The following haemodynamic  and metabolic data were obtained from a patient admitted to the ICU with sepsis.

Pulmonary artery catheter data:

CI                                         4.2L/min/m2 
DO2                                     900 ml/min 
VO2                                     190 ml/min

Indirect calorimetry data:

VO2                                     220 ml/min 
VCO2                                  290 ml/min

a)  Why   is   the   VO2     different   between   the   two   methods?   (Assume   no measurement errors).

b)  What  changes  in  patient  management  will  you  consider  based  on  the indirect calorimetry data?

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College Answer

a)  Why   is   the   VO2     different   between   the   two   methods?   (Assume   no measurement errors).

Indirect calorimetry also measures lung oxygen consumption.

b)  What  changes  in  patient  management  will  you  consider  based  on  the indirect calorimetry data?

A high RQ suggests excess carbohydrate load. Reduce caloric intake and consider changing to a higher fat intake.

Discussion

Indirect calorimetry and the reverse Fick method are discussed in detail elsewhere.

The VO2, or the oxygen consumption, is different between the caloric cart and the PA catheter because the PA catheter measurement (of systemic oxygen extraction) neglects the lungs as a oxygen user. But the lung does use oxygen- in this particular patient, 30ml/min are used.

The RQ is calculated using the VO2 and the VCO2 (RQ = VCO2/VO2).

In this case, it comes to 1.31 - which is well in excess of what one would expect (0.8). The college suggests that a high carbohydrate foodsource may be responsible, and that the patient should go on a fat and protein enriched diet.

References

LITFL has an excellent summary dedicated to indirect calorimetry. I stole a couple of their references.

Holdy, Kalman E. "Monitoring energy metabolism with indirect calorimetry: instruments, interpretation, and clinical application." Nutrition in Clinical Practice 19.5 (2004): 447-454.

Flancbaum, Louis, et al. "Comparison of indirect calorimetry, the Fick method, and prediction equations in estimating the energy requirements of critically ill patients." The American journal of clinical nutrition 69.3 (1999): 461-466.

Weir, JB de V. "New methods for calculating metabolic rate with special reference to protein metabolism." The Journal of physiology 109.1-2 (1949): 1.

McClave, Stephen A., Robert G. Martindale, and Laszlo Kiraly. "The use of indirect calorimetry in the intensive care unit." Current Opinion in Clinical Nutrition & Metabolic Care 16.2 (2013): 202-208.

Lev, Shaul, Jonathan Cohen, and Pierre Singer. "Indirect calorimetry measurements in the ventilated critically ill patient: facts and controversies—the heat is on." Critical care clinics 26.4 (2010): e1-e9.

Fraipont, Vincent, and Jean-Charles Preiser. "Energy Estimation and Measurement in Critically Ill Patients." Journal of Parenteral and Enteral Nutrition 37.6 (2013): 705-713.