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Noninvasive Methods of Fluid Status Assessment in Critically Ill Patients
Objectives
This chapter will:
- 1.
Discuss the importance of fluid status assessment in the setting of the intensive care unit and the correlation between fluid overload and adverse outcomes in critically ill patients.
- 2.
Describe noninvasive methods to assess fluid status (i.e., physical examination, fluid balance recording, chest radiography, ultrasonography techniques) and their role in the management of intensive care patients.
- 3.
Focus on bioimpedance methods to estimate fluid status and present the results from the most recent studies.
Accurate fluid management in patients admitted to the intensive care unit (ICU) is still one of the most challenging and important tasks for critical care physicians. Although the advantages of early fluid resuscitation have been recognized, several studies have demonstrated that fluid administration beyond the correction of hypovolemia is associated with adverse outcomes in critically ill patients. Despite progress in the standard intensive care, the assessment of fluid status is still complex and requires an in-depth knowledge of body fluid homeostasis. Nowadays, the treatment of critically ill patients in the setting of the ICU requires a multidisciplinary approach to achieve better outcomes, mainly because many of these patients present with problems such as acute kidney injury and fluid and electrolyte disorders.
Fluid overload (FO) is a very common situation in patients among ICUs all around the world; some studies have reported prevalence between 62% and 64.8%, and it is widely accepted that FO is associated with an increase in mortality in critically ill patients.
In the acute setting of shock, as well as in the acute congestive decompensated states, physical examination may respectively provide signs of hypovolemia or venous congestion that should lead to prompt treatment. Nevertheless, physical examination should be aided by other tools to improve assessment of fluid status and guide therapeutic decisions. Although the gold standard method is isotope dilution, unfortunately it is difficult to perform in critically ill patients because of fluid sequestration and abnormal penetration of tracers into cells. In this chapter, we discuss noninvasive methods to assess fluid status and its role in the management of ICU patients. Starting from physical examination, fluid balance recording, and chest radiography (CRX), we list ultrasonography techniques that are explained in depth in another book section and then focus on bioimpedance methods.
Physical Examination and Fluid Balance Recording
Physical Examination
The first tool that every physician has to evaluate a patient's volume status is the history and physical examination. Several symptoms and signs can be found depending if the patient is hypovolemic or hypervolemic ( Table 135.1 ). Many of the studies performed in patients presenting with FO were done in heart failure patients; the most prevalent symptom in these patients was dyspnea, present in 87% to 93% of evaluated individuals. In the context of the critically ill, in whom symptoms are difficult to evaluate, edema and weight gain were the most recognized signs.
TABLE 135.1
Summary of Symptoms and Signs Detectable in Hypovolemic and Hypervolemic Patients
| HYPOVOLEMIA | HYPERVOLEMIA |
|---|---|
Symptoms
|
Symptoms
|
Signs
|
Signs
|
Nevertheless, although it is an easy and rapid way of assessment, clinical examination has several limitations. For example, significant volume overload can occur without edema, edema and intravascular volume depletion can coexist, and the presence of edema can have a wide range of addition contributing causes.
There is a significant difference when comparing clinical evaluation with invasive volume assessment techniques. Duane evaluated the accuracy of diagnosing volume overload with Swan-Ganz catheter, echocardiography, and clinical assessment in ICU patients with hypotension, pulmonary edema, or both. He found a sensitivity of 100%, 77%, and 40%, respectively, with a false-positive rate of 0, 62%, and 21%. Because of these and other findings, the assessment of volume status in ICU patients should be aided by other noninvasive and invasive methods. To achieve this objective, cumulative fluid balance (CFB) recording, serial body weight (BW) measurements, and CRX are used commonly to complete the task.
Fluid Balance Recording
Fluid balance charts are used frequently by nurses in the ICU setting to assess the patient's fluid status. In these charts, the type and amount of fluid administered and lost by each patient are recorded every day. Using this method, daily fluid balance is defined as the arithmetical difference between fluid administered (e.g., intravenous fluids, blood products, enteral fluids) and fluid lost (e.g., urine output, blood losses, enteral losses, drain losses). CFB can be defined as the sum of the daily fluid balances over a determined period of time. Most studies in the literature calculated FO by dividing the CFB by the ICU admission weight; this value is expressed as a percentage. Fluid balance charts are used widespread and, in some ICUs around the globe, they constitute the only way to estimate FO in critically ill patients. Although useful, fluid balance recording has many limitations, and it can result in inaccurate data. Arithmetic errors, complicated recording formats, and ignoring insensible losses (IL) (IL = 10 mL/kg of BW/day; if body temperature is higher than 37.8°C, add 500 mL/day) are some of the most common problems.
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