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Ann Thorac Surg 1999;68:34-39
© 1999 The Society of Thoracic Surgeons

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Original Articles

Effects of hypothermic and normothermic cardiopulmonary bypass on brain oxygenation

^a Department of Anesthesiology, Saitama Prefectural Ohara-Cardiovascular Center, Saitama, Japan

Address reprint requests to Dr Kadoi, Department of Anesthesiology and Reanimatology, Gunma University, School of Medicine, 3-39-22 Showa-machi, Maebashi, Gunma 371, Japan

	Abstract

Top Abstract Introduction Patients and methods Results Comment References

 
Background. In this study, we assessed the effects of normothermia and hypothermia during cardiopulmonary bypass (CPB) both on internal jugular venous oxygen saturation (SjvO₂) and the regional cerebral oxygenation state (rSO₂) estimated by near infrared spectroscopy (NIRS).

Methods. Thirty patients scheduled for elective coronary artery bypass graft surgery (CABG) were randomly divided into two groups. Group 1 (n = 15) underwent surgery for normothermic (> 35°C) CPB, and group 2 (n = 15) underwent surgery for hypothermic (30°C) CPB, and alpha-stat regulation was applied. A 4.0-French fiberoptic oximetry oxygen saturation catheter was inserted into the right jugular bulb to continuously monitor the SjvO₂ value. To estimate the rSO₂ state, a spectrophotometer probe was attached to the mid-forehead. SjvO₂ and rSO₂ values were then collected simultaneously using a computer.

Results. Neither the cerebral desaturation time (duration during SjvO₂ value below 50%), nor the ratio of the cerebral desaturation time to the total CPB time significantly differed (normothermic group: 18 ± 6 min, 15 ± 6%; hypothermic group: 17 ± 6 min, 13 ± 6%, respectively). The rSO₂ value in the normothermic group decreased during the CPB period compared with the pre-CPB period. The rSO₂ value in the hypothermic group did not change throughout the perioperative period.

Conclusions. These findings suggest that near infrared spectroscopy might be sensitive enough to detect subtle changes in regional cerebral oxygenation.

	Introduction

Top Abstract Introduction Patients and methods Results Comment References

 
Central nervous system (CNS) complications continue to be a major cause of morbidity and mortality after cardiac surgery [1]. Neuropsychological dysfunction after cardiopulmonary bypass (CPB) has been reported in as much as 79% of patients during the early postoperative period [2]. The brain is sensitive to ischemic injury at normothermia, but at hypothermia the tolerance to ischemic event is more than at normothermia [3]. Therefore, moderately (28°C to 30°C) hypothermic CPB has frequently been applied to cardiac surgery to protect the brain against ischemic events.

Normothermic CPB has recently been used in cardiac surgery. With the advent of warm heart surgery [4], the neuroprotective role of hypothermic CPB has come under increasing scrutiny [5–7]. Martin and associates [6] reported a threefold greater stroke rate and a significantly higher incidence of perioperative neurological dysfunction in patients who underwent normothermic surgery. In contrast, the Warm Heart Investigations (WHI) [7] reported no difference in the incidence of strokes. This discrepancy might be due in part to the definition of neurological dysfunction [8] or to the absence of a decisive neurological monitor of cerebral ischemia.

Cook and associates [9] demonstrated that patients undergoing normothermic CPB were at greater risk for cerebral desaturation, which was defined as a low saturation value in internal jugular bulb venous blood. Internal jugular venous oxygen saturation (SjvO₂) indicates the global balance of cerebral blood flow (CBF) and the cerebral metabolic rate (CMRO₂), and it has been widely used to estimate the adequacy of flow/metabolism coupling in the brain during the perioperative period and in the intensive care unit [10]. However, Cook and associates [9] suggested that the SjvO₂ monitor should be insensitive to regional ischemic events.

Near infrared spectroscopy (NIRS) is a noninvasive technique that enables physicians to continuously monitor alterations in regional cerebral tissue oxygenation [11]. Recently, NIRS has been used to detect brain ischemia in patients with head injuries or those undergoing carotid endarterectomy in clinical practice [12].

The aim of this study was to compare these two methodologies in terms of assessing cerebral oxygen saturation in patients undergoing normothermic and hypothermic CPB.

	Patients and methods

Top Abstract Introduction Patients and methods Results Comment References

 
After obtaining the approval of the ethical committee of our institution, informed consent of 30 consecutive patients who underwent elective coronary artery bypass graft surgery were studied. The patients were randomly divided into two groups: group 1 (n = 15) underwent normothermic CPB, and group 2 (n = 15) underwent hypothermic CPB. The demographic data of groups 1 and 2 are shown in Table 1. None of the patients had pulmonary, renal, or hepatic disease. Furthermore, none had diabetes mellitus or clinical or laboratory evidence of cerebral vascular disease.

To estimate the regional cerebral oxygenation (rSO₂) state, a spectrophotometer probe (INVOS 3100; Somanetics, Troy, MI) (distances between the light source and two receivers were 3 cm and 4 cm) was attached to the mid-forehead with adhesive and a rubber strap. The rSO₂ value was then collected in the computer along with the SjvO₂.

The partial pressure of the arterial and jugular venous blood gases were analyzed using a Stat Profile Ultmita (NOVA biomedical Co, Boston, MA). All patients were ventilated with 100% oxygen, and the end-tidal CO₂ was monitored (Ultima; Datex, Helsinki, Finland) and maintained between 35 and 40 mm Hg. After anesthesia induction, propofol 4 mg/kg/h was infused using a syringe pump and continued until the patients arrived in the intensive care unit. Muscular relaxation was maintained by the intermittent administration of vecuronium. No volatile anesthetic was administrated. Rectal and nasopharyngeal temperatures were continuously monitored (Hewlett Packard, Andover, MA). The tympatic membrane temperature was also continuously monitored by Mon-a-Therm (Mallinckrodt Co, St. Louis, MO).

CPB was primed with a crystalloid, non-glucose-containing solution, and a nonpulsatile pump flow rate of 2.2 to 2.5 L/min/m was maintained. A membrane oxygenator and a 40-µm arterial line filter were used, and PaCO₂ uncorrected for temperature was adjusted to normocapnic levels (35 to 40 mm Hg) by varying fresh gas flow to the membrane oxygenator (alpha-stat regulation).

The target nasopharyngeal temperatures were 30°C and > 35°C for the hypothermic and normothermic groups, respectively.

Hematocrit was maintained at > 0.20 during CPB, with the addition of blood as necessary. Phenylephrine infusions were used during CPB to maintain mean arterial pressures (MAP) of 50–90 mm Hg.

Intermittently antegrade blood cardioplegia was administrated at 37°C for the normothermic group and at 5°C for the hypothermic group. Distal coronary anastomoses and proximal anastomoses were performed during a single aortic cross-clamp.

Hemodynamic parameters, arterial and jugular venous blood gases, and regional cerebral oxygenation state were measured as follows. Normothermic group: 1) after the induction of anesthesia and before the start of surgery; 2) at the onset of CPB; 3) 20 minutes after the CPB; 4) 40 minutes after the CPB; 5) 60 minutes after the CPB; 6) at the cessation of CPB, and 7) at the end of the operation. Hypothermic group: 1) after the induction of anesthesia and before the start of surgery; 2) at the onset of CPB; 3) just after the cooling to 30°C; 4) during stable hypothermia at 30°C; 5) at the end of stable hypothermia; 6) at 33°C during rewarming; 7) just after the rewarming to 36°C; 8) at the cessation of CPB; and 9) at the end of the operation.

Intraoperative epiaortic ultrasonography confirmed that none of the patients had moderate or severe atherosclerotic lesions in the ascending aorta. We defined cerebral desaturation state as a SjvO₂ value below 50%, as described by Cook and associates [9]. We identified the number of patients with cerebral desaturation and the desaturating time by stored computer data.

Statistical analysis
All data are expressed as means ± SEM. Changes in mean values were compared with the baseline values using the repeated measures analysis of variance. The parameters obtained from the normothermic and the hypothermic groups were compared using an unpaired t test. Cerebral desaturation at the three time points was analyzed using Fisher’s exact test. Statistical significance was set at p < 0.05.

	Results

Top Abstract Introduction Patients and methods Results Comment References

 
Table 1 shows demographic data from the two groups. Age, height, weight, left ventricular (LV) ejection fraction, phenylephrine dosage, aortic clamping time, total CPB time, and catecholamine dosage did not significantly differ between 2 groups.

The oxygen saturation values measured by the optical catheter (SjvO₂) and in sampled blood measured by the blood gas analyzer were compared. SjvO₂ and oxygen saturation in sampled blood correlated well (y = -0.194 + x, r² = 0.979, p < 0.0001).

Change in the rSO₂ value from the baseline and in the SjvO₂ value was correlated in the normothermic group (Fig 1) (r² = 0.49, p < 0.01). In the normothermic group, the rSO₂ value decreased during CPB compared with the pre-CPB period (Fig 2A), whereas the SjvO₂ value decreased to less than 50% only at an early period during CPB (Table 2).

View larger version (15K): [in this window] [in a new window]   Fig 1. The change in NIRS and in SjvO2 from baseline in the hypothermic group. Horizontal axis represents the fraction that follows: (baseline % saturation minus actual % saturation)/baseline % saturation. Vertical axis represents the fraction that follows: baseline value minus actual value. There was a good correlation between two values.

View larger version (15K): [in this window] [in a new window]   Fig 2. The time course change in NIRS value from the baseline at normothermic (A) and hypothermic (B) groups. Vertical axis represents the fraction that follows: baseline value minus actual value. Values are means ± SEM. *p < 0.05 compared with the baseline value.

	Comment

Top Abstract Introduction Patients and methods Results Comment References

 
In this study, we found that the regional cerebral oxygenation states estimated by NIRS did not significantly change during the perioperative period in the hypothermic group. In contrast, regional cerebral oxygenation states slightly, but significantly, decreased during CPB compared with pre-CPB periods in normothermic group. Cerebral desaturation indicated by SjvO₂ was only observed at an early period during normothermic CPB, and SjvO₂ increased during hypothermic CPB.

Some investigators have reported SjvO₂ changes during normothermic or hypothermic CPB [9, 13–15]. Cook and associates [9] demonstrated cerebral desaturation during CPB in 54% of patients in a normothermic group and 12% of those in a hypothermic group. However, they did not compare the duration when the SjvO₂ value decreased to below 50%. We continuously measured SjvO₂ values and compared the duration of desaturation state in patients undergoing normothermic or hypothermic CPB. We observed a cerebral desaturation state during early CPB in the normothermic group. In contrast, the hypothermic group was in this state during rewarming. These results were essentially compatible with those of Cook and associates [9]. Croughwell and associates [15] also observed cerebral desaturation during rewarming, and that this state is closely related to postoperative neurological disorders. Although cerebral desaturation was more prevalent during normothermic than hypothermic CPB, many investigators have found no differences in the incidence of postoperative neurological disorders between these two procedures [2, 4, 16]. In our study, more normothermic than hypothermic patients underwent cerebral desaturation during CPB. These results are in agreement with published findings. However, cerebral desaturation time did not differ in our study. This result is difficult to explain. However, subsequent work by Croughwell and associates found SjvO₂ to have only a minor independent effect on outcome [17]. Furthermore, Hindman [18] suggested that cerebral desaturation is puzzling because whether or not it influences neurological outcome after cardiac surgery was unclear. Dexter and Hindman [14] recently reported that high SjvO₂ value during hypothermic CPB, which we also observed, indicated the impaired oxygen transfer from hemoglobin to the brain. They also suggested that during normothermia, increased oxygen extraction would reduce the SjvO₂ value, but that this reduction does not indicate the imbalance of cerebral blood flow and metabolism [14]. Furthermore, Murkin and associates [19] suggested that neurological outcome could be worse in patients with higher SjvO₂ value during CPB. Thus, cerebral oxygenation during CPB should not be assessed only by SjvO₂ value.

NIRS has been used as a noninvasive, real-time, online monitor to determine cerebral oxygenation state in humans and other animals [11, 12]. We reported that regional cerebral oxygenation change during induced hypotension could be only detected using a NIRS monitor [20]. Until now, few reports have described rSO₂ alterations during normothermic or hypothermic CPB [19]. The present study shows that changes in regional cerebral oxygenation during normothermic CPB were detected using a NIRS monitor, whereas such change could only be detected at an early period by the SjvO₂ monitor. In contrast, during hypothermic CPB, regional cerebral oxygenation did not change according to the NIRS monitor, whereas the SjvO₂ value increased. This observation indicated that cerebral oxygenation was adequately preserved during hypothermia. However, Sapire and colleagues [21] reported cerebral oxygenation measured by NIRS decreased during hypothermic CPB. This discrepancy might be attributable to differences in the method of anesthesia. Newman and associates [22] demonstrated that propofol infusion protects the brain against cerebral ischemia during CPB. These observations suggested that NIRS should be useful to detect subtle changes in regional cerebral oxygenation.

Correlation between NIRS and SjvO₂ monitoring for detecting cerebral events remains controversial [22, 23]. That SjvO₂ monitoring can determine global cerebral saturation is generally accepted [10]. In contrast, NIRS monitoring is considered to determine the chromophore concentration change in the small region of the cortex where the NIRS probe is positioned [11]. Tateishi and associates [24] described a close correlation between the values indicated by NIRS and those by SjvO₂ monitors in patients with head injury. Furthermore, Sapire and associates [21] reported that the correlation between the two measurements was highly significant during hypothermic CPB. However, McCormick and associates [25] reported that the nonhomogeneous distribution of blood and activity in the brain would reduce the correlation between SjvO₂ and NIRS monitoring. Lewis and associates [23] claimed that the NIRS monitor was not clinically useful for patients with head injury. These discrepancies may be attributed to the differences in the study background, such as patient demographics, mode of anesthesia, and temperature management during CPB. These reports imply that physicians showed understanding of the methodological bases of these measurements, when processing data obtained from these machines in a clinical setting.

In conclusion, we found one short period of significant decrease in SjvO₂ value that occurred approximately 20 min after the start of normothermic CPB. However, rSO₂ value measured by NIRS was significantly decreased at all time during CPB. These findings suggest that NIRS might be sensitive enough to detect subtle changes in regional cerebral oxygenation.

	References

Top Abstract Introduction Patients and methods Results Comment References

 

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Personal Folders Download to citation manager Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Kadoi, Y. Articles by Fujita, N. Search for Related Content PubMed PubMed Citation Articles by Kadoi, Y. Articles by Fujita, N.