Critical Care

The evolution of usable indirect calorimetry

Patrick Sweeney
Product Manager
Critical Care Monitoring
Datex-Ohmeda

The article also available in PDF: 102 KB

The medical device industry is committed to developing innovations that contribute to easier care based on constant dialogue with end users. However, providing tools for the intensive care environment is at the same time becoming increasingly challenging, largely because today’s advanced technology provides enormous scope for new applications.

This, in turn, highlights the need to help users gain a full understanding of the application areas of a device, and to assess the real impact of a device on outcomes in the intensive care environment. The usefulness of a medical device, for all its advantages on paper, is limited in practice if the data it provides is difficult to decipher, or if the device requires specialists with the time and understanding to apply and process the data in the clinical environment.

For a number of years the trend in the industry has been to create smaller, more modular devices which can be applied to a greater number of patients, by a greater number of professionals. It then becomes necessary to ensure that data gathered by these devices can be easily interpreted and integrated with other physiological data to gain full advantage from it.

The evolution of Datex-Ohmeda’s Deltatrac stand-alone metabolic monitor into today’s M-COVX monitoring module provides a good example of this trend in practice.

Deltatrac, the gold standard in the 1980s

Datex-Ohmeda’s Deltatrac metabolic monitor came to the forefront in the mid- to late-1980s as the gold standard in indirect calorimeters. It provides VO2, VCO2, energy expenditure, respiratory quotient values and substrate analysis.

The Deltatrac uses a mixing chamber to collect expired gases. It then analyzes these gases using a paramagnetic oxygen sensor for FiO2 and FeO2, and calculates the difference between FiO2 and FeO2. The Deltatrac also measures the CO2 concentration of inspired and expired gases using an infrared sensor. A constant flow generator is incorporated to produce a known total (constant) flow. The Deltatrac data and values on both ventilated and non-ventilated patients are updated every minute after a recommended period necessary for reaching a steady state.

The Deltatrac was generally used to assess the nutritional requirements of intensive care patients by providing energy expenditure and respiratory quotient data due to the nature of the measurement and because the pulmonary artery catheter was still routinely used in intensive care patients. Since it also provided substrate analysis (the amount of fats, proteins and carbohydrates being metabolized) it could be used to identify and tailor feeding requirements for patients. Very often it is still used to determine carbohydrate metabolism in difficult-to-wean patients.

Towards the ideal monitor

Certain attributes are desirable when attempting to create the ‘ideal’ monitoring system (1). The device must:

• Ensure low risk of harming the patient
• Be inexpensive
• Provide accurate data
• Be pertinent to patient management
• Be practical, and
• Ensure good reproducibility of data

The Deltatrac covers a number of the attributes prescribed by Tobin (1) for an ideal monitoring system. These primarily fall in the areas of risk, expense and accuracy (2, 3). Because the Deltatrac was non-invasive, it could be used between patients, and the lifetime of the monitor was very long (there are some 5,000-10,000 units still in use). Gas exchange and metabolic function results obtained from the Deltatrac are highly reproducible and accurate if it is used according to certain prescribed conditions.

However, all these advantages came under scrutiny for three basic reasons. First, its use of a mixing chamber made the device bulky. Second, its ability to integrate the information it provided with other physiological information was poor since any correlation or presentation of its data had to be separate or done by hand.

The third limitation, and perhaps one of the greatest of any stand-alone monitor for specialized and focused measurement in intensive care, was the need for a dedicated user responsible for operating the device and interpreting the data. This specialist, usually a dedicated nurse, clinical dietician, respiratory therapist, ICU technologist or research fellow, was then responsible for charting the data or conveying the information to other members of the care team before it could be used for the benefit of the patient.

The operator also needed time to allow a steady state be attained for the ventilator settings (a ‘steady state’ in this context is a period with no changes to ventilator parameters and no large changes in patient physiological status). This diminished the Deltatrac’s applicability in routine care and therefore its value in patient care was largely overlooked.

M-COVX, towards the ‘ideal’ device

The advent of modular monitoring provided an excellent opportunity to rethink the Deltatrac technology to provide users with full access to its acclaimed advantages while removing the limitations described above.

Modular medical monitoring has, in most cases, made it possible to measure and investigate quickly and effectively the physiological parameters dictated by the patient’s disease state or increased risk for certain sequelae. It has also provided continuity of information through the care process and the ability to report this information in a standardized way.

Datex-Ohmeda’s M-COVX is a module for monitoring respiratory gas exchange and pulmonary mechanics. This module is based on the measurement of CO2 and O2 concentrations, flow and pressure at the patient Y-piece, and it integrates these to obtain gas exchange values.

The COVX module is designed for use in the CS/3 and AS/3 monitors and the System 5 family of monitors. As a modular form of the Deltatrac, it enables breath-by-breath measurement of VO2, VCO2, energy expenditure and respiratory quotient calculation. It is meant to be used continuously so that up to 72 hours of trended values can be viewed in conjunction with the trends of all other monitored parameters.

Weighing only 1.6 kg, the module can be integrated into patient monitoring as quickly as any other modular measurement parameter (e.g. SpO2, ECG) and reports can be generated and information stored for later reference.

The COVX module enables modular monitoring of gas exchange and indirect calorimetry. Unlike the Deltatrac, however, it does not employ the collection of gases in a mixing chamber. Instead, it uses a paramagnetic oxygen sensor to measure the O2 curve and an infrared sensor to measure the CO2 curve at the patient airway.

How the M-COVX works

A D-Lite+ sensor located at the patient airway measures flows and sophisticated algorithms are then used to compensate for time delays and distortion of gas concentrations. Finally, the waveforms are reconstructed breath by breath and made to coincide so that the following equations are made possible (see Figure 1 and below):

Figure 1. Calculation of volume-averaged concentrations.

To obtain the oxygen consumption (VO2) of a patient, the inhaled and exhaled amounts of oxygen are measured and the amount exhaled is then subtracted from the amount inhaled. Since a curve is being constructed these values are found by multiplying each tidal volume piece (dv) by the corresponding gas concentration in that piece:

The same is true for carbon dioxide production (VCO2), except that inspired values are subtracted from the expired values:

Using the inspiratory (MVi) and expiratory (MVe) minute volumes and the volume-averaged inspiratory and expiratory concentrations fi and fe, these equations can then be written:

To make results less sensitive to error in volume measurement, the well-known Haldane transformation is then applied. This entails taking advantage of the fact that the patient is not consuming or producing nitrogen. Therefore, the amount of nitrogen inhaled equals the amount of nitrogen exhaled:

VO2 and VCO2 are then written using inspiratory minute volume (MVi):

or they can also be written using expiratory minute volume (MVe):

where:

Greater insight in routine monitoring

The result is a module providing gas exchange and indirect calorimetry measurements that can be used for a prolonged period of time on a greater number of patients. The COVX module has been shown to be just as accurate as the Deltatrac when used in the clinical environment (4). In addition to comparable accuracy the module reacts faster to changes in patient metabolic status. The COVX module follows changes breath by breath but averages them over 60 seconds to reduce variations in measurement.

The application area of gas exchange measurements can now also extend to the operating theater where accurate measurement of gas exchange has been shown to be of value in a variety of applications (5, 6).

However, perhaps the greatest advantage of this module is its ability to provide the members of a multi-disciplinary team with greater insight in routine monitoring (Fig. 2). This means that the module provides usable information on nutrition and energy requirements as well as a comprehensive analysis of ventilation and oxygen transport of patients with cardio-respiratory problems. If it can also be shown to help improve patient management and influence outcomes in the areas of failure to wean from mechanical ventilation, sepsis, tissue response to vaso-active drugs and other therapies, then the questions of pertinence and cost will have been addressed.

Figure 2. The value of modular indirect calorimetry in a bedside monitor can best be seen when data from other parameters are compared with those from the COVX module. Here we see that at approx. 12:05 VO2 and VCO2 from the COVX module react to changes in patient status before other parameters. The generation of breath-by-breath data, updated at one-minute intervals, makes comparison of such changes possible

Since indirect calorimetry can now be easily added to a bedside patient monitor as quickly as any other routine clinical measurement, the COVX module fulfills virtually all of the criteria for the ‘ideal’ monitor, especially when coupled with the device’s accuracy and ease of use.

Edited with permission from Patrick Sweeney’s article ‘Monitoring the situation’ published in HES International, May 2001.

(Published in WINDOW magazine 2002 No. 1.)

References:

1. Tobin MJ. Respiratory monitoring in the intensive care unit. American Review of Respiratory Disease 1988;138:1625-42.
2. 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;26:1200-7.
3. Raurich Puigdevall JM, Ibanez Juve J. Energy expenditure at rest: indirect calorimetry vs the Fick principle (in Spanish). Nutrición Hospitalaria 1998;13:303-8.
4. McLellan S, Walsh T, Lee A. Clinical evaluation of a new gas exchange monitor in mechnically ventilated patients. Abstracts of the 12th ESICM Annual Congress; 1999 Oct 3-6; Berlin, Germany. Intensive Care Medicine 1999;25 Suppl 1:S6.
5. Walsh TS, Hopton P, Garden OJ, Lee A. Effect of graft reperfusion on haemodynamics and gas exchange during liver transplantation. British Journal of Anaesthesia 1998;81:311-6.
6. Hickey S, Gaylor JD, Kenny GN. In vitro uptake and elimination of isoflurane by different membrane oxygenators. Journal of Cardiothoracic and Vascular Anesthesia 1996;10:352-5.
   

Last updated: 1 April 2002Created
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