Monitoring enteral nutrition efficacy in ICU patients: is there a place for the Brix value-derived fraction of enteral formula in gastric residual volume?

Introduction

Achieving nutritional targets by means of enteral nutrition in critically ill (ICU) patients has been shown to be difficult and unpredictable. In this regard, the amount of enteral formula lost in gastric residual volume (GRV) remains unknown. In previous studies performed in a non-ICU population, bedside monitoring of the specific gravity (or relative density) of GRV with a refractometer (expressed as Brix value) has been shown to be an accurate tool to measure the concentration of enteral nutrition (% EF) in GRV [1, 2]. The aim of this study is to evaluate the use of the Brix value in monitoring the concentration of enteral feeding in GRV of patients in the ICU.

Methods

The specific gravity, pH and clinical aspect of GRV in fasting ICU patients were examined. The Brix value of gastric fluid from fasting critically ill patients was measured with a refractometer. One observer measured the Brix value of each sample in duplo. A third measurement was performed by an independent observer. The enteral nutrition was diluted in GRV in vitro to establish the relation between measured Brix value and % EF by linear regression analysis. Factors influencing the measured Brix value were analyzed in a linear regression analysis.

Results

The mean ± SD Brix value for GRV of 51 fasting ICU patients was 2.9 ± 1.1 (ranging from 0.0 to 6.0). The inter-observer and intra-observer variabilities were 8.4 and 3.2%. The relation between Brix value and % EF was significantly affected by gastric fluid pH before mixing with nutrition. Analyzing all measurements with simple linear regression revealed the following linear relation (P < 0.0001): % EF = (Brix value - 2.368/0.185).

Conclusions

The hand-held refractometer is a good tool for monitoring the overall nutritional efficacy in an enteral nutrition strategy measuring and discarding GRV. The linear relation between % EF in GRV and measured Brix value described in earlier non-ICU studies was confirmed in ICU patients. However, the large baseline Brix variation in ICU patients and unpredictable impact of the pH of GRV makes the Brix value less suitable for the management of nutrition in individual patients.