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262 Heart Rate Variability in Ischemic
Heart Disease and Diabetes

iologic properties of the myocardium and promote (normal to normal RR) intervals over a 24-hour pe-
arrhythmogenesis, regardless of the mechanism in- riod reflects total HRV, but in the literature SDNN
volved (enhanced automaticity, triggered activity, or is often reported over a brief (often five minutes)
reentry). Some of the arrhythmogenic effects of high measuring period.
sympathetic activity are related to the adverse effects • SDANN in milliseconds - Standard deviation of
of tachycardia, such as ischemia, while others result the average NN intervals for all of the five-minute
from heterogeneity of ventricular repolarization.
intervals in a 24-hour continuous ECG recording.
Increased sympathetic activity can cause tachycar- However, this measure is not meaningful in a brief
dia, which can result in myocardial ischemia, but recording.
myocardial ischemia, independent of the underlying • pNN50 and related - Percent NN intervals >50 ms
mechanism, usually increases sympathetic activity different from the prior interval is often available
while decreasing efferent vagal activity. Additional- on commercial Holter HRV reports. pNN50 is ex-
ly, myocardial ischemia triggers a release of norepi- tremely sensitive to uneven beat detection and/
nephrine from epicardial sympathetic nerves and an or incorrect beat morphology labeling.
increase in its local myocardial concentrations due
to the high extracellular potassium concentrations • rMSSD in milliseconds - Root mean square of dif-
in the ischemic regions, resulting in regional depo- ferences between successive NN intervals; essen-
larization and repolarization, which is an important tially the average absolute value of the change in
precondition for the development of reentrant activity NN interval between beats. rMSSD is also sensi-
and the precipitation of ventricular fibrillation tive to uneven beat detection and/or incorrect beat
morphology labeling.
The parasympathetic nervous system: The parasym- Frequency domain HRV
pathetic nervous system, through vagal innervation,
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may exert important antiarrhythmic effects by reduc- • Total power (TP) in ms – TP captures the total vari-
ing the heart rate and counteracting the proarrhyth- ance in HRV.
mic effects of sympathetic nervous system activity • Ultra-low frequency power (ULF) in ms – ULF cap-
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. Furthermore, the parasympathetic nervous system tures the magnitude of underlying rhythms in heart
plays a large role in regulating the inflammatory re- rate at frequencies of every five minutes to once in
sponse, and RR variability is inversely related to the 24 hours.
production of many inflammatory markers
• Very low frequency power (VLF) in ms – VLF cap-
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tures the magnitude of underlying oscillations in
HRV METHODOLOGY, DEFINITIONS, AND the heart rate pattern at frequencies between ev-
NORMAL VALUES — ery 25 seconds and every five minutes (0.003 to
Heart rate variability (HRV) is derived from intervals 0.04 Hz).
between normal heart beats (NNs) and can be quan- • Low-frequency power (LF) in ms – LF captures the
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tified by many methods : magnitude of heart rate oscillations in the range of
• Time domain measures three to nine cycles per minute (0.04 to 0.15 Hz).
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• Frequency domain measures • High-frequency power (HF) in ms – HF captures
heart rate oscillations in the range of 9 to 24 cy-
• Heart rate turbulence cles per minute, which is the range of typical adult
• Nonlinear/complexity-based measure respiratory frequencies (0.15 to 0.40 Hz).
Time domain HRV • LF/HF ratio – (unitless) Often referred to as the
“sympathovagal” balance.
• Average NN in milliseconds (and/or heart rate in Heart rate turbulence: Heart rate “turbulence” (HRT)
beats per minute) - Not a measure of variability per is a relatively recent HRV parameter that evaluates
se, but average heart rate is an important marker the oscillation (shortening then lengthening) in NN
for cardiac autonomic function that is available intervals associated with a temporary loss of cardi-
from any commercial software. Daytime and ac output in the presence of a ventricular premature
nighttime average heart rates are also generally beat (VPB) . Two measures have been calculated,
available, and often hourly heart rates can be turbulence onset (TO) and turbulence slope (TS), us-
obtained as well.
ing a composite of the responses to all VPBs on the
• SDNN in milliseconds - Standard deviation of NN recording.

GCDC 2017