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Utility of near-infrared spectroscopic measurements during deep hypothermic circulatory arrest
Utility of Near-Infrared Spectroscopic Measurements During Deep Hypothermic Circulatory Arrest Toshiharu Shin’oka, MD, Georg Nollert, MD, Dominique Shum-Tim, MD, Adre du Plessis, MD, and Richard A. Jonas, MD Departments of Cardiovascular Surgery and Neurology, Children’s Hospital, and Departments of Surgery and Neurology, Harvard Medical School, Boston, Massachusetts
Background. Near-infrared spectroscopy (NIRS) is used to monitor cerebral oxygenation during cardiac surgery. However, interpretation of the signals is controversial. The aim of the study was to determine which NIRS variable best correlated with brain damage as assessed by animal behavior and neurohistologic score and to compare the accuracy of NIRS and magnetic resonance spectroscopy (MRS) in predicting brain injury. Methods. Forty 5-week-old piglets underwent 60 minutes of deep hypothermic circulatory arrest (DHCA) at 15°C. Changes in brain adenosine triphosphate (ATP), phosphocreatine (PCr), and intracellular pH (pHi) were determined by MRS and correlated to changes in oxygenated hemoglobin (HbO2), deoxygenated hemoglobin (Hb), and oxidized cytochrome a,a3 (CytOx) NIRS signals. Brains were fixed on day 4 and examined using a neurohistologic score.
Results. Reductions in CytOx and HbO2 values were correlated closely with decreases in ATP, PCr, and pHi. The changes in CytOx and PCr showed the strongest correlation (r ⴝ 0.623). Maximal CytOx reduction during DHCA of more than ⴚ25 M * differential pathlength factor (DPF) predicted brain damage with a sensitivity of 100% and a specificity of 75%. The histologic score was also correlated with a decrease in ATP (r ⴝ ⴚ0.52 for CytOx; r ⴝ ⴚ0.32 for ATP); HbO2, PCr, and pHi showed no correlations. Conclusions. Reduction in CytOx correlates with decreased brain energy state and predicts histologic brain injury after DHCA with a high sensitivity. These data suggest that the level of CytOx could be a very important predictor of brain damage during DHCA. (Ann Thorac Surg 2000;69:578 – 83) © 2000 by The Society of Thoracic Surgeons
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under conditions similar to those used in neonatal cardiac surgery [5, 6]. We now report the correlation between NIRS and MRS measurements by combining results from these two earlier studies with the exact same operative protocol to overcome the statistical limitations of small experimental numbers. Intraoperative changes in MRS and NIRS measurements, which may be indicators of the extent of ischemic metabolic changes, were compared with neurologic and histologic outcome.
ear-infrared spectroscopy (NIRS) is a noninvasive optical monitoring technique that can provide information on changes in cerebral oxyhemoglobin (HbO2), deoxyhemoglobin (Hb), and oxidized cytochrome a,a3 (CytOx) concentrations in brain tissues [1]. The usefulness of these measurements has been questioned because the measurements are relative, and CytOx provides a small signal [2]. In addition, studies that directly compare these measurements with changes in cerebral energetics are limited [3, 4]. We have developed a survival piglet model of deep hypothermic circulatory arrest (DHCA) in which we measured cerebral adenosine triphosphate (ATP), phosphocreatine (PCr), and intracellular pH (pHi) with P31 nuclear magnetic resonance spectroscopy (MRS) in parallel with NIRS measurements [5, 6]. Neurologic evaluation was continued for 4 days, at which time the brain was fixed for histology. We previously used this experimental model to evaluate the relations of neurologic outcome with variations of perfusate hematocrit or perfusate colloid oncotic pressure Accepted for publication July 12, 1999. Address reprint requests to Dr Jonas, Department of Cardiovascular Surgery, Children’s Hospital, 300 Longwood Ave, Boston, MA 02115; e-mail: [email protected].
© 2000 by The Society of Thoracic Surgeons Published by Elsevier Science Inc
Material and Methods Experimental Preparation All animals received humane care in compliance with the Principles of Laboratory Animal Care formulated for the National Society for Medical Research and the Guide for the Care and Use of Laboratory Animals prepared by the National Academy of Science (NIH Publication No. 86-23, revised in 1985). Details of the experimental preparations have been described previously [5, 6]. Briefly, 40 5-week-old Yorkshire piglets were anesthetized with intraperitoneal sodium methohexital (45 mg/kg) and intubated. Anesthesia was maintained by continuous infusion of fentanyl (25 0003-4975/00/$20.00 PII S0003-4975(99)01322-3
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g 䡠 kg⫺1 䡠 h⫺1), midazolam (0.2 mg 䡠 kg⫺1 䡠 h⫺1), and pancuronium (0.2 mg 䡠 kg⫺1 䡠 h⫺1) throughout the entire experiment except during the period of circulatory arrest. Before surgery, a 3.0-cm diameter surface coil was sutured on the scalp overlying the cerebral hemispheres, and a pair of fiberoptic optodes for NIRS measurements was attached to the head of the animal. The optode spacing was 3.0 to 3.5 cm in the coronal plane. After systemic heparinization (300 IU/kg), an 8F arterial cannula and a 24F venous cannula were inserted into the right femoral artery and right atrium, respectively. The animal was then placed in a 40-cm diameter horizontal-bore superconducting 4.7 T magnet (Oxford Research System, Oxford, England) and subjected to hypothermic cardiopulmonary bypass (CPB; 40 minutes of cooling, pH-stat strategy, hematocrit between 0.10 and 0.30) and 60 minutes of circulatory arrest at 15°C nasopharyngeal. Upon termination of the experiment after 45 minutes of rewarming, the piglet was weaned off CPB and decannulated outside the magnet. Protamine (6 mg/kg) was administered intravenously. Immediately after decannulation, the animal was repositioned in the bore for 3 hours for MRS and NIRS data collection. After that period, all incisions were closed in a sterile fashion and the animal was left intubated during the first 12 hours postoperatively.
Postoperative Management Postoperatively, all animals remained fully sedated (fentanyl 25 g 䡠 kg⫺1 䡠 h⫺1, midazolam 0.2 mg 䡠 kg⫺1 䡠 h⫺1), paralyzed (pancuronium 0.2 mg 䡠 kg⫺1 䡠 h⫺1), intubated, and monitored (arterial pressure, heart rate) continuously for 12 hours after surgery. At this time chest tubes were removed, infusions discontinued, and the animals weaned from ventilator and extubated.
Data Collection MAGNETIC RESONANCE SPECTROSCOPY. Phosphorus 31 magnetic resonance spectra were acquired on a spectrometer built by the Francis Bitter Magnet Laboratory at the Massachusetts Institute of Technology (MIT) with the Oxford horizontal-bore 4.7 T magnet and surface coil. Each spectrum was the average of 128 acquisitions (1,024 complex data points, repetition rate of 4.65 seconds, spectrum width 5,000 Hz). Total acquisition time was 10 minutes. Peak areas of inorganic phosphate, creatine phosphate, and beta-nucleoside triphosphate were determined by Lorentzian curve fitting and peak integration (NMRI Software, New Methods Research, East Syracuse, NY). Changes in ATP concentration were assessed from the beta-nucleoside triphosphate peak area. The inorganic phosphate, creatine phosphate, and ATP data are reported as percentage of the baseline obtained over 10 minutes before the initiation of CPB. The pHi in the brain was calculated from the chemical shift of the inorganic phosphate peak relative to the creatine phosphate peak. Details of methods employed have been described previously [4, 5].
The optodes transmitted and detected laser light at four near-infrared wavelengths (776 nm, 828 nm, 848 nm, and 913 nm) and connected to
NEAR-INFRARED SPECTROSCOPY.
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a near-infrared spectrometer (NIRO-500, Hamamatsu Photonics KK, Hamamatsu, Japan), which calculated concentration changes in HbO2, Hb, total hemoglobin (tHb), and CytOx. These data were recorded continuously every 30 seconds from the initiation of anesthesia induction until 3 hours after the termination of CPB. NEUROLOGIC EVALUATIONS. Neurologic and behavioral evaluations were performed at 24-hour intervals. Neurologic scoring data were adapted from the neurologic deficit score (NDS) and overall performance category (OPC) scale developed at the University of Pittsburgh [7, 8]. In the NDS system, a score of 100 was assigned to each of five general components (central nervous system function, respiration, motor and sensory function, level of consciousness, and behavior). A total score of 500 indicates brain death, whereas a score of 0 is considered normal. The OPC score assessed outcome in five categories: 1 ⫽ normal, 2 ⫽ moderate disability, 3 ⫽ severe disability, 4 ⫽ coma, 5 ⫽ brain death. NDS and OPC determinations were agreed upon by two members of the laboratory team blinded to the perfusion strategies. All animals were observed closely for seizure activity.
The brain of the animal was fixed with perfusion of 4% paraformaldehyde on postoperative day 4. Histologic changes of the brain were evaluated by a pathologist who was unaware of the experimental procedure. After fixation, the brain was cut into eight to ten coronal slabs that were embedded in paraffin. Seven micrometer sections were stained with hematoxylineosin. A standardized list of 24 of the major gray and white matter structures was examined following the nomenclature for porcine neuroanatomy. The structures were scored according to their location within three broad categories (neocortex, hippocampus, and caudate) as described by Yoshikawa [9]. Histologic changes were rated on an arbitrary scale: 0 ⫽ no damage, 1⫹ ⫽ isolated damaged neurons, 2⫹ ⫽ small clusters of damaged neurons, 3⫹ ⫽ large cluster of injured neurons, 4⫹ ⫽ completely damaged neurons, and 5⫹ ⫽ frank cavitated lesions with necrosis. HISTOLOGIC EVALUATIONS.
Statistical Analysis The two experimental studies analyzed were performed according to exactly the same surgical protocol by the same surgeon (T.S.). Data acquisition was performed in the same fashion using the same devices. Furthermore, the original data were accessible for every experiment. Given these circumstances, we are of the opinion that we can treat the two experiments as one to increase the statistical power for the analyses. All results were analyzed by a statistical analysis software package (SPSS for Windows, Version 7.0, SPSS Inc, Chicago, IL). Parametric correlation coefficients according to Pearson were calculated to analyze the MRS and NIRS data. Nonparametric correlation coefficients according to Spearman were used for the correlation analyses of the abovementioned parameters with the NDS, OPC, and histologic score. A p value less than 0.05 was considered statistically significant.
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Fig 1. Time course of adenosine triphosphate (ATP), phosphocreatine (PCr), and intracellular pH (pHi) during the experiments. ATP values are slightly reduced during cooling and drop with the onset of deep hypothermic circulatory arrest (DHCA). ATP values decrease further without reaching a plateau and increase again with rewarming. PCr and pHi values rise with the beginning of cardiopulmonary bypass (CPB). Both parameters decrease quickly during DHCA with PCr storages becoming depleted after 30 minutes of DHCA. With reperfusion PCr and pHi increase and exceed baseline values. (CytOx ⫽ oxidized cytochrome a,a3; DPF ⫽ differential pathlength factor; HbO2 ⫽ oxygenated hemoglobin.)
Results Course of Near-Infrared and Magnetic Resonance Spectroscopic Parameters The parameters showed different characteristics during the cooling phase. HbO2, PCr, and pHi increased significantly, whereas ATP and CytOx decreased. During DHCA all parameters showed a decline. PCr was not detectable at the end of DHCA. After the onset of reperfusion all parameters recovered to baseline values and in part exceeded those (PCr, HbO2). Figures 1 and 2 show the courses of NIRS and MRS parameters during the experiments in detail.
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Fig 2. Time course of oxidized cytochrome a,a3 (CytOx) and oxygenated hemoglobin (HbO2) during the experiments. CytOx values decrease with the onset of cooling and drop with the start of deep hypothermic circulatory arrest (DHCA). They continue to decrease slowly during arrest and reach a plateau at 20 minutes. They increase with rewarming and reperfusion and exceed baseline values at the end of cardiopulmonary bypass (CPB). HbO2 values increase with the onset of cooling on CPB. Deoxygenation takes place immediately after beginning of DHCA. HbO2 rises after start of reperfusion and exceeds baseline values. (ATP ⫽ adenosine triphosphate; PCr ⫽ phosphocreatine; pHi ⫽ intracellular pH.)
⫺0.52 for CytOx, r ⫽ ⫺0.32 for ATP; see Table 2). Assuming a threshold in CytOx reduction for ischemic brain damage of ⫺25 M * differential pathlength factor (DPF), sensitivity of the CytOx signal was 100% and specificity was 70%. The ATP signal showed a sensitivity for ischemic brain damage of 75% and a specificity of 38% at a threshold of 40% of baseline ATP (Fig 6). However, HbO2, Hbt, PCr, and pHi showed no correlations to the histologic score. The neurologic score on the first postoperative day (1 POD) was related significantly to the histologic score.
Comment
Neurologic Results and Histologic Changes
Technical Limitations of Near-Infrared Spectroscopy
The piglets showed only mild neurologic deficits after 60 minutes of DHCA that were most severe on the first postoperative day and regressed completely within 4 days. The OPCs showed also only mild impairments and had normalized by the third postoperative day (Fig 3). Light microscopy revealed mild cerebral neuronal damage on the fourth postoperative day. The average maximum brain damage score was 0.7 ⫾ 0.3. The injury was most pronounced in the neocortex and caudate nucleus (Fig 4).
NIRS is a noninvasive optical method with the capability to measure changes in concentrations of chromophores in tissue [1]. These chromophores absorb the light emit-
Correlation Between Near-Infrared and Magnetic Resonance Spectroscopic Data Relations between NIRS and MRS data at all time points are shown in Table 1 and Figure 5. The changes in ATP, PCr, and pHi showed only slight correlations with Hb or tHb. In contrast, reductions in CytOx or HbO2 were closely correlated with decreases in ATP, PCr, and pHi. The changes in CytOx and PCr showed the closest correlation (r ⫽ 0.623, p ⬍ 0.0001).
Correlation Between Histologic Data and Other Data The histologic score was negatively correlated with minimal CytOx and minimal ATP values during DHCA (r ⫽
Fig 3. Neurologic damage. Postoperatively the piglets were neurologically examined daily by two persons. Sixty minutes of deep hypothermic circulatory arrest produced deficits in the neurologic deficit score (NDS; 0 ⫽ normal; 500 ⫽ brain death) and in the overall performance categories (OPC; 1 ⫽ normal; 5 ⫽ brain death), which were mild and most pronounced on the day of extubation (first postoperative day [POD 1]). On POD 4 the piglets had recovered completely.
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Fig 4. Histopathologic brain damage. After perfusion fixation on postoperative day 4 defined brain regions were analyzed by light microscopy. For assessment a qualitative score system was used: 0 ⫽ no damage, 1⫹ ⫽ isolated damaged neurons, 2⫹ ⫽ small clusters of damaged neurons, 3⫹ ⫽ large cluster of injured neurons, 4⫹ ⫽ completely damaged neurons, 5⫹ ⫽ frank cavitated lesions with necrosis. The highest score in a region is shown. On average 60 minutes of deep hypothermic circulatory arrest caused only mild brain damage.
ted by a NIRS device dependent on their concentrations and the pathlength of the light within the tissue; this relation is well known as the Beer-Lambert law. Oxygenated and deoxygenated hemoglobin have well-defined absorption peaks at 758 nm and 929 nm respectively; at 798 nm the absorption of light of these two chromophores is equal. Cytochrome a,a3 shows a broad absorption peak with a maximum at 830 nm; the absorption spectrum of reduced cytochrome a,a3 is completely flat. Therefore, the measurement of the oxygenation status of hemoglobin can be performed in a relatively simple fashion, whereas the assessment of cytochrome aa3 requires elaborate algorithms and use of multiple wavelengths [10 –12]. The validity of monitoring cerebral oxygenation at a cellular level by measurements of the cytochrome aa3 signal with a NIRS machine that does not give absolute values has been doubted [13, 14]. The pathlength of near-infrared light in brain tissue is influenced by factors such as optode placement, age of the patient, geometry of the skull, brain temperature, edema, and pH. Some of these factors certainly change during our experiments and may even have a different influence on every single Table 1. Pearson’s Correlation Coefficients for Intraoperative Data Obtained by Near-Infrared Spectroscopy and Nuclear Magnetic Resonance Spectroscopya
ATP PCr pHi
CytOx
Hb
HbO2
tHb
0.565 0.623 0.523
⫺0.166 ⫺0.233 ⫺0.193
0.483 0.573 0.501
0.260 0.288 0.252
a
Measurements obtained by near-infrared spectroscopy: CytOx, Hb, HbO2, and tHb. Measurements obtained by nuclear magnetic resonance spectroscopy: ATP, PCr, and pHi. All correlation values were significant at the 0.0001 level (2-tailed). ATP ⫽ adenosine triphosphate; CytOx ⫽ oxidized cytochrome aa3; Hb ⫽ deoxygenated hemoglobin; HbO2 ⫽ oxygenated hemoglobin; PCr ⫽ phosphocreatine; pHi ⫽ intracellular pH; tHb ⫽ total hemoglobin.
Fig 5. Correlation between intraoperative magnetic resonance spectroscopic (MRS) and near-infrared spectroscopic (NIRS) measurements. Forty piglets underwent 40 minutes of cooling, 60 minutes of deep hypothermic circulatory arrest at 15°C, and 45 minutes of rewarming. Changes in brain adenosine triphosphate (ATP) (A) and phosphocreatine (PCr) (B) were determined by MRS every 10 minutes during cardiopulmonary bypass and correlated to changes in the oxidized cytochrome a,a3 (CytOx) NIRS signal. Changes in the high energy phosphates are measured and expressed as percent of baseline values. The variation in the cytochrome signal is described as a change from baseline values in relative chromophore concentration. The value for every measurement at a specific time is shown as mean and standard error of mean. CytOx showed the highest correlation to PCr changes and also demonstrated significant correlation to ATP and intracellular pH variations (see Table 1). (DPF ⫽ differential pathlength factor)
wavelength used for the calculation of the CytOx signal. In addition, the Hamamatsu device (NIRO-500) detects only the change in redox state, but not the absolute value of the redox state. Therefore, the detected correlation between the CytOx values and outcome could be influenced strongly by the changes in pathlength and the scattering properties of the brain. However, a major change in pathlength itself could be an indicator of brain damage. Therefore, it remains uncertain whether a reduction in the CytOx value reflects a decrease of cerebral CytOx or just a change in pathlength, but as is demonstrated in this study, it does indicate brain damage.
Clinical Validation of the Near-Infrared Spectroscopic Signal NIRS technology has been applied in various clinical fields such as neonatology, neurosurgery, and cardiac surgery. It has been used to monitor cerebral oxygen-
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Table 2. Spearman’s Correlation Coefficients for Aggregated Intraoperative Data and Outcome Variablesa Outcome Variables
Intraoperative Variable CytOx (M)
Hb (M)
HbO2 (M)
tHb (M)
ATP (%)
PCr (%)
pHi (%)
Neurologic Deficit Score
Overall Performance Categories
Aggregate Function
Day 1
Day 2
Day 1
Day 2
Histologic Score
Min Max Ave Min Max Ave Min Max Ave Min Max Ave Min Max Ave Min Max Ave Min Max Ave
⫺0.206 0.303 ⫺0.004 0.029 0.051 0.072 0.043 0.005 0.011 ⫺0.060 0.101 0.080 ⫺0.315 ⫺0.251 ⫺0.318 ⫺0.152 0.197 ⫺0.094 0.148 ⫺0.421b ⫺0.442c
⫺0.324 0.049 ⫺0.180 ⫺0.021 ⫺0.100 ⫺0.065 0.018 ⫺0.247 ⫺0.128 ⫺0.200 ⫺0.040 ⫺0.117 ⫺0.155 ⫺0.026 ⫺0.065 0.050 0.249 ⫺0.036 0.128 ⫺0.512b ⫺0.290
⫺0.234 0.115 ⫺0.104 0.157 0.049 0.090 0.086 0.074 0.108 0.094 0.109 0.198 ⫺0.219 ⫺0.343 ⫺0.301 ⫺0.121 0.018 ⫺0.210 0.300 ⫺0.560b ⫺0.479b
⫺0.075 0.047 ⫺0.036 0.060 ⫺0.190 ⫺0.075 0.255 ⫺0.038 0.072 0.127 ⫺0.084 0.000 0.115 ⫺0.113 ⫺0.057 0.136 0.252 ⫺0.005 0.299 ⫺0.486b ⫺0.147
⫺0.524b ⫺0.188 ⫺0.439c 0.198 0.123 0.120 0.063 ⫺0.069 0.030 0.037 0.103 0.174 ⫺0.326c ⫺0.329c ⫺0.330c ⫺0.218 ⫺0.129 ⫺0.260 ⫺0.030 ⫺0.118 ⫺0.363c
a
Measurements obtained by near-infrared spectroscopy: CytOx, Hb, HbO2, and tHb. Measurements obtained by nuclear magnetic resonance spectroscopy: ATP, PCr, and pHi. To correlate outcome with intraoperative parameters, aggregate functions for intraoperative data were used: minimum b c Correlation values were significant at the 0.01 level (2-tailed). Correlation values were (Min), maximum (Max), and average (Ave) values. significant at the 0.05 level (2-tailed). Hb ⫽ deoxygenated hemoglobin; ATP ⫽ adenosine triphosphate; CytOx ⫽ oxidized cytochrome a,a3; PCr ⫽ phosphocreatine; pHi ⫽ intracellular pH; tHb ⫽ total hemoglobin.
ation during cardiac or vascular surgery associated with DHCA. Impaired cerebral oxygen metabolism has been detected by decreased CytOx [15, 16]. Although the correlation between NIRS data and neuropsychologic deficits after cardiac surgery should be strongly assumed, data are very limited [17, 18]. The CytOx signal has been validated in a clinical study showing high correlations to factors of operative management, eg, pH, pCO2, hematocrit, and temperature [19].
Correlations Between Near-Infrared and Magnetic Resonance Spectroscopic Data This metaanalysis of two experimental studies has revealed important correlations between CytOx and nucleoside triphospate, PCr, and pHi. These results are consistent with a previous study from our institution demonstrating this relation in piglets submitted to different stages of hypoxia [3]. We hypothesized that the relation between high energy phosphates and CytOx values might not be linear. However, assumption of a linear curve fit revealed the highest correlation when compared with other nonpolynomial curves. The values for Hb and tHb showed poor correlation to MRS data. In comparison to the CytOx measurements, HbO2 values showed similar, but lower correlation to high energy phosphates.
HbO2 ⫽ oxygenated hemoglobin;
Correlations Between Near-Infrared and Magnetic Resonance Spectroscopic Data and Histology The lowest CytOx value recorded during DHCA proved to be a highly sensitive predictor of histologic brain damage as assessed by light microscopy after 4 days. The predictive value of CytOx was much higher than that of ATP. It is reasonable to assume that a combination of the level and the duration of hypoxia/ischemia causes brain damage. However, the minimum CytOx value was a better predictor for neurologic damage than the average CytOx value during DHCA, duration of reduced CytOx values, or the integral of decreased CytOx values over time. This indicates that there might be a threshold of brain deoxygenation and only the duration of severe ischemia/hypoxia exceeding this deoxygenation threshold causes injury. Other NIRS factors such as HbO2 and Hb failed to show any correlation with histology. This finding supports the thesis that the value of these signals during episodes of increased oxygen affinity to hemoglobin because of hypothermia and alkalosis is doubtful. As these signals are highly related to the brain venous saturation [20], it had been assumed that they provide an index of the adequacy of global cerebral oxygenation [21]. A theoretical approach by computer modeling as well as clinical and experimental findings led to the hypothesis
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Fig 6. Correlation between adenosine-triphosphate (ATP) and oxidized cytochrome a,a3 (CytOx) measurements and neurologic outcome. On postoperative day 4 the brain of the animal was fixed with perfusion of 4% paraformaldehyde. Histologic changes were rated on an arbitrary scale (see Material and Methods). The most severe injury in a brain region of an animal was correlated to the minimum values of CytOx (A) and ATP (B) recorded during the operation. All animals with visible structural brain damage had shown severe changes in the CytOx signal and—less pronounced—the ATP signal. Correlation coefficients are shown in Table 2. (DPF ⫽ differential pathlength factor.)
that high regional tissue oxygen levels estimated by NIRS values during CPB might indicate decreased offloading of oxygen from hemoglobin and not necessarily adequate tissue oxygenation [4, 15, 17, 19, 22].
Conclusion This study indicates that the CytOx signal measured by the NIRO-500 spectrometer might be a better predictor for neuronal damage than cerebral high energy phosphates. In addition, NIRS monitoring is noninvasive, continuous, cost effective, and applicable during operation. It may become the method of choice in brain monitoring during DHCA to predict the safety limit of circulatory arrest. We are very grateful to Miles Tsuji, MD, Department of Neurology, St Joseph’s Hospital, Wisconsin, and David H. Holtzman, MD, PhD, Department of Neurology, Children’s Hospital Boston, for a critical review of the paper and their thoughtful suggestions.
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2. Nollert G, Shin’oka T, Jonas RA. Near infrared spectrophotometry in cardiac surgery. Thorac Cardiovasc Surg 1998;46: 167–75. 3. Tsuji M, Naruse H, Volpe J, Holtzman D. Reduction in cytochrome aa3 measured by near-infrared spectroscopy predicts cerebral energy loss in hypoxic piglets. Pediatr Res 1995;37:253–9. 4. Matsumoto H, Oda T, Hossain MA, Yoshimura N. Does the redox state of cytochrome aa3 reflect brain energy level during hypoxia? Simultaneous measurements by near infrared spectrophotometry and 31P nuclear magnetic resonance spectroscopy. Anesth Analg 1996;83:513– 8. 5. Shin’oka T, Shum-Tim D, Jonas RA, et al. Higher hematocrit improves cerebral outcome after deep hypothermic circulatory arrest. J Thorac Cardiovasc Surg 1996;112:1610–21. 6. Shin’oka T, Shum-Tim D, Laussen PC, et al. Relative roles of colloid oncotic pressure and hematocrit in determining outcome after deep hypothermic circulatory arrest. Ann Thorac Surg 1998;65:155– 64. 7. Safar P, Griswold SE, Aaagenes P. Long-term animal models for the study of global brain ischemia (GBI). In: Wanguier A, Barger M, Anery WK, eds. Protection of tissues against hypoxia. Amsterdam: Elsevier Biomedical, 1982:147–70. 8. Tisherman SA, Safar P, Radovsky A, Peitzman A, Sterz F, Kuboyama K. Therapeutic deep hypothermic circulatory arrest in dogs: a resuscitation modality for hemorrhagic shock with “irreparable” injury. J Trauma 1990;30:836– 47. 9. Yoshikawa T. Atlas of the brains of domestic animals. University Park: Pennsylvania State University Press, 1968. 10. Wray S, Cope M, Delpy DT, Wyatt JS, Reynolds EO. Characterization of the near infrared absorption spectra of cytochrome a,a3 and haemoglobin for the non-invasive monitoring of cerebral oxygenation. Biochim Biophys Acta 1988;933:184–92. 11. Edwards AD, Brown GC, Cope M, et al. Quantification of concentration changes in neonatal human cerebral oxidized cytochrome oxidase. J Appl Physiol 1991;71:1907–13. 12. Delpy DT, Arridge SR, Cope M, et al. Quantitation of pathlength in optical spectroscopy. Adv Exp Med Biol 1989; 248:41– 6. 13. Matcher SJ, Elwell CE, Cooper CE, Cope M, Delpy DT. Performance comparison of several published tissue near-infrared spectroscopy algorithms. Anal Biochem 1995;227:54– 68. 14. Skov L, Greisen G. Apparent cerebral cytochrome aa3 reduction during cardiopulmonary bypass in hypoxaemic children with congenital heart disease. A critical analysis of in vivo near infrared spectrophotometric data. Physiol Meas 1994; 15:447–57. 15. Du Plessis AJ, Newburger J, Jonas RA, et al. Cerebral oxygen supply and utilization during infant cardiac surgery. Ann Neurol 1995;37:488–97. 16. Greeley WJ, Bracey VA, Ungerleider RM, et al. Recovery of cerebral metabolism and mitochondrial oxidation state is delayed after hypothermic circulatory arrest. Circulation 1991;84(Suppl III):400– 6. 17. Nollert G, Mo¨hnle P, Tassani-Prell P, et al. Postoperative neuropsychological dysfunction and cerebral oxygenation during cardiac surgery. Thorac Cardiovasc Surg 1995;43:260– 4. 18. Kurth CD, Steven JM, Nicolson SC. Cerebral oxygenation during pediatric cardiac surgery using deep hypothermic circulatory arrest. Anesthesiology 1995;82:74– 82. 19. Nollert G, Mo¨hnle P, Tassani-Prell P, Reichart B. Determination of cerebral oxygenation during cardiac surgery. Circulation 1995;92(Suppl II):327–33. 20. Deeb GM, Jenkins E, Bolling SF, et al. Retrograde cerebral perfusion during hypothermic circulatory arrest reduces neurologic morbidity. J Thorac Cardiovasc Surg 1995;109: 259– 68. 21. Daubeney PEF, Pilkington SN, Janke E, Charlton GA, Smith DC, Webber SA. Cerebral oxygenation measured by nearinfrared spectroscopy: comparison with jugular bulb oxymetry. Ann Thorac Surg 1996;61:930– 4. 22. Dexter F, Hindman BJ. Computer simulation of brain cooling during cardiopulmonary bypass. Ann Thorac Surg 1994;57: 1171– 8.
Background. Near-infrared spectroscopy (NIRS) is used to monitor cerebral oxygenation during cardiac surgery. However, interpretation of the signals is controversial. The aim of the study was to determine which NIRS variable best correlated with brain damage as assessed by animal behavior and neurohistologic score and to compare the accuracy of NIRS and magnetic resonance spectroscopy (MRS) in predicting brain injury. Methods. Forty 5-week-old piglets underwent 60 minutes of deep hypothermic circulatory arrest (DHCA) at 15°C. Changes in brain adenosine triphosphate (ATP), phosphocreatine (PCr), and intracellular pH (pHi) were determined by MRS and correlated to changes in oxygenated hemoglobin (HbO2), deoxygenated hemoglobin (Hb), and oxidized cytochrome a,a3 (CytOx) NIRS signals. Brains were fixed on day 4 and examined using a neurohistologic score.
Results. Reductions in CytOx and HbO2 values were correlated closely with decreases in ATP, PCr, and pHi. The changes in CytOx and PCr showed the strongest correlation (r ⴝ 0.623). Maximal CytOx reduction during DHCA of more than ⴚ25 M * differential pathlength factor (DPF) predicted brain damage with a sensitivity of 100% and a specificity of 75%. The histologic score was also correlated with a decrease in ATP (r ⴝ ⴚ0.52 for CytOx; r ⴝ ⴚ0.32 for ATP); HbO2, PCr, and pHi showed no correlations. Conclusions. Reduction in CytOx correlates with decreased brain energy state and predicts histologic brain injury after DHCA with a high sensitivity. These data suggest that the level of CytOx could be a very important predictor of brain damage during DHCA. (Ann Thorac Surg 2000;69:578 – 83) © 2000 by The Society of Thoracic Surgeons
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under conditions similar to those used in neonatal cardiac surgery [5, 6]. We now report the correlation between NIRS and MRS measurements by combining results from these two earlier studies with the exact same operative protocol to overcome the statistical limitations of small experimental numbers. Intraoperative changes in MRS and NIRS measurements, which may be indicators of the extent of ischemic metabolic changes, were compared with neurologic and histologic outcome.
ear-infrared spectroscopy (NIRS) is a noninvasive optical monitoring technique that can provide information on changes in cerebral oxyhemoglobin (HbO2), deoxyhemoglobin (Hb), and oxidized cytochrome a,a3 (CytOx) concentrations in brain tissues [1]. The usefulness of these measurements has been questioned because the measurements are relative, and CytOx provides a small signal [2]. In addition, studies that directly compare these measurements with changes in cerebral energetics are limited [3, 4]. We have developed a survival piglet model of deep hypothermic circulatory arrest (DHCA) in which we measured cerebral adenosine triphosphate (ATP), phosphocreatine (PCr), and intracellular pH (pHi) with P31 nuclear magnetic resonance spectroscopy (MRS) in parallel with NIRS measurements [5, 6]. Neurologic evaluation was continued for 4 days, at which time the brain was fixed for histology. We previously used this experimental model to evaluate the relations of neurologic outcome with variations of perfusate hematocrit or perfusate colloid oncotic pressure Accepted for publication July 12, 1999. Address reprint requests to Dr Jonas, Department of Cardiovascular Surgery, Children’s Hospital, 300 Longwood Ave, Boston, MA 02115; e-mail: [email protected].
© 2000 by The Society of Thoracic Surgeons Published by Elsevier Science Inc
Material and Methods Experimental Preparation All animals received humane care in compliance with the Principles of Laboratory Animal Care formulated for the National Society for Medical Research and the Guide for the Care and Use of Laboratory Animals prepared by the National Academy of Science (NIH Publication No. 86-23, revised in 1985). Details of the experimental preparations have been described previously [5, 6]. Briefly, 40 5-week-old Yorkshire piglets were anesthetized with intraperitoneal sodium methohexital (45 mg/kg) and intubated. Anesthesia was maintained by continuous infusion of fentanyl (25 0003-4975/00/$20.00 PII S0003-4975(99)01322-3
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g 䡠 kg⫺1 䡠 h⫺1), midazolam (0.2 mg 䡠 kg⫺1 䡠 h⫺1), and pancuronium (0.2 mg 䡠 kg⫺1 䡠 h⫺1) throughout the entire experiment except during the period of circulatory arrest. Before surgery, a 3.0-cm diameter surface coil was sutured on the scalp overlying the cerebral hemispheres, and a pair of fiberoptic optodes for NIRS measurements was attached to the head of the animal. The optode spacing was 3.0 to 3.5 cm in the coronal plane. After systemic heparinization (300 IU/kg), an 8F arterial cannula and a 24F venous cannula were inserted into the right femoral artery and right atrium, respectively. The animal was then placed in a 40-cm diameter horizontal-bore superconducting 4.7 T magnet (Oxford Research System, Oxford, England) and subjected to hypothermic cardiopulmonary bypass (CPB; 40 minutes of cooling, pH-stat strategy, hematocrit between 0.10 and 0.30) and 60 minutes of circulatory arrest at 15°C nasopharyngeal. Upon termination of the experiment after 45 minutes of rewarming, the piglet was weaned off CPB and decannulated outside the magnet. Protamine (6 mg/kg) was administered intravenously. Immediately after decannulation, the animal was repositioned in the bore for 3 hours for MRS and NIRS data collection. After that period, all incisions were closed in a sterile fashion and the animal was left intubated during the first 12 hours postoperatively.
Postoperative Management Postoperatively, all animals remained fully sedated (fentanyl 25 g 䡠 kg⫺1 䡠 h⫺1, midazolam 0.2 mg 䡠 kg⫺1 䡠 h⫺1), paralyzed (pancuronium 0.2 mg 䡠 kg⫺1 䡠 h⫺1), intubated, and monitored (arterial pressure, heart rate) continuously for 12 hours after surgery. At this time chest tubes were removed, infusions discontinued, and the animals weaned from ventilator and extubated.
Data Collection MAGNETIC RESONANCE SPECTROSCOPY. Phosphorus 31 magnetic resonance spectra were acquired on a spectrometer built by the Francis Bitter Magnet Laboratory at the Massachusetts Institute of Technology (MIT) with the Oxford horizontal-bore 4.7 T magnet and surface coil. Each spectrum was the average of 128 acquisitions (1,024 complex data points, repetition rate of 4.65 seconds, spectrum width 5,000 Hz). Total acquisition time was 10 minutes. Peak areas of inorganic phosphate, creatine phosphate, and beta-nucleoside triphosphate were determined by Lorentzian curve fitting and peak integration (NMRI Software, New Methods Research, East Syracuse, NY). Changes in ATP concentration were assessed from the beta-nucleoside triphosphate peak area. The inorganic phosphate, creatine phosphate, and ATP data are reported as percentage of the baseline obtained over 10 minutes before the initiation of CPB. The pHi in the brain was calculated from the chemical shift of the inorganic phosphate peak relative to the creatine phosphate peak. Details of methods employed have been described previously [4, 5].
The optodes transmitted and detected laser light at four near-infrared wavelengths (776 nm, 828 nm, 848 nm, and 913 nm) and connected to
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a near-infrared spectrometer (NIRO-500, Hamamatsu Photonics KK, Hamamatsu, Japan), which calculated concentration changes in HbO2, Hb, total hemoglobin (tHb), and CytOx. These data were recorded continuously every 30 seconds from the initiation of anesthesia induction until 3 hours after the termination of CPB. NEUROLOGIC EVALUATIONS. Neurologic and behavioral evaluations were performed at 24-hour intervals. Neurologic scoring data were adapted from the neurologic deficit score (NDS) and overall performance category (OPC) scale developed at the University of Pittsburgh [7, 8]. In the NDS system, a score of 100 was assigned to each of five general components (central nervous system function, respiration, motor and sensory function, level of consciousness, and behavior). A total score of 500 indicates brain death, whereas a score of 0 is considered normal. The OPC score assessed outcome in five categories: 1 ⫽ normal, 2 ⫽ moderate disability, 3 ⫽ severe disability, 4 ⫽ coma, 5 ⫽ brain death. NDS and OPC determinations were agreed upon by two members of the laboratory team blinded to the perfusion strategies. All animals were observed closely for seizure activity.
The brain of the animal was fixed with perfusion of 4% paraformaldehyde on postoperative day 4. Histologic changes of the brain were evaluated by a pathologist who was unaware of the experimental procedure. After fixation, the brain was cut into eight to ten coronal slabs that were embedded in paraffin. Seven micrometer sections were stained with hematoxylineosin. A standardized list of 24 of the major gray and white matter structures was examined following the nomenclature for porcine neuroanatomy. The structures were scored according to their location within three broad categories (neocortex, hippocampus, and caudate) as described by Yoshikawa [9]. Histologic changes were rated on an arbitrary scale: 0 ⫽ no damage, 1⫹ ⫽ isolated damaged neurons, 2⫹ ⫽ small clusters of damaged neurons, 3⫹ ⫽ large cluster of injured neurons, 4⫹ ⫽ completely damaged neurons, and 5⫹ ⫽ frank cavitated lesions with necrosis. HISTOLOGIC EVALUATIONS.
Statistical Analysis The two experimental studies analyzed were performed according to exactly the same surgical protocol by the same surgeon (T.S.). Data acquisition was performed in the same fashion using the same devices. Furthermore, the original data were accessible for every experiment. Given these circumstances, we are of the opinion that we can treat the two experiments as one to increase the statistical power for the analyses. All results were analyzed by a statistical analysis software package (SPSS for Windows, Version 7.0, SPSS Inc, Chicago, IL). Parametric correlation coefficients according to Pearson were calculated to analyze the MRS and NIRS data. Nonparametric correlation coefficients according to Spearman were used for the correlation analyses of the abovementioned parameters with the NDS, OPC, and histologic score. A p value less than 0.05 was considered statistically significant.
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Fig 1. Time course of adenosine triphosphate (ATP), phosphocreatine (PCr), and intracellular pH (pHi) during the experiments. ATP values are slightly reduced during cooling and drop with the onset of deep hypothermic circulatory arrest (DHCA). ATP values decrease further without reaching a plateau and increase again with rewarming. PCr and pHi values rise with the beginning of cardiopulmonary bypass (CPB). Both parameters decrease quickly during DHCA with PCr storages becoming depleted after 30 minutes of DHCA. With reperfusion PCr and pHi increase and exceed baseline values. (CytOx ⫽ oxidized cytochrome a,a3; DPF ⫽ differential pathlength factor; HbO2 ⫽ oxygenated hemoglobin.)
Results Course of Near-Infrared and Magnetic Resonance Spectroscopic Parameters The parameters showed different characteristics during the cooling phase. HbO2, PCr, and pHi increased significantly, whereas ATP and CytOx decreased. During DHCA all parameters showed a decline. PCr was not detectable at the end of DHCA. After the onset of reperfusion all parameters recovered to baseline values and in part exceeded those (PCr, HbO2). Figures 1 and 2 show the courses of NIRS and MRS parameters during the experiments in detail.
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Fig 2. Time course of oxidized cytochrome a,a3 (CytOx) and oxygenated hemoglobin (HbO2) during the experiments. CytOx values decrease with the onset of cooling and drop with the start of deep hypothermic circulatory arrest (DHCA). They continue to decrease slowly during arrest and reach a plateau at 20 minutes. They increase with rewarming and reperfusion and exceed baseline values at the end of cardiopulmonary bypass (CPB). HbO2 values increase with the onset of cooling on CPB. Deoxygenation takes place immediately after beginning of DHCA. HbO2 rises after start of reperfusion and exceeds baseline values. (ATP ⫽ adenosine triphosphate; PCr ⫽ phosphocreatine; pHi ⫽ intracellular pH.)
⫺0.52 for CytOx, r ⫽ ⫺0.32 for ATP; see Table 2). Assuming a threshold in CytOx reduction for ischemic brain damage of ⫺25 M * differential pathlength factor (DPF), sensitivity of the CytOx signal was 100% and specificity was 70%. The ATP signal showed a sensitivity for ischemic brain damage of 75% and a specificity of 38% at a threshold of 40% of baseline ATP (Fig 6). However, HbO2, Hbt, PCr, and pHi showed no correlations to the histologic score. The neurologic score on the first postoperative day (1 POD) was related significantly to the histologic score.
Comment
Neurologic Results and Histologic Changes
Technical Limitations of Near-Infrared Spectroscopy
The piglets showed only mild neurologic deficits after 60 minutes of DHCA that were most severe on the first postoperative day and regressed completely within 4 days. The OPCs showed also only mild impairments and had normalized by the third postoperative day (Fig 3). Light microscopy revealed mild cerebral neuronal damage on the fourth postoperative day. The average maximum brain damage score was 0.7 ⫾ 0.3. The injury was most pronounced in the neocortex and caudate nucleus (Fig 4).
NIRS is a noninvasive optical method with the capability to measure changes in concentrations of chromophores in tissue [1]. These chromophores absorb the light emit-
Correlation Between Near-Infrared and Magnetic Resonance Spectroscopic Data Relations between NIRS and MRS data at all time points are shown in Table 1 and Figure 5. The changes in ATP, PCr, and pHi showed only slight correlations with Hb or tHb. In contrast, reductions in CytOx or HbO2 were closely correlated with decreases in ATP, PCr, and pHi. The changes in CytOx and PCr showed the closest correlation (r ⫽ 0.623, p ⬍ 0.0001).
Correlation Between Histologic Data and Other Data The histologic score was negatively correlated with minimal CytOx and minimal ATP values during DHCA (r ⫽
Fig 3. Neurologic damage. Postoperatively the piglets were neurologically examined daily by two persons. Sixty minutes of deep hypothermic circulatory arrest produced deficits in the neurologic deficit score (NDS; 0 ⫽ normal; 500 ⫽ brain death) and in the overall performance categories (OPC; 1 ⫽ normal; 5 ⫽ brain death), which were mild and most pronounced on the day of extubation (first postoperative day [POD 1]). On POD 4 the piglets had recovered completely.
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Fig 4. Histopathologic brain damage. After perfusion fixation on postoperative day 4 defined brain regions were analyzed by light microscopy. For assessment a qualitative score system was used: 0 ⫽ no damage, 1⫹ ⫽ isolated damaged neurons, 2⫹ ⫽ small clusters of damaged neurons, 3⫹ ⫽ large cluster of injured neurons, 4⫹ ⫽ completely damaged neurons, 5⫹ ⫽ frank cavitated lesions with necrosis. The highest score in a region is shown. On average 60 minutes of deep hypothermic circulatory arrest caused only mild brain damage.
ted by a NIRS device dependent on their concentrations and the pathlength of the light within the tissue; this relation is well known as the Beer-Lambert law. Oxygenated and deoxygenated hemoglobin have well-defined absorption peaks at 758 nm and 929 nm respectively; at 798 nm the absorption of light of these two chromophores is equal. Cytochrome a,a3 shows a broad absorption peak with a maximum at 830 nm; the absorption spectrum of reduced cytochrome a,a3 is completely flat. Therefore, the measurement of the oxygenation status of hemoglobin can be performed in a relatively simple fashion, whereas the assessment of cytochrome aa3 requires elaborate algorithms and use of multiple wavelengths [10 –12]. The validity of monitoring cerebral oxygenation at a cellular level by measurements of the cytochrome aa3 signal with a NIRS machine that does not give absolute values has been doubted [13, 14]. The pathlength of near-infrared light in brain tissue is influenced by factors such as optode placement, age of the patient, geometry of the skull, brain temperature, edema, and pH. Some of these factors certainly change during our experiments and may even have a different influence on every single Table 1. Pearson’s Correlation Coefficients for Intraoperative Data Obtained by Near-Infrared Spectroscopy and Nuclear Magnetic Resonance Spectroscopya
ATP PCr pHi
CytOx
Hb
HbO2
tHb
0.565 0.623 0.523
⫺0.166 ⫺0.233 ⫺0.193
0.483 0.573 0.501
0.260 0.288 0.252
a
Measurements obtained by near-infrared spectroscopy: CytOx, Hb, HbO2, and tHb. Measurements obtained by nuclear magnetic resonance spectroscopy: ATP, PCr, and pHi. All correlation values were significant at the 0.0001 level (2-tailed). ATP ⫽ adenosine triphosphate; CytOx ⫽ oxidized cytochrome aa3; Hb ⫽ deoxygenated hemoglobin; HbO2 ⫽ oxygenated hemoglobin; PCr ⫽ phosphocreatine; pHi ⫽ intracellular pH; tHb ⫽ total hemoglobin.
Fig 5. Correlation between intraoperative magnetic resonance spectroscopic (MRS) and near-infrared spectroscopic (NIRS) measurements. Forty piglets underwent 40 minutes of cooling, 60 minutes of deep hypothermic circulatory arrest at 15°C, and 45 minutes of rewarming. Changes in brain adenosine triphosphate (ATP) (A) and phosphocreatine (PCr) (B) were determined by MRS every 10 minutes during cardiopulmonary bypass and correlated to changes in the oxidized cytochrome a,a3 (CytOx) NIRS signal. Changes in the high energy phosphates are measured and expressed as percent of baseline values. The variation in the cytochrome signal is described as a change from baseline values in relative chromophore concentration. The value for every measurement at a specific time is shown as mean and standard error of mean. CytOx showed the highest correlation to PCr changes and also demonstrated significant correlation to ATP and intracellular pH variations (see Table 1). (DPF ⫽ differential pathlength factor)
wavelength used for the calculation of the CytOx signal. In addition, the Hamamatsu device (NIRO-500) detects only the change in redox state, but not the absolute value of the redox state. Therefore, the detected correlation between the CytOx values and outcome could be influenced strongly by the changes in pathlength and the scattering properties of the brain. However, a major change in pathlength itself could be an indicator of brain damage. Therefore, it remains uncertain whether a reduction in the CytOx value reflects a decrease of cerebral CytOx or just a change in pathlength, but as is demonstrated in this study, it does indicate brain damage.
Clinical Validation of the Near-Infrared Spectroscopic Signal NIRS technology has been applied in various clinical fields such as neonatology, neurosurgery, and cardiac surgery. It has been used to monitor cerebral oxygen-
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Table 2. Spearman’s Correlation Coefficients for Aggregated Intraoperative Data and Outcome Variablesa Outcome Variables
Intraoperative Variable CytOx (M)
Hb (M)
HbO2 (M)
tHb (M)
ATP (%)
PCr (%)
pHi (%)
Neurologic Deficit Score
Overall Performance Categories
Aggregate Function
Day 1
Day 2
Day 1
Day 2
Histologic Score
Min Max Ave Min Max Ave Min Max Ave Min Max Ave Min Max Ave Min Max Ave Min Max Ave
⫺0.206 0.303 ⫺0.004 0.029 0.051 0.072 0.043 0.005 0.011 ⫺0.060 0.101 0.080 ⫺0.315 ⫺0.251 ⫺0.318 ⫺0.152 0.197 ⫺0.094 0.148 ⫺0.421b ⫺0.442c
⫺0.324 0.049 ⫺0.180 ⫺0.021 ⫺0.100 ⫺0.065 0.018 ⫺0.247 ⫺0.128 ⫺0.200 ⫺0.040 ⫺0.117 ⫺0.155 ⫺0.026 ⫺0.065 0.050 0.249 ⫺0.036 0.128 ⫺0.512b ⫺0.290
⫺0.234 0.115 ⫺0.104 0.157 0.049 0.090 0.086 0.074 0.108 0.094 0.109 0.198 ⫺0.219 ⫺0.343 ⫺0.301 ⫺0.121 0.018 ⫺0.210 0.300 ⫺0.560b ⫺0.479b
⫺0.075 0.047 ⫺0.036 0.060 ⫺0.190 ⫺0.075 0.255 ⫺0.038 0.072 0.127 ⫺0.084 0.000 0.115 ⫺0.113 ⫺0.057 0.136 0.252 ⫺0.005 0.299 ⫺0.486b ⫺0.147
⫺0.524b ⫺0.188 ⫺0.439c 0.198 0.123 0.120 0.063 ⫺0.069 0.030 0.037 0.103 0.174 ⫺0.326c ⫺0.329c ⫺0.330c ⫺0.218 ⫺0.129 ⫺0.260 ⫺0.030 ⫺0.118 ⫺0.363c
a
Measurements obtained by near-infrared spectroscopy: CytOx, Hb, HbO2, and tHb. Measurements obtained by nuclear magnetic resonance spectroscopy: ATP, PCr, and pHi. To correlate outcome with intraoperative parameters, aggregate functions for intraoperative data were used: minimum b c Correlation values were significant at the 0.01 level (2-tailed). Correlation values were (Min), maximum (Max), and average (Ave) values. significant at the 0.05 level (2-tailed). Hb ⫽ deoxygenated hemoglobin; ATP ⫽ adenosine triphosphate; CytOx ⫽ oxidized cytochrome a,a3; PCr ⫽ phosphocreatine; pHi ⫽ intracellular pH; tHb ⫽ total hemoglobin.
ation during cardiac or vascular surgery associated with DHCA. Impaired cerebral oxygen metabolism has been detected by decreased CytOx [15, 16]. Although the correlation between NIRS data and neuropsychologic deficits after cardiac surgery should be strongly assumed, data are very limited [17, 18]. The CytOx signal has been validated in a clinical study showing high correlations to factors of operative management, eg, pH, pCO2, hematocrit, and temperature [19].
Correlations Between Near-Infrared and Magnetic Resonance Spectroscopic Data This metaanalysis of two experimental studies has revealed important correlations between CytOx and nucleoside triphospate, PCr, and pHi. These results are consistent with a previous study from our institution demonstrating this relation in piglets submitted to different stages of hypoxia [3]. We hypothesized that the relation between high energy phosphates and CytOx values might not be linear. However, assumption of a linear curve fit revealed the highest correlation when compared with other nonpolynomial curves. The values for Hb and tHb showed poor correlation to MRS data. In comparison to the CytOx measurements, HbO2 values showed similar, but lower correlation to high energy phosphates.
HbO2 ⫽ oxygenated hemoglobin;
Correlations Between Near-Infrared and Magnetic Resonance Spectroscopic Data and Histology The lowest CytOx value recorded during DHCA proved to be a highly sensitive predictor of histologic brain damage as assessed by light microscopy after 4 days. The predictive value of CytOx was much higher than that of ATP. It is reasonable to assume that a combination of the level and the duration of hypoxia/ischemia causes brain damage. However, the minimum CytOx value was a better predictor for neurologic damage than the average CytOx value during DHCA, duration of reduced CytOx values, or the integral of decreased CytOx values over time. This indicates that there might be a threshold of brain deoxygenation and only the duration of severe ischemia/hypoxia exceeding this deoxygenation threshold causes injury. Other NIRS factors such as HbO2 and Hb failed to show any correlation with histology. This finding supports the thesis that the value of these signals during episodes of increased oxygen affinity to hemoglobin because of hypothermia and alkalosis is doubtful. As these signals are highly related to the brain venous saturation [20], it had been assumed that they provide an index of the adequacy of global cerebral oxygenation [21]. A theoretical approach by computer modeling as well as clinical and experimental findings led to the hypothesis
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Fig 6. Correlation between adenosine-triphosphate (ATP) and oxidized cytochrome a,a3 (CytOx) measurements and neurologic outcome. On postoperative day 4 the brain of the animal was fixed with perfusion of 4% paraformaldehyde. Histologic changes were rated on an arbitrary scale (see Material and Methods). The most severe injury in a brain region of an animal was correlated to the minimum values of CytOx (A) and ATP (B) recorded during the operation. All animals with visible structural brain damage had shown severe changes in the CytOx signal and—less pronounced—the ATP signal. Correlation coefficients are shown in Table 2. (DPF ⫽ differential pathlength factor.)
that high regional tissue oxygen levels estimated by NIRS values during CPB might indicate decreased offloading of oxygen from hemoglobin and not necessarily adequate tissue oxygenation [4, 15, 17, 19, 22].
Conclusion This study indicates that the CytOx signal measured by the NIRO-500 spectrometer might be a better predictor for neuronal damage than cerebral high energy phosphates. In addition, NIRS monitoring is noninvasive, continuous, cost effective, and applicable during operation. It may become the method of choice in brain monitoring during DHCA to predict the safety limit of circulatory arrest. We are very grateful to Miles Tsuji, MD, Department of Neurology, St Joseph’s Hospital, Wisconsin, and David H. Holtzman, MD, PhD, Department of Neurology, Children’s Hospital Boston, for a critical review of the paper and their thoughtful suggestions.
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