Monitoring and managing blood pressure under general anesthesia (Proceedings)

Hypotension is a very common complication in the anesthetized patient, especially when the patient is maintained with inhalant anesthetics.  Blood pressure is simple to measure in the anesthetized patient, and is very helpful to monitor depth of anesthesia and overall patient welfare.  It is used as an estimate of tissue perfusion.  Hypotension is generally defined as a mean arterial pressure less than 60 mmHg.  Inhalant anesthetics, such as isoflurane or sevoflurane, contribute to hypotension by vasodilation and reduction of cardiac output.  When hypotension is present in the anesthetized patient, there is concern that vital organs and tissues are not receiving sufficient oxygen to support their needs.

Blood pressure monitoring is a simple method of detecting and controlling hypotension.  Blood pressure can be monitored by direct or indirect means.  Direct blood pressure monitoring utilizes a catheter placed in a peripheral artery and is connected via fluid filled tubing to a transducer, which converts the pressure wave to an electronic signal.  Direct arterial pressure monitoring is considered a more accurate method of blood pressure measurement than the indirect, cuff-based methods.  Indirect methods include Doppler technology, which measures systolic blood pressure, or the oscillometric monitors, which measure the oscillation or movement of the vessel wall.  Both are dependent on the cuff selected for accuracy.  The width of the cuff should be 40% of the circumference of the limb that it is wrapped around.  Too large a cuff will result in a blood pressure reading that is lower than actual and too small a cuff will give a reading that is too high.  The oscillometric units will give systolic, mean and diastolic blood pressure numbers.  It is important to remember that the indirect methods are not as accurate as direct measurement, but very helpful to monitor trends.

Tissue perfusion is a critical component of good anesthetic practice.  With this in mind, blood pressure numbers should be evaluated as an estimate only.  There is no reason to believe that an animal with very high measured blood pressure has better tissue perfusion than an animal with low normal blood pressure numbers, and just the opposite may occur.  A good understanding of the pharmacology of common anesthetic agents is helpful in interpreting what blood pressure numbers may mean.  If hypotension is identified, a systematic approach to managing the situation is helpful.

First off---vaporizer settings should be evaluated.  There is a direct, linear correlation between the amount of inhalant administered and the degree of hypotension seen.  In the majority of cases, the higher the vaporizer setting, the lower the blood pressure.  Small adjustments in the vaporizer can make big differences in the patient's blood pressure.  In the same vein, the use of agents that decrease the vaporizer setting needed while not causing a lot of additional hypotension can be very helpful.  Small, incremental doses of opioids can be given to decrease the vaporizer setting needed.  The use of sedatives and analgesics as premeds are another example of MAC sparing techniques that can work to improve blood pressure.

Fluid management is the next step in hypotension treatment.  “Normal” anesthesia fluid rates are 10 ml/kg/hr of crystalloid fluids.  Crystalloid fluids disseminate within the extracellular fluid.  Colloids such as Dextran 70 or Hetastarch can be very helpful in managing hypotension, as they remain within the intravascular space.  There are many different “recipes” for mixtures of crystalloid and colloid fluids for managing hypovolemia.  Nonetheless, it is important to remember that administration of fluids to manage circulating peripheral volume can be very helpful to improve low blood pressure, as every patient anesthetized with inhalants experiences vasodilation and decreased venous return.  Care should be taken with patients at risk for volume overload.

If lowering the vaporizer setting and administering a robust fluid load do not correct the problem, sympathomimetic therapy may be warranted.  There are a variety of catecholemines available.  It makes sense to use inotropic agents that improve contractility and cardiac output, since inhalant anesthetics diminish contractility.  It is necessary to have adequate circulating volume before administering inotropes, since increasing contractility will not significantly improve cardiac output if there is not sufficient venous return.

If hypotension remains significant, and the above steps have not improved the situation, then vasopressor therapy is warranted.  Vasoconstriction will increase blood pressure numbers, but may not necessarily improve tissue perfusion.

Table 1. Dosages for Sympathomimetic Therapy Drug Dose Dobutamine 1-20 µg/kg/min Dopamine 1-10 µg/kg/min Ephedrine 0.05-0.3 mg/kg Epinephrine 0.2-1.0 µg/kg/min Phenylephrine 0.5-1.0 µg/kg/min Norepinephrine 0.1-0.5 µg/kg/min