Preventing and treating hypotension (Proceedings)


Preventing and treating hypotension (Proceedings)

Nov 01, 2010


Discuss the physiological ramifications of hypotension

          1. Describe the methods used to detect hypotension
          2. Discuss the approach to treating hypotension in the anesthetized patient.
     • Hypotension is one of the most common adverse events associated with general anesthesia
     • Blood pressure is used as an indicator of adequate blood flow to tissues
     • Blood pressure monitoring is an important part of patient assessment-especially when
          1. anesthesia time will be prolonged
          2. an animal has concurrent disease (i.e., liver, kidney, hepatic, cardiac)
          3. blood loss is expected as part of a procedure
          4. cardiovascular stability is questionable
          5. the procedure is an emergency procedure
     • Hypotension can lead to morbidity and mortality
               Morbidity associated with hypotension may NOT be recognized by the veterinarian.

Physiology and Physics:

One of the most important assessments a veterinarian can make is whether or not oxygen delivery is adequate. Unfortunately, it is not possible to easily or directly assess oxygen delivery in our patients. Oxygen delivery can be defined as follows:

Oxygen Delivery = Blood Flow x Blood Oxygen Content

Pulmonary function is certainly and important contributor to oxygen delivery, but this presentation will focus on the blood flow and its relationship to blood pressure.

Blood flow is most commonly quantified using Doppler ultrasound monitoring (transthoracic or esophageal), lithium dilution methodology, or thermodilution techniques. In general, these techniques are expensive, technically demanding, or invasive. Thus, measurement of blood flow is limited in clinical veterinary medicine. Instead, we make assumptions about cardiac output from blood pressure measurements.

Blood pressure is related to blood flow as follows:

Mean Arterial Blood Pressure = Blood Flow x Vascular Resistance

Note that blood pressure is related to blood flow. This is the reason for using the measurement as an indicator of perfusion. However, vascular resistance also determines blood pressure. Thus, it possible to have an increase in blood pressure in spite of a decrease in blood flow. Consider the administration of acepromazine compared to the dexmedetomidine in small animal patients as an illustration of the disconnect between changes in blood flow and blood pressure: In general, acepromazine is associated with vasodilation and hypotension, but with minimal decreases in cardiac output. Dexmedetomidine is associated with vasoconstriction, decreased heart rate, and increased blood pressure. Although dexmedetomidine administration causes increased blood pressure, it may also decrease organ and total blood flow.

In general, normal blood pressure in awake dogs and cats is considered to be similar to that seen in human beings (systolic blood pressure = 120 mm Hg, diastolic blood pressure = 70 mm Hg). In actuality, blood pressure is probably somewhat higher in our awake, normal dogs and cats. In anesthetized animals, systolic pressure is considered to be inadequate if it is less than 80-90 mm Hg. Mean blood pressures lower than 60-70 mm Hg are considered inadequate in anesthetized small animal patients.

Blood Pressure Measurement

Indirect Methods

     • Heart Sounds
          o Auscultation of heart sounds using a stethoscope or esophageal stethoscope can be used to assess blood pressure
          o Very subjective and inaccurate
          o Important monitoring tool:
               > Heart rate
               > Heart rhythm
               > The absence of heart sounds is certainly an important clinical observation!
     • Palpation of Peripheral Pulse
          o Cheap, easy, noninvasive
          o Subjective
          o Pulse pressure is determined by the difference between systolic and diastolic pressure
               > May not necessarily reflect mean blood pressure