MONITORING: OLD AND NEW METHODS
Traditional methods for monitoring perioperative intravenous fluid administration are not reliable.1 Other than increases in postoperative body weight, there have been few accurate indicators of excessive fluid administration.2 Traditional static methods for monitoring fluid therapy, such as central venous pressure (CVP), ventricular end diastolic pressure, and pulmonary capillary wedge pressure, have been shown to be unreliable and generally late indicators of changes in blood volume, fluid responsiveness, and fluid overload (see sidebar "Perioperative fluid therapy dilemmas" in Rethinking your approach to perioperative fluid therapy).1,3-6
Trends in cardiac filling pressures, including CVP, can indicate when too much fluid is administered too quickly but are very dependent upon concurrent changes in heart rate, ventricular function, and venous capacitance, which are frequently altered by disease and anesthesia.1,3
A new method
The plethysmographic variability index (PVI) measures the dynamic changes in perfusion index (PI) over respiratory cycles and is calculated as follows:
PVI = (PImax – PImin)/PImax × 100
The PVI is accurate and far less expensive than esophageal Doppler measurement of variations in stroke volume.4 The greater the PVI, the more likely the patient will respond to fluid administration.
The use of PVI-based goal-directed fluid management in surgical patients reduces the volume of intraoperative fluid infused and decreases intraoperative and postoperative lactate concentrations.8 PVI was less accurate in predicting fluid responsiveness during spontaneous breathing than during mechanical ventilation, but PVI was still better than CVP, at least in people.9
An automatic PVI measurement
The Masimo pulse oximeter (Masimo Corp.) uses an algorithm to continuously quantify changes in pulse volume (i.e. PVI) (Figure 1). PVI is better than CVP for determining the animal's response to fluid therapy.6
Keep in mind that not all animals respond (show a decreased PVI) to fluid administration (volume expansion). Sick, depressed, and debilitated animals; animals with reduced cardiac function due to acquired disease; or animals that are overly depressed by anesthetic drugs may not be capable of increasing cardiac output when venous return (preload) increases.4,5,9-11
INDIVIDUALZING FLUID THERAPY
The dangers of hypovolemia and hypotension
Intraoperative hypovolemia (loss of > 10% of the blood volume) results in tissue hypoperfusion, which leads to metabolic acidosis and increased morbidity and mortality.12,13
Absolute hypovolemia refers to the actual loss of volume from the extracellular space, such as from blood loss. Furthermore, the stress of major surgery and anesthesia can alter fluid distribution to the body's fluid compartments and promote salt (sodium, chloride) and water retention, which may take five to 10 days to return to normal.
Relative hypovolemia (blood volume is normal but insufficient because of widespread vasodilation) is caused by diseases (e.g. sepsis, trauma) or drugs (e.g. anesthetics) that produce vasodilation and results in dilatation of the intravascular space and a decrease in the effective circulating blood volume.
Both absolute and relative hypovolemia cause hypotension. Absolute hypovolemia requires fluid replacement, while relative hypovolemia requires careful consideration of its cause (anesthesia, trauma, sepsis) and the side effects of intravenous fluid administration. Note that fluids do not correct vasodilatation.
Five factors to consider
1. Preoperative fluid loss and fluid loading
Basal fluid water requirements depend on metabolic rate and rarely exceed 1 to 2 ml/kg/hr at room temperature (70 F) in dogs and cats.14 Any attempt to restore estimates of fluid deficit due to dehydration with a crystalloid immediately preoperatively (within one to four hours of the surgery) or intraoperatively almost always leads to tissue edema. Simple dehydration (i.e. loss of water alone) results in proportional reductions of both interstitial fluid and plasma volume. The dehydrated interstitium absorbs the crystalloid solutions that are infused and decreases their effectiveness to produce plasma volume expansion.
Arterial blood pressure and tissue perfusion (mucous membrane color, capillary refill time) should be normalized and stabilized in animals that are moderately to severely dehydrated, if time permits, before the animals are anesthetized. And only 75% to 80% of the dehydration deficit should be replaced during the 24 hours before anesthesia, to avoid fluid overload. The fluid deficit due to dehydration can gradually be replaced over 24 to 48 hours after surgery.19
2. Fasting loss
Mature dogs and cats can be fasted for six to eight hours; however, for older and younger animals, two to three hours of fasting before surgery is more appropriate.20,,21 Except for medical or behavioral reasons, access to water should be allowed until the preanesthetic medication is administered. An additional 1 to 2 ml/kg of crystalloid can be added to the base fluid administration rate for each hour the animal does not have access to water before surgery and can be administered during the first hour of surgery to replace water losses if water has been withheld.14
3. Insensible loss
Insensible fluid loss in people and animals is generally less than 1 to 2 ml/kg/hr during anesthesia.9,14 Evaporative losses from surgically traumatized tissues are more difficult to assess but have been experimentally determined to range from 2 to 30 ml/hr in nonexteriorized and exteriorized viscera, respectively.22 Surgically manipulated tissues do not accumulate marked amounts of fluid unless they are severely inflamed or traumatized.
These insensible fluid losses, taken together with basal and fasting losses, suggest that initial rates for intravenous fluid replacement to otherwise normal healthy dogs and cats rarely need to exceed 5 ml/kg/hr (basal + fasting + insensible). In those circumstances in which greater rates of fluid administration are required (> 10 to 30 ml/kg/hr), the fluid should be administered during the first hour and decreased to the basal rate while accounting for blood loss thereafter.23
4. Surgical and traumatic loss
Fluid loss from the vascular compartment is either absolute (e.g. hemorrhage) or relative (e.g. vasodilatation). Although conceptually thought to be a myth, third space losses can also be either absolute or relative.24 Loss of intravascular fluid to a third space can be either anatomical or nonanatomical.25 Anatomical third space loss represents the accumulation of fluid within the interstitium, whereas nonanatomical third space losses include fluid accumulation in traumatized tissue, the bowel, and the peritoneal cavity. Nonanatomical third space losses contain protein concentrations similar to that of plasma and are lost from further extracellular exchange. Therefore, they represent an absolute loss of protein-containing fluid.
Blood loss during anesthesia should be estimated or quantified during surgical procedures, considered in relation to the animal's hemoglobin concentration, and replaced as soon as possible.26 Timely fluid resuscitation reduces later volume requirements. Fluid loss into surgically traumatized tissues can be highly variable and is independent of the animal's weight. Fluid accumulation at bowel anastomotic surgical sites is minimal and has been experimentally determined to depend on the rate of fluid (crystalloid) administration and ranges from 5 to 10 ml.27
Blood loss can be replaced with adequate amounts of crystalloid, colloid, or blood (see sidebar "A guide to optimal perioperative fluid therapy"). The amount of crystalloid required to effectively replace blood loss is difficult to determine and usually exceeds the traditional 3:1 replacement ratio derived from estimates for the distribution of water between the intravascular and interstitial fluid compartments.28-30 Experimental data in isoflurane-anesthetized dogs and clinical studies in people suggest that crystalloid fluid replacement ratios ranging from 4 to 5 ml or greater for each milliliter of lost blood (4:1 or 5:1) are more efficacious than a 3:1 ratio, depending on the amount of blood loss and whether concurrent hypotension exists.
Administering large volumes of crystalloid (> 30 ml/kg) during the first hour of anesthesia to replace blood loss is more likely to produce hemodilution, fluid accumulation at the surgical site, and interstitial edema.23,29,30 Hemodilution decreases hemoglobin and protein concentrations and can produce coagulation defects.33-35 It is important to monitor hemoglobin concentrations and coagulation and treat dilutional coagulopathy when it occurs (e.g. with fresh plasma: 5 to 15 ml/kg). Hemoglobin (should be > 7 g/dl) and total protein (should be > 4 g/dl) concentrations should be measured before and during surgery to minimize oxygen delivery deficits and interstitial edema. Either a colloid or blood (recommended if the hemoglobin concentration is < 5 to 7 g/dl) is a more rational and effective choice for replacement of blood loss.
Reducing or terminating anesthesia, when possible, and co-administering fluids with vasopressors or inotropes are the best methods for managing hypotension caused by anesthetic drugs.
Hypotension in dogs and cats is one of the most frequent complications of general anesthesia and is related to the animal's health status.20,36-41 The development of hypotension during anesthesia in a normal healthy dog or cat is more a testament to the animal's physical status and the anesthetist's competency than to blood loss or the vasodilatory and negative inotropic effects of anesthetic drugs. But keep in mind that all anesthetic drugs can induce hypotension, particularly when administered to dehydrated, sick, depressed, or debilitated animals.
Absolute hypovolemia (e.g. blood loss) requires fluid replacement therapy with a crystalloid, colloid, or blood, depending on the animal's hemoglobin concentration (Table 2). The question is how to treat hypotension due to relative hypovolemia during anesthesia. The answer is determined by its principal cause (e.g. vasodiliatation, poor cardiac function).
Anesthetic drugs. Relative hypovolemia (vasodilatation) is most likely due to the vasodilating effects of anesthetic drugs on venous vascular tone and suppression of compensatory homeostatic mechanisms.3 Isoflurane, sevoflurane, and propofol inhibit sympathetic compensatory homeostatic mechanisms and directly interfere with smooth muscle contraction (e.g. splenic contraction), thereby decreasing venous vascular tone and increasing venous capacitance.42,43 These effects combine to decrease venous return, cardiac filling pressures, cardiac output, and arterial blood pressure and are as important, if not more important, than their arterial vasodilatation effects.3
Conventional rates of fluid administration, whether crystalloid or colloid, may be ineffective for restoring normal arterial blood pressure unless anesthetic depth is reduced.5,33 An experimental study in isoflurane-anesthetized dogs demonstrated that arterial blood pressure improved only when anesthetic depth was decreased.5 Notably, the fluid administration rate in this study was six times greater (60 ml/kg/hr) than the currently suggested conventional rate (10 ml/kg/hr).5 This rate of crystalloid administration is likely to produce hemodilution and hypervolemia unless administered for a very short time. Clearly, more practical and effective recommendations are required for the intraoperative treatment of hypotension in anesthetized animals.
Toward this end, we have administered 3 to 5 ml/kg of tetrastarch over one to three minutes to isoflurane-anesthetized dogs and produced an immediate and sustained increase in arterial blood pressure. The magnitude of the beneficial effect was directly related to the depth of anesthesia (i.e. deeper level then less effect). Similar but shorter duration effects were produced when lactated Ringer's solution was administered. The relatively short duration of fluid administration decreases the potential for fluid overload (e.g. for a 10-kg dog, 30 ml of crystalloid or colloid over one to three minutes). If arterial blood pressure does not increase, then the administration of a catecholamine (e.g. vasoconstrictor, inotrope) should be considered (Table 2).
Catecholamines. Catecholamines increase heart rate and cardiac contractility, and some are potent vasoconstrictors (e.g. dopamine, norepinephrine). Notably, catecholamines alter the intravascular volume-expanding effects of fluid therapy.44,45 Drugs that activate alpha receptors (e.g. dopamine, norepinephrine, and phenylephrine) and induce vasoconstriction reduce blood volume expansion, while those that activate beta receptors augment blood volume expansion.46 Since optimization of blood pressure and blood flow is the goal in a hypotensive anesthetized animal, the administration of a mixed alpha-beta agonist (e.g. dopamine) is the more rational therapeutic choice for maintaining arterial blood pressure.46
Final considerations. Finally, it should be emphasized that anesthetic-induced or -associated hypotension is best treated by decreasing or stopping the administration of anesthetic drugs. A potent analgesic (e.g. fentanyl) can be administered to reduce the requirement for inhalant or injectable anesthesia.47 A vasoconstrictor (e.g. dopamine, norepinephrine) or positive inotrope (e.g. dopamine, dobutamine) may be required to restore and maintain blood vessel tone or cardiac contractile function, blood flow, and arterial blood pressure, respectively, in animals that do not respond to fluid therapy due to vasodilatation or poor cardiac contractile function.48
Perioperative fluid therapy in animals is not innocuous and should be considered identical to drug therapy—individualized and goal-directed. Differences in the types of fluids (crystalloid or colloids) available determine their effects upon electrolyte and acid-base balance and their ability to restore or maintain arterial blood pressure and tissue perfusion.
Isotonic balanced crystalloids maintain water requirements, blood osmolality, tissue perfusion, and acid-base balance in otherwise normal healthy surgical candidates. Colloid solutions are more effective for treating low arterial blood pressure, poor tissue perfusion, and blood loss. Both crystalloids and colloids are dilutional to substances; they do not contain (e.g. hemoglobin) and should be dosed in order to prevent excessive hemodilution (hemoglobin < 7 g/dl) and avoid fluid overload. Thus, accurate assessment of fluid status (i.e. fluid responsiveness) and careful definition of targets at all stages of the perioperative period are needed to improve clinical outcomes.
Fluid administration is essential for optimizing tissue perfusion. The determination of PVI helps to predict fluid therapy responsiveness and to guide fluid replacement during anesthesia in dogs and cats.
The author thanks Dava Cazzolli, DVM, DACVECC; Jaime Chandler, DVM, DACVECC; Michelle Albino, LVT, CVT (Anesth); and Yukie Ueyama, DVM, for their assistance in abstracting data.
William Muir, DVM, MS, PhD, DACVA, DACVECC
338 W. 7th Ave.
Columbus, OH 43201
For references, see dvm360.com/FluidTherapy3Refs.