume resuscitation, “third spacing”) without a reliable report of body weight at proper hydration
What are the contraindications for Indirect Calorimetry?
Air leak (chest tubes, cuff leak, any other leak in ventilator circuit, leaks around face masks, canopies, etc.)
ECMO
HD (during and for several hours afterwards)
For mechanically vented patients, a fraction of inspired oxygen >60%
For spontaneously breathing patients:
- Reliance on any supplemental oxygen (cannula, face mask, etc.)
- Inability to cooperate with the measurement
- Claustrophobia or any anxiety about the measurement
What is the amount of time that IC measurements are useful for in critically ill, mechanically ventilated patients?
3-4 days
What is the maximum BMI that was used when the Mifflin - St. Jeor equation was being studied?
Maximum BMI of 42
How is the accuracy rate of the Mifflin-St. Jeor equation affected when applied to obese people?
It falls somewhat. Accuracy of 75% in people with BMI 30 or higher, compared to 87% in people with BMI less than 30
What are the 2 earliest and most common ways to predict RMR in critically ill patients?
Using a ratio of kcal to kg of body weight OR calculate healthy RMR (with Harris-Benedict or Mifflin-St. Jeor) and multiply by a stress factor.
In which populations has the Penn State Equation been demonstrated to be accurate?
Morbidly obese (80% accuracy up to a BMI of at least 80)
Brain injuries (72% accuracy)
Barbiturate coma (73% accuracy)
In which populations does the Penn State Equation lose accuracy?
Low BMI (63% accuracy when BMI is <20.5)
Cystic fibrosis (58% accuracy; many of these patients had very low BMI which might be the underlying reason for the low accuracy rate)
VCO2 stands for:
Volume of CO2 production
VCO2 is calculated as:
The breath-by-breath exhaled volume multiplied by the fraction of expired CO2, averaged over a minute, and then aggregated to a 24-hour period
VO2 stands for:
Oxygen consumption
