Factors that can affect ANS

A change in the heart rate from start to finish of the test can distort the reading. Hence, we recommend a 10 minutes settling period prior to measurement. A standing posture increases the LF component in HRV due to sympathetic predominance, whereas supine posture enhances the HF vagal component. Controlled respiration at frequencies less than normal breathing rate enhances vagal modulation of HRV. Respiratory sinus arrythmia (vagal tone) increases during the night and decreases in the morning.

In contrast, the baroreflex mediated HRV decreases during the night and exhibits a rapid early morning increas. During sleep there is a higher sympathetic tone during REM sleep and a higher vagal tone during non-REM sleep. The latter is probably due to slower and more regular breathing in non-REM sleep. Vagal tone was significantly decreased during continuous performance suggesting that attending to environmental demand would lead to a decreased in vagal tone.

Effects Of Drugs

Angiotensin converting enzyme inhibitors augment low frequency HRV in dogs (Akselrod et al 1985). Several studies have shown that tricyclic antidepressants decrease HRV, whereas paroxetine and fluvoxamine (newer selective serotonin reuptake inhibitors) did not (Rechlin 1994; Jacobsen et al 1984). It is likely that the suppressant effect of the tricyclic antidepressants is mediated by their anti-cholinergic effects. One would predict that any drug having this effect should reduce HRV due to inhibition of vagal tone.

Beta-blockers such as propranalol have been found to reduce the low frequency component of HRV without influence on high frequency (Akselrod et al 1985). In normotensive adults, atenolol augments the vagal component of HRV (Cook et al 1991). In patients with hypertension atenolol was found to reduce both high and low frequency components of HRV. (Guzzetti et al 1988). A similar decrease in sympathetic activity was seen in post myocardial infarction (MI) patients with metoprolol (Bekheit et al 1990).

Calcium-channel blockers have various effects on HRV. While diltiazem reduces low frequency HRV to the same extent as beta-blockers in post MI patients, nifedipine does not. (Bekheit et al 1990). This may explain the reduction of mortality post-MI by beta-blockers and diltiazem but not calcium antagonists, such as nifedipine (Muller et al 1984; Multicentre diltiazem post-infection trial research group 1988).

Other drugs that negatively influence HRV, are sedatives, analgesics and anaesthetics. The HRV reduction by these drugs is thought to be established predominantly through central nervous system depression. Diazepam and other benzodiazepines attenuated the vagal component of HRV and are thought to do so by a CNS action at the GABA receptor (Adinoff et al 1992; Halliwill and Billman 1992).

HRV decreases with age with attenuation of the vagal component in adults (Shannon et al 1987; Lipsitz et al 1990). Others have shown more pronounced HRV decline in the low frequency band (Jennings & Mack, 1984). However, the ratio between high and low frequency HRV appears to remain stable with age (Conny et al 1993).

Increased levels of fitness were found to be associated with increased HRV in 15-83 year olds compared with sedentary controls (De Meersman 1993). In athletes at rest the vagal component of HRV was higher and the low frequency (sympathetic) band lower. Furthermore the LF/HF ratio returned to pre-exercise levels within 5 minutes in athletes compared with 15 minutes to notice any decrease in this ratio in controls.

Patients with congestive heart failure and coronary artery disease had an increased LF/HF ratio due to attenuated vagal tone and augmented sympathetic tone (Sopher et al, 1990). In essential hypertension an enhanced sympathetic activity and reduced vagal activity has been found (Goldstein 1983).

Patients with polyneuropathy due to diabetes mellitus, chronic alcoholism or Guillain-Barré Syndrome also have decreased HRV.

In diabetics there is evidence that parasympathetic damage occurs more commonly than sympathetic damage and this may occur before clinical symptoms of neuropathy are evident. (Conny et al 1993). Diabetic patients with chronic renal failure have a strong reduction in HRV at all frequency ranges, predominantly of the PS (Zoccali et al 1982; Axelrod et al 1987; Cloarec-Blanchard et al 1992).

Smoking is associated with an acute and transient decrease in cardiac vagal tone. Heavy smoking can cause long term decreases in cardiac vagal function (Hayano et al 1990).

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