Nutritional Adequacy in Mechanically Ventilated Patients
Background: Inadequate nutritional intake in critically ill patients can lead to complications resulting in increased mortality and healthcare costs. Several factors limit adequate nutritional intake in intensive care unit patients given enteral feedings.
Objective: To examine the adequacy of enteral nutritional intake and the factors that affect its delivery in patients receiving mechanical ventilation.
Methods: A prospective, descriptive design was used to study 60 patients receiving enteral feedings at target or goal rate. Energy requirements were determined for the entire sample by using the Harris-Benedict equation; energy requirements for a subset of 25 patients were also determined by using indirect calorimetry. Energy received via enteral feeding and reason and duration of interruptions in feedings were recorded for 3 consecutive days.
Results: Mean estimated energy requirements (8996 kJ, SD 1326 kJ) and mean energy intake received (5899 kJ, SD 3058 kJ) differed significantly (95% CI 3297-3787; P < .001). A total of 41 patients (68.3%) received less than 90% of their required energy intake, 18 (30.0%) received within ± 10%, and 1 (1.7%) received more than 110%. Episodes of diarrhea, emesis, large residual volumes, feeding tube replacements, and interruptions for procedures accounted for 70% of the variance in energy received ( P < .001). Procedural interruptions alone accounted for 45% of the total variance. Estimated energy requirements determined via indirect calorimetry and mean energy received did not differ.
Conclusions: Most critically ill patients receiving mechanical ventilation who are fed enterally do not receive their energy requirements, primarily because of frequent interruptions in enteral feedings.

Adequate nutritional support is crucial in the prevention and treatment of malnutrition in critically ill patients. Patients in the intensive care unit (ICU) who cannot take food orally require either enteral or parenteral nutritional support. Enteral nutrition is generally preferred over parenteral nutrition because the former is associated with a lower incidence of infectious and noninfectious complications, reduced cost, and a decreased length of hospital stay. Despite these benefits, meeting the full nutritional requirements of critically ill patients by giving enteral feedings can be difficult. Patients who require prolonged mechanical ventilation are at particular risk for underfeeding and overfeeding. Several factors potentially limit enteral intake in critically ill patients, including gastrointestinal intolerance of enteral tube feedings, displacement or obstruction of the feeding tube, and interruption of tube feedings for tests and procedures. Gastrointestinal intolerance of tube feedings (eg, large gastric residual volumes, nausea and vomiting, ileus, abdominal distention, and diarrhea) is a major factor limiting adequate enteral intake in patients. Displacement or obstruction of feeding tubes also markedly limits the adequacy of enteral intake. In order to reduce the risk of aspiration of enteral formula, feeding is routinely withheld in patients with unstable hemodynamic conditions and in preparation for surgical or diagnostic procedures, weaning, and various nursing care activities.

Accurately assessing nutritional requirements and monitoring the adequacy of nutritional intake in critically ill patients can help ensure that complications associated with underfeeding or overfeeding are avoided. Mathematical estimates of energy requirements are the most common method of determining energy needs in these patients. Traditionally, the Harris-Benedict equation (HBE) has been the accepted standard for determining the energy requirements of critically ill patients. In 1919, Harris and Benedict reported the results of a study done in healthy adult volunteers to determine daily energy expenditure. Regression equations were calculated for men and women on the basis of body weight, age, and height. The HBE was derived from this regression analysis to provide estimates of the basal energy expenditure for healthy persons at rest. In order to account for the increased energy expenditure during acute illness and injury, disease-specific equations and stress factors that account for activity and injury have been developed to improve the accuracy of the HBE in hospitalized patients. Despite these modifications for critically ill patients, the accuracy of the HBE is limited in ventilator-dependent patients, patients who are either morbidly obese or severely malnourished and underweight, transplant patients, and patients with marked fluid overload, ascites, extensive limb amputations, or paraplegia.

Using indirect calorimetry to measure energy expenditure is an alternative method of estimating nutritional requirements that is more accurate than the HBE in critically ill patients. In this method, energy expenditure is calculated indirectly by measuring pulmonary gas exchange. Strict adherence to measurement criteria is required because of the numerous factors that affect energy expenditure. Technical limitations and the associated costs of indirect calorimetry limit its usefulness in measuring energy expenditure in critically ill patients. Unstable hemodynamic conditions, agitation, cuff leaks, and increased oxygen requirements may preclude use of indirect calorimetry. Because of the limitations of using indirect calorimetry, the HBE remains the most commonly used method for assessing energy requirements in critically ill patients.

Recent studies indicated that repeated interruptions of enteral tube feeding result in significant underfeeding in critically ill patients. Adam and Batson found that ICU patients received only 76% of the patients' daily energy requirements with enteral tube feeding, primarily because of gastrointestinal intolerance and elective withholding of feedings for procedures. In a similar study, McClave et al8 found that ICU patients received only 52% of the patients' energy requirements with enteral tube feeding and that 66% of the interruptions in tube feedings were avoidable. In both studies, the authors assumed that the estimated energy requirement of ICU patients was 105 to 146 kJ/kg per day (25–35 kcal/kg per day), a significant underestimation of energy requirements in critically ill patients. Neither group of investigators used either the HBE or indirect calorimetry to estimate energy needs.

Although both groups reported the type and frequency of interruptions of tube feedings, only McClave et al documented the duration of interruptions attributable to each reason for cessation of tube feedings. In the study by McClave et al, for each patient, the study period began as soon as the feeding tube was inserted. Therefore, the patients were adjusting to enteral feedings. To date, no investigators waited until patients had achieved goal rates for enteral feedings to begin measurements.

In the study reported here, we assessed the adequacy of enteral intake in ICU patients receiving mechanical ventilation once the prescribed tube feeding rate had been achieved. We also determined the factors affecting the delivery of enteral feedings in these patients. The primary aims of the study were to compare the number of kilojoules patients received via enteral feedings with the patients' estimated energy requirements as indicated by the HBE and by indirect calorimetry and to determine the factors that affect the delivery of enteral feedings in critically ill patients.