Page 416 - Cardiac Nursing

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392 P A R T III / Assessment of Heart Disease
low HRV, nonsustained ventricular tachycardia, and baroreflex Although the derangement of ANS function in heart failure is
sensitivity led to a 22-fold increase in the risk for sudden death or well recognized, the ability of HRV measures to aid in the risk as-
40
sustained VT. Findings from the Women’s Health Initiative, a sessment of patients with this disorder is mixed. One of the fac-
prospective, population-based study of postmenopausal women, tors complicating the interpretation of HRV patterns in heart
identify a reduction in HRV as one of the five major ECG abnor- failure is the impact of respiratory patterns. Cheyne–Stokes respi-
mality predictors for mortality. 25 ration and oscillatory breathing pattern (characterized by cyclic
Sudden unexpected death syndrome is a leading cause of death changes in ventilation without apnea) are common in CHF and
in young men of Southeast Asian descent. Although nighttime are associated with significant reductions in mixed SNS and PSNS
ventricular fibrillation typically precedes the cardiac arrest, the activity (low LF power). 41 Interestingly, this severe LF power de-
pathophysiology of this disorder is not known. HRV analysis in- crease contrasts with the increase in SNS activity found in patients
dicates reduced 24-hour HRV and reduced circadian variation in with obstructive sleep apnea, hypoxia, and hypercapnia. For pa-
HRV, with very low-nighttime HRV in survivors of this syndrome tients with obstructive sleep apnea, the LF:HF is considered the
compared with controls. 26 best estimator of the apnea/hypopnea index, a measure of disor-
der severity. 42
Angina Despite these confounding factors, the use of HRV measures in
heart failure is reported to be helpful in the evaluation of risk for
27
Low HRV is associated with poor prognosis in stable angina and malignant cardiac events. HRV is a significant predictor for sudden
unstable angina. 19,28 Low HRV, including reduced total power death. 35–38 Based on a multivariate survival model, risk of sudden
37
and reduced HF, LF, and very low-frequency components, provide death in patients with CHF was strongly predicted by HRV. Re-
strong and independent predictors of cardiac death but not non- searchers collected ECG data during 8 minutes of controlled
fatal MI in stable angina pectoris. 27 Low HRV in patients pre- breathing and found that reduced sympathetic predominance (LF
2
senting with unstable angina increases the risk of either cardiac power 13 ms ) in patients with CHF was associated with a rela-
death or nonfatal MI within 12 months, 19 with patients having tive risk of 3.7 for sudden death compared to patients with sym-
SDNN, HF, and LF power values in the lowest quartile at in- pathetic input above this level. Of patients with CHF, those pa-
creased risk for in-hospital death. 28 tients presenting with values of SDNN less than 65.3 milliseconds
were reported to be at significantly greater risk for sudden death. 35
Hypertension In another study, mortality and hospitalization caused by deterio-
ration of CHF were predicted by HRV. In this case, SDNN ( 75
Individuals with HTN 29 and those at high risk for HTN 30,31 milliseconds) provided significant and independent predictive
exhibit abnormal HRV patterns. HRV is reduced in individuals value in addition to the standard risk indices of left ventricular
with essential HTN compared with healthy control subjects, as re- ejection fraction and peak oxygen intake. 36
flected in significantly reduced values for SDNN, SDANN, Another cardiac condition exhibiting disturbed ANS func-
32
pNN50, and rmsSD. These findings are consistent with results tioning is aortic regurgitation. Low SDANN significantly pre-
from the Framingham Heart Study (N 1,919), a prospective dicted risk of death or progression to aortic valve repair in a study
epidemiological study of coronary risk factors. Singh et al. 31 of 50 asymptomatic or minimally symptomatic patients with
found significant reductions in time domain measures of HRV in chronic severe aortic regurgitation. 43
men and women with HTN compared with normotensive sub-
jects. Hypertensive patients also exhibit a lower than normal vagal Cardiac Surgery, Heart
tone (low HF). 31,32 Transplantation, and Other
Some HTN studies indicate a significant increase in mixed Invasive Procedures
SNS and PSNS activity, reflected by increased resting LF
power. 32,33 However, results from the Framingham Heart Study Cardiovascular surgery has been shown to have an impact on
indicate reduced LF activity in patients with HTN and suggest HRV patterns. Results from the Cardiac Arrhythmia Suppression
that low vagal tone is a strong risk factor for the development of Trial indicate that HRV is significantly reduced in post-MI pa-
HTN in men. 31 Methodological differences in covariate adjust- tients after coronary artery bypass graft (CABG) surgery and that
ment may have contributed to inconsistent study findings. this reduction in HRV is not associated with increased mortality. 44
The normal circadian pattern of HRV is disturbed in patients This finding contrasts with the increased risk of mortality seen in
with HTN. In normotensive individuals, the nighttime fall in post-MI Cardiac Arrhythmia Suppression Trial patients who did
blood pressure is paralleled by a corresponding reduction in the not undergo CABG surgery but who did exhibit reduced HRV,
mixed PSNS and SNS activity marker LF. This nocturnal drop in specifically reduced SDANN. This important finding may help to
LF power is not as great in subjects with HTN. 29 explain the lower than predicted mortality rates seen in some post-
CABG surgery patients. Similar reductions in HRV were reported
Heart Failure in a Danish study of CABG patients without a recent MI and with
ejection fractions of 0.36 0.07. 45 These researchers found sig-
Patients with congestive heart failure (CHF) have reduced HRV. 34 nificantly reduced HRV immediately post-CABG and at the
The most consistently reported finding is a reduction in 6-month follow-up. Furthermore, improvement in myocardial
SDNN. 35–38 Some researchers report a significant early increase in function seen after surgery was not associated with post-CABG
sympathetic predominance (high LF:HF) using a paced canine measures of HRV.
model of CHF. 39 This initial SNS surge appears to be lost as the Heart transplant patients exhibit significantly reduced HRV
34
34
condition worsens. Reduced vagal activity (low HF) and reduced immediately after surgery. This reduction continues to be signif-
total power have also been reported in patients with CHF. 34 icantly reduced 2 years after surgery despite evidence of a return to