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Assessment of dCA during CPB
expressed as % change per mmHg. The phase lag between CBFV and ABP as
defined by phase angle shift is regarded to be the dominant and most relevant
parameter to quantify dCA 29. Theoretically, phase may range from –$ to +$
radians. In awake human in supine resting position, a positive phase angle
around 0.9 ± 0.3 indicates normal autoregulation 15. From the estimated trans-
fer function a step response can be calculated. According to the method intro-
duced by Tieckset al 50 dCA can also be evaluated by assessing the autoregula-
tion index (ARI). ARI is calculated by fitting the first 5 seconds of the step
response function resulting from the transfer function analysis in the same way
as described previously 15.
By changing paCO2 and measuring the mean CBFV at each level of paCO2, cere-
bral carbon dioxide reactivity can be evaluated. CO2R is expressed as the percent
change in SctO2, and blood flow velocity in the middle cerebral artery per mmHg
change in paCO2. The mean values of five minute recording of SctO2/CBFV at
each paCO2 level are used to calculate both absolute and percentage change in
CBFV and SctO2 using the formulae proposed by Kadoiet al 21.
SctO (40) / SctO (30)
CO2 RNIRS,hypo ? 2 2 μ100%
(40 / 30) μ SctO2 (40)
SctO (50) / SctO (40)
CO2 RNIRS,hyper ? 2 2 μ100%
(50 / 40) μ SctO2 (40)
CBFV (40) / CBFV (30)
CO2 RTCD,hypo ? μ100%
(40 / 30) μ CBFV (40)
CBFV (50) / CBFV (40)
CO2 RTCD,hyper ? μ100%
(50 / 40) μ CBFV (40)
where SctO2and CBFV are the values measured for the specified level of paCO2.
Statistics
Statistic analyses were performed using the Statistical Package for Social Sci-
ences (SPSS) version 16.0 for Windows. Data normality was tested using the
Kolgomorov-Smirnov test. If normally distributed, the paired Student’s t-test
with Bonferroni correction was used to compare results between different levels
of paCO2 otherwise the Wilcoxon signed-rank test was used.
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