Methods to validate the accuracy of an indirect calorimeter in the in-vitro setting

Feeding should be individualized in the critically ill patients

PURPOSE OF REVIEW

Any critical care therapy requires individual adaptation, despite standardization of the concepts supporting them. Among these therapies, nutrition care has been repeatedly shown to influence clinical outcome. Individualized feeding is the next needed step towards optimal global critical care.

RECENT FINDINGS

Both underfeeding and overfeeding generate complications and should be prevented. The long forgotten endogenous energy production, maximal during the first 3 to 4 days, should be integrated in the nutrition plan, through a slow progression of feeding, as full feeding may result in early overfeeding. Accurate and repeated indirect calorimetry is becoming possible thanks to the recent development of a reliable, easy to use and affordable indirect calorimeter. The optimal timing of the prescription of the measured energy expenditure values as goal remains to be determined. Optimal protein prescription remains difficult as no clinically available tool has yet been identified reflecting the body needs.

SUMMARY

Although energy expenditure can now be measured, we miss indicators of early endogenous energy production and of protein needs. A pragmatic ramping up of extrinsic energy provision by nutrition support reduces the risk of overfeeding-related adverse effects.

Evaluation of the accuracy and precision of a new generation indirect calorimeter in canopy dilution mode

BACKGROUND & AIMS

Indirect calorimetry (IC) is the only way to measure in real time energy expenditure (EE) and to optimize nutrition support in acutely and chronically ill patients. Unfortunately, most of the commercially available indirect calorimeters are rather complex to use, expensive and poorly accurate and precise. Therefore, an innovative device (Q-NRG®, COSMED, Rome, Italy) that matches clinicians' needs has been developed as part of the multicenter ICALIC study supported by the two academic societies ESPEN and ESICM. The aim of this study was to evaluate the accuracy and intra- and inter-unit precision of this new device in canopy dilution mode in vitro and in spontaneously breathing adults.

METHODS

Accuracy and precision of oxygen consumption (VO₂) and carbon dioxide production (VCO₂) measurements were evaluated in vitro and in 15 spontaneously breathing healthy adults by interchanging three Q-NRG® units in a random order. In vitro validation was performed by gas exchange simulation using high-precision gas mixture and mass flow controller. Accuracy was calculated as error of measured values against expected ones based on volume of gas infused. Respiratory coefficient (RQ) accuracy was furthermore assessed using the ethanol-burning test. To evaluate the intra- and inter-unit precisions, the coefficient of variation (CV% = SD/Mean*100) was calculated, respectively, from the mean ± SD or the mean ± SD of the three mean values of VO₂, VCO₂, RQ and EE measured by each Q-NRG® units. In vivo accuracy measurement of the Q-NRG® was assessed by simultaneous comparison with mass spectrometry (MS) gas analysis, using Bland-Altman plot, Pearson correlation and paired t-test (significance level of p = 0.05).

RESULTS

In vitro evaluation of the Q-NRG® accuracy showed measurement errors <1% for VO₂, VCO₂ and EE and <1.5% for RQ. Evaluation of the intra- and inter-unit precision showed CV% ≤1% for VO₂ and EE and CV% ≤1.5% for VCO₂ and RQ measurements, except for one Q-NRG® unit where CV% was 2.3% for VO₂ and 3% for RQ. Very good inter-unit precision was confirmed in vivo with CV% equal to 2.4%, 3%, 2.8% and 2.3% for VO₂, VCCO₂, RQ and EE, respectively. Comparison with MS showed correlation of 0.997, 0.987, 0.913 and 0.997 for VO₂, VCO₂, RQ and EE respectively (p ≤ 0.05). Mean deviation of paired differences was 1.6 ± 1.4% for VO₂, -1.5 ± 2.5% for VCO₂, -3.1 ± 2.6% for RQ and 0.9 ± 1.4% for EE.

CONCLUSION

Both in vitro and in vivo measurements of VO₂, VCO₂, RQ and EE on three Q-NRG® units showed minimal differences compared to expected values and MS and very low intra- and inter-unit variability. These results confirm the very good accuracy and precision of the Q-NRG® indirect calorimeter in canopy dilution mode in spontaneously breathing adults.

In vitro validation of indirect calorimetry device developed for the ICALIC project against mass spectrometry

RATIONALE

Accurate evaluation of the energy needs is required to optimize nutrition support of critically ill patients. Recent evaluations of indirect calorimeters revealed significant differences among the devices available on the market. A new indirect calorimeter (Q-NRG^®, Cosmed, Roma, Italy) has been developed by a group of investigators supporting the international calorimetry study initiative (ICALIC) to achieve ultimate accuracy for measuring energy expenditure while being easy to use, and affordable. This study aims to validate the precision and the accuracy of the Q-NRG^® in the in-vitro setting, within the clinically relevant range for adults on mechanical ventilation in the ICU. Mass spectrometry is the reference method for the gas composition analysis to evaluate the analytic performances of the Q-NRG^®.

METHODS

The accuracy and precision of the O₂ and CO₂ measurements by the Q-NRG were evaluated by comparing the measurements of known O₂ and CO₂ gas mixtures with the measurements by the mass spectrometer (Extrel, USA). The accuracy and precision of the Q-NRG^® for measurements of VO₂ (oxygen consumption) and VCO₂ (CO₂ production) at clinically relevant ranges (150, 250 and 400 ml/min STPD) were evaluated by measuring simulated gas exchange under mechanically ventilated setting at different FiO₂ settings (21-80%), in comparison to the reference measurements by the mass spectrometer-based mixing chamber system.

RESULTS

The measurements of gas mixtures of predefined O₂ and CO₂ concentrations by the Q-NRG^® were within 2% accuracy versus the mass spectrometer measurements in Passing Bablok regression analysis. In a mechanically ventilated setting of FiO₂ from 21 up to 70%, the Q-NRG^® measurements of simulated VO₂ and VCO₂ were within 5% difference of the reference mass spectrometer measurements.

CONCLUSION

In vitro evaluation confirms that the accuracy of the Q-NRG^® indirect calorimeter is within 5% at oxygen enrichment to 70%; i.e. maximum expected for clinical use. Further recommendations for the clinical use of the Q-NRG^® by will be released once the ongoing multi-center study is completed.

Indirect calorimetry in critically ill mechanically ventilated patients: Comparison of E-sCOVX with the deltatrac

BACKGROUND & AIMS

Indirect calorimetry is recommended to measure energy expenditure (EE) in critically ill, mechanically ventilated patients. The most validated system, the Deltatrac® (Datex-Ohmeda, Helsinki, Finland) is no longer in production. We tested the agreement of a new breath-by-breath metabolic monitor E-sCOVX® (GE healthcare, Helsinki, Finland), with the Deltatrac. We also compared the performance of the E-sCOVX to commonly used predictive equations.

METHODS

We included mechanically ventilated patients eligible to undergo indirect calorimetry. After a stabilization period, EE was measured simultaneously with the Deltatrac and the E-sCOVX for 2 h. Agreement and precision of the E-sCOVX was tested by determining bias, limits of agreement and agreement rates compared to the Deltatrac. Performance of the E-sCOVX was also compared to four predictive equations: the 25 kcal/kg, Penn State University 2003b, Faisy, and Harris-Benedict equation.

RESULTS

We performed 29 measurements in 16 patients. Mean EE-Deltatrac was 1942 ± 274 kcal/day, and mean EE-E-sCOVX was 2177 ± 319 kcal/day (p < 0.001). E-sCOVX overestimated EE with a bias of 235 ± 149 kcal/day, being 12.1% of EE-Deltatrac. Limits of agreement were -63 to +532 kcal/day. The 10% and 15% agreement rates of EE-E-sCOVX compared to the Deltatrac were 34% and 72% respectively. The bias of E-sCOVX was lower than the bias of the 25 kcal/kg-equation, but higher than bias of the other equations. Agreement rates for E-sCOVX were similar to the equations. The Faisy-equation had the highest 15% agreement rate.

CONCLUSION

The E-sCOVX metabolic monitor is not accurate in estimating EE in critically ill mechanically ventilated patients when compared to the Deltatrac, the present reference method. The E-sCOVX overestimates EE with a bias and precision that are clinically unacceptable.

Innovations in energy expenditure assessment

PURPOSE OF REVIEW

Optimal nutritional therapy has been associated with better clinical outcomes and requires providing energy as closed as possible to measured energy expenditure. We reviewed the current innovations in energy expenditure assessment in humans, focusing on indirect calorimetry and other new alternative methods.

RECENT FINDINGS

Although considered the reference method to measure energy expenditure, the use of indirect calorimetry is currently limited by the lack of an adequate device. However, recent technical developments may allow a broader use of indirect calorimetry for in-patients and out-patients. An ongoing international academic initiative to develop a new indirect calorimeter aimed to provide innovative and affordable technical solutions for many of the current limitations of indirect calorimetry. New alternative methods to indirect calorimetry, including CO2 measurements in mechanically ventilated patients, isotopic approaches and accelerometry-based fitness equipments, show promises but have been either poorly studied and/or are not accurate compared to indirect calorimetry. Therefore, to date, energy expenditure measured by indirect calorimetry remains the gold standard to guide nutritional therapy.

SUMMARY

Some new innovative methods are demonstrating promises in energy expenditure assessment, but still need to be validated. There is an ongoing need for easy-to-use, accurate and affordable indirect calorimeter for daily use in in-patients and out-patients.