Authors

1. Davies, Hugh
2. Leslie, Gavin
3. Morgan, David

Abstract

Review objective: The objective of this review is to determine the effectiveness of daily fluid balance charting in comparison to the measurement of body weight when used in guiding fluid therapy for critically ill adult patients.

More specifically, the objectives are to identify:

* The accuracy and utility of estimating body fluid status by charting daily fluid balance totals

* The accuracy and utility of estimating body fluid status by daily body weight measurements

* The utility of both approaches for estimation of changes in body fluid status

* Any special considerations required for patients who are oliguric and at risk of fluid overload

Background: Fluid therapy plays an important role in the management of the critically ill adult patient.1 The requirement to resuscitate the patient and restore organ perfusion can result in large volumes of fluid being given. In several studies the effect of fluid overload has been reported to delay recovery and decrease patient survival.2-4 When delivering fluid therapy the monitoring of a patient's input and output is routinely charted to estimate the patient's fluid balance status.

Fluid overload occurs when the fluid used to expand the intravascular fluid compartment causes depression of ventricular function due to over filling, and impaired capillary permeability allows fluid to leave the circulation and re-distribute into the extravascular space.5 This situation is exacerbated when the response to subsequent episodes of hypotension results in impaired kidney function reducing the excretion of sodium and water while more fluid is being administered. In some situations the patient may require invasive hemodynamic monitoring to estimate the adequacy of circulatory volume.6 However, the patient's body fluid status will be largely estimated by non-invasive methods of physical examination, daily or more frequent fluid balances and measurement of body weight changes.7 It is important to know the body fluid status of patients so that the following appropriate initial resuscitation measures (additional fluid administration, fluid restriction and fluid removal) can be implemented to maintain adequate circulatory volume, prevent fluid overload and achieve homeostasis.

The charting of fluid input and output each hour and the calculation of daily fluid balance totals is a routine nursing activity. It is one of many non-invasive methods commonly used in the intensive care unit (ICU) to assist with estimating body fluid status. Although the method of subtracting recorded fluid output from the patient's input is mathematically a simple procedure, management of the critically ill patient is complex, making the process of charting daily fluid balances difficult and time consuming. This includes the recording of multiple infusions of fluid such as liquid nutrition, blood products and intravenous medications. Recording hourly urinary output and loss of other body fluids, such as fluid lost from surgical drains, also requires this diligence. The charting of fluid balance totals is open to error in the documentation of inputs and outputs or when fluid balance totals are calculated incorrectly by the nurse, particularly when large numbers of calculations are involved.8,9 A pocket calculator can reduce inaccuracies in addition and subtraction which may occur when paper based charting is used to record fluid balance totals.10 The option of using a computer based spreadsheet within a clinical information system removes calculation errors, but it does not eliminate the problem of missed or double entry errors being inadvertently entered on the bedside monitor.11

The prospect of fluid balance inaccuracies is further confounded if corrections are not made for water which is lost from the body by insensible fluid losses, for example breathing and fever.12 Obvious losses of fluid such as urine and surgical drains can be measured but insensible fluid loss is dependent upon a number of variables which cannot be predicted accurately, such as fluid loss in expired air. These problems associated with recording inputs and outputs in the ICU patient combine to make the charting of fluid balances less and less accurate, thus increasing the magnitude of clinical significance when errors are repeated over several days.

In view of the potential for errors in the calculation of fluid balance totals and the problem of accounting for insensible fluid losses, measurement of body weight changes is an alternative non-invasive method commonly used for estimating body fluid status.13 Its accuracy over fluid balance totals assumes a one kilogram change in body weight equates to a one liter gain or loss in body fluid volume.14 The measurement of body weight allows the detection of fluid overload when the gains and losses which occur daily are evaluated according to the weight of fluid present in the body. Over a short period of time (less than a week) changes in body weight are almost entirely the result of alterations in body fluid volume. In critically ill patients who stay longer than a week this method becomes less reliable as body weight becomes more likely to be influenced by muscle and fat loss as well as bone demineralization.15 Since the average length of stay in ICU has been reported to be 3.69 days,16 daily weighing can be a good indicator of fluid overload for the majority of patients, and particularly as a trend for all patients, irrespective of length of stay.

Weighing the incapacitated adult ICU patient makes the measurement of body weight changes challenging in a practical sense, as compared to the recording of accurate fluid balance totals. Obtaining a reliable body weight measurement requires the development of practice guidelines and compliance by nursing staff to follow procedures to ensure accuracy is achieved.13 The bed must be tarred (zeroed) before the patient is placed on top and before body weight measurements are recorded. Several members of staff are required when slings and hoists are used to lift and weigh patients, a procedure that may not always be possible to organize or tolerable for the critically ill patient. The introduction of beds with built in scales can reduce the extra workload involved in weighing patients but can be unreliable if the nurse forgets to remove items not attached when the bed is first tarred. It was shown in one study that a series of consecutive successful body weight measurements using weight enabled beds was not achieved in a third of the patients surveyed.17

Notwithstanding the challenges of charting, daily fluid balances and recording body weight changes, both methods are used in ICU to guide fluid therapy. Underlying the practice of each method is the assumption that over several days fluid overload has been identified in the critically ill patient. 18 In view of the potential for errors and the problem of accounting for insensible fluid losses, the weight based method of monitoring body fluid status is seen by some as the more accurate method of identifying fluid overload.19,20 This is despite several studies showing a poor correlation between measured body weight changes which is then reflected on the fluid balance chart.14,17 An initial search of literature has found no systematic review of findings from studies that have evaluated the effectiveness of daily fluid balance charting in comparison to the measurement of body weight when used in guiding fluid therapy in the critically ill adult patient.