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AUTONOMIC DYSFUNCTION

Assessment of Autonomic Function

The increased operative risk of patients with autonomic dysfunction that may occur with aging and diabetes^[448] makes the diagnosis of autonomic neuropathy extremely important. A panel of five tests of cardiovascular function has been developed to evaluate autonomic function in diabetic patients.^[449] The tests include heart rate responses to the Valsalva maneuver, standing up, and deep breathing and blood pressure responses to standing up and sustained handgrip. The tests involving changes in heart rate measure injury to the parasympathetic system and precede changes in the measures of blood pressure that reflect sympathetic injury. Early autonomic dysfunction is defined as a single abnormal or two borderline-abnormal results on the tests involving changes in heart rate. Definite involvement comes when two of the tests of changes in heart rate are abnormal. Severe dysfunction is defined as abnormalities in the blood pressure assessments. The application of these standards requires that the investigator understand the proper techniques for performing the five tests and the expected results in patients without autonomic neuropathy ( Table 16-13 ). If patients demonstrate an increase in heart rate of more

TABLE 16-13 -- Noninvasive tests for assessing the autonomic nervous system

Clinical Examination	Technique	Normal Value
Parasympathetic
HR response to Valsalva	The seated subject blows into a mouthpiece (maintaining a pressure of 40 mm Hg) for 15 seconds. The Valsalva ratio is the ratio of the longest R-R interval (which comes shortly after the release) to the shortest R-R interval (which occurs during the maneuver).	Ratio of >1.21
HR response to standing	HR is measured as the subject moves from a resting supine position to standing. Normal tachycardic response is maximal around the 15th beat after rising. A relative bradycardia follows that is most marked around the 30th beat after standing. The response to standing is expressed as the 30:15 ratio and is the ratio of the longest R-R interval around the 30th beat to the shortest R-R interval around the 15th beat.	Ratio of >1.04
HR response to deep breathing	The subject takes six deep breaths in 1 minute. The maximum and minimum heart rates during each cycle are measured, and the mean of the differences (maximum HR-minimum HR) during three successive breathing cycles is taken as the maximum-minimum HR.	Mean difference >15 BPM
Sympathetic
BP response to standing	The subject moves from resting supine to standing, and the standing SBP is subtracted from the supine SBP.	Difference <10 mm Hg
BP response to sustained handgrip	The subject maintains a handgrip of 30% of the maximum squeeze for up to 5 minutes. The blood pressure is measured every minute, and the initial DBP is subtracted from the DBP just before release.	Difference >16 mm Hg
BP, blood pressure; BPM, beats per minute; DBP, diastolic blood pressure; HR, heart rate; SBP, systolic blood pressure.

than 20 beats/min with a vigorous handgrip, it can be assumed they are incompletely β-blocked. The simplicity and effectiveness of this clinical assessment has led to its use in the evaluation of patients with nondiabetic causes of autonomic dysfunction.