Departments of 1 Anaesthesiology and 2 Cardiovascular Surgery, Graduate School of Medicine, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8655, Japan
Corresponding author. E-mail: hayashida-todai@umin.ac.jp
Accepted for publication: January 13, 2003
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Abstract |
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Methods. We measured BIS and cerebral haemoglobin saturation (SrO2) by near-infrared spectroscopy in 10 children undergoing cardiac surgery.
Results. We noted 14 episodes of simultaneous decreases in SrO2 and BIS during acute hypotension in five children. An acute decrease in BIS, which coincided with a decrease in SrO2 suggesting a reduction in cerebral blood flow, was associated with acute slowing of the raw EEG waveforms.
Conclusions. Our findings suggest that an acute decrease in BIS during acute hypotension indicates cerebral hypoperfusion, and that cerebral hypoperfusion caused by hypotension may occur frequently during paediatric cardiac surgery.
Br J Anaesth 2003; 90: 6948
Keywords: monitoring, bispectral index; monitoring, electroencephalography; surgery, paediatric; surgery, cardiovascular
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Introduction |
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Case reports |
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Case 4: at the start of, during and immediately after CPB, four episodes when SrO2 and BIS decreased at the same time during acute hypotension (Fig. 1);
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Discussion |
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In five of the 10 children, we found simultaneous acute decreases in SrO2 and BIS during acute hypotension. An acute decrease in SrO2 suggests an acute reduction in cerebral blood flow (CBF) or cerebral oxygen supply unless cerebral oxygen consumption changes acutely.5 Acute decrease in BIS in our patients occurred with acute slowing of the EEG. The acute EEG slowing is a sign of cerebral hypoperfusion, and can be detected within seconds of severe hypotension, cardiac arrest or carotid occlusion using other processed EEG methods.610 Our findings suggest that if a change in BIS is not drug induced, an acute decrease in BIS indicates cerebral hypoperfusion, particularly when it accompanies acute hypotension and a decrease in SrO2.
A decrease in jugular venous haemoglobin saturation (SjO2) to less than 50% is generally considered to indicate cerebral hypoperfusion.11 Values of SrO2 remained above 60% in most of our patients, however, even when cerebral hypoperfusion was indicated by acute EEG slowing or a decrease in BIS. The relatively high SrO2 values during cerebral hypoperfusion might result in part from the lack of calibration of the NIRS oximeter in children, since an absolute value of SrO2 can be obtained only after calibration based on some assumptions. The clinical value of an NIRS oximeter is thus limited to tracking trends of SrO2.12 13 In addition, SrO2 may remain higher than SjO2 because SrO2 represents the weighted average of haemoglobin saturation of arterial, capillary and venous blood within a volume of tissue whereas SjO2 indicates venous haemoglobin saturation.13 14 For example, even when SjO2 is 50%, SrO2 can be 62.5% if SaO2 is nearly 100%, assuming that blood in the cerebral vasculature is three-quarters in the venous bed and one-quarter in the arterial bed (SrO2=0.75xSjO2+ 0.25xSaO2).12 For these reasons, cerebral hypoxia may be present when the SrO2 value is relatively greater than the SjO2 value.
Cerebral hypoperfusion indicated by acute EEG slowing or an acute BIS reduction occurred most commonly at the start of CPB. Acute haemodilution might contribute to development of cerebral perfusion at this time point because of reduced arterial pressure and reduced oxygen carrying capacity associated with haemodilution. More directly, bloodless prime being flushed through the brain could decrease electrical activity at the onset of CPB.
Cerebral hypoperfusion caused by acute hypotension occurred often in the five children aged 3 yr or younger but not in the five older children. Only in the younger children did SrO2 change in parallel with changing arterial pressure throughout surgery, suggesting that CBF depended on arterial pressure in these younger children. Although data regarding the development of cerebral autoregulation in humans are lacking,15 our findings suggest that cerebral autoregulation is immature during infancy. With an abrupt reduction in cerebral perfusion pressure, blood flow will decrease for a brief period (12 min) before autoregulation restores CBF.16 The immature autoregulatory system may take more time to restore CBF, and thus a decrease in CBF that is long and severe enough to cause decreased cerebral electrical activity can occur more easily in younger children. Even in younger children, however, SrO2 tended to return towards a normal level, and decreases in BIS lasted no longer than 3 min, even when arterial pressure remained at a reduced level. Therefore, it was likely that cerebral autoregulation acted slowly to restore CBF and EEG during persistent hypotension in the younger children.
One case report described an acute profound reduction in BIS following hypovolaemic cardiac arrest during adult cardiac surgery.17 We found that in children undergoing cardiac surgery, cerebral hypoperfusion can occur not only following severe haemodynamic changes such as cardiac arrest but also following less severe changes in arterial pressure such as hypotension at the onset of CPB.
We cannot determine the saturation level at which cerebral ischaemia will occur with an NIRS oximeter alone.13 14 By combining SrO2 and BIS, we can establish if a reduction of CBF indicated by a decrease of SrO2 is sufficient to slow the EEG by following BIS. Conversely, we can determine whether a decreased BIS value is caused by decreased CBF rather than other causes (e.g. deepened anaesthesia or hypothermia) by noticing changes in SrO2.
The manufacturer of the device notes clearly that BIS is not intended as a monitor of ischaemia. Further studies are required to assess the adequacy of such use.17 In our patients, however, development of and recovery from cerebral hypoperfusion could be conveniently monitored with the BIS EEG monitor. This simple-to-use monitor of brain function may indicate hypoxia and recovery of cerebral electrical activity in response to circulatory changes during anaesthesia and surgery, especially when used in combination with an NIRS oximeter.
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