Targeting the autonomic nervous system: measuring autonomic function and novel devices for heart failure management.

Neurohumoral activation, in which enhanced activity of the autonomic nervous system (ANS) is a key component, plays a pivotal role in heart failure. The neurohumoral system affects several organs and currently our knowledge of the molecular and systemic pathways involved in the neurohumoral activation is incomplete. All the methods of assessing the degree of activation of the autonomic system have limitations and they are not interchangeable. The methods considered include noradrenaline spillover, microneurography, radiotracer imaging and analysis of heart rate and blood pressure (heart rate variability, baroreceptor sensitivity, heart rate turbulence). Despite the difficulties, medications that affect the ANS have been shown to improve mortality in heart failure and the mechanism is related to attenuation of the sympathetic nervous system (SNS) and stimulation of the parasympathetic nervous system. However, limitations of compliance with medication, side effects and inadequate SNS attenuation are issues of concern with the pharmacological approach. The newer device based therapies for sympathetic modulation are showing encouraging results. As they directly influence the autonomic nervous system, more mechanistic information can be gleaned if appropriate investigations are performed at the time of the outcome trials. However, clinicians should be reminded that the ANS is an evolutionary survival mechanism and therefore there is a need to proceed with caution when trying to completely attenuate its effects. So our enthusiasm for the application of these devices in heart failure should be controlled, especially as none of the devices have trial data powered to assess effects on mortality or cardiovascular events.