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C HAPTER 1 7 / Heart Rate Variability 395
enhances vagal activity (increased HF, reduced LF, increased Depression
HF:LF). 107 Depression is a significant risk factor for the development of car-
Alcohol restriction also impacts HRV patterns. Compared diovascular disease and for the increased risk of cardiac mortality
with their usual drinking patterns (70.1 4.6 mL/day), 3 weeks and morbidity in those with existing cardiovascular conditions (see
of alcohol restriction (19.1 2.5 mL/day) in habitual drinkers reviews by Carney et al. 118 and Musselman et al. 119 ) One possible
led to reduced heart rate, increased HRV, and increased indices of explanation is that depressive disorders may stimulate harmful in-
PSNS activity. 108 Findings support ANS activity recovery even af- fluences on the ANS by increasing SNS activity and/or reducing
ter chronic high alcohol consumption. vagal tone. 118 Consistent with this hypothesis, lower HRV and re-
duced vagal tone (low HF) are found in depressed compared with
Caffeine nondepressed individuals with CHD 120 and those with acute coro-
Caffeine causes an acute increase in systemic vascular resistance nary syndrome. 121 The severity of depression in cardiac patients is
through the blocking of central antiadrenergic adenosine recep- negatively associated with HRV. 120,122–124 The Enhancing Recov-
tors. This acute increase in blood pressure is compensated for by ery in Coronary Heart Disease study found that low HRV was a sta-
a brief reduction in heart rate and a probable increase in vagal pre- tistically significant mediator of the effect of depression on survival
dominance. This prediction is in keeping with findings of a sym- after acute MI. 124 Cardiac care patients successfully treated for se-
pathetic rebound (higher LF:HF) in people who consume coffee vere depression exhibit enhanced HRV and reduced heart
after at least 2 hours of abstinence from caffeine compared with rate. 121,122 Collectively, these findings underscore the need for
individuals who do not consume caffeine regularly. 81 In patients prompt assessment and treatment of depression in cardiac care.
with Type 1 diabetes, 2 weeks of moderate (500 mg/day) caffeine
intake enhanced parasympathetic activity, potentiating reduced Diabetes
cardiovascular risk. 109 More work is needed to fully determine the Autonomic neuropathy is a common and serious complication
short-term and long-term impact of caffeine on HRV. of diabetes. Measures of HRV can aid in the early diagnosis and
treatment of this ANS dysfunction and are recommended as
Acute and Chronic Conditions components of standard diabetic care. Findings from the Fram-
ingham Heart Study indicate that individuals with diabetes mel-
Pain litus have significantly lower HRV (SDNN), LF and HF power,
Acute pain is clinically seen to increase heart rate and decrease and LF:HF compared to those with normal fasting glucose
HRV. Interventions that foster good pain management are ex- (p ( .005). 125 Results also show a strong negative association
pected to enhance HRV and reflect a decreased activation of the between these HRV measurements and the level of fasting blood
SNS. Exploration of HRV patterns related to chronic pain is lim- glucose across all subjects. These findings support a blunting of
ited and frequently confounded by cardiac pathology. HRV pat- ANS activity and a relative SNS dominance in patients with di-
terns across sleep cycles are not different between patients with abetes and in those with impaired blood glucose regulation.
chronic low back pain and healthy controls. 110 Standard measures Not surprisingly, patients with diabetes exhibiting low HRV
of HRV are not significantly different for patients characterized by have an increased risk for CHD. 126 Based on a 9-year follow-up,
either successful or unsuccessful pain reduction after treatment of subjects with diabetes with impaired autonomic function have
a herniated disc. 111 These findings suggest that pain is not a mod- nearly double the risk of mortality than do the general popula-
ifying factor for HRV. More work is needed, however, before con- tion 127 although others have reported only trends toward in-
clusions regarding the relationship between HRV patterns and creased mortality risk in patients with diabetes with low HRV as
chronic pain can be drawn. measured by time domain approaches (minimum–maximum RR
difference; SDANN, CV). 128
Brain Injury Lowered HRV has been linked to inflammation, and specifi-
The ability to perform continuous and noninvasive assessments of cally, increased plasma concentrations of C-reactive protein in the
ANS function with HRV has made this a valued additional ap- general population 129 and with both C-reactive protein 130 and in-
proach for neurological injury assessment and prognosis. terleukin-6 131 in patients with Type 1 diabetes.
Traumatic brain injury is associated with ANS dysfunction Overall, diabetes and poor glucose regulation and accompany-
marked by reduced HRV and reduced LF and HF power. 112 ing inflammatory factors appear to signal serious risk for the de-
Biswas et al. 113 studied children with brain injury. Factors that velopment and/or exacerbation of cardiovascular conditions.
predicted poorer outcome, such as a low Glasgow Coma Scale
score (3–4 versus 5–8), higher intracranial pressure ( 30 Chronic Obstructive Pulmonary Disease
mm Hg), and decreased cerebral perfusion pressure ( 40 mm Individuals with COPD exhibit ANS disturbance as measured by
Hg), were associated with PSNS dominance as reflected by low HRV, although the nature of the dysfunction reported is not con-
LF:HF. Profound vagal dominance is characteristic of patients sistent. 132–134 Stein et al. 132 studied young PiZ 1 -antitrypsin-
with brain injury who progress to brain death. 113,114 deficient COPD patients and found significant decreases in al-
Stroke patients exhibit cardiovascular regulatory impairment most all HRV parameters. Severity of ANS disturbance was
in both ANS branches. Total power, LF power, and HF power are directly related to clinical severity as measured by forced expira-
all reduced, 115–117 although an increase in the LF:HF supports tory volume in one second (FEV 1 ). Vagal activity was signifi-
relative sympathetic dominance. 117 Injury in the region of the in- cantly reduced during the daytime only. Other researchers report
sula (especially the right) is strongly associated with ANS instabil- that compared with control subjects, COPD patients exhibit in-
ity and sudden death. 117 Within the brainstem, only medullary creased HF activity during the daytime 134 or abnormally reduced
stroke injury is associated with depressed ANS activity (reduced nocturnal vagal activity. 133 Stein et al. 132 studied younger pa-
LF and HF power). 116 tients with more severe cases and, unlike the two other studies