Few topics are as controversial as the “ideal” fluid rate during surgery. To clarify the situation, we talked to Bill Muir III, DVM, Ph.D., Dipl. ACVA, Dipl. ACVECC, and Chief Medical Officer of the Animal Medical Center in New York City.
Why is intraop fluid therapy so debated?
Perioperative and more specifically intraoperative fluid therapy continue to be enigmatic and controversial subjects due to varying opinions regarding the type and amount of fluid to administer, inadequate monitoring techniques and most importantly a lack of evidence defining “best practice.”
Conventional intraoperative fluid administration for most is “recipe based” (ml/kg/hr) and administered at rates that cannot be justified either physiologically or medically. Contemporary rates of intraoperative fluid administration (10-15 ml/kg/hr) are defended based on insensible water losses, the conjectured severity of surgically induced tissue trauma and anesthesia-induced increases in intravascular volume (vasodilatation).
Regardless, little, if any, evidence has been generated to support these recommendations.
What do anesthesiologists do in humans?
Recent evidence generated by studies conducted in humans and experimental animals suggest that a more “restrictive,” rather than “liberal,” approach for fluid administration should be practiced. This practice likely decreases morbidities associated with excessive fluid administration, such as pulmonary gas exchange impairment, coagulopathy, hypothermia, ileus and delayed wound healing.
These studies are further supported by data demonstrating that:
- Insensible water loss during anesthesia is low, rarely exceeding 0.5 ml/kg/hr
- The extravasation of fluid due to “average” surgical trauma is less than 1 ml/kg
- The administration of a crystalloid or colloid to counteract the effects of anesthetic induced vasodilatation (i.e. relative hypovolemia) is frequently ineffective and cannot be guaranteed
In addition, the effects of anesthesia on intraoperative fluid administration are largely unknown.
Don’t intraoperative fluids improve hemodynamics, such as blood flow and tissue perfusion?
Yes, but during anesthesia these effects are more likely to be linked to changes in the rheological properties of blood than to a “volume effect.” The nagging questions that most veterinary surgeons and anesthetists ask remain:
- What is the ideal fluid to administer during surgery?
- At what rate?
- How should this rate be adjusted to compensate for anesthesia (e.g. hypotension) or surgery associated events (e.g. blood loss)?
Some of these questions remain unanswered but several are clear:
- Conventional rates for fluid administration (10-15 ml/kg/hr) during surgery are not justifiable and become excessive the longer they are administered
- Recipe-based fluid therapy regimens should be replaced by goal-directed approaches
- Fluid therapy should be procedure-specific
- Increased rates of fluid administration are most effective if administered when needed (demand-related), not before or after
A hemoabdomen (splenic tumor rupture) may require crystalloids or colloids to fight hypotension.
The last point, timing of fluid administration, highlights a key clinical caveat: Fluid therapy should be continuously monitored.
So what’s a concerned practitioner to do?
First, fluids should be considered as drugs: they produce pharmacologic effects dependent on their composition, dose, metabolism and elimination. Since not all fluids are the same (crystalloids vs. colloids vs. blood product), the IV administration of identical doses should not be expected to produce the same pharmacologic response.
Second, the composition of the fluid should be considered and selected based on the patient’s requirements (physical status). The administration of saline (0.9 percent NaCl), for example, is inappropriate for animals suspected or known to have metabolic acidosis or hypokalemia. Lactated Ringers solution is hypo-osmotic relative to plasma, so it is contraindicated in animals that are hypoproteinemic or as a diluent for blood.
Third, fluid selection and administration should be procedure specific. Insensible losses, and fluid requirements associated with surgically induced trauma (tissue handling) suggest fluid administration rates during operation closer to 2-3 ml/kg/hr (< 5 ml/kg/hr), rather than the current conventional rates of 10-15 ml/kg/hr published in most texts and clinical proceedings
Blood loss should be treated in a 1:1 ration with blood or a colloid. The administration of a crystalloid to correct anesthesia-associated hypotension is temporary at best and often ineffective, although their rheological effects do have the potential to improve blood flow, as mentioned above. This suggests that alternative or adjunctive techniques that help to minimize the need (and effects) for inhalant or injectable anesthesia are employed particularly in aged and high risk patients.
Can you give us some examples?
Using a Constant Rate Infusion (CRI) is a great modality. The infusion of an opioid (fentanyl, hydromorphone, morphine) and/or ketamine during inhalant anesthesia is known to improve analgesia and reduce inhalant anesthetic effects (e.g. hypotension) and requirements up to 50 percent.
Fluid overload is especially risky in small patients, such as this 6-week-old kitten with a femur fracture. Photos Courtesy of Dr. Phil Zeltzman
You talked earlier about monitoring fluid administration. What did you mean?
Fluid administration should be administered when needed; every patient must be closely monitored. Fluid responsiveness should be continuously monitored, using arterial blood pressure, central venous pressure, and dynamic indices for more specific, sensitive and predictive information regarding fluid responsiveness.
These indices include pulse pressure variability, systolic pressure variation and plethysmographically-derived pulse variability index. Pulse variability index has recently been incorporated into a multiparameter monitoring device (Masimo Corp.; Radical 7) for clinical use.
What is the role of fluids to treat hypotension?
Hypotension due to vasodilatation is more effectively treated by reducing the amount of inhalant or injectable anesthetic administered (see above) or by administering a vasoconstrictive drug (ex. dopamine), than using a crystalloid. This becomes particularly important in sick or high-risk patients. Blood loss is most effectively treated by administering whole blood or a colloid (e.g. VetStarch) in a 1:1 ratio. The administration of a crystalloid for this purpose is highly variable, less effective and likely to result in fluid overload.