Gas Exchange

Indirect calorimetry - Do we need it?

Dr. Alistair Lee, FRCA
Edinburgh Royal Infirmary
Edinburgh, Scotland

Article also available in PDF: 25 KB

The ability to use indirect calorimetry routinely in the critically ill presents a formidable technical challenge that should not be underestimated. Reliable, accurate and easy to use monitors, able to cope with the environment of the ventilated patient on high concentrations of oxygen, have only been available from the late 1980s. The technology is relatively expensive and in most centers has been viewed as a research tool, although recent developments have reduced costs and make routine use a practical proposition. Is it likely that more Intensive Care Units will adopt indirect calorimetry as a standard component of patient monitoring?

Indirect calorimetry has been used to determine the energy expenditure of patients at different phases of their illness. A diverse group of conditions has been studied. There are variations between patients, and with time in an individual patient, when considering energy and nutritional requirements. Overfeeding and underfeeding are both recognized as undesirable. In animal models of sepsis overfeeding has been associated with a significant increase in mortality. Underfeeding contributes to protein loss, and the lack of evidence of detrimental effects from short-term starvation in the critically ill does not necessarily equate to lack of harm. Only by direct measurement can we accurately determine a patient's needs. It is well known that sepsis, surgery, other injuries and burns may affect resting energy expenditure, but it is increasingly recognized that other conditions and their complications may influence metabolic requirements and need to be considered. Malnourished patients with severe liver disease have an increased resting energy expenditure and provide a difficult enough nutritional problem. With the development of ascites, there is a further significant increase in resting energy expenditure, which will contribute to the development of the protein energy malnutrition common in these patients. Patients with end stage liver disease and higher resting energy expenditure before liver transplantation have a poorer prognosis. Patient assessment and determination of prognosis preoperatively is only possible with a full understanding of this complexity.

While standard formulae may provide a starting point for appropriate nutritional support, there is much to be learned about energy and nutritional requirements. Only by frequent measurement of a patient's individual needs will we be in a position to provide correct nutritional support to the critically ill. This alone would appear to warrant the routine use of indirect calorimetry in the Intensive Care Unit.

Ventilatory support of the critically ill patient has undergone a quiet revolution in recent years. It is recognized that mechanical ventilation itself has the potential to cause or exacerbate lung injury. Modern ventilatory strategies endeavor to limit ventilator induced lung injury. We have progressed from a consideration of extracorporeal carbon dioxide removal, which permits very low volume ventilation, to permissive hypercapnia. This similarly permits reduced ventilation with limitation of barotrauma. The positive pressure provided by the ventilator may not be the most harmful effect and the concept of volutrauma with increased shearing effects on lung tissue may be of greater significance. This has led to the frequent adoption of ventilator settings, which include relatively high levels of end expiratory pressure, together with small tidal volumes. Elimination of carbon dioxide may be influenced by this respiratory pattern. In these changing circumstances, it is essential that indirect calorimetry is applied with care and it can make a positive contribution to ventilator adjustment and ability to wean the patient from mechanical support. The increased carbon dioxide production associated with high carbohydrate loads may be one factor, which influences the ease of weaning.

Throughout the last two decades many Intensive Care clinicians have focused on the relationship between global oxygen delivery and oxygen consumption in the critically ill. Oxygen consumption has been routinely determined by a method based on the Fick principle using a pulmonary artery catheter. A positive relationship between oxygen delivery and oxygen consumption has been found in septic patients and in hyperdynamic patients with acute liver failure implying a failure of oxygen supply. This relationship has disappeared in the recovery phase of patients who have been restudied using the same methodology. Therapy has been tailored to improve oxygen delivery, but when pursued aggressively this has led to an increase in mortality. It is now generally accepted that the Fick methodology of calculating oxygen consumption is flawed when used in the hyperdynamic patient. The simultaneous measurement of oxygen consumption, using indirect calorimetry as well as Fick derived calculations, has produced differing results. While there is some increase in oxygen consumption when oxygen delivery is increased using dobutamine rather than fluids, the use of indirect calorimetry in human volunteer experiments has shown that this is due to the potent calorigenic effect of dobutamine. Indirect calorimetry has proved effective in elucidating the true relationship between oxygen delivery and consumption in a group of patients that are notoriously difficult to study.

The effective use of indirect calorimetry in the critically ill depends on careful attention to detail. Careful maintenance, calibration and use during steady state conditions are important prerequisites. It is likely that indirect calorimetry will become an increasingly appreciated and valuable monitoring tool for the Intensive Care clinician in years to come.

References

1. Griffiths RD. Feeding the critically ill – should we do better? Intensive Care Med 1997:23:246
2. Elia M. Changing concepts of nutrient requirements in disease: implications for artificial nutritional support. The Lancet 1995: 345: 1279
3. Dolz C, Raurich JM, Ibanez J et al. Ascites increases the resting energy expenditure in liver cirrhosis. Gastroenterology 1991: 100: 738
4. Liposky JM, Nelson LD. Ventilatory response to high caloric loads in critically ill patients. Critical Care Medicine 1994: 22: 796
5. Walsh TS, Hopton P, Lee A. A comparison between the Fick method and indirect calorimetry for determining oxygen consumption in patients with fulminant hepatic failure. Critical Care Medicine 1998: 26:1200
6. Bhatt SB, Hutchinson RC, Tomlinson B et al. Effect of dobutamine on oxygen supply and uptake in healthy volunteers. British Journal of Anaesthesia 1992: 69 :298.

Last updated: 1 March 2001Created
Legal notice	© GE Healthcare 2008 ISSN 1795-6269 (Web) ISSN 1795-6277 (CD)	Webmaster