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Indirect Calorimetry: A Case-Based Approach for Patients with Sepsis, ARDS and Covid-19

For critically ill patients, customized nutrition is ideal, but precisely determining energy intake is challenging. Utilizing indirect calorimetry takes the guesswork out of feeding to improve outcomes and reduce risks.

Acute illnesses, such as sepsis, acute respiratory distress syndrome (ARDS) and Covid-19, increase stress on a patient’s metabolism, making it challenging to determine their precise caloric needs. Additionally, critical illnesses can alter energy expenditure (EE), further impacting a patient’s energy needs.
Unfortunately, reliance on the predictive equations that are often used to estimate energy expenditure can lead to either over- or underfeeding, both of which carry numerous risks to patient safety and recovery.

These issues are why hospitals are increasingly turning to a valuable nutrition assessment tool known as indirect calorimetry or IC to take the guesswork out of feeding patients with acute illnesses and more precisely manage their nutritional needs.

In this article, we will discuss the challenges that come with estimating energy expenditure in patients with sepsis, ARDS and Covid-19, as well as the dangers of over- and underfeeding. We will also take a look at why predictive equations fail to provide the precise measurements needed to design an appropriate feeding regimen and how IC can solve these issues.

Clinician holding an IV bag full of fruit

Predictive Equations Fail to Predict EE in Acutely Ill Patients

Patients suffering from infections, such as sepsis, become hypermetabolic and their metabolism changes frequently, leading to metabolic stress. For elderly patients, low energy expenditure has also been observed in cases of acute sepsis.

Further challenges in accurately predicting nutritional requirements arise in mechanically ventilated patients. This can be due to inflammatory response, the disease process itself, as well as multiple other variables. These patients also experience metabolic variations1, including higher EE, as well as protein catabolism, leading to a loss of muscle and lean body mass.

Carefully monitoring and delivering the proper nutrition support to these patients is critical to achieve optimal outcomes.

Yet, while predictive equations, like the Harris-Benedict Equation, have been traditionally used to predict nutrition needs, they have numerous drawbacks that make their use less than optimal. Since there are over 200 equations hospitals can use, there is little, if any, consensus to rely on in choosing the best one. Because of this, patient feeding practices can vary widely. This makes it easy to see why using these equations to determine nutrition needs is so often inaccurate, with patient’s estimated resting energy expenditure (REE) differing significantly from their true REE2.

This also sheds light on the reason why 43% of patients in the ICU are malnourished3.

This malnourishment can increase infection complications4, force patients to remain on a ventilator longer, lengthen stay, increase morbidity and mortality and raise costs.

Of course, malnourishment isn’t the only risk patients with acute illnesses, such as sepsis, ARDS and Covid-19 face since overfeeding comes with consequences of its own5, such as to lipogenesis, hyperglycemia, and exacerbated respiratory failure.

All of this highlights the need for a more reliable method to predict EE in order to provide accurate nutritional support.

The Gold Standard in the Measurement of Nutritional Needs

Unlike those predictive equations, IC is not simply an estimate. It’s an objective measurement that allows nutrition to be tailored to fit each patient’s individual needs.

According to the Society of Critical Care Medicine (SCCM) and the American Society for Parenteral and Enteral Nutrition (ASPEN)’s 2016 Guidelines for the Provision and Assessment of Nutrition Support Therapy in the Critically Ill Patient6, “Indirect calorimetry should be used to determine energy requirements when available.”

IC monitors both the consumption of oxygen and the production of carbon dioxide. By measuring this respiratory gas exchange, indirect calorimetry accurately assesses energy metabolism and so determines nutritional needs.

The goal of measuring the expiration of carbon dioxide and the inspiration of oxygen is to calculate the resting energy expenditure (REE) and the respiratory quotient (RQ).

The REE is used to determine how much to feed a patient, while the RQ offers validation of the results.

Energy Target Recommendations Using Indirect Calorimetry

To design an appropriate feeding program, the variations in patient’s response to metabolic stress over time must be considered.

Critical illnesses, such as sepsis, ARDS and Covid-19 are characterized by three phases7:

  • The stress phase which includes hemodynamic instability, hypometabolism, surging counter-regulatory hormones, and insulin resistance
  • The catabolic phase characterized by fever, hypercatabolism and increased oxygen demands
  • The anabolic phase which occurs upon resolution of the stress and catabolic phases and can last for months.

Because of the variations in the nutrition needs in each of these phases, feeding should be implemented using the following recommendations as recommended by research published in the journal Critical Care8:

  • Hypocaloric nutrition (not exceeding 70%) should be administered during the early acute phase of illness.
  • After day 3, caloric delivery can be increased up to 80 – 100% of measured EE.
  • Post ICU nutrition should target 125% of indirect calorimetry.
  • Post hospital discharge, increase to 150% of indirect calorimetry.

Since the introduction of sedatives and muscle relaxants, as well as small changes in body temperature can affect energy expenditure, frequent monitoring using IC will provide the most accurate picture of evolving nutrition requirements.

Improving Feeding to Reduce Mortality

A recent meta-analysis of ICU patients guided by indirect calorimetry found that this precision nutrition, based on each patient’s individual needs rather than less accurate predictive formulas, resulted in a 23% lower in-hospital mortality9.

Using IC allows physicians and dieticians to precisely design macro- and micronutrient delivery to help favorably modulate immune responses and achieve meticulous glycemic control. Together, these may serve to reduce disease severity, diminish complications, decrease length of stay in the ICU and favorably impact patient outcomes.


  • Indirect calorimetry is the gold standard for determining energy expenditure in patients with sepsis, ARDS and Covid-19 and is preferred over predictive equations.
  • The measured REE is an objective, patient-specific caloric reference that offers the most accurate method of determining energy metabolism.
  • Energy expenditure varies widely between patients and changes throughout the patient journey.
  • Continuous monitoring through indirect calorimetry versus intermittent measurements helps ensure patient nutritional needs are met in order to reduce mortality and improve outcomes.


[1] Michael T. Vest, Emma Newell, Mary Shapero, Patricia McGraw, Claudine Jurkovitz, Shannon L. Lennon, Jillian Trabulsi, Energy balance in obese, mechanically ventilated intensive care unit patients, Nutrition, Volume 66, 2019, Pages 48-53, ISSN 0899-9007, (

[2] Xian X, Quach A, Bridgeman D, Tsow F, Forzani E, et al. (2015) Personalized Indirect Calorimeter for Energy Expenditure (EE) Measurement. Glob J Obes Diabetes Metab Syndr 2(1): 004-008. DOI: 10.17352/2455-8583.000007

[3] Reid CL. Nutritional requirements of surgical and critically-ill patients: do we really know what they need? Proc Nutr Soc. 2004 Aug;63(3):467-72. doi: 10.1079/pns2004312. PMID: 15373959.

[4] Powers J, Samaan K. Malnutrition in the ICU patient population. Crit Care Nurs Clin North Am. 2014 Jun;26(2):227-42. doi: 10.1016/j.ccell.2014.01.003. PMID: 24878208.

[5] Catherine J Klein, Gena S Stanek, Charles Wiles, Overfeeding Macronutrients to Critically Ill Adults: Metabolic Complications, Journal of the American Dietetic Association, Volume 98, Issue 7, 1998, Pages 795-806, ISSN 0002-8223, (

[6] Taylor, Beth E. RD, DCN1; McClave, Stephen A. MD2; Martindale, Robert G. MD, PhD3; Warren, Malissa M. RD4; Johnson, Debbie R. RN, MS5; Braunschweig, Carol RD, PhD6; McCarthy, Mary S. RN, PhD7; Davanos, Evangelia PharmD8; Rice, Todd W. MD, MSc9; Cresci, Gail A. RD, PhD10; Gervasio, Jane M. PharmD11; Sacks, Gordon S. PharmD12; Roberts, Pamela R. MD13; Compher, Charlene RD, PhD14 and the Society of Critical Care Medicine and the American Society of Parenteral and Enteral Nutrition Guidelines for the Provision and Assessment of Nutrition Support Therapy in the Adult Critically Ill Patient, Critical Care Medicine: February 2016 - Volume 44 - Issue 2 - p 390-438 doi: 10.1097/CCM.0000000000001525

[7] Secombe P, Harley S, Chapman M, Aromataris E. Feeding the critically ill obese patient: a systematic review protocol. JBI Database System Rev Implement Rep. 2015 Oct;13(10):95-109. doi: 10.11124/jbisrir-2015-2458. PMID: 26571286.

[8] van Zanten ARH, De Waele E, Wischmeyer PE. Nutrition therapy and critical illness: practical guidance for the ICU, post-ICU, and long-term convalescence phases. Crit Care. 2019 Nov 21;23(1):368. doi: 10.1186/s13054-019-2657-5. PMID: 31752979; PMCID: PMC6873712.

[9] Duan, JY., Zheng, WH., Zhou, H. et al. Energy delivery guided by indirect calorimetry in critically ill patients: a systematic review and meta-analysis. Crit Care 25, 88 (2021).

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