Indirect Calorimetry

Indirect calorimetry measures oxygen consumption and carbon dioxide production to calculate energy expenditure. The respiratory quotient, which is the ratio of carbon dioxide produced to oxygen consumed, indicates which fuels are being oxidised.

It is the reference method for measuring resting energy expenditure. Predictive equations are often inaccurate in critical illness, obesity and malnutrition, and measurement allows nutrition to be matched to actual needs.

Usage. It is used in intensive care, for patients with obesity or malnutrition, in metabolic disease, and in research.

Indications. Prolonged mechanical ventilation, difficult weaning, suspected over- or underfeeding, major burns, obesity, severe malnutrition, and unexplained failure to respond to nutritional support.

Preparation and technique. The patient rests quietly for 20 to 30 minutes in a thermoneutral room, after a fast of about 4 to 5 hours where feeding allows. Gas exchange is measured through a canopy hood, face mask or the ventilator circuit. A steady state is defined as a period of about 5 minutes in which oxygen consumption and carbon dioxide production vary by less than 10%. Resting energy expenditure is calculated from the Weir equation.

Contraindications and precautions. In ventilated patients, an inspired oxygen fraction over 60%, air leaks (chest drain, cuff leak, bronchopleural fistula) and continuous renal replacement therapy reduce accuracy. Agitation and movement raise energy expenditure.

Interpretation.

• Resting energy expenditure is compared with the predicted value and with the energy delivered.
• A respiratory quotient near 0.7 indicates fat oxidation, near 0.85 mixed fuel use, and near 1.0 predominant carbohydrate oxidation.
• A respiratory quotient over 1.0 suggests overfeeding with lipogenesis. A value under 0.7 usually reflects a measurement error or a metabolic disturbance such as ketosis.

Causes of high energy expenditure. Sepsis, burns, trauma, fever, hyperthyroidism, and agitation.

Causes of low energy expenditure. Sedation, hypothermia, hypothyroidism, starvation, and neuromuscular blockade.

Critical values and findings needing immediate action. A respiratory quotient over 1.0 with raised carbon dioxide production in a patient with respiratory failure requires prompt reduction of carbohydrate load and review of ventilation.

Limitations and artefacts. The test measures a short period, which may not represent a full day. Equipment requires calibration, and leaks distort values.

Follow up tests. Serum prealbumin, C-reactive protein, urea and nitrogen balance, glucose, triglycerides, and blood gases.

Special groups. Children and neonates need devices suited to low flows. Patients with obesity are measured against actual weight, not adjusted weight.