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


Zijlstra N, ten Dam SM, Hulshof PJ, Ram C, Hiemstra G, de Roos NM. 24-hour indirect calorimetry in mechanically ventilated critically ill patients. Nutr Clin Pract. 2007; 22(2): 250-255.

PubMed ID: 17374800
Study Design:
Diagnostic, Validity or Reliability Study
C - Click here for explanation of classification scheme.
Quality Rating:
Neutral NEUTRAL: See Quality Criteria Checklist below.
Research Purpose:

To investigate whether brief measurements of indirect calorimetry at any time of the day would give valid estimates of 24-hour energy expenditure (EE).

Inclusion Criteria:
  • Admission to the intensive care unit (ICU) Internal Medicine and Toxicology of the University Medical Centre Utrecht in the Netherlands
  • Received mechanical ventilation (more than 24 hours) for pulmonary or neurologic disease on a volume-cycled ventilator
  • Obtaining an informed consent not obligatory for this study (data were collected as part of routine care).
Exclusion Criteria:
  • Patients connected to another volume-cycled ventilator (ventilator could not be properly connected with the calorimeter)
  • Disturbed electrolyte balance due to diarrhea or dialysis
  • Body mass index less than 18kg/m2 or more than 40kg/m2
  • Pregnancy
  • Normally receiving mechanical ventilation at home.
Description of Study Protocol:


  • Patients were recruited from November 2004 until mid-January 2005 from the University Medical Centre Utrecht in the Netherlands
  • Patients were included after admission to the intensive care unit (ICU).


Diagnostic, validity or reliability study.

Blinding Used

Implied with measurements.


All patients were continuously fed enteral nutrition and energy expenditure was measured by indirect calorimetry every minute for 24 hours

Statistical Analysis

  • Results are expressed as mean ±SD. Statistical analysis of the data was carried out using the statistical software program GraphPad Prism 4.02 for Windows.
  • A one-way repeated measures ANOVA was used to test whether there was a difference in energy expenditure during the morning (6:00 a.m. to noon), afternoon (noon to 6:00 p.m.) evening (6:00 to midnight) and night (midnight to 6:00 a.m.)
  • The maximal error was calculated for each patient and the average of these maximal errors was plotted against measurement time. Non-linear regression was used to curve-fit the data to a power equation (y=axb).


Data Collection Summary:

Timing of Measurements

  • Weight and height were measured by any of the nurses on the day of admission
  • Measurements made continuously for 24 hours.

Dependent Variables

  • Energy expenditure: Measured by indirect calorimetry for 24 consecutive hours preferably on day two or three after admission. All measurements were performed by two trained persons under supervision of a nurse

  • Respiratory gas exchange was measured using a Deltatrac II metabolic monitor. This is an open-system indirect calorimeter for measurement of both oxygen consumption (VO2, ml per minute) and carbon dioxide production (VCO2, ml per minute). The monitor was set to record O2 consumption (ml per minute) and CO2 production (ml per minute) every minute

  • Urinary nitrogen (Un) in 24-hour urine was collected during the day of the calorimetric measurement on a Vitros 250 Chemistry system
  • Weight was measured with a weighing bed and height with a measuring tape while the subject was in supine position
  • Heart rate, respiratory rate and minute ventilation
  • Rectal body temperature, or when this was not possible, axillary or groin temperature.

Independent Variables

  • All patients were continuously fed enteral nutrition
  • The patients' severity of illness was evaluated by the Acute Physiology and Chronic Health Evaluation score on the day of admission (APACHE II).
Description of Actual Data Sample:
  • Initial N: 12 patients (six male, six female)
  • Attrition (final N): 12 patients (six male, six female)
  • Age: Mean age 67±18 years
  • Anthropometrics: Mean weight 155±19lbs, mean height  67±2.8 inches, mean BMI 24.2±3.1kg/m2
  •  Location: The Netherlands.
Summary of Results:

Key Findings

  • Mean ±SD EE was 1,658±279kcal per day (6,941±1,167kJ per day)
  • Within patients, EE during the day fluctuated by 234kcal in the most constant patient to 1,190kcal in the least constant patient, with a mean fluctuation of 521kcal (12 patients)
  • No statistically significant difference (P=0.53) in mean EE between morning (6:00 a.m. to noon, 1,676kcal), afternoon (noon to 6:00 p.m., 1,642kcal), evening (6:00 p.m. to midnight, 1,658kcal) and night (midnight to 6:00 a.m., 1,655kcal) was found
  • A two-hour instead of a 24-hour measurement resulted in a maximal error of 128kcal (536kJ), which was less than 10% of the average EE. The maximal error decreased with longer interval times.




Author Conclusion:

Single measurements of indirect calorimetry can replace 24-hour measurements if performed for at least two hours. The time of day (morning, afternoon, evening or night) does not seem to be critical. Measurements should be repeated when the clinical status of the patient changes.

Funding Source:
University/Hospital: University Medical Centre Utrecht, Netherlands
Reviewer Comments:

The authors noted that the patient population was too small to test for relations between variability in body temperature and fluctuations in energy expenditure, the same applies to variability in other factors, heart rate or handling of the patients.

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? Yes
  2.2. Were criteria applied equally to all study groups? Yes
  2.3. Were health, demographics, and other characteristics of subjects described? Yes
  2.4. Were the subjects/patients a representative sample of the relevant population? No
3. Were study groups comparable? Yes
  3.1. Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT) Yes
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? Yes
  3.3. Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.) Yes
  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")? Yes
4. Was method of handling withdrawals described? Yes
  4.1. Were follow-up methods described and the same for all groups? Yes
  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%.) Yes
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? Yes
  4.4. Were reasons for withdrawals similar across groups? Yes
  4.5. If diagnostic test, was decision to perform reference test not dependent on results of test under study? Yes
5. Was blinding used to prevent introduction of bias? Yes
  5.1. In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate? No
  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.) Yes
  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? Yes
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? Yes
  6.2. In observational study, were interventions, study settings, and clinicians/provider described? N/A
  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? Yes
  6.6. Were extra or unplanned treatments described? Yes
  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? Yes
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? Yes
  7.7. Were the measurements conducted consistently across groups? Yes
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)? No
  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? No
9. Are conclusions supported by results with biases and limitations taken into consideration? Yes
  9.1. Is there a discussion of findings? Yes
  9.2. Are biases and study limitations identified and discussed? Yes
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