Postoperative fluid management

Increasing importance is now being placed upon the maintenance of normovolemia as well as electrolyte balance in the postoperative phase, as this is associated with a reduction in complication rates and hospital length of stay.[29,30] However, despite this recommendation it is not uncommon for patients in the postoperative period to receive 5–10 liters of fluid with large sodium loads in the first 24 hours following surgery. This is often related to errors in prescribing, with studies suggesting this to occur in up to 20% of patients. Excess fluid administration in the postoperative period is further compounded by the metabolic response to trauma precipitated by surgery which causes the retention of salt and water in order to maintain intravascular volumes. This excess water and sodium load can take several weeks to be offloaded in the postoperative period. Excessive fluid administration in the intra- and postoperative period can have multisystem complications following surgery, including respiratory (increased pneumonia), gastrointestinal (decreased mesenteric blood flow, prolonged ileus, increased anastomotic leak rates, and splanchnic edema), mobility (increased peripheral edema), and general patient status (increased nausea, impaired wound healing, and impaired cognitive abilities). In contrast, underhydration in the postoperative period is known to have significant hemodynamic effects including decreased venous return and preload, increased blood viscosity, and decreased tissue perfusion.

Once oral intake is reestablished postoperatively, intravenous fluid administration should be tailored to intake requirements, with cessation at the earliest opportunity, encouraging early postoperative mobility. Intravenous fluids should only be recommenced if clinically indicated. ERAS guidelines advocate rapid treatment of postoperative nausea and vomiting (PONV) in order to encourage early oral intake, with prophylaxis indicated in specific circumstances. ERAS guidelines recommend the avoidance of nasogastric tubes following evidence suggesting that their placement increases the incidence of pneumonia and time to passage of flatus but makes no difference to length of hospital stay.[31,32] Recommencement of normal diet as early as possible following surgery should be a major target of any ERAS protocol, with evidence that delay in commencement of feeding is associated with increased hospital length of stay and infectious complications.[33]

A frequently employed technique in ERAS protocols is the use of thoracic epidural analgesia (TEA), particularly following open surgery, owing to superior analgesic quantities in the first 72 hours following surgery.[34] This technique, however, often results in a degree of sympathetic blockade, reducing venous tone and capacitance, and resulting in fluid-resistant hypotension. It is key that when managing epidural-induced hypotension (EIH), a combination of judicious intravenous fluid administration as well as vasopressor use is adopted to prevent significant fluid overload (Figure 18.2) which is associated with worsening of clinical outcomes.[35]

Figure 18.2

Postoperative algorithm for the management of epidural-induced hypotension (EIH). BP, blood pressure; CVP, central venous pressure; GCS, Glasgow coma scale; HR, heart rate; MAP, mean arterial pressure; NG, nasogastric; OR, operating room; RR, respiratory rate; UO, urine output.

With permission from Ref. [45].

The vast majority of the evidence for ERAS programs originates from the speciality of colorectal surgery, and thus its extrapolation to other fields of surgery is not without limitations.

Choice of fluid

A vast amount of research has been conducted into the potential risks and benefits associated with the type of fluid infused (colloid versus crystalloid; balanced versus unbalanced).

“Normal” 0.9% saline has historically been used as a maintenance fluid method and has previously been the most commonly prescribed crystalloid in the UK and USA, using 10 and 200 million liters annually. However, excessive administration of “normal saline,” owing to its extra-physiological content of both sodium and chloride (10% and 50% higher than the content in extracellular fluid, respectively), is now understood to result in a significant hyperchloremic acidosis which may result in reduced renal cortical perfusion. This acidosis has been found to be associated with a significant reduction in mean renal artery flow velocity and perfusion [36] as well as increased postoperative morbidity and mortality [37,38] when compared with a balanced crystalloid, even in healthy volunteers. Bolus administration of 0.9% saline versus balanced crystalloid solutions in healthy human volunteers is associated with slower fluid excretion but equivalent blood volume expansion,[36,39] therefore increasing the likelihood of prolonged fluid retention and resultant edema. In addition, human studies have shown that sodium balance remains abnormal for up to 2 days following infusion of 0.9% saline, as the mechanisms for sodium excretion are dependent upon prolonged suppression of the renin–angiotensin–aldosterone system. No clinical studies have found 0.9% saline to be superior to balanced crystalloids, and two randomized controlled trials [40,41] have shown saline to be associated with an increased side-effect profile versus Ringer’s lactate. ERAS guidelines [1–5] now place much emphasis upon the administration of balanced crystalloid as the maintenance fluid of choice, reinforced by GIFTASuP, which recommend the use of balanced crystalloids in the majority of clinical settings. The main exceptions to this are patients with high output from the upper gastrointestinal tract (vomiting, high volume nasogastric drainage) which may result in hypochloremic alkalosis, and neurosurgical patients in whom hypo-osmolar fluid administration may be harmful.

The administration of colloid versus crystalloid fluids remains a much debated topic. Colloid fluids are of an increased molecular size; as such, they are retained in the intravascular compartment more readily and therefore provide increased expansion of this space. This is particularly useful in the hypo-volemic patient, in whom colloid administration results in a greater ability to expand intravascular volume, although excessive administration results in damage to the endothelial glycocalyx. Much research into GDT has utilized hydroxyethyl starch (HES) for bolus fluid infusion, but owing to concerns raised about HES from level I evidence regarding the risk of increased mortality [42,43] as well as acute kidney injury rates,[44] this fluid is no longer in use in Europe. Recent evidence has suggested that balanced crystalloid (Hartmann’s solution) and colloid (6% HES) as the bolus agent in GDT result in equivalent rates of morbidity and coagulopathy, suggesting this may become increasingly utilized in a GDT strategy.

Summary

Optimal fluid management in the setting of an ERAS program is a careful balance of pre-, intra-, and postoperative factors, all aiming for “zero balance” of both water and salt. The range of balanced fluid administration is fairly narrow, with both under- and overhydration having significant harmful effects on the surgical patient. ERAS guidelines have recommended the preferential use of balanced crystalloids as the fluid of choice in this setting rather than 0.9% saline, excessive administration of which is associated with detrimental effects.