Page 412 - Cardiac Nursing

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CHAPTER
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H H H H Heart Rate Variability
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Diana E. McMillan / Robert L. Burr
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He ar ra te v ar ia bi li ty ( ( HR V) ) i th e be at t o be at e undderstandinng of the phhy isiological mecha inisms associated witth
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Heartt rate variability (HRV) is he beat-to-beat variation of thhe
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cardiac cycle that results, in large part, from the interaction of sp specific HRV measures, anaalysiis of HRV provides significaantt clin-
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sympathetic and parasympathetic inputs to hthe sinus nodde. he ic ical predictive usefulness wiithin cardiac care.
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te term “arrhythmia” fofttenn carriess negative connotationss, and many y
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serious disturbances of heart rhythm and waveform morphology
are malignant. However, some variia ion in he ime between suc- HRV MEASUREMENT
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cessive beats is normal, reflecting a healthy heart and healthy au-
tonomic nervous system (ANS). A moderate amount of respira-
tory sinus arrhythmia (RSA), for example, is viewed as evidence of HRV measures are statistical or mathematical summaries of
good cardiovascular health. In addition to short-term or beat-to- within-subject variation in beat-to-beat heart period or instanta-
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beat variation, a healthy individual also exhibits a marked circa- neous heart rate. This section summarizes the principles behind
dian or 24-hour variation in heart rate. some of the more common HRV measures.
Measures of HRV provide clinicians and researchers with a
noninvasive, practical, reproducible, sensitive, and dynamic in- General Considerations
sight into the autonomic neural regulation of the heart. These
measures are increasingly popular in cardiac care and are recog- The current diversity of HRV measures and nomenclature is par-
nized as important diagnostic tools for risk identification in a wide tially caused by the relative novelty and rapid proliferation of these
range of cardiovascular conditions and health conditions that pre- methods. It also reflects simultaneous independent development
dispose cardiac complications. in several distinct disciplines by clinical researchers with very dif-
This chapter provides a basic overview of the mechanisms of ferent purposes and very different typical sources of heart rhythm
HRV, the approaches used in measuring HRV, and guidance for information. Most HRV measures are so strongly correlated with
the interpretations of these measurements. Current research re- each other that they are nearly redundant. However, no one sub-
lated to HRV patterns in common cardiovascular conditions and set of HRV measures so consistently outperforms all the others, in
in health conditions predisposing cardiac complications is pre- all circumstances, that clear choices can be made. There have been
sented. General health history factors that can influence HRV pat- several attempts at standardization of HRV measures and nomen-
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terns are discussed. The chapter concludes with a brief review of clature, but the recommendations have not been universally ac-
pharmacological and nonpharmacological interventions and their cepted, particularly in the interdisciplinary literature relevant to
impact on HRV patterns. nursing.
Practically, the instantaneous heart period must be defined from
a series of discrete events corresponding to the beating of the heart.
This discrete event series itself is usually derived from fiducial fea-
MECHANISMS OF HRV tures of the raw electrocardiograph (ECG) waveform. The arrival
time of a beat, in particular the time interval from the previous beat,
The beat-to-beat variation of the cardiac electrical signal expressed provides us with somewhat irregularly spaced information about
in normal sinus rhythm is termed HRV and is considered to be an short-term fluctuations in heart rhythm, and by inference, the dy-
index of ANS balance and imbalance. The time between succes- namic autonomic control of the heart. 4,5
sive beats is governed by the intrinsic firing rate of the sinoatrial Despite the variety of purposes motivating HRV analysis, the
(SA) node and the modulation of the SA node firing rate by input primary goal is usually to compute some within-subject or within-
from the ANS. The input of the ANS is based on the relative con- condition indices of heart rhythm variation to make some quali-
tributions of the two ANS branches: the sympathetic nervous sys- fied inferences, not about the heart organ itself, but about the
tem (SNS) and the parasympathetic nervous system (PSNS). sympathetic and parasympathetic neural traffic impinging on the
Thus, HRV does not reflect absolute sympathovagal input, but SA node of the heart. Thus it would be ideal to base the definition
rather the relative dominance and interaction of these two ANS of the interbeat heart period on the interval between adjacent P
branches. PSNS activity normally dominates under conditions of waves to reflect as closely as possible the statistics of the firing of
rest and restoration. SNS activity predominance is associated with the SA pacemaker node. 6,7 However, the P-P interval is much
increased physiological arousal. harder to empirically define than the R-R interval, particularly
Complicating the interpretation of HRV indices aimed at iden- from noisy low-frequency Holter recordings of ambulatory sub-
tification of these respective ANS inputs are neural and nonneural jects. Most HRV studies, and essentially all of those performed us-
factors that can modify the SA node firing rate. These factors in- ing ambulatory ECG monitoring technology, use the R-R interval
clude the central nervous system integration of cardiac neural input, as the fundamental metric. Although it is conventional to speak of
positive feedback from sympathetic afferents, and negative feedback heart period as specific to a particular beat, an R-R interval is ac-
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from baroreceptors and vagal afferents. Despite our incomplete tually a measurement of the time interval between the R waves of
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