Vol. III: Treatment of Hypotension During Anesthesia

The volume and type of fluids needed to correct hypotension are different for every patient. The electrolyte status, surgical procedure, and expected losses need to be considered before choosing a fluid.

• Crystalloids (e.g., lactated Ringer’s solution, Normosol) can be given at a bolus rate of 10 to 20 ml/kg.
• Synthetic colloids (e.g., hetastarch, dextran-70) or plasma can be given to help increase oncotic pressure and maintain vascular volume.
• In patients that have suffered blood loss, packed red blood cells or fresh whole blood is the best choice for volume replacement.

When to Consider Inotropic Support

If the patient has received an appropriate volume of fluids and remains hypotensive, then it is time to consider inotropic support. Inotropes increase cardiac contractility, thereby increasing cardiac output and arterial blood pressure.

Ephedrine for Blood Pressure Support

Ephedrine is very effective in raising blood pressure as it increases cardiac contractility and causes peripheral vasoconstriction. It can be given intravenously as a bolus at a dose of 0.1 to 0.25 mg/kg.

The effects of ephedrine are variable, but it is useful in anesthetized patients during short procedures (e.g., dentals, tumor removals).

Dopamine as a Constant-Rate Infusion

Dopamine has a very short half-life; therefore, it needs to be administered as a constant-rate infusion (CRI). The dose for dopamine is 5 to 10 μg/kg/min.

A quick and easy method of mixing dopamine using a Buretrol (drip size = 60 drops/ml) is as follows:

• Add 6 mg of dopamine to 100 ml of D5W in the Buretrol.
• The rule of thumb for this combination is: ml/kg/hr = μg/kg/min.

This format clearly organizes the content into sections, making it easier to follow and understand the information about fluid therapy and inotropic support for hypotensive patients.

The combination of dobutamine and dopamine given as a CRI is the inotropic cocktail of choice at Iowa State University Veterinary Hospital. Both drugs are calculated at 5 to 10 μg/kg/min and mixed together in the same syringe with D5W. A fluid rate of 1 ml/kg/hr is calculated for the patient. The calculation for the inotropes is:

• Dose (μg) x Body weight (kg) x 60/1,000 = mg/hr of each drug

Monitoring blood pressure has become routine in many practices and provides the opportunity to not only detect hypotension but to administer proper treatment.