Virtus
Metabolic
Monitor
Simple and Cost Efficient Indirect Calorimetry
What is Indirect Calorimetry?
Indirect Calorimetry allows continuous and non-invasive measurement of oxygen consumption (V̇O₂), carbon dioxide production (V̇CO₂), and calculation of respiratory quotient (RQ) and resting energy expenditure (REE) for mechanically ventilated patients
The gas exchange measurement is captured through a single non-invasive flow sensor and gas sampler at the patient’s airway and led to the Monitor. Inside the Monitor, a paramagnetic sensor is used to measure the O₂ curve and an infrared bench for the CO₂ curve. Both measurements are based on the side-stream principle. For each measured breath-by-breath O₂ and CO₂ curve, the V̇O₂ and V̇CO₂ is calculated and once every minute the display is updated. The respiratory quotient (RQ) and the resting energy expenditure (REE) is calculated from V̇CO₂ and V̇O₂ once every minute and updated to the display also once every minute.
Oxygen consumption
Indirect calorimetry measures oxygen consumption (V̇O₂) as the uptake of oxygen from the respiratory gases. Acute changes in ventilation, hemodynamics, and physical activity may induce wide variations in the V̇O₂ measured by any method. Since V̇O₂ can be measured continuously, the transient changes in the measured V̇O₂ can be readily observed in prolonged measurements.
Under aerobic conditions, V̇O₂ depends on the metabolic activity of the tissues. At a given metabolic rate, the substrates of energy metabolism also have an impact on the V̇O₂, since the amount of oxygen required to produce the same amount of energy from different substrates varies.
If the amount of oxygen delivered to the tissues is inadequate for metabolic needs, tissue oxygen consumption becomes dependent on oxygen delivery and anaerobic metabolism with lactic acid production will ensue. During anaerobic metabolism, the V̇O₂ measured from the respiratory gases does not reflect the tissue oxygen needs, since an oxygen debt develops in the tissues. When aerobic conditions are restored, the oxygen debt will be reflected as increased oxygen consumption.
Carbon dioxide production
Indirect calorimetry measures carbon dioxide production (V̇CO₂) as the production of CO₂from the respiratory gases.
In a steady state, V̇CO₂ depends on the metabolic activity of the tissues and, similar to V̇O₂, on the substrates of the energy metabolism.
The time required for the stabilization varies widely and ranges from 30 to 120 minutes. Continuous measurement of gas exchange facilitates the verification of a steady state
Respiratory Quotient (RQ)
The ratio between V̇CO₂ and V̇O₂ is called the respiratory quotient (RQ), when measured in steady state conditions. In non-steady state conditions, the expression “respiratory exchange ratio” is more appropriate. Assuming steady state conditions, the RQ reflects the mixture of substrates utilized by the energy metabolism.
The RQ of carbohydrate is 1, the RQ of fat 0.7 and the RQ of protein approximately 0.81
For clinical purpose, major shifts in substrate oxidation will be reflected in the total RQ, as measured directly from the respiratory gases.
Interpretation of total RQ from respiratory gas exchange measurements, assuming a steady state gas exchange – to verify steady state, a recording of at least 30 minutes is necessary.
Increased glucose oxidation will be observed as an RQ approaches 1, whereas increased fat oxidation will result in an RQ approaches 0.7.
A steady state RQ above 1 indicates fat synthesis and is a clinical rarity, associated with excessive carbohydrate feeding. Even in these conditions, the RQ rarely exceeds 1.3.
A steady state RQ below 0.7 is also a rarity, but may occur during ketosis, if the ketone bodies are incompletely oxidized and excreted into the urine.
RQ-values exceeding 1 and below 0.7 should be carefully examined for measurement errors and the lack of a steady state. The most common causes of RQ-values >1 or <0.7 are changes in ventilation.
Energy expenditure (REE)
The energy expenditure cannot be directly measured by indirect calorimetry, but has to be calculated from the measured gas exchange variables and protein oxidation. The relative contribution of protein oxidation to the total energy expenditure is small even in catabolic conditions, and can be estimated or ignored without inducing a major error in the estimated energy expenditure.
Resting normal values for V̇O₂ and V̇CO₂ vary according to the body size, age, and sex of the patient.
Several studies have consistently demonstrated that actual energy expenditure is difficult to predict in the hospitalized patients in general, and specifically, in the mechanically ventilated intensive care patient. (see e.g.Boullata et al 2007, Kross et al 2012).
An increase in energy expenditure will be reflected as a proportional increase in both V̇O₂ and V̇CO₂. Temporary increase of up to 200% can occur due to e.g. shivering and convulsions. Clinical conditions associated with hypermetabolism, for example, injury and sepsis, typically increase the energy expenditure by 25-40%, but the variability between patients is high.
Patients with severe pulmonary pathology and impairment of respiratory mechanics may have markedly increased work of breathing: the oxygen cost of breathing can be up to 20% of whole body V̇O₂, whereas it normally represents less than 5% of the total V̇O₂.