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from the radial artery by a tonometer (Colin Medical
Instruments) at 1000 Hz. Data were channelled into
a bed-side laptop after A/D-conversion and stored
for analysis. Simultaneously, breathing excursions
and a standard ECG were monitored. Data were
sampled for a 30-min period the day before surgery,
24h and seven days after surgery on the ICU. Care
was taken to perform the measurements during the
same time of the day in each patient. From the
recorded data the beat-to-beat intervals as well as the
beat-to-beat systolic and diastolic values were
extracted; premature beats, artifacts and noise were
excluded using an adaptive filter considering the
instantaneous variability.
Baroreflex Sensitivity (BRS): Dual Sequence
Method (DSM).
Using the DSM, the most relevant
parameters for estimating the spontaneous baroreflex
(BR) are the slopes as a measure of sensitivity. The
DSM is based on standard sequence methods with
several modifications: Two kinds of BBI responses
were analyzed: bradycardic (an increase in systolic
blood pressure (SBP)) that causes an increase in the
following beat-to-beat-intervals (BBI) and
tachycardic fluctuations (a decrease in SBP causes a
decrease in BBI). Both types of fluctuations were
analyzed both in a synchronous and in a 3-interbeat-
shifted mode. The bradycardic fluctuations primarily
represent the vagal spontaneous BR analysis of the
tachycardic fluctuations represent the delayed
responses of heart rate (shift 3) assigned to the
beginning slower sympathetic regulation. The
following parameter groups are calculated by DSM:
(1) the total numbers of slopes in different sectors
within 30 min; (2) the percentage of the slopes in
relation to the total number of slopes in the different
sectors; (3) the numbers of bradycardic and
tachycardic slopes; (4) the shift operation from the
first (sync mode) to the third (shift 3 mode)
heartbeat triple; and (5) the average slopes of all
fluctuations. DSM parameters are defined as
described by Malberg et al (European Heart Journal,
1996).
Heart rate Variability (HRV).
Respecting the
suggestions by the Task Force HRV (Malberg et al.,
2003), the following standard parameters are
calculated from the time series: MeanNN (mean
value of normal beat-to-beat intervals): Is inversely
related to mean heart rate. sdNN (standard deviation
of intervals between two normal R-peaks): Gives an
impression of the overall circulatory variability.
Rmssd (root mean square of successive RR-
intervals):
600
650
700
750
800
850
85 105 125 145
SBP [mmHg]
RRI [ms]
Avera
ge
Slope
Slope
s>20
ms/m
mHg
Slope
Figure 1: Schematic representation of the two main
baroreflex parameters estimated by the Dual Sequence
Method: the average slope (dotted line) of all baroreflex
sequences as well as the total number of baroreflex slopes
above 20 ms/mmHg (thick lines). The thin lines symbolize
all baroreflex slopes below 20 ms/mmHg.
Higher values indicate higher vagal activity.
Shannon (the Shannon entropy of the histogram):
Quantification of RR-interval distribution. Apart
from the time-domain parameters mentioned above,
the HRV analysis focused on high-frequency
components (HF, 0.15-0.4 Hz, high values indicate
vagal activity) and low-frequency components (LF,
0.04-0.15 Hz, high values indicate sympathetic
activity). The following ratios were considered: LFn
– the normalized low frequency (LFn=LF/(LF+HF)),
HP/P - the to the total power P normalized high
frequency as well as LP/P - the P-normalized low
frequency.
Nonlinear Dynamics.
New parameters can be
derived from methods of nonlinear dynamics, which
describe complex processes and their interrelations.
These methods provide additional information about
the state and temporal changes in the autonomic
tonus. Several new measures of non-linear dynamics
in order to distinguish different types of heart rate
dynamics as proposed by Kurths were used. The
concept of symbolic dynamics is based on a coarse-
graining of dynamics. The difference between the
current value (BBI or systolic blood pressure) and
the mean value of the whole series is transformed
into an alphabet of four symbols (0; 1; 2; 3).
Symbols '0' and '2' reflect low deviation (decrease or
increase) from mean value, whereas '1' and '3' reflect
a stronger deviation (decrease or increase over a
predefined limit, for details see Voss et al.
Subsequently, the symbol string is transformed to
'words' of three successive symbols explaining the
nonlinear properties and thus the complexity of the
system.
DELAYED RECOVERY OF CARDIOVASCULAR AUTONOMIC FUNCTION AFTER MITRAL VALVE SURGERY -
Evidence for Direct Trauma?
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