We studied a bioimpedance device that provides a quantitative determination of ECW/TBW to prospectively determine whether this parameter is associated with adverse outcomes in a heterogeneous medical and surgical ICU population. BIA devices measure the volume within body compartments and may help clinicians objectively determine the extracellular (intravascular and interstitial) volume of a patient.
We observed that the mean ECW/TBW ratio recorded within the first 24 hr of admission to the ICU was elevated in > 80% of patients despite the mean cumulative fluid balance being only 0.6 L positive at this time. This likely reflects the volume the patient received in the operating room or emergency department prior to their arrival in the ICU. At our centre, the fluid administered prior to ICU admission is not consistently accounted for in the cumulative fluid balance calculations. This highlights a potential advantage of BIA measurements, specifically to provide an objective point-of-care measurement of volume status, which may reduce dependence on recordings of fluid balance (or lack thereof) during transitions of care.
The ECW/TBW ratio measurements remained elevated throughout the observation period despite an early rise and eventual fall in body weight and cumulative fluid balance. Although there were only a small number of patients measured at day 7, this observation may reflect the underlying pathophysiology of critical illness. It has been shown that ICU patients rapidly lose lean muscle mass, and thus their ICW becomes proportionally smaller and their ECW becomes larger. In this circumstance, the ECW accounts for a larger percentage of the TBW and this translates into a higher ECW/TBW ratio.11 Thus in practice, as the patient is “de-resuscitated” to a normal volume status clinically, a persistent increase in ECW/TBW ratio may reflect loss of muscle mass. The other possibility is sub-clinical hypervolemia, which can be measured by bioimpedance but cannot be detected by clinical assessment.
Our data show that a higher ECW/TBW ratio on day 1 of admission to the ICU is associated with increased number of ventilator days independent of severity of illness as measured by the APACHE II score. This finding of “early” volume overload prolonging time on mechanical ventilation has been recently reported in critically ill children.12 It is well documented that excess volume leads to interstitial tissue edema and organ dysfunction in critically ill patients.13,14 In the kidney, interstitial tissue edema leads to impaired venous outflow (and eventually arterial inflow) causing renal ischemia.13 We hypothesized that we would observe an increased incidence of AKI requiring RRT in those who were volume overloaded,15,16 but event rates at the end of the study were too low to determine this.
While others have shown increased mortality in the setting of volume overload8,17 the power of our small study was perhaps too low to detect any association between these variables. Samoni et al. studied 125 patients from a medical/surgical ICU and found a significant association between elevated bioimpedance vector analysis (BIVA) measurements and mortality.18 Basso et al. studied 64 medical/surgical patients using BIVA and found, similar to our study, that most patients were hypervolemic on admission, and that this persisted during their ICU stay.19 These investigators observed a significant correlation between maximum bioimpedance-measured hydration status and mortality. Yang et al. studied 140 medical patients and reported that bioimpedance-measured overhydration on the third admission day to ICU was an independent predictor of hospital death.20
A significant finding in our study pertains to the feasibility of performing BIA measurements in the critically ill population, thus far not discussed in any other published study using a bioimpedance device. We encountered many logistical barriers that interfered with the proper use of the BIA device at the bedside. The presence of medical equipment on patients’ hands and lower extremities precluded BIA measurement in a significant number of patients and contributed to lower recruitment rates. In addition, all BIA devices require an accurate body weight, which changes daily in this patient population and is influenced by the presence of medical equipment on the patient’s bed (tubing, drains, pillows, casts, etc.). If an accurate weight is not obtained, the accuracy of the BIA measurement is diminished.
Bioelectrical impedance analysis devices have other limitations, namely the inability to distinguish between intravascular and interstitial volume in the extracellular compartment. It is well known that in sepsis, the distribution of fluid in body compartments is altered, with capillary leak promoting fluid movement from the vascular into the interstitial space.21 The inability to determine a patient’s intravascular volume is a source of clinical uncertainty and our study does not address questions regarding the assessment of cardiac output, volume responsiveness, or organ perfusion in this population.
This study has many strengths, specifically its prospective nature, multiple BIA measurements over time, and the use of a novel technology to measure volume status in a critically ill population. Limitations include the small study size, which may have resulted in low statistical power to detect an association of the ECW/TBW ratio at day 1 with mortality, the heterogeneous mix of ICU patients, the challenges related to the logistics of performing measurements in all patients, and the lack of direct clinical applicability of the results.