EE: Duration of Measurement (Steady State) (2013)

Citation:

Cunningham KF, Aeberhardt LE, Wiggs BR, Phang T. Appropriate interpretation of indirect calorimetry for determining energy expenditure of patients in intensive care units. Am J Surg. 1994; 167: 547-549.

PubMed ID: 8185046
 
Study Design:
Retrospective Cohort Study
Class:
B - Click here for explanation of classification scheme.
Quality Rating:
Neutral NEUTRAL: See Quality Criteria Checklist below.
Research Purpose:
  • To determine whether energy expenditure determined from five minutes of measurement with less than 5% variation was similar to energy expenditure determined from a longer measurement period
  • To determine what duration of energy expenditure measurements with coefficients of variations of 5% to 20% is required to achieve a standard error of the mean of 3% (by generating a statistical model using computer stimulation).
Inclusion Criteria:


 

Intubated patients in intensive care unit who were being assessed for nutrition support.

Exclusion Criteria:

 

Not meeting inclusion criteria.

 

Description of Study Protocol:

Recruitment

Retrospective review of results of indirect calorimetry for 47 consecutively intubated patients in the intensive care unit.

Design

Correlational study.

Statistical Analysis

  • Pearson correlation coefficient to indicate relationship between the mean of the five-minute period and the mean of the entire study period
  • Percent difference between the two values for each patient determined according to methods described by Bland et al
  • Statistical model using a computer simulation was generated to determine what duration of energy expenditure measurements with coefficients of variation of 5% to 20% is required to achieve a standard error of the mean of 3%:
    • Energy expenditure was assumed to be randomly distributed about a mean value
    • Random data were generated within each specified magnitude of coefficient of variation (±5%, 10%, 15% and 20% of the mean value), representing a one-minute value of energy expenditure
    • A random number was generated repeatedly for each data set within the specified magnitude of coefficient of variation and a "rolling" calculation of the mean and standard error of the mean of all previously generated random numbers was performed, representing consecutive minutes of measurement of energy expenditure
    • The generation of random numbers within a specified magnitude of coefficient of variation was stopped once the standard error of the mean was 3%
    • For each specified magnitude of coefficient of variation, 1,000 such data sets were generated and the minutes of measurement required to achieve a 3% standard error of the mean in 95% of cases were noted.
Data Collection Summary:

Timing of Measurements

One measurement time, on weekday mornings from 8:00 A.M. to 12:00 P.M.

Dependent Variables

  • Difference between the short mean and long mean of energy expenditure measurement:
    • Short mean: Energy expenditure determined from five minutes of measurement with less than 5% variation in minute-to-minute measurements
    • Long mean: Energy expenditure determined from a longer measurement period  (average duration of 31±13 minutes)
  • Correlation between between short mean and long mean.

Independent Variables

  • Measured REE [(VO2 L per minute), VCO2 (L per minute; ml per kg per minute), RQ, ventilation (L per minute)]
    • IC type: Deltatrac Metabolic Monitor
    • Equipment of Calibration: Medical-grade gas (96% O2 and 4% CO2)
    • Coefficient of variation using std gases: Yes
    • Number of measures within the measurement period: Not specified; to continue measuring energy expenditure until a period of five consecutive minutes of readings with a coefficient of variation of 5% or less occurs
    • Were some measures eliminated? Not stated
    • Were a set of measurements averaged? Yes, five consecutive minutes of readings were averaged when stable measurements occurs (coefficient of variation of 5% or less)
    • Coefficient of variation in subjects measures? Yes
    • IC measurements were made on week-day mornings from 8:00 A.M. to 12:00 P.M. All measurements were obtained by one person
    • Patients were not purposefully sedated, but sedation was used for routine care
    • Nursing interventions (i.e., suctioning, turning or dressings) were not allowed during the study periods.

 

 

Description of Actual Data Sample:
  • Initial N: N=47 patients in intesive care unit (gender not reported)
  • Final N: N=47
  • Location: Vancouver, British Columbia, Canada.
Summary of Results:


 

  • The correlation coefficient was 0.983, P<0.00001, for the relationship between the mean of the five-minute period and the mean of the entire study period
  • The difference between short mean and long mean was less than 5% for 42 of 47 patients; the difference was more than 10% for two patients
  • The difference between the mean values of the five-minute period and the entire study period was -2.6±60kcal per day (mean ±SD; P was not significant)
  • The statistical model showed that for energy expenditure variation of 5%, 10%, 15% and 20%, the necessary durations of measurement to achieve a 3% error were 3, 9, 16 and 25 minutes.

Statistical Model

  • The measurement duration required to achieve a 3% standard error of the mean in 95% of cases is five minutes for a 5% coefficient of variation
  • It increases to 25 minutes for a 20% coefficient of variation.
Author Conclusion:
  • We found that the mean of a five-minute period of measurement of energy expenditure with a CV of 5% or less is the same as the mean of a longer measurement period of 31±13 minutes
  • A 25-minute measurement of energy expenditure is required to achieve a 3% SEM with variation coefficients as high as 20%
  • We concluded that energy expenditure may be determined using the mean of a five-minute period of measurement if variation in that measurement is less than 5%. Larger variation requires longer period of time.
  • Even with measurement variability as great as ±20%, a reliable value of energy expenditure is assured with measurement duration of 25 minutes
  • Variations of minute-to-minute readings are associated with patient agitation, restlessness and movement.
Funding Source:
University/Hospital: University of British Columbia (Canada)
Reviewer Comments:

Agree with authors’ conclusions based on the study design, well conducted statistical test, and the valid statistical model. 

Strengths

  • Good measurement reliability and validity, appropriate statistical test
  • The process of computer simulation to estimate the duration of measurements was well explained. 

 

Generalizability/Weaknesses

  • The measurement recommendation can only be applied to critically ill patients. MD referral bias related to request for nutrition support assessment.
  • Demographic information was not reported in detail
  • Did not discuss potential intervening factors for the two patients whose CV was higher than 10%.
Quality Criteria Checklist: Primary Research
Relevance Questions
  1. Would implementing the studied intervention or procedure (if found successful) result in improved outcomes for the patients/clients/population group? (Not Applicable for some epidemiological studies) Yes
  2. Did the authors study an outcome (dependent variable) or topic that the patients/clients/population group would care about? Yes
  3. Is the focus of the intervention or procedure (independent variable) or topic of study a common issue of concern to dieteticspractice? Yes
  4. Is the intervention or procedure feasible? (NA for some epidemiological studies) Yes
 
Validity Questions
1. Was the research question clearly stated? Yes
  1.1. Was (were) the specific intervention(s) or procedure(s) [independent variable(s)] identified? Yes
  1.2. Was (were) the outcome(s) [dependent variable(s)] clearly indicated? Yes
  1.3. Were the target population and setting specified? Yes
2. Was the selection of study subjects/patients free from bias? No
  2.1. Were inclusion/exclusion criteria specified (e.g., risk, point in disease progression, diagnostic or prognosis criteria), and with sufficient detail and without omitting criteria critical to the study? No
  2.2. Were criteria applied equally to all study groups? ???
  2.3. Were health, demographics, and other characteristics of subjects described? No
  2.4. Were the subjects/patients a representative sample of the relevant population? ???
3. Were study groups comparable? N/A
  3.1. Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT) N/A
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? N/A
  3.3. Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.) N/A
  3.4. If cohort study or cross-sectional study, were groups comparable on important confounding factors and/or were preexisting differences accounted for by using appropriate adjustments in statistical analysis? N/A
  3.5. If case control study, were potential confounding factors comparable for cases and controls? (If case series or trial with subjects serving as own control, this criterion is not applicable.) N/A
  3.6. If diagnostic test, was there an independent blind comparison with an appropriate reference standard (e.g., "gold standard")? N/A
4. Was method of handling withdrawals described? N/A
  4.1. Were follow-up methods described and the same for all groups? N/A
  4.2. Was the number, characteristics of withdrawals (i.e., dropouts, lost to follow up, attrition rate) and/or response rate (cross-sectional studies) described for each group? (Follow up goal for a strong study is 80%.) N/A
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? N/A
  4.4. Were reasons for withdrawals similar across groups? N/A
  4.5. If diagnostic test, was decision to perform reference test not dependent on results of test under study? N/A
5. Was blinding used to prevent introduction of bias? No
  5.1. In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate? N/A
  5.2. Were data collectors blinded for outcomes assessment? (If outcome is measured using an objective test, such as a lab value, this criterion is assumed to be met.) N/A
  5.3. In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded? N/A
  5.4. In case control study, was case definition explicit and case ascertainment not influenced by exposure status? N/A
  5.5. In diagnostic study, were test results blinded to patient history and other test results? N/A
6. Were intervention/therapeutic regimens/exposure factor or procedure and any comparison(s) described in detail? Were interveningfactors described? Yes
  6.1. In RCT or other intervention trial, were protocols described for all regimens studied? N/A
  6.2. In observational study, were interventions, study settings, and clinicians/provider described? Yes
  6.3. Was the intensity and duration of the intervention or exposure factor sufficient to produce a meaningful effect? Yes
  6.4. Was the amount of exposure and, if relevant, subject/patient compliance measured? Yes
  6.5. Were co-interventions (e.g., ancillary treatments, other therapies) described? N/A
  6.6. Were extra or unplanned treatments described? N/A
  6.7. Was the information for 6.4, 6.5, and 6.6 assessed the same way for all groups? Yes
  6.8. In diagnostic study, were details of test administration and replication sufficient? N/A
7. Were outcomes clearly defined and the measurements valid and reliable? Yes
  7.1. Were primary and secondary endpoints described and relevant to the question? Yes
  7.2. Were nutrition measures appropriate to question and outcomes of concern? Yes
  7.3. Was the period of follow-up long enough for important outcome(s) to occur? Yes
  7.4. Were the observations and measurements based on standard, valid, and reliable data collection instruments/tests/procedures? Yes
  7.5. Was the measurement of effect at an appropriate level of precision? Yes
  7.6. Were other factors accounted for (measured) that could affect outcomes? ???
  7.7. Were the measurements conducted consistently across groups? N/A
8. Was the statistical analysis appropriate for the study design and type of outcome indicators? Yes
  8.1. Were statistical analyses adequately described and the results reported appropriately? Yes
  8.2. Were correct statistical tests used and assumptions of test not violated? Yes
  8.3. Were statistics reported with levels of significance and/or confidence intervals? Yes
  8.4. Was "intent to treat" analysis of outcomes done (and as appropriate, was there an analysis of outcomes for those maximally exposed or a dose-response analysis)? N/A
  8.5. Were adequate adjustments made for effects of confounding factors that might have affected the outcomes (e.g., multivariate analyses)? N/A
  8.6. Was clinical significance as well as statistical significance reported? Yes
  8.7. If negative findings, was a power calculation reported to address type 2 error? N/A
9. Are conclusions supported by results with biases and limitations taken into consideration? No
  9.1. Is there a discussion of findings? Yes
  9.2. Are biases and study limitations identified and discussed? No
10. Is bias due to study's funding or sponsorship unlikely? Yes
  10.1. Were sources of funding and investigators' affiliations described? Yes
  10.2. Was the study free from apparent conflict of interest? Yes