Departments of 1 Anaesthesiology and 2 Hygiene, Nara Medical University, 840 Shijo-cho, Kashihara, Nara, 634-8521, Japan
* Corresponding author. Present address: Department of Anaesthesiology, National Cardiovascular Centre, Fujishirodai 5-7-1, Suita, Osaka, 565-8565, Japan. E-mail: nkenji{at}mva.biglobe.ne.jp
Accepted for publication October 22, 2004.
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Abstract |
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Methods. Forty-two patients with pre-donated autologous blood were randomly assigned to receive propofol (Group P) or sevoflurane/nitrous oxide (Group S) anaesthesia. A fibreoptic catheter was placed in the jugular bulb to measure . A cerebral oximeter, INVOS 4100S was used to monitor
. Arterial and jugular bulb blood samples were drawn simultaneously at: (i) 10 min after the start of operation, (ii) after 400 ml of blood loss, (iii) after 800 ml of blood loss, (iv) just before the transfusion of pre-donated autologous blood, and (v) after 400 ml transfusion.
Results. Mean (SD) control values of in Group P were significantly lower than those in Group S (55 (8)% vs 71 (10)%, respectively; P<0.05), whereas there was no significant difference in control values of
between the two groups. During the operation, haemoglobin (Hb) concentrations significantly deceased in the both groups compared with control values (from 9.8 to 7.6 g dl1 in Group P and from 9.9 to 8.0 g dl1 in Group S). During a reduction in Hb concentration,
values remained unchanged in both groups, whereas
values significantly decreased in both groups (from 57 to 51% in Group P and from 59 to 52% in Group S).
Conclusion. The results indicated that, although the changes in and
during a reduction in haemoglobin concentration were similar under propofol and sevoflurane/nitrous oxide anaesthesia, the changes in
were not parallel to those in
. The discrepancy of the results in
and
may make the interpretation of their values difficult during haemodilution.
Keywords: circulation, haemodilution ; monitoring, jugular venous bulb oxygen saturation ; monitoring, near infrared spectroscopy
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Introduction |
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Near infrared spectroscopy (NIRS) provides a non-invasive optical monitoring technique assessing regional cerebral oxygen saturation (). Several studies have proven the efficacy of NIRS during neurosurgical and cardiac surgical areas.713 Recent evidences have indicated that
can decrease as haemoglobin concentration decreased during haemodilution. Torella and colleagues14 have demonstrated that
values decreased significantly during normovolemic haemodilution to a target Hb concentration of 11 g dl1 and had a significant positive correlation with Hb concentrations. Lassnigg and colleagues15 also reported that a decrease in Hb concentration (from 11.7 to 8.5 g dl1) during the onset of cardiopulmonary bypass (CPB) induced a significant reduction in oxyhemoglobin (O2Hb) measured by NIRS. These data suggested that
can decrease even during mild haemodilution and changes in
may differ from those in
during haemodilution. In the present study we therefore tested the hypothesis that the changes in
may not be parallel to those in
during haemodilution. In addition, recent data suggested that cerebral oxygen balance during the operation could vary depending on the anaesthetics used.
values have been shown to be lower under propofol compared with isoflurane or sevoflurane anaesthesia.1619 We therefore compared the changes in
and
during changes in haemoglobin concentration between patients under propofol and isoflurane/nitrous oxide anaesthesia.
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Methods |
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Routine monitoring equipment included a radial artery catheter for direct arterial blood pressure measurement, a pulse oximeter, and an electrocardiograph. End-tidal carbon dioxide () tension and end-tidal concentration of sevoflurane were measured using a CAPNOMAC multi-gas analyser (Hewlett-Packard, Andover, MA, USA). The tympanic membrane temperature was also continuously monitored by Mon-a-Therm (Mallinckrodt Co., St Louis, MO, USA) and maintained between 35.5°C and 36.5°C using a warming blanket.
Cerebral oximeter, INVOS 4100S (Somanetics, Troy, MI, USA) was used to monitor cerebral oxygen saturation. For the measurements, the cerebral oximeter probe was placed on the right forehead, with the caudal border 1 cm above the eyebrow with the medial edge at the midline. This position places the light source and sensors away from the frontal sinus. To measure
for the assessment of the ratio of cerebral oxygen delivery to demand, a fibreoptic catheter (U 440, Oximetrix, Abbott Critical Care System, Abbott Laboratory, North Chicago, IL, USA) was placed in the right jugular venous bulb. Catheter position was verified by radiography in the anteriorposterior projection. Measurements were performed at the following 5 points: (i) 10 min after the start of operation, (ii) after 400 ml of blood loss, (iii) after 800 ml of blood loss, (iv) just before the transfusion of pre-donated autologous blood, and (v) after 400 ml transfusion. At each measurement, arterial and jugular venous bulb bloods were collected and cerebral oxygenation data were simultaneously measured. During the operation, blood loss was calculated from swab weights and discard suction volumes every 10 min and the calculated blood loss was replaced by the same amount of 6% hydroxyethylated starch to avoid hypovolaemia.
For estimation, cerebral oxygenation state and cerebral oxygen extraction ratio (COER) were calculated using the following equations:
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Statistics
Data are expressed as mean (SD). Demographic variables between the groups were compared using un-paired t-test or 2-test. Haemodynamic variables and cerebral oxygen parameters were analysed using two-way ANOVA with repeated measurement (intergroup comparison) and one-way ANOVA with repeated measurement (intragroup comparison). Post-hoc analysis using multiple independent sample t-tests with Bonferroni correction was performed where significant differences occurred. A preliminary estimate of sample size was based on an expected 10% reduction in
. With a type I error of 0.05 and a type II error of 0.2, the required sample size was 1719 patients in each group. We estimated dropout rate as 10%. Therefore we assigned 21 patients randomly to Groups P and S, respectively. During measurements, patients who required vasodilator drugs were excluded from the study. Sample size was determined by G power (the software is freely available from the website: http://www.psycho.uni-duesseldorf.de/aap/projects/gpower/index.html). Data were analysed using the SPSS-PC statistical software program (Version 11.0.1: SPSS, Inc., Chicago, IL, USA).
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Results |
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In contrast to the results of , there was no significant difference in control values of
between the two groups (Group P, 57 (10)%; Group S, 59 (9)%). During a reduction in Hb concentration,
values were gradually decreased in both groups.
values just before the transfusion of pre-donated blood (Group P, 51 (8)%; Group S, 52 (10)%) were significantly lower compared with control values in both groups (P<0.05).
Figure 2 shows the relationship between values and Hb concentrations, and between
values and Hb concentrations. There is a significant positive correlation between
values and Hb concentrations (Group P, r=0.37, P<0.001; Group S, r=0.46, P<0.001), whereas there are no significant correlation between
values and Hb concentrations (Group P, r=0.12, P=0.26; Group S, r=0.19, P=0.07).
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Discussion |
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Although available data on changes in and
during mild to moderate haemodilution are limited in humans, previous data are consistent with the results obtained in the present study, in which the reduction in mean Hb concentrations from 9.8 to 7.6 g dl1 in Group P and from 9.9 to 8.0 g dl1 in Group S did not affect
values, but significantly reduced
values. Shapira and colleagues5 evaluated the safety of haemodilution combined with induced hypotension with
in patients under isoflurane anaesthesia and demonstrated that
values remained unchanged during mild to moderate haemodilution with a haematocrit: from 35.6 to 20%. In contrast, Torella and colleagues14 have demonstrated that
values decreased significantly during acute normovolaemic haemodilution to a target haemoglobin of 11 g dl1 in aortic surgery and a significant positive correlation between Hb concentrations and
values were noted. Lassnigg and colleagues15 also reported that O2Hb measured by NIRS significantly decreased during the onset of CPB, in which Hb concentrations were decreased from 11.7 to 8.5 g dl1.
The reasons of discrepancy in changes of and
during haemodilution are unknown. However, possible explanations are as follows. First,
is an indirect indicator of global cerebral oxygen use and has high specificity and low sensitivity of cerebral ischaemia. Normal
values may not reflect focal areas of ischaemia, although low
values were indicative of low flow.20 In contrast, monitoring
to detect cerebral ischaemia had high sensitivity and specificity.8 Therefore, it seems likely that
changes might reflect regional oxygen imbalance during mild to moderate haemodilution, although
did not detect any changes in global cerebral oxygen balance. In fact, Hino and colleagues21 demonstrated that regional cerebral oxygen extraction fraction (OEF) of cortical grey matter significantly increased (41.7 to 43.3%) during a mild reduction in Hb concentration (14.3 to 12.6 g dl1) using positron emission tomography in human volunteers. Morimoto and colleagues22 also reported that brain tissue oxygen tension gradually decreased as Hb concentration decreased, as the increases in CBF and oxygen extraction could only partially compensate for the decreased oxygen transport during haemodilution.
Second, the algorithm to estimate might lead to an overestimated reduction in
values during haemodilution. A modified BeerLambert law has been used to estimate
and O2Hb in NIRS and contains a factor of pathlength. Lassnigg and colleagues15 suggested that low arterial Hb concentration leads to an increase in optical pathlength and an overestimation of the decrease in cerebral O2Hb. In fact, Kurth and colleagues23 demonstrated that optical pathlength increased linearly with decreasing Hb concentration in the perfusate to the brain. As pathlength factors are assumed to be constant in the BeerLambert law, the increase in pathlength factors may lead overestimation of changes in
.
Although the control values of were significantly lower under propofol than sevoflurane anaesthesia,
values did not change significantly in either group during haemodilution in the present study. Previous studies1619 have reported that
values were lower under propofol anaesthesia compared with isoflurane or sevoflurane anaesthesia. Our results were compatible with those in previous studies. Different effects of these anaesthetics on cerebral blood flow might have caused the differences of
values between the two groups.16 However, any differences in absolute
values between the two groups were not observed. Previous studies showed that there was a significant correlation between percentage changes in
and
values, but there was a wide limit of agreement between absolute
and
values.14 24 These were consistent with the result of our study. However, we previously reported that tissue oxygen index (TOI), one of
, that use the algorithm independent of pathlength factors, were significantly lower under propofol anaesthesia than under isoflurane anesthesia.25 Although exact mechanisms are unknown, pathlength factors might affect the absolute values of
.
There are several limitations in the present study to merit comments. First, we did not measure CBF and CMRO2, which may limit the interpretation of the results because we could not differentiate between the changes of flow and oxygen consumption. Second, although we tried to maintain normovolaemia, we did not have haemodynamic parameters including pulmonary artery pressure and central venous pressure proving that we achieved this goal. In fact, inaccuracy of calculating blood loss may have prevented us to achieve this goal. Third, all patients in this study were free from cerebral pathology. It remains therefore unknown how and
would have responded during haemodilution in patients with a cerebral pathology. Fourth, only mild haemodilution was assessed in the present study. During severe haemodilution, the changes in
and
may be different from those obtained in the present study. To clarify these points, further studies will be necessary.
In summary, we compared the changes in and
values during a mild changes in Hb concentration under propofol and sevoflurane/nitrous oxide anaesthesia. Although
values remained unchanged,
values significantly decreased as Hb concentration decreased and positive correlation between
values and Hb concentrations was observed. These results suggest that the changes in
were not parallel to those in
during haemodilution and retransfusion. In clinical situations, the discrepancy between the results of
and
makes their interpretation difficult. Further studies will be necessary to clarify this point.
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