Advancements in Critical Care

Indirect calorimetry - Practical applications

Chris L. Harris RRCP, RRT
Clinical leader- Respiratory therapy
London Health Sciences Center, University Campus
London, Ontario, Canada

The article also available in PDF: 90KB

[Editor’s note: This paper accompanies the Clinical Window presentation at the 16th ESICM Annual Congress in Amsterdam, Netherlands 5-8 October 2003. See also CWWJ’s Internet podium area, where you find author’s presentation slides.]

Introduction

Since the early 1980’s, the application of Indirect Calorimetry for determining oxygen consumption (V02) and carbon dioxide production (VC02) through metabolic measurements continues to increase. The accuracy of the devices used to obtain these measurements has also become more reliable as the methods for obtaining this information have greatly improved [1,2]. From analyzing expiratory gases with mass spectrometry, to the more recent technology of utilizing mixing chambers, metabolic monitors have now evolved to a small bedside module enabling the clinician to have accurate metabolic measurements for a wide variety of clinical conditions.

Some of the initial metabolic monitors were large and cumbersome, as they measured expired respiratory gases collected in a large mixing chamber. They required varying stabilization times after each Fi02 change in order to achieve accurate data [9]. With the development of a bedside metabolic module (M-COVX), which is incorporated into the patient monitoring system, the need of collecting gases in a sample chamber is no longer necessary. This new technology uses a mathematical integration of flow and time synchronized continuous gas sampling and has been shown to be comparable to standard metabolic monitors [1].

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 has been demonstrated to be unreliable in the malnourished and critically ill patient [6]. Correction factors were therefore developed for various clinical conditions to compensate for the inaccurate estimation of REE [7]. However, these values are approximations, and are based on measurements of healthy individuals. They are not capable of determining the true REE in each critically ill patient. Elevated energy expenditure and negative nitrogen balance on the other hand characterize the metabolic derangement of the critically ill patient and these two values correlate with the severity of illness and the extent of injury. [8]

Considerations for metabolic monitoring in mechanical ventilation

Despite all the advances in metabolic measurements, several clinical and physiological factors can influence the accuracy 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.
  
  A steady state can be define as a period of time after the patient has stabilized from any changes and will not incur further changes in their treatment that may effect gas exchange or increase metabolism. The patient must be motionless, with consistent respiratory rates and volumes.[8]
• As Haldane Transformation is used in the VO2 calculations, monitoring should be limited to patients using less then 85% oxygen. A stable delivery FiO2 must be achieved.
• Air leaks of endotracheal tube cuffs or circuit connections, and chest tubes may result in false values.
• Metabolic monitors require routine calibration to ensure accuracy [2,8].

Practical tips for metabolic monitoring with the M-COVX

When using any type of new equipment, it takes time to learn all the finer points of the device. Several variables can contribute to inaccurate information being collected. When using any metabolic monitor the clinician must consider the above guidelines to ensure a steady state has been achieved to ensure accuracy. There are some practical considerations of which are also good to bear in mind [12]:

1. Correct length of sampling line is important
   - Sampling lines require specific lengths and diameters for accurate reporting of gas and flow.
   - Datex-Ohmeda only recommends a 2-meter sampling line [Ed.].
2. Free tubing with no obstructions
   - Water in a line (active humidity, secretions) or kinked tubing should be noted.
   - M-COVX should be disconnected during nebulization of medications [Ed.]
3. Calibration and accuracy of measurement
   - Calibration => within last six months or according to company specifications
   - Always check that correct calibration gas is used. Please note that he module must finish its self-calibration prior to connecting to the patient.
   - Adequate VO2/VCO2 values cannot be obtained when N2O/O2 mixtures are used in ventilation or when anesthetic agents are used [Ed].
4. Sampling sensor (D-lite) in correct position in the patient circuit.
   - When HME/HMEF is used it must be placed between the D-lite sensor and the patient.
   - When monitoring pediatric patients with small (15-300 ml) tidal volumes Pedi-lite or Pedi lite+ sensors should be used, and sensor type "Pedi" selected in the monitor menu [Ed.].
5. Ventilator function
   - Leak in the circuit or Flow-by being used results in inaccurate volume measurement
   - High PEEP or ventilating pressures may influence the gas analyzers
   - High Fi02 (above 85%) will increase the sensitivity of the Haldane Transformation to error.
   - High (over 35/min) respiratory rates, high by-pass flow, HFV or BiPAP ventilation do not allow adequate time for accurate sampling

When to use indirect calorimetry?

Indirect Calorimetry has many clinical applications. From assessing a patient’s metabolic responses to illness, to measuring the basic nutrition and energy requirements, the applications for this type of technology continue to grow. The bedside clinician must know not only how to obtain accurate results, but also must have a general understanding of how the values obtained can influence the course of the patient’s care plan.

Nutrition
Malnutrition is a common problem in the critically ill patient [10]. Severe malnutrition is associated with a reduction in the strength of respiratory muscles. This may lead to prolonged 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 may 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 also be determined. Optimizing a patient’s nutritional status prior to a weaning trial, which includes modifying the composition and amount of feeding according to the patient’s needs, may contribute to overall success.

Ventilatory weaning

When initiating a weaning trial, it is helpful to incorporate continuous measurements of VO2 and VCO2. While these values are being monitored, reductions in ventilator support can be implemented while observing VO2 as an indicator of the metabolic 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 in VCO2 and/or an increase in dead space (Vd/Vt). The underlying cause for each variable may lead to other management interventions or investigations as well as a better understanding of why the patient is unable to wean from the ventilator.

Sepsis/Trauma/Surgery/Burns

In the critically ill patient, stress imparts a hypermetabolic state. This results in an increase in the stress hormone release (epinephrine, cortisol, glucagons etc.) into the body. The increase in hormone levels results in an elevation of the energy expenditure and increase in glucose, lipid and protein consumption. With monitoring of metabolic values of the patient and the changes that occur as a result of management interventions, an additional piece of information is provided allowing the clinician a more complete picture of the patient’s physiological state. Not only can nutrition be tailored to suit the patient, but monitoring of the metabolic state will demonstrate acute changes in the patient’s metabolism and derangement of gas exchange which may be attributable to a number of factors (i.e., sepsis, fever, acute lung injury.)

Therapeutic targets
With certain therapeutic interventions, the results can be monitored with the V02 and VC02 values obtained through metabolic monitoring. By observing these continuous readings the effective change on oxygen consumption and carbon dioxide production can be observed in order to achieve an optimal effect. Examples include sedation, heating/cooling a patient, response to inotropic support etc.

Summary

Although the initial development of metabolic monitors was for assessing nutritional regimes, today’s technology with its ability to have instantaneous values at the bedside allows for many more potential areas of diagnostic and therapeutic modalities. Many opportunities exist to measure whether or not a determined ventilatory management is adequate. With the progression of new and varied ventilators and modes of ventilation, permissive hypercapnia, and protective lung strategies for example, more research is required to validate the applicability of Indirect Calorimetry to the critically ill patient 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. Makita K, Nunn JF, Royston B. Evaluation of metabolic measuring instruments for use in critically ill patients. Crit Care Med. 1990 Jun;18(6):638-44
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
8. Brandi LS, Bertolini R, Calafa M, Indirect Calorimetry in Critically Ill patient: Clinical Applications and Practical Advice, Nutrition 1997; 13: 349-358
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, Johnson FW, 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
12. Chapter Gas exchange in the User guide for critical care monitor by Datex-Ohmeda
    

Last updated: 1 November 2003Created
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