There has been a lot of discussion in the past 10 years about how much fluid to deliver to a patient during general anesthesia and, even though we might like to take a simple formulaic approach, I think that the actual answer is a lot more nuanced than, say, 3 mL/kg/hr or 10 mL/kg/hr.
Recommended anesthetic fluid rates have decreased since I graduated, lo' these many years ago, but I am not actually sure that this reduced rate always makes good sense. Veterinarians tend to focus only on fluid therapy during anesthesia as opposed to taking a holistic approach that integrates pre- and post-anesthetic fluid management. I will even go out on a limb and say that I care less about the fluid rate (within the typical range of fluid rates) and more about the total volume delivered. More about that later....
Intravenous fluids are traditionally prescribed during anesthesia to replace any pre-existing deficits and ongoing losses, thereby maintaining an effective circulating blood volume and tissue oxygen delivery. Maintaining tissue perfusion can make healthy patients feel better faster and go home more quickly. In an old human study, patients undergoing minor outpatient surgeries with general anesthesia felt less nauseated, less thirsty, less drowsy, and overall had a better perioperative experience when they had some fluids (2 L crystalloids, which is ~28 mL/kg if all people are 70 kg until proven otherwise) vs no IVF therapy (Keane & Murry 1986). More recent human studies delineate similar outcomes, with improved satisfaction and reduction in post-operative nausea and vomiting for outpatient procedures with larger doses of crystalloid (30 mL/kg vs 10 mL/kg) (Goodarzi et al. 2006, Apfel et al. 2012).
Some amount of fluid in the perianesthetic time period is good, especially in healthy patients.
The Bad & The Ugly:
In humans, a positive fluid balance after major surgery delays normal GI function, increases pulmonary dysfunction and the need for ventilation, can increase the risk of surgical site infection, keeps people in the hospital longer, and can even increase mortality (Brandstrup et al. 2003, Holte et al. 2007). This type of evidence has led to a debate in the literature regarding “liberal” vs “restrictive” perianesthetic fluid approaches, with the “liberal” approach generally being in disfavor in recent years. As veterinarians, we have probably all seen cases that went south due to fluid overload. Cats can be particularly tricky because they may have undetected heart disease that means they cannot handle excess volume being infused during a routine anesthetic event (e.g 10 mL/kg/hr during a 4 hour dental procedure would result in that cat receiving basically a full blood volume). It should be noted that the definition of “liberal” and “restrictive” perioperative fluid therapy varies quite a bit amongst studies and there is a lot of overlap.
On the other hand, rushing a patient into an anesthetic and surgical event without optimizing their volume status preoperatively can lead to acute kidney injury, cerebral hypoperfusion, and even death post-operatively. Short of overt fluid deficits, the recent swing towards “restrictive” fluid therapy can result in morbidity in some patients, as indicated by a large randomized controlled clinical study called the RELIEF trial: Restrictive versus Liberal Fluid Therapy for Major Abdominal Surgery (2018). Patients undergoing major abdominal procedures requiring at least a 3 day hospital stay and who had an increased risk of post-operative complications (including age >70 years, heart disease, diabetes, renal disease, obesity etc) were randomized to receive a “restrictive” or “liberal” perianesthetic fluid regimen. The “restrictive” regimen was designed to achieve a net zero fluid balance. In short, these patients received a 5 mL/kg bolus of crystalloids at induction, 5 mL/kg/hr intraoperatively, and then 0.8 mL/kg/hr postoperatively for the first 24 hours, ending up receiving on average a total volume of 3.7 L of fluids in that time (~50 mL/kg over 24 hours). The patients in the “liberal” fluid therapy group received a bolus of 10 mL/kg at induction, 8 mL/kg/hr intranesthetically, and 1.5 mL/kg/hr afterwards, receiving an average of 6 L of crystalloids in the first 24 hours (~85 mL/kg total). Interestingly, there was no difference between groups in disability free survival up to 1 year postoperatively; however, acute kidney injury occurred more commonly in the “restrictive” group with a relative risk of 1.71. Of course, this population of patients and their perianesthetic management is quite different from the average veterinary patient undergoing anesthesia but I think there are some overarching concepts that still apply. In particular, I think this study highlights the need to individualize fluid therapy as well as the fact that we may have swung the pendulum to far in the "restrictive" direction.
Too much fluid volume delivered can be bad or ugly; but too little fluid volume delivered can also be bad or ugly.
So what is the “right” volume? Pinpointing the right amount, without going over (I call this "The Price is Right" approach), can be tough and entire textbooks have been written on the theory and practice of perianesthetic fluid delivery. I have a couple of thoughts about the prescription of perioperative fluid therapy:
1) Balanced isotonic crystalloids are usually the best choice for fluid type, e.g LRS, plasmalyte-A, Hartmann’s, etc.
2) If your patient is healthy and has been routinely NPO (6 hours from a light meal and water removed no more than 2 hours prior), you can use faster fluid rates to begin with and then decrease the rate, depending on the length of the procedure.
3) Keep the total dose delivered in mind, including fluid boluses. A cat who receives 3 mL/kg/hr during a 15 minute spay has not gotten all that much, and depending on preoperative hydration, may benefit from continued fluids post-operatively, or from receiving SQ fluids. Remember healthy humans presenting for outpatient procedures feel better faster if they get 20 - 30 mL/kg of fluids. Stay on the lower side with cats.
4) In healthy animals who are hydrated prior to anesthesia and do not experience hemorrhage or other major fluid shifts, 20 – 30 mL/kg of crystalloids is probably plenty and consideration should be given to drastically decreasing fluid rates. If the patient is hypotensive, now would be a good time to think about pharmacologic manipulation with dopamine or norepinephrine.
5) Unhealthy or unstable patients need an individualized fluid therapy plan that is reassessed often on the basis of physical exam and physiologic findings. Below is a graphic that I find particularly helpful when thinking about perianesthetic fluid therapy decisions.
This is just the way I think about coming up with a perianesthetic fluid plan. Here’s what I think we can agree on: excessive fluid therapy is bad, inadequate fluid therapy is bad, and finding the sweet spot can be a bit of a guessing game.
I’m interested to hear how you approach the prescription of fluids in the perianesthetic time period.
Apfel CC, Meyer A, Orhan-Sungur M, et al. (2012). Supplemental intravenous crystalloids for the prevention of postoperative nausea and vomiting: quantitative review. Br J Anaesth. 108(6):893-902.
Brandstrup B, Tønnesen H, Beier-Holgersen R, et al. (2003) Danish Study Group on Perioperative Fluid Therapy. Effects of intravenous fluid restriction on postoperative complications: comparison of two perioperative fluid regimens: a randomized assessor-blinded multicenter trial. Ann Surg. 238(5):641-8.
Della Rocca G, Vetrugno L, Tripi G, et al. (2014) Liberal or restricted fluid administration: are we ready for a proposal of a restricted intraoperative approach? BMC Anesthesiol. 14:62.
Goodarzi M, Matar MM, Shafa M, et al. (2006) A prospective randomized blinded study of the effect of intravenous fluid therapy on postoperative nausea and vomiting in children undergoing strabismus surgery. Paediatr Anaesth. 16(1):49-53.
Keane PW, Murray PF. (1986) Intravenous fluids in minor surgery. Their effect on recovery from anaesthesia. Anaesthesia. 41(6):635-7.
Holte K, Foss NB, Andersen J, et al. (2007) Liberal or restrictive fluid administration in fast-track colonic surgery: a randomized, double-blind study. Br J Anaesth. 99:500-8.
Myles PS, Bellomo R, Corcoran T, et al. (2018) Australian and New Zealand College of Anaesthetists Clinical Trials Network and the Australian and New Zealand Intensive Care Society Clinical Trials Group. Restrictive versus Liberal Fluid Therapy for Major Abdominal Surgery. N Engl J Med. 378(24):2263-2274.
Schol PB, Terink IM, Lance MD, et al (2016) Liberal or restrictive fluid management during elective surgery: a systematic review and meta-analysis. J Clin Anes. 35:26-39.