Fluid therapy in anesthesia (Proceedings)

All anesthetic drugs act as cardiovascular and respiratory depressants on varying levels, and they can compromise a patient's homeostasis at unpredictable times in unpredictable ways. A patient must maintain adequate blood pressure for the duration of any surgical procedure because prolonged anesthetic hypotension can have devastating effects such as renal failure or brain damage. Proper fluid therapy is imperative in maintaining blood pressure under anesthesia. There are many different fluid options, and the goal of fluid therapy is similar to that of anesthesia, which is to maximize benefits while minimizing side effects. A thorough knowledge of fluid products, benefits, and side effects is essential to accomplish this.

Physiology of fluid therapy

There are 3 major fluid compartments; intravascular, interstitial and intracellular. Fluid movement from the intravascular to interstitial and intracellular compartments occurs at the capillary membrane. This membrane is freely permeable to water and small molecular weight particles. The interstitial compartment is the space between the capillaries and the cells. Fluids support the cells within the interstitial space. The intracellular compartment is separated from the interstitial space by a cell membrane. This membrane is freely permeable to water but not small or large molecular weight particles.

Fluids to be administered must concentrate within the body fluid compartment where the volume deficit lies. Crystalloids such as Lactated Ringers solution are water based solutions with small molecular weight particles that are freely permeable to the capillary membrane. Colloids such as 6% Hetastarch are water based solutions that are of a larger molecular weight that are too large to freely pass across the capillary membrane. Colloids are thought of as intravascular volume replacement fluids and crystalloids as interstitial volume replacement solutions.

Sympathomimetics such as Dopamine, and Dobutamine are indicated when the patient is unresponsive to vigorous fluid therapy so their blood pressure and, therefore, tissue perfusion has not returned to acceptable levels.

Intra-operative fluid status can be assessed with blood pressure monitoring. There are three values considered when measuring blood pressure. They are as follows:

      1. Systolic pressure, which is the pressure in the arteries produced by contraction of the ventricles and subsequent propulsion of blood through the aorta and major arteries. It is the highest pressure exerted throughout the cardiac cycle, and should be above 90mmHg during anesthesia.

      2. Diastolic pressure, which is the pressure in the arteries when the heart is resting between contractions. It is the lowest pressure exerted throughout the cardiac cycle.

      3. Mean arterial pressure (MAP), which is the average pressure in the arteries over the cardiac cycle. It is considered the most important of the blood pressure values because it is the best indicator of perfusion to vital organs. This value is usually provided with an oscillometric monitor during a surgical procedure. The MAP during anesthesia should range between 70-90 mmHg, and a MAP below 60 indicates decreased perfusion to internal organs.

A Doppler ultrasonic flow detector does not provide an accurate MAP so decisions should be based on the systolic value.

Hypotension is defined as a blood pressure value that is below normal limits. Causes of anesthetic hypotension include excessive anesthetic depth, excessive vasodilation, cardiac insufficiency, and excessive blood loss (hypovolemia). Adequate preoperative fluid resuscitation should be considered for emergent surgical procedures such as a splenectomy secondary to a hemoabdomen, Caesarian section, or gastric dilitation and volvulus (GDV). These patients are at more risk for anesthetic hypotension due to hemodynamic changes such as massive blood loss in the case of a hemoabdomen, the rapid expansion of the aorta when compression is relieved after the uterus is externalized during a Caesarian section, or the lack of circulation from the pressure abdominal distension puts on the caudal vena cava from a GDV. The shock dose for crystalloids is 40-90 ml/kg in dogs and 20-60 ml/kg in cats. The full shock dose is rarely necessary so fluids should be administered incrementally.

Treatments for anesthetic hypotension

If a patient is becoming hypotensive, the anesthetist should try lowering the patient's inhalant anesthetic first. Inhalants are potent vasodilators that will decrease blood pressure. If that is not effective, treatments are listed below:

      1. Bolus crystalloids at 10-20 ml/kg up to 3 times

      2. Administer Hetastarch, a colloid, at 5 ml/kg in dogs and 2.5 ml/kg in cats over 5 minutes up to 3 times, or 20 ml/kg rapid infusion once. Use fluid therapy with extreme caution if the animal has cardiac disease as fluid overload is a major concern. The anesthetist can use sympathomimetic agents in conjunction with lower fluid rates for cardiac patients.

      3. Administer sympathomimetic agents as follows:

          • Dopamine is administered at a dose of 5-13 mcg/kg/min. It can be used in cats to improve blood pressure as it acts at alpha and beta adrenergic receptors to increase blood pressure. Some do not consider it as effective since cats lack the DA1 receptor, and use Dobutamine instead.

          • Dobutamine is indicated in dogs and cats with hypotension from poor cardiac contractility. The dose range is 4.4-15.4 mcg/kg/min. Reduce the dose or discontinue use if tachycardia and arrhythmias occur.

Dopamine and Dobutamine are both inotropic agents meaning they will strengthen muscle contractions. Dobutamine is a rapid acting parenteral agent that will cause the heart to have stronger beats. Dopamine, on the other hand, is a catecholamine that has different uses at different doses. Lower doses (0.5-3 mcg/kg/min) dilate renal mesenteric coronary and vascular beds, which makes it useful in oliguric renal failure. It is the higher doses listed above that cause systemic increases in peripheral vascular resistance to treat hypotension.

      4. Phenylephrine, an alpha-adrenergic agonist, is indicated for hypotension when cardiac stimulation is not desirable. It is contraindicated in hypertension and ventricular tachycardia, Use with caution in patients that are geriatric, hyperthyroid, bradycardic, and have cardiac disease. The dose is 1-3 mcg/kg/min

      5. If this therapy is ineffective, administer Vasopressin, a pressor agent, at 1-4 microunits/kg/min. Vasopressin is antidiuretic hormone (ADH), and it is able to cause vasoconstriction independent of adrenergic stimulation as opposed to most catecholamine vasopressors. This allows for lower doses of other vasopressors with more results, which can help successfully treat hypotension and poor tissue perfusion while minimizing adverse drug reactions.

As time is often of the essence if the anesthetic patient is hypotensive, and not responding to therapy, a quick way to calculate a CRI is:

(Drug dose in mcg/kg/min) * (Body weight in kg) = Drug in mg to put in 250 ml fluid bag. Administer at 15 ml/hr.

Hypertension

Hypertension is defined as a blood pressure value that is above normal limits. Though it is far less common than hypotension in anesthesia, there are preexisting conditions that can exacerbate the condition, therefore it should be monitored for and treated appropriately. Some preexisting conditions in which to consider hypertension include renal failure, hyperthyroidism, diabetes mellitus, pheochromocytoma, and hyperadrenocorticism (Cushing's disease). A systolic value of 200 mmHg is the minimum to consider treatment. Intervention can start with increasing the inhalant anesthesia to induce vasodilation, and lowering intravenous fluid administration. If that is ineffective, Nitroprusside can be administered at a dose of 0.5mcg/kg/min IV in cats and 1-2 mcg/kg/min IV in dogs. Acepromazine, a tranquilizer, can also be administered intravenously at a dose range of 0.05-0.1 mg/kg.

References

Battaglia. Andrea. Small Animal Emergency and Critical Care: A Manual for the Veterinary Technician. W.B. Saunders Company. 2001.

Gaynor, James. Blood Pressure Monitoring: What You Don't Know Can Hurt Your Patients—A Practical Approach. Proceedings Western Veterinary Conference 2007.

Kirby, Rebecca. Shock and Resuscitation: "Be a Shock Buster...!". Proceedings World Veterinary Congress 2008.

Plumb, Donald. Veterinary Drug Handbook. Fifth Edition. Wiley-Blackwell. 2005.