Background-Heart rate variability is thought to measure autonomic modu
lation, but the relation has never been demonstrated directly in human
s. Aim-To test the hypothesis that increased low frequency heart rate
variability reflects sympathetic reinnervation after cardiac transplan
tation. Patients-24 cardiac transplant recipients at the time of routi
ne surveillance coronary angiography two Or more years after cardiac t
ransplantation, and 10 controls with normal coronary arteries undergoi
ng angiography for investigation of chest pain. Setting-Regional cardi
othoracic centre. Methods-Sympathetic effector function at the sinus n
ode was assessed by measuring the fall in cycle length for two minutes
after injection of tyramine to the artery supplying the sinus node. H
eart rate variability was measured from three-minute RR interval seque
nces at rest, during metronomic respiration, and before and after atro
pine. Results-The logarithm of the low frequency component of heart ra
te variability during metronomic respiration was linearly related to t
he logarithm of the change in cycle length after injection of tyramine
(R-2 = 0.28, P = 0.007). Absolute units more accurately reflected sym
pathetic effector function than did normalised units or the ratio of l
ow frequency to high frequency. Atropine did not affect high frequency
heart rate variability in transplant recipients. Conclusions-The low
frequency component of heart rate variability is directly related to s
ympathetic reinnervation to the sinus node.