How should autonomic functions be evaluated pre-operatively?

Pre-operative Evaluation of Autonomic Functions

Pre-operative autonomic function testing should be performed in a dedicated laboratory by specialists trained in autonomic function testing and interpretation, with standardized tests including heart rate variability to deep breathing, Valsalva maneuver, and active orthostatism to detect cardiac autonomic neuropathy that may increase perioperative hemodynamic instability risk. 1, 2

Core Autonomic Function Tests

• Cardiovagal function tests should include heart rate variability during deep breathing, active orthostatism, and Valsalva maneuver, which primarily assess parasympathetic function 2
• Tests should be performed with continuous beat-to-beat blood pressure and ECG monitoring in a quiet environment with temperature controlled between 21-23°C 2, 1
• Patients should fast for 3 hours before testing and avoid nicotine, caffeine, theine, or taurine-containing drinks on the day of examination 2, 1
• Test results should be interpreted according to age-specific reference values due to physiological reduction in heart rate variability with aging 2, 1

Patient Selection for Pre-operative Autonomic Testing

• Testing is particularly indicated for Type 1 diabetic patients with disease duration ≥5 years and all Type 2 diabetic patients, especially those with microangiopathic complications 2
• Patients with symptoms suggesting orthostatic or post-prandial hypotension or episodes of serious unfelt hypoglycemia should undergo testing 2
• Patients with known neurodegenerative diseases, Parkinsonism, or peripheral neuropathies benefit from pre-operative autonomic evaluation 1
• Patients with prolonged QTc interval (>440 ms) on ECG should undergo autonomic testing 2

Clinical Implications of Autonomic Dysfunction

• Cardiac autonomic neuropathy (CAN) is associated with increased risk of perioperative hemodynamic instability and vasopressor requirements 2, 3
• Preoperative decrease in respiratory heart rate variability (respiratory sinus arrhythmia) predicts perioperative hemodynamic instability 2
• Patients with autonomic dysfunction may have unpredictable and paradoxical responses to anesthetic agents 3
• Both general and regional anesthesia significantly affect sympathetic tone, with greater effects in patients with pre-existing autonomic dysfunction 2

Additional Tests for Comprehensive Evaluation

• If CAN is detected, ambulatory blood pressure monitoring should be performed to identify nocturnal non-dipping patterns 2
• QT interval should be measured on standard ECG; if prolonged, 24-hour ECG monitoring is indicated to detect paroxysmal ventricular rhythm disorders 2
• Sudomotor function can be evaluated with quantitative sudomotor axon reflex test (QSART) or thermoregulatory sweat test to assess postganglionic sympathetic function 4, 5
• Adrenergic function should be evaluated through blood pressure and heart rate responses to Valsalva maneuver and head-up tilt 4, 6

Perioperative Management Considerations

• Drugs that may induce orthostatic hypotension should be avoided in patients with detected CAN 2
• Patients with autonomic dysfunction are at increased risk when receiving phenylephrine, as noted in FDA labeling under "Risk in Patients with Autonomic Dysfunction" 7
• Perioperative vasopressor requirements correlate with the degree of dysautonomia, requiring careful titration 2
• Patients with CAN have increased risk of myocardial infarction (which may be painless), increased cardiovascular events, and sudden death 2

Testing Pitfalls and Caveats

• Avoid testing during acute illness, stressful conditions, fever, infection, or dehydration 1
• Testing should not be performed during hypoglycemia or marked hyperglycemia 1
• Tests should be performed at least 2 hours after short-acting insulin administration 1
• Consider appropriate washout of interfering drugs, particularly diuretics, sympatholytic agents, and psychoactive drugs 1
• No single autonomic function test provides comprehensive assessment; different clinical questions require different test batteries 2, 1