Discussion
Most clinicians, primarily RDNs, use weight-based formulae or predictive equations as part of nutrition assessment to determine estimated REE to help guide nutrition prescription. When using equations, one must be cognizant regarding the inaccuracies of much of the anthropometric data used in the calculation. It has been practiced in this facility to obtain recumbent length measures. The most reliable weight available should be used: the admission weight (usually the lowest documented weight) or the usual body weight or documented dry weight. As discussed, it remains disputed in the literature as to whether or not predictive equations are accurate enough in day-to-day determination of energy requirement. IC is uncommon, and even rare, outside of academic centers, due in part to the high cost of acquiring the indirect calorimeter and the need for skilled and trained personnel to conduct studies.11 Poor reimbursement by insurance for tests performed has also been considered a barrier to using IC.8 As a result, predictive equations have been recommended as surrogates for IC, despite accuracies which range from 0% to 77%.15 Unfortunately, those healthcare professionals who do not receive a strong education in nutrition or are not familiar with critical care medicine do not recognize these formulae were created based on healthy individuals. So although the predictive formulae are reasonably accurate for reasonably healthy individuals, it is often incorrectly assumed these equations are also accurate in patients with significant medical illness.16 This misconception is further reinforced by the ASPEN-SCCM Guidelines, which recommend a weight-based formula due to its simplicity over more complicated predictive equations, when IC is not available.2 The Bland-Altman plots of the predictive equations and weight-based formulae illustrate that although across a group of patients these calculations may be reasonable estimates collectively, for an individual patient it is not possible to know the direction or magnitude of the error (figures 1 and 2).
IC studies are reimbursable and have specific assigned Current Procedural Terminology (CPT) codes and reimbursements (ie, Medicare reimbursement CPT code 94690 “Exhaled Air Analysis”) which can offset the costs associated with its purchase and necessary disposables (ie, tubing, filters, gas).17 As determined in the study by Heyland et al18, caloric adequacy may contribute to prolonged ICU survival. Underfed patients are also likely to experience poor wound healing and increased risk of nosocomial infections.19 Overfeeding, although arguably a less common occurrence in ICU, is also detrimental as it is associated with prolonged ventilation weaning, increased risk of infection, and hyperglycemic events.8 Therefore, it is imperative to provide the optimum energy dose for the critically ill patients to ensure the best outcomes. The usefulness of IC continues to be debated as prediction equations are seen as easier and less costly to use. IC calculates REE by measuring gas exchange between whole-body oxygen and carbon dioxide.19 The mean level of hypermetabolism in trauma patients has been reported to be as high as 116% to 158%.20 Factors that may influence REE include injury (ie, burn, trauma, surgery), temperature (presence or absence of fever), diet-induced thermogenesis, sepsis or infection, sedation, agitation, and potentially when family or friends visit the patient at bedside.8 Patients on extreme ends of the BMI scale, those with significant fluid accumulation, or those with amputations provide a challenge when using predictive equations as most equations use body weight as a variable.19 BMI does not consistently correlate with body composition, which is an important determinant of energy expenditure due to variations in the metabolic activities of tissues, such as the skeletal muscle as compared with adipose.21 22 Findings by Janssen et al23 indicate that men have more skeletal muscle than women (38.4% vs. 30.6%) and that these gender differences are greater in the upper body. Independent of gender, aging has been found to be associated with a decrease in skeletal muscle mass that is explained, in large measure, by a decrease in the lower body occurring after the fifth decade of life. Equations attempt to take these differences into account; however, body composition is highly variable, directly affecting mREE.23
The patients that were included in this study had an average core temperature slightly above the normal range (38.3°C±0.84°C), which may explain why the average kcal/kg for mREE was higher than HBE and MSJ, but more closely related to PSU. PSU incorporates temperature as part of the equation. Disease severity may play a role in increased mREE, but diagnoses or disease severity data were not collected in this study. Estimation formulae cannot factor in other issues such as metabolic conditions and effects of medications (ie, paralytics, steroids, beta blockers). This makes the choice of a stress factor less of an educated guess and more of a random multiplier that gives a false sense of accuracy.
Clinicians may not fully realize the importance of accurate dosing of nutrition support.24 A retrospective analysis of prospective study data from an international sample of ICUs examined the relationship between caloric provision and mortality in critically ill patients requiring long-term ventilation. It was found that only 0.8% of approximately 8000 cases used IC.18 A total of 475 patients out of 1223 study participants met the criteria for further analysis. Of these 475 patients, 36% died. Patients who received less than estimated/measured caloric requirements had a significantly shortened survival time than those who were considered to be adequately fed. In this study, nutritional adequacy was categorized as low (<50%), moderate (≥50% and <80%), and high (≥80%). This study shed light on the importance of appropriate dosing of calories. However, Heyland et al admitted there was no standardized method used between study participants’ determination of energy requirements (IC vs. numerous predictive equations).18 The researchers detailed this as a major limitation of the study.
The strengths of this study include that IC testing was limited to two skilled RDNs, performed by the same Quark RMR metabolic cart that was regularly calibrated. The limitations include measurements obtained from a single center with a fairly low number of patients, in a retrospective fashion, and that all patients were intubated and mechanically ventilated. As the number of ICU patients assessed increases, it will be possible to repeat the process of this study in specific conditions, such as traumatic brain injury, acute decompensated congestive heart failure, or acute sepsis and septic shock. Future studies will consider the acuity of the patients studied based on diagnoses and the Acute Physiology and Chronic Health Evaluation II scoring to determine disease severity. Using these data, a multivariate regression analysis might be able to determine if severity of illness condition affects the comparison of IC with commonly used predictive equations. Serial studies may allow assessment of how mREE changes throughout different phases of illness/injury. Finally, the use of the IC canopy would allow expansion of the use of IC to patients who are not mechanically ventilated.