By Sheri Colberg, PhD
Diabetes-related damage to the central nervous system (autonomic neuropathy) can result in silent ischemia, hyperthermia, or lightheadedness with standing and during exercise.
The presence of cardiac autonomic neuropathy (CAN) impairs exercise tolerance and lowers maximal heart rate4. Slower heart rate recovery after physical exertion is associated with greater mortality risk. Moderate-intensity aerobic training can improve autonomic function and shift the autonomic balance back toward more balance between the sympathetic and parasympathetic function in individuals with and without CAN2,5,6. In fact, regular exercise training increases heart rate variability, suggesting that there is a shift in the cardiac sympathovagal balance in favor of parasympathetic dominance in individuals with diabetes. Long-term physical training may be an effective means to reverse the autonomic dysregulation seen in T2D5,6. Improvements, however, may be evident only following acute submaximal exercise7.
Physical Activity Precautions and Recommendations
When autonomic dysfunction in any form is present, a conservative approach to exercise is recommended. Given the likelihood of silent ischemia, resting and exercise heart rate alterations, and blood pressure abnormalities, individuals with CAN should have physician approval and possibly undergo stress testing to screen for cardiovascular abnormalities before commencing exercise2.
Although most aerobic and resistance activities are suitable for individuals with autonomic dysfunction, when certain symptoms are present, they are advised to avoid rapid changes in movement that may result in fainting and to engage in longer warm-up and cooldown periods, especially for more intense resistance or aerobic training. If postural hypotension is present, inadequate heart rate and blood pressure and responses may occur with incremental work; therefore, these individuals should focus on lower intensity activities in which mild changes in both are more easily tolerated and that lessen ventricular ectopy.
Exercise Intensity and Duration
For individuals with autonomic neuropathy, increases in physical activity levels must be approached with caution because of the role of the autonomic nervous system in hormonal and cardiovascular regulation during exercise. Furthermore, exercise intensity should be monitored by means other than heart rate alone (such as subjective ratings of perceived exertion) because it may no longer rise as much as expected or be the best way to monitor intensity unless maximal heart rate is determined8. Intensity may be accurately prescribed using the heart rate (HR) reserve method (a percentage of the difference between maximal and resting HR, added to the resting value) to approximate oxygen consumption during submaximal exercise with maximal HR directly measured, rather than estimated, for better accuracy2,8. In addition, perceived exertion ratings may be used8. Physical stamina required to engage in longer duration activities may be compromised; therefore, duration of physical activity should progress slowly.
Exercise with Cardiac Symptoms
When CAN is present, the heart rate response is abnormal at rest, when standing, and when during strain related to holding the breath (Valsalva maneuver)8. Blood pressure responses can be abnormal when changing positions or performing isometric exercise. Moreover, the potential for exercise-related dehydration is a concern, as is impaired thermoregulation during activities in environmental extremes, and extra fluids may need to be consumed to protect against both dehydration and hyperthermia. Care must be taken with all components of the exercise prescription.
Exercise with Orthostatic Hypotension
When orthostatic hypotension is present, individuals may experience more erratic blood pressure responses to physical activity. Accordingly, they would benefit from monitoring their blood pressure during physical activity to determine whether different postures (e.g., sitting, standing, reclining, supine) affect their symptoms and ability to exercise9. Dehydration may exacerbate symptoms, particularly with sweating and fluid losses during exercise, so ensuring adequate hydration at all times is critical as well.
Exercise with Gastroparesis
The presence of any of the symptoms of gastroparesis can make physical activity more difficult to perform. Medications and foods must be balanced as part of the exercise prescription to minimize the impact of these symptoms10. Eating large meals before exercise should be avoided, as it could result in delayed emptying of food; therefore, only small food portions before exercise are recommended. Individuals should use rapidly absorbed glucose tablets to treat hypoglycemia and when blood glucose levels decrease to 100 mg/dl to prevent severe hyperglycemia.
Table 19.3. Exercise Recommendations for Autonomic Neuropathy