Respiratory Support in Critical Care

Weaning With Indirect Calorimetry

Chris L. Harris RRCP, RRT
Clinical Leader – Respiratory Therapy
London Health Sciences Centre, University Campus
London, Ontario, Canada

The article also available in PDF: 57 KB

See the author giving the Clinical Window presentation at the 23rd ISICEM congressin in Brussels (March 2003).

Introduction

Obtaining metabolic measurements by Indirect Calorimetry for determining oxygen consumption (VO2) and carbon dioxide production (VCO2) has increased in popularity among clinicians since the early 1980’s. Not only have the methods of obtaining this information improved, but also the accuracy of the devices has become more reliable [1,2]. From analyzing expiratory gases with mass spectrometry, and from the more recent technology utilizing mixing chamber like in the DeltatracTM (Datex-Ohmeda), metabolic monitors have now evolved to a small, bedside module enabling the bedside clinician to have accurate metabolic measurements for a wide variety of clinical conditions.

What Information can be obtained?

The patient’s energy expenditure can be determined by inserting gas exchange measurements into an equation developed by Weir [3]. The information gathered by indirect Calorimetry not only determines the resting energy expenditure (REE) as a guide to appropriate nutritional support, but also allows the clinician to tailor ongoing nutritional support to meet the patient’s need [4]. With subsequent measurements, adequacy and appropriateness can further be evaluated. In addition to the nutritional information, the relationship between oxygen delivery (DO2) and VO2 can be assessed, as can the cost of breathing. These can be used as a guide to weaning success and outcome [4].

Since 1919, the Harris-Benedict Equation [5] has been used to predict a normal, nourished individual’s REE, but this is unreliable in the malnourished patient [6]. Correction factors have been developed for various clinical conditions [7]. However, these values are approximations, and have been based on measurements of healthy individuals, not the critically ill patient. Elevated energy expenditure and a negative nitrogen balance on the other hand usually characterize Intensive Care Unit patients. These two values correlate with the severity of illness and the extent of injury.

Considerations for Metabolic Monitoring

Despite all of the advances in metabolic measurements, several clinical and physiological factors can influence the results of the gas exchange measurements. Some of the guidelines to be considered include:

• A steady state condition must be present to ensure that the gas exchange measurement is equivalent to the tissue gas exchange
• As Haldane Transformation is used in the VO2 calculations, monitoring should be limited to patients using less then 60% oxygen.
• A stable FiO2 must be achieved.
• Air leaks around endotracheal tubes or through chest tubes may result in false values.
• Metabolic monitors require routine calibration to ensure accuracy [8].

Metabolic monitors of the past were large and cumbersome, as they measured expired respiratory gases collected in a mixing chamber. They required varying stabilization times to achieve accurate data after a change in gas concentration [9]. Paramagnetic oxygen sensors could be used to measure the FiO2 and FeO2 values, and an infrared sensor to measure the CO2.

The development of a bedside metabolic module (M-COVX) for a critical care patient monitor has replaced the need of collecting gases in a sample chamber. This new technology uses a mathematical integration of flow and time synchronized continuous gas sampling. This new module has been shown to be comparable to standard metabolic monitors [1].

How can indirect calorimetry contribute to weaning?

Malnutrition is a common entity in the critically ill patient [10]. Severe malnutrition is associated with a reduction in the strength of respiratory muscles. This may lead to ventilatory dependence, increased risk of infection, and an increase in hospital morbidity and mortality. Overfeeding can result in metabolic, hepatic, and cardiopulmonary complications, including hypercapnia and increased minute ventilation requirements, which reduce the ability to wean. Since the Respiratory Quotient (RQ) is the ratio of VCO2/VO2, we can determine what energy sources are at work. Underfeeding, which results in the use of endogenous fat stores, should result in decreases in the RQ. Overfeeding promotes lipogenesis, which can cause an increase in the RQ. With the additional measurement of urinary urea excretion, the distinction between protein oxidation and the relative contributions of fat and carbohydrates can be determined. Optimizing a patient’s nutritional status prior to a weaning trial including the composition and amount of feeding can contribute to overall success.

When initiating a weaning trial and incorporating indirect calorimetry, continuous measurements of the VO2 and VCO2 are available. With these values being monitored, reductions in ventilator support can be implemented while observing the VO2 for an increase in the oxygen cost of breathing [11]. When monitoring a patient’s increase in minute ventilation, consideration of the reason for the acute increase in ventilatory demand must be determined. For example, this increase may be as a result of increase VCO2 and/or an increase in lung dead space (Vd/Vt). The reason for each variable may lead to a further understanding of why the patient is unable to wean from the ventilator.

Summary

Bedside monitoring of accurate gas exchange in critically ill patients is now possible. Although the initial development of metabolic monitors was for monitoring nutritional regimes, today’s technology allows for many potential areas for diagnostic and therapeutic modalities. In fact, there are new opportunities to investigate the information measured for ventilatory management of the patient. With the progression of new and varied ventilators and modes, permissive hypercapnia, and protective lung strategies, more research is required to validate the applicability of indirect calorimetry in each of these areas, and to utilize this technology to its fullest potential.

References

1. McLellan S, Walsh T, Lee A. Clinical evaluation of a new gas exchange monitor in mechanically ventilated patients. Abstracts of the 12th ESICM Annual congress (Berlin, Germany). Intensive Care Med. 1999; 25, Suppl. 1:S6
2. Wells JC, Fuller NJ. Precision and accuracy in a metabolic monitor for indirect calorimetry. Eur J Clin Nutr 1998 Jul; 52(7): 536-40
3. de V Weir JB: New methods for calculating metabolic rate with special reference to protein metabolism. J Physiol 1949; 109: 1-9
4. AARC Clinical Practice Guideline. Metabolic Measurement using Indirect Calorimetry during Mechanical Ventilation, Respir Care 1994: 39(12): 1170-1175
5. Harris JA, Benedict FG: A Biometric Study of Basal Metabolism in Man. Washington DC, Carnegie Institute of Washington, Publ 279, 1919
6. Roza AM, Shizgal HM: The Harris-Benedict equation reevaluation: Resting energy requirements and the body cell mass. Am J Clin Nutr 1984; 40: 168-182
7. Bursztein S, Elwyn D, Askanazi J, Kinney J: Energy Metabolism, Indirect Calorimetry, and Nutrition (pp. 17-21). Williams and Wilkins, 1989
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9. Brandi LS, Bertolini, R, Santini L, Cavani S: Effects of Ventilator Resetting on indirect Calorimetry measurement in critically ill surgical patient, Crit Care Med 1999; 27(3): 459-60,
10. Driver AG, Le Brun M, Iatrogenic malnutrition in patients receiving ventilatory support. JAMA 1980; 244-2195
11. Mitsuoka M, Kinninger KH, Jacobson KL, JohnsonFW, Burns DM: Utility of measurement of oxygen cost of breathing in predicting success or failure in trials of reduced mechanical ventilatory support. Resp Care 2001; 46(9): 90
    

Last updated: 1 March 2003Created
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