Complementary Alternative Cardiovascular Medicine

Chapter 8 / Meditation and CVD 127
unchanged in either longitudinal or acute TM studies. These include
testosterone, insulin, T3, T4, and luteinizing hormone (LH) (33,35).
Studies investigating the impact of meditation on neurotransmitters
suggest that the regular practice of meditation has a significant impact
on the sympathetic-adrenal medulla system. Michaels and colleagues
first reported a nonsignificant acute decrease in catecholamine levels
during TM (36). Subsequently, in an advancement of this research,
Infante and colleagues conducted a controlled study of catecholamine
plasma levels, which considered the impact of circadian cycles and time
of day. Infante and colleagues found significantly lower morning and
evening norepinephrine levels and morning epinephrine levels in the
TM group than in the control subjects (37). Another controlled study
investigating the urinary metabolite of serotonin, 5-hydroxyindoleaceatic
acid (5-HIAA) found significant increases after 30 min of practicing TM.
In addition, throughout the experiment, 5-HIAA levels were higher in
meditators than they were in rest subjects (38).
AUTONOMIC EFFECTS
There have been numerous studies investigating the acute and longterm
impact of meditation on autonomic activity. The overall results
indicate decreased activation and include such findings as increased
galvanic skin resistance (GSR), decreased spontaneous electrodermal
response (EDR), and decreased heart rate (8,9,27,28). A meta-analysis
of 31 studies comparing physiological differences between TM and rest
control groups indicated that those subjects who had been practicing TM
for a long time exhibited significantly lower baseline levels of spontaneous
skin resistance responses, respiration rate, heart rate, and plasma
lactate before meditation than comparison subjects did before rest (39).
Yet another group of studies investigated the impact of meditation on
recovery from stressful stimulation. Several researchers have reported
significantly faster habituation or return to baseline (as measured by
GSR, EDR, and heart rate) after exposure to stressful stimuli in meditators
as compared to controls (40,41).
HEMODYNAMIC EFFECTS
Hemodynamic variables, such as systolic and diastolic blood pressures
cardiac output, and total peripheral resistance (TPR) have also
been a focus of study regarding how they are affected by the practice of
meditation. Individuals who practice TM have lower resting systolic and
diastolic blood pressures than matched controls (42–44). Because blood
pressure is the product of cardiac output and TPR, changes in these
variables have also been studied during meditation. An investigation of