Underestimating fluid balance may lead to continued or increased administration of intravenous fluids, which if monitored incorrectly could result in circulatory overload. Excess intravenous fluid administration is not the only cause of circulatory overload, which can also result from acute renal failure, heart failure or excessive sodium intake (Rhoda et al. [151]).

In health, homeostatic mechanisms exist to compensate and redistribute excess fluids; however, in ill health, these mechanisms are often inadequate, leading to increasing circulatory volumes. As the volume within the circulatory system rises, so does the hydrostatic pressure, which, when excessive, results in leaking of fluid from the vessels into the surrounding tissues. This is evident as oedema, initially apparent in the ankles and legs (Figure 8.10) or buttocks and sacrum if the patient is in bed. This can progress to generalized oedema, where even the tissues surrounding the eyes become puffy and swollen. A bounding pulse and an increased blood pressure are also signs of fluid overload, as are an increased cardiac output and raised central venous pressure (Gross et al. [63]).

One of the most dangerous symptoms of fluid overload is pulmonary oedema, which occurs when rising hydrostatic pressure within the vessels leads to congestion within the pulmonary circulation, causing fluid to leak into the lungs and pulmonary tissues (Chioncel et al. [35]). This presents with respiratory symptoms, including shortness of breath, increased respiratory rate, a cough (Tidy [180]) (often associated with pink, frothy sputum), and finally reduced oxygen saturations due to inadequate gaseous exchange at the alveolar level (Tidy [180]). Left untreated, this can be fatal as the lungs fail to provide essential cells and organs with oxygen; this eventually leads to organ dysfunction and then failure.

Cardiac dysfunction can result from fluid overload, not only from the reduced availability of oxygen to the cardiac cells due to the pulmonary oedema, but also from the increase in volume, which causes the cardiac muscle to stretch, leading to cell damage and inability to contract effectively (Tidy [180]).

Treatment of hypervolaemia involves restricting fluid intake, monitoring electrolytes and using diuretics in an attempt to offload some of the excess fluid (Thomsen et al. [179]). Vasodilators may also be considered to reduce the pressure in the vessels in patients with congestive heart failure. If these mechanisms fail, it may be necessary to use renal replacement therapy to drive the fluid out of the circulation.

In some cases, fluid overload is part of the disease process. However, with effective monitoring, fluid balance recording and assessment, it may be possible to avoid the devastating complications.

Dehydration implies a negative fluid balance: when the fluid output exceeds the fluid intake (Jevon [72]). Overestimation of the fluid balance may lead to inadequate replacement of lost fluids. Dehydration can, however, be caused by a loss of fluids to ‘third spaces’ such as ascites or lost due to a reduction in colloid osmotic pressure (hypoalbuminaemia) (Frost [55]) – losses that are not easy to account for. Fluid balance charts should therefore always be used in association with physical assessment of the patient, weight measurement and laboratory results.

There are three categories of dehydration (Mentes and Kang [101]) – isotonic, hypertonic and hypotonic – each related to the type of fluid and solutes lost. Isotonic describes the loss of both water and sodium from the ECF; hypertonic is excessive loss of water only, which leads to a rise in ECF sodium, causing a shift in fluid from the intracellular space to the extracellular. Hypotonic dehydration results from excessive sodium loss, particularly with the overuse of diuretics or in the case of high‐output ileostomies (Chan et al. [33], Welch [187]).

Dehydration can ultimately cause a reduction in circulating volume (Tortora and Derrickson [183]). As with any change in a homeostatic state, in health the body has the ability to compensate but in ill health these mechanisms are often inadequate. Untreated dehydration will quickly lead to a drop in blood pressure and a rise in heart rate (to compensate for the fall in blood pressure). A fall in blood pressure will initially lead to inadequate renal perfusion, causing a rise in metabolites, acidosis, acute kidney injury and eventual toxaemia. If this is untreated, other organs will suffer from poor perfusion, possibly resulting in ischaemia, organ dysfunction and eventually organ failure (Adam et al. [1]).

Additional signs and symptoms of dehydration are thirst, weight loss, decreased urine output, dry skin and mucous membranes, fatigue and increased body temperature (Rhoda et al. [151]).

Treatment of dehydration includes the replacement of lost fluid and electrolytes but caution must be exercised. If the dehydration is mild, slower fluid replacement is advised, in order to prevent further complications in shifts in electrolytes. However, if hypovolaemia exists with the signs and symptoms of circulatory shock, low blood pressure and organ dysfunction, aggressive fluid replacement is advised (Jevon [72], NICE [122]). By restoring circulatory volume with fluid administration or resuscitation, renal perfusion can be improved and acute kidney injury may be prevented (Perner et al. [139]).

Acute kidney injury (which can be due to a multitude of causes) is a common feature in the acutely or critically unwell patient and will require even more careful approaches to monitoring and maintenance of fluid balance (Goldstein [60]). The National Confidential Enquiry into Patient Outcome and Death (NCEPOD [109]) reviewed the care delivered to patients with acute kidney injury and found that 50% received suboptimal care. NICE responded to these findings by producing guidance for the prevention, detection and management of acute kidney injury (NICE [120]). The relevant recommendations from this guidance include the need for reliable monitoring of urine output and early recognition of oliguria. The KDIGO ([75]) grading system also reinforces the need for accurate and reliable fluid balance monitoring, particularly monitoring urine output (Table 8.6). All assessments, investigations and results require the healthcare professional undertaking them to have an appreciation and understanding of the implications of the findings so they can act accordingly.