Introduction to Autonomic Dysfunction David Roy Godden, MS CRNA Keck School of Medicine Department of Anesthesia

ANS Definition Autonomic nervous system (ANS) dysfunction is known as dysautonomia. The autonomic nervous system regulates unconscious body functions, including heart rate, blood pressure, temperature regulation, gastrointestinal secretion, and metabolic and endocrine responses to stress such as the "fight or flight" syndrome. As regulating these functions involves various and multiple organ systems, dysfunctions of the autonomic nervous systems encompass various and multiple disorders.

So What She says! Why should I care about autonomic dysfunction? What patient population is this condition most prevalent in? How do I know if my patients have it? What are the anesthetic implications of this condition?

Are you interested in BP control? The generation of blood pressure requires two components. What are they? BP = CO X SVR. Autonomic Nervous system function controls CO and SVR. Orthostatic BP and HR measurements as an example.

ANS Blood Pressure Control Nervous system control of BP is achieved by reflexes that originate in the baroreceptors, the chemoreceptors and the low pressure receptors in the pulmonary circulation. All of these peripheral receptors are located outside of the brain.

Baroreceptor Negative Feedback Pressure System The baroreceptors of the internal carotid and aortic arch modulate blood pressure in a delicate buffer system. Internal carotid stretch initiates afferent firing of Hering’s nerve to the glossopharyngeal nerve up to the tractus solitarus in the medulla. Aortic arch stretch initiates firing of the afferent vagus to the tractus solitaus as well. These signals to the medulla initiate secondary signals to the vagoconstrictor center of the medulla and excite the vagal parasympathetic center. Next!

What now Batman? Stimulation of the vagal parasympathetic center causes efferent vagal impulses to the periphery. Effect is to cause vasodilatation of veins and arterioles Also a causes a decrease in heart rate and contractility = decrease in BP and stretch. Alrighty then, what happens with a hypotensive patient – a decrease in aortic stretch and then?

Normal orthostatic changes Sudden standing from a supine position causes me to get a little dizzy how about you? Normal in hypovolemic young people – right? Sudden standing causes a dramatic drop in preload, CO, aortic stretch and internal artery stretch. The decrease in stretch of the baroreceptors decreases firing of the afferent glossopharyngeal and vagal nerves to the medulla and results in a strong sympathetic discharge throughout the body. The negative feedback Buffer Baroreceptor System at work to maintain homeostasis.

Denerved Rabbits Draw chart stupid!

Hypotension: answer to question #1 Hypotension for this discussion is defined as a drop in mean arterial blood pressure of 30% or more or a Systolic BP less than 85.

Diabetics: answer to question #2 30% of diabetic patients require intraoperative vasopressors compared with 7% of patients without diabetes. (Huang, C. and Kuok, C., 2006 Pre-operative measurement of heart rate variability predicts hypotension during general anesthesia. ACTA Anesesthesiology Scandinavia).

Diabetics and neuropathy A few sequelae of diabetes mellitus is peripheral neuropathy, gastroparesis, cardiac nervous system impartment and autonomic nervous system dysfunction. Denerved Rabbits?

Heart Rate Variability Yea Right! What is the one test at the bedside to assess autonomic dysfunction? Heart rate variability and blood pressure changes with orthostatic measurements.

Measurements of HR variability E:I ratio is the ratio of the mean of the maximal longest R- R interval during deep expiration to the mean of the minimal or shortest R-R interval during deep inspiration. Valsalva maneuver in a supine pt asked to forcibly exhale for 15 seconds against a fixed resistance with an open glottis. The valsalva ratio is the longest R-R divided by the shortest R-R occurring within 45 seconds of peak heart rate. Orthostatics or the 30:15 ratio is a test of HR response to standing from a supine position. The R-R intervals peak at a maximal rate generally around the 15 th beat after standing and slows at around the 30 th beat in normal individuals. The 30:15 ratio is the ratio of the longest R-R interval after standing to the shortest R-R interval.

Hypotension predictability In Keyl’s study in the cardiac surgery patient with diabetes MOST of the patients showed a lack of HR variability preoperatively but did not have greater incidence of hypotension post induction. In Huang’s study measuring HR variability as an indicator of post induction hypotension 30% of DM patients needed vasopressors as opposed to only 7% of patients with normal Studies are inconclusive but warn of greater incidence of hypotension in the diabetic population with autonomic dysfunction. Denerved Rabbits.

What we can do about it. Greater awareness of the potential for post induction hypotension in the diabetic population as in those on chronic ACEi therapy. Same mechanism kind of. Not able to vasoconstrict appropriately besides being dry as popcorn. Fluid therapy prior to induction in the diabetic population – consider it. Vasopressor therapy for those not able to mount an appropriate vasoconstriction response – DM pts. Further study applies always.

References Ellis, H. Anatomy For Anesthetists. 8th ed. Oxford, England. Blackwell Publishing: Huang, C., & Kuok, C. Pre-operative measurement of heart rate variability predicts hypotension during general anesthesia. Acta Anaesthesiology Scandinavia 2006; 50: Keyl, C., & Lemberger, P. Cardiovascular autonomic dysfunction and hemodynamic response to anesthetic induction in patients with coronary artery disease and diabetes mellitus. Anesthesia & Analgesia 1999; 88: Morgan, E., Mikhail, M., & Murray, M. Clinical Anesthesiology 4th ed. New York, NY. McGraw Hill Companies Stoelting, R., & Miller, R. Basics of Anestesia 5 th ed. New York, NY. Churchill Livingstone Elsevier 2007