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Carbogen inhalation increases oxygen transport to hypoperfused brain tissue in patients with occlusive carotid artery disease
BR A IN RE S EA RCH 1 3 04 ( 20 0 9 ) 9 0 – 95
available at www.sciencedirect.com
www.elsevier.com/locate/brainres
Research Report
Carbogen inhalation increases oxygen transport to hypoperfused brain tissue in patients with occlusive carotid artery disease: Increased oxygen transport to hypoperfused brain Mahmoud Ashkanian a,⁎, Albert Gjedde a,b , Kim Mouridsen a , Manouchehr Vafaee b , Kim Vang Hansen b , Leif Østergaard a , Grethe Andersen c a
Center of Functionally Integrative Neuroscience (CFIN), Aarhus University Hospital, Norrebrogade 44, Bygn. 30, 8000 Aarhus C, Denmark PET Center, Aarhus University Hospital, Denmark c Department of Neurology, Aarhus University Hospital, Denmark b
A R T I C LE I N FO
AB S T R A C T
Article history:
Hyperoxic therapy for cerebral ischemia reduces cerebral blood flow (CBF) principally from
Accepted 18 September 2009
the vasoconstrictive effect of oxygen on cerebral arterioles. Based on a recent study in
Available online 25 September 2009
normal volunteers, we now claim that the vasodilatory effect of carbon dioxide predominates when 5% CO2 is added to inhaled oxygen (the mixture known as carbogen).
Keywords:
In the present study, we measured CBF by positron emission tomography (PET) during
Carbogen
inhalation of test gases (O2, carbogen, and atmospheric air) in healthy volunteers (n = 10) and
Oxygen
in patients with occlusive carotid artery disease (n = 6). Statistical comparisons by an
Cerebral hypoperfusion
additive ANOVA model showed that carbogen significantly increased CBF by 7.51 ± 1.62 ml/
Internal carotid artery disease
100 g/min while oxygen tended to reduce it by −3.22 ± 1.62 ml/100 g/min. A separate analysis
CBF
of the hemisphere contralateral to the hypoperfused hemisphere showed that carbogen
PET
significantly increased CBF by 8.90 ± 2.81 ml/100 g/min whereas oxygen inhalation produced no reliable change in CBF (−1.15 ± 2.81 ml/100 g/min). In both patients and controls, carbogen was as efficient as oxygen in increasing SaO2 or PaO2 values. The study demonstrates that concomitant increases of CBF and SaO2 are readily obtained with carbogen, while oxygen increases only SaO2. Thus, carbogen improves oxygen transport to brain tissue more efficiently than oxygen alone. Further studies with more subjects are, however, needed to investigate the applicability of carbogen for long-term inhalation and to assess its therapeutic benefits in acute stroke patients. © 2009 Elsevier B.V. All rights reserved.
1.
Introduction
Hyperoxic therapy (>21% oxygen) is widely used in medical practice, such as short-term administration in emergency
medicine and chronic treatment of obstructive pulmonary disease. Trials have tested the applicability of normobaric (Flynn and Auer, 2002; Singhal et al., 2002; Kim et al., 2005; Singhal, 2007; Flynn and Auer, 2002) and hyperbaric (Schabitz
⁎ Corresponding author. Fax: +0045 8949 4400. E-mail address: [email protected] (M. Ashkanian). 0006-8993/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.brainres.2009.09.076
BR A IN RE S E A RCH 1 3 04 ( 20 0 9 ) 9 0 – 9 5
et al., 2004; Lou et al., 2004; Badr et al., 2001) oxygen therapy in salvaging ischemic brain tissue. Conflicting results reported by these studies have, however, called into question the efficacy of oxygen treatment in patients with cerebral ischemia. Importantly, some studies have shown that pure oxygen inhalation lowers cerebral blood flow (CBF) (Rostrup et al., 1995; Watson et al., 2000). Tissue oxygenation depends not only on the oxygen content of the blood (SaO2) but also on blood flow to the tissue. Previous studies have shown that the gas mixture of 5% CO2 and 95% O2 (i.e., carbogen) raises blood flow to normal brain tissue (Macey et al., 2007) and tumors (Kaanders et al., 1998; Siemann, 1998). Recently, we showed that carbogen differs from oxygen by causing concomitant improvement of CBF and SaO2 in healthy adults (Ashkanian et al., 2008). The purpose of the present study was to determine whether carbogen improves tissue oxygenation also in hemodynamically impaired brain tissue. CBF of normal gray matter (GM) is approximately 60 ml/100 g/ min. Whereas ischemia refers to a restriction in blood supply with resultant neuronal damage or dysfunction, hypoperfusion is used in the literature as a general term covering a wide spectrum of graded reduction of CBF. The most sensitive early sign of hypoperfusion is protein synthesis, which is inhibited by 50% at cortical CBF around 55 ml/100 g/min, corresponding to a CBF reduction by 8%. In our study, tissue hypoperfusion is defined as CBF at least 10% below the average baseline CBF for GM in the control group. Protein synthesis is completely suppressed at blood flows below 35 ml/100 g/min. At lower flow rates, glucose utilization transiently increases before it sharply declines at CBF below 25 ml/100 g/min (Hossmann, 2006). Here, we use positron emission tomography (PET) to measure CBF changes and SaO2 levels in patients with occlusive carotid artery disease as well as in control subjects during
91
inhalation of three different gases, namely, carbogen, oxygen, and atmospheric air (baseline).
2.
Results
As expected, inhalation of oxygen caused CBF in GM to decline in patients as well as control subjects, while inhalation of carbogen caused CBF to increase from baseline. Fig. 1 illustrates the CBF changes that occurred in both hemispheres (ipsilateral and contralateral to the hypoperfused side) in response to each of the three different gases in one patient compared to average changes in the control group (n = 10). Average CBF values of the hypoperfused regions (patients) are shown in Fig. 2, with the average CBF values of the healthy controls shown for comparison. No significant interactions were found between group and condition ( P = 0.25), implying similar effects of oxygen and carbogen in the two groups. Comparisons in the additive ANOVA model showed that carbogen significantly increased CBF (7.51 ± 1.62 ml/100 g/min; P < 0.01), while CBF tended to decline following oxygen inhalation (−3.22 ± 1.62 ml/100 g/min; P = 0.06). We found that carbogen raised average CBF in all patients relative to baseline, whereas oxygen caused CBF to decrease in 4 of 6 patients. In the control group, carbogen increased CBF in 9 of 10 subjects, whereas oxygen decreased CBF in 9 of 10 of them. Separate analysis of the hemisphere contralateral to the hypoperfused hemisphere showed that CBF rose by 8.90 ± 2.81 ml/100 g/min (P = 0.01) in the carbogen condition compared to no reliable change in the oxygen condition (−1.15 ± 2.81 ml/100 g/min). As with the ipsilateral hemisphere, carbogen increased average CBF in all cases relative to baseline, while oxygen caused CBF to decrease in 4 of 6 cases.
Fig. 1 – CBF changes in one patient with occlusive carotid artery disease compared to averaged CBF changes in ten healthy controls.
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Fig. 2 – Quantitative changes of CBF during inhalation of carbogen and oxygen relative to atmospheric air (baseline) in patients (ipsilateral and contralateral hemisphere, n = 6) and controls (n = 10). In patients as well as controls, carbogen was as efficient as oxygen to increase SaO2 and PaO2 levels. In the patients, the mean SaO2 rose significantly from 95.2 ± 1.75% at baseline to 99.65 ± 0.19% (P < 0.01) and 99.62 ± 0.19% (P < 0.01) during inhalation of oxygen and carbogen, respectively, and the mean PaO2 increased from 10.0 ± 1.0 kPa at baseline to 65.6 ± 9.9 (P < 0.01) kPa and 71.8 ± 11.0 (P < 0.01) kPa, respectively. In control subjects, the mean SaO2 increased from 97.2 ± 0.65% at baseline to 99.89 ± 0.16% (P < 0.01) and 99.8 ± 0.18% (P < 0.01) during inhalation of oxygen and carbogen, respectively, whereas the mean PaO2 increased from 11.6 ± 0.79 kPa at baseline to 59.3 ±16.7 (P < 0.01) kPa and 61.4 ± 17.1 (P < 0.01) kPa, respectively.
3.
Discussion
The present study shows that carbogen significantly raised CBF, while oxygen inhalation tended to lower CBF in healthy subjects and patients with occlusive carotid artery disease. Since no interactions emerged between groups and conditions, we pooled the results of patients and controls for statistical analysis in an additive model. The effect of carbogen on CBF augmentation was observed more consistently than the usually deleterious effect of oxygen, although oxygen paradoxically raised CBF in 1 of 10 controls and 2 of 6 patients. In patients, inhalation of carbogen and oxygen affected CBF in both hemispheres (the hypoperfused and the contralateral side). While oxygen lowered CBF in 4 of 6 patients, carbogen enhanced CBF in all. Concomitant measurements of arterial blood gases showed that in patients as well as controls, carbogen was as efficient as oxygen in increasing SaO2 and PaO2 levels.
3.1. Effect of oxygen versus carbogen on cerebral oxygenation The amount of oxygen carried by the cerebral circulation depends on oxygen content of the blood as well as CBF.
Therapeutic effects of oxygen therapy in patients with ischemic–hypoxic brain tissue is explained by the relief of negligible mitochondrial oxygen tensions by even minimal increases of arterial oxygen tension that facilitate oxygen diffusion into the intracellular space (Hempel et al., 1977). However, studies on the ability of normobaric (Singhal et al., 2002; Kim et al., 2005; Singhal, 2007; Flynn and Auer, 2002) and hyperbaric (Schabitz et al., 2004; Lou et al., 2004; Badr et al., 2001) oxygen therapy to salvage ischemic brain tissue have had conflicting results that call into question the efficacy of oxygen inhalation for patients with cerebral ischemia. Some researchers have reported that treatment of stroke patients with oxygen supplementation actually worsens clinical outcome (Rusyniak et al., 2003; Mickel et al., 1987; Bromont et al., 1989). It is well-known that administration of pure oxygen decreases CBF in healthy volunteers (Watson et al., 2000). On the other hand, because CO2 acts as a vasodilator, its addition to oxygen may counteract the oxygen-dependent reduction in CBF (Iscoe and Fisher, 2005). In a previous PET study, we showed that carbogen caused concomitant increases of CBF and SaO2 in healthy adults (Ashkanian et al., 2008). It is noteworthy that the ability of carbogen to increase oxygenation of normal brain tissue was recently demonstrated also by fMRI in healthy children (Macey et al., 2007). Likewise, oxygensensitive microelectrodes have shown that carbogen increases optic disc PaO2 significantly more than hyperoxia in minipigs (Petropoulos et al., 2005).
3.2.
Paradoxical vasoreactivity to oxygen versus carbogen
The acute cerebrovascular responses (CVRs) to CO2 and O2 in ischemic brain tissue do not always follow the rules observed in nonischemic tissues. As CVRs may be exhausted in ischemic regions and maintained in nonischemic zones, there is a risk that inhaled CO2 redirects circulating blood away from ischemic tissue, jeopardizing oxygen delivery in affected tissue (i.e., steal phenomenon). On the other hand, inhalation of pure oxygen may, in some cases, result in a paradoxical vasodilation, possibly because of reversed vasomotor reactivity (Nakajima et al., 1983). However, we observed no evidence of stealing in the present study, as carbogen increased CBF of both hemispheres of all patients. In contrast, 2 of 6 patients had a paradoxical reaction to oxygen in the form of elevated CBF, and in the control group, oxygen inhalation increased CBF in one of ten subjects. The observation that oxygen decreased CBF in controls but had a less deleterious effect on CBF in patients is interesting, because it indicates an underlying pathology in mechanisms controlling CBF in this patient population. The physiological mechanisms underlying vascular reactivity to O2 and CO2 are, however, poorly understood.
3.3.
Possible limitations for clinical use of carbogen
The present study involved patients with cerebral hypoperfusion due to severe carotid stenosis, yet with some vascular reserve capacity. The results of this study may therefore not apply to patients with ischemic brain tissue. Future studies of carbogen must determine whether vascular adaptations limit therapeutic effects of long-term administration, since acute CVRs are not always maintained during various types of
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prolonged treatments (Christensen, 1974). In addition, adverse effects or discomfort due to inhaling CO2 may limit the clinical use of carbogen, although we did not observe significant changes in pulse, blood pressure, SaO2, ventilation, alertness/consciousness, or anxiety during inhalation of carbogen, except for one subject who reported mild headache. Accumulation of CO2 due to a higher dose or longer inhalation time would be undesirable in certain clinical situations, such as patients with elevated intracranial pressure. In particular, long-term use of CO2 inhalation may require safety studies. The subjects in our study were exposed to only a few minutes of inhalation of carbogen mixture with 5% CO2. Our results therefore cannot rule out possible side effects of long-term inhalation of carbogen. Further studies with a larger number of subjects are needed to confirm the beneficial effects of carbogen on oxygen content of cerebral circulation and to investigate possible toxic effects of long-term use of carbogen on different organ systems including brain (e.g., intracranial pressure), heart (e.g., heart rate, cardiac output), lungs (e.g., respiratory frequency, tidal volume), and kidney (diuresis, acidity, perfusion).
± 2.45 years), were recruited by Department of Neurology at Aarhus University Hospital. The diagnosis was made by MR and ultrasonic evaluation of carotid arteries. Based on diagnostic as well as clinical findings, patients with suspected misery perfusion (a clinical condition where an increase in oxygen extraction fraction is caused by a decrease in cerebral blood flow, presumably due to a reduction in cerebral perfusion pressure and disturbance of cerebral autoregulation) were recruited to the study. Patient data regarding age, gender, symptoms, imaging, and diagnosis are summarized in Table 1. Exclusion criteria included history of stroke, current cardiovascular, pulmonary or hematological disease, smoking, pregnancy, breastfeeding, and contraindications to MRI (e.g., pacemaker, metal prosthesis, claustrophobia). All subjects underwent medical (including ECG) and neurological examinations before and after participation in the project. National Institutes of Health Stroke Scale (NIHSS) rating was performed.
4.1.2. 3.4.
Summary
Concomitant increases of CBF and SaO2 are readily obtained with carbogen, which is far more effective than oxygen in improving oxygenation of hypoperfused brain tissue. The present results are, however, insufficient to comment on possible therapeutic benefits of carbogen in acute stroke patients. Further studies are needed to (1) verify the present findings, (2) further test the applicability of carbogen in ischemic brain tissue, (3) investigate possible adverse side effects of CO2 during long-term use of carbogen, and (4) assess whether prolonged inhalation of carbogen results in vascular or metabolic adaptations.
4.
Experimental procedures
4.1.
Subjects
4.1.1.
Patients
Six patients with occlusive carotid artery disease (five men and one women), aged between 40 and 80 years (average: 66.5
Healthy subjects
Ten healthy volunteers (5 men and 5 women), aged between 40 and 80 years (mean and SD: 53.4 ± 9.4 years), were recruited by advertisement in a local newspaper. Exclusion criteria included the same conditions as in patients. All subjects underwent medical (including ECG) and neurological examinations before and after participation in the project. National Institutes of Health Stroke Scale (NIHSS) rating was performed, and only subjects with an NIHSS = 0 were included. MRI scans were evaluated by neuroradiologists to exclude the presence of overt brain abnormalities. Written informed consent was obtained from both patients and controls, as approved by the Research Ethics Committee of County Aarhus.
4.2.
PET measurements
PET scans were acquired with a ECAT EXACT HR 47 (CTI/ Siemens) whole-body tomograph, operated in 3D mode, with a transverse resolution of 3.6–7.4 mm and an axial resolution of 4.0–6.7 mm. Images were reconstructed as 128 × 128 matrices of 2 × 2 mm pixels using filtered back-
Table 1 – Patient data describing age, gender, symptoms, and diagnostic imaging findings. Patients: age, gender
Symptoms
(1) 63 years old, male
Episodic left-sided hemiparesis
(2) 58 years old, male
Intermitting blindness (amourosis fugax), right-sided Episodic left-sided hemiparesis Episodic left-sided hemiparesis
(3) 78 years old, male (4) 66 years old, male (5) 67 years old, female (6) 68 years old, male
Episodes of amourosis fugax, right-sided Intermitting amourosis fugax and left-sided hemiparesis
Imaging
Diagnosis
PWI, DWI, and MRA Ultrasound PWI, MRA, Ultrasound
Occluded right ICA and stenotic right MCA
PWI, MRA, Ultrasound DWI, MRA, PET-Diamox test MRA, Ultrasound MRA, Ultrasound
Occluded right ICA, stenotic right ACA Occluded right ICA, stenotic right MCA, Reduced reduced CVR Occluded right ICA and CCA. Occluded right ICA and stenotic left ICA
90% stenosis of right ICA
PWI: perfusion-weighted imaging, DWI: diffusion-weighted imaging, MRA: magnetic resonance angiography, ICA: internal carotid artery, ACA: anterior carotid artery, CCA: common carotid artery, CVR: cerebrovascular reserve capacity.
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projection with a 0.5 cycles−1 ramp filter full width half maximum (FWHM), followed by smoothing with a 6-mm Gaussian filter, resulting in an isotropic resolution of 7 mm. The reconstructed images were corrected for random and scattered events, detector efficiency variations, and dead time. Tissue attenuation correction scans were performed using a rotating 68Ge source. The head of each subject was comfortably immobilized in a customized head holder (Vac-Lock; MED-TECH). Catheters were inserted in the left radial artery for arterial blood sampling and right antecubital vein for tracer injection. The arterial blood radioactivity was measured by an automated blood sampling system and then corrected for external delay and dispersion caused by bolus distortion in the sampling catheter. Dynamic 3-min emission recordings consisting of 21 frames were initiated upon bolus intravenous injection of [15O]-H2O (500 MBq).
4.3.
Study design
In this study, one session of PET measurements consisted of three [15O]-H2O scans; one baseline during which subjects inhaled normal atmospheric air followed by two consecutive measurements in which subjects inspired each of the test gases (100% O2, 5% CO2 + 95% O2) in a random order. Subjects were informed of test procedures and possible side effects and were familiarized with the scan room, personnel, and use of face mask. During the entire experiment, the subjects were breathing via a mask connected to a gas administration unit, which supplied the subjects with inhalation gas at a rate about 7 l/min. PET scans were obtained at intervals of 10 min while subjects rested in a supine position on the scanner bed. Ninety seconds prior to each PET scan, the subjects started breathing the test gas, which was delivered to the mask by the gas application unit. Recordings were obtained with eyes open, and the subjects were instructed to fixate their gaze on a cross-hair during the recordings.
4.4.
Subject monitoring
Throughout the entire experiment, the subjects were monitored by medical staff, who noted reports of possible discomfort (e.g., dyspnea, anxiety, headache) or observations of altered consciousness/alertness, pulse, and SaO2. Standard biochemical parameters were measured prior to the study, and arterial blood gas samples were analyzed at the end of each scan. Blood pressure and pulse were monitored continuously and were recorded throughout the experiment. To compare effects of oxygen and carbogen on oxygen content of the blood, we measured the steady-state oxygen saturation (SaO2) and oxygen tension (PO2) in arterial blood samples during administration of each gas.
4.5.
T1-weighted MR was acquired with a 3. 0-T Signa Excite GE imager using a 3D IR-FSPGR sequence (256 × 256 matrix, TE = min full, TI = 750, slice thickness = 1.2 mm). Standard T2-FLAIR images were acquired to rule out other cerebral pathology.
4.6.
Data analysis
4.6.1.
Coregistration
The T1-weighted MRI images of the brain of each subject were co-registered to an MR template defined by the brain of 85 young adults in Talairach space, using a combination of linear and nonlinear transformations (Grabner et al., 2006). Each summed PET emission recording was then linearly co-registered to the corresponding MR image using automated algorithms by registering the first summed PET recording to MR and then each sequential summed PET recording to the first PET recording. PET images were subsequently re-sampled to standard stereotaxic space.
4.6.2.
Volumes of interest
Using arterial time–activity curves corrected for delay and dispersion, we calculated parametric maps of CBF with the single-step, two-compartment, three-weighted integration method (Ohta et al., 1992, 1996). The corrections apply a global adjustment of the timing difference between the measured arterial curve and the PET scan. We searched the absolute CBF values in the parametric maps. Baseline scans were used to identify hypoperfused cortical regions in the affected hemisphere. Volumes of interest (VOIs) containing hypoperfused lesions were defined as regions with CBF values 10% below the average baseline CBF values for grey matter (GM) in the control group. Multiplying the outlined VOIs with a GM mask produced GM-VOIs, which encompassed the GM portion of the hypoperfused area. All operations were performed in Talairach space. For each test gas, we extracted and averaged CBF values by applying the generated GM-VOIs to the corresponding co-registered parametric CBF maps. Data from a previous PET study (Ashkanian et al., 2008) with identical inhalation and scanning routine in ten healthy volunteers were used as control. We analyzed the effect of oxygen and carbogen on CBF in ischemic and control GM using a mixed-effects model. Subjects were considered as a random factor to model the correlation between measurements performed on the same individual.
Sources of funding This study was supported by The Danish National Research Foundation, The Danish Medical Research Council, The Velux Foundation, and The Toyota Foundation.
MRI measurements
Conflicts on interest disclosures Brain MRI scans of each subject were obtained on a separate day for PET image co-registration. High-resolution
There is no conflict of interest in this study.
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Acknowledgments We wish to thank Michael Geneser for helping with MRI protocol, the technicians and chemists at PET Centre at Aarhus University Hospital for their technical support, and Dr. Donald Smith for his linguistic revision of the manuscript.
REFERENCES
Ashkanian, M., Borghammer, P., Gjedde, A., Ostergaard, L., Vafaee, M., 2008. Improvement of brain tissue oxygenation by inhalation of carbogen. Neuroscience 156, 932–938. Badr, A.E., Yin, W., Mychaskiw, G., Zhang, J.H., 2001. Dual effect of HBO on cerebral infarction in MCAO rats. Am. J. Physiol. Regul. Integr. Comp. Physiol. 280, R766–R770. Bromont, C., Marie, C., Bralet, J., 1989. Increased lipid peroxidation in vulnerable brain regions after transient forebrain ischemia in rats. Stroke 20, 918–924. Christensen, M.S., 1974. Acid–base changes in cerebrospinal fluid and blood, and blood volume changes following prolonged hyperventilation in man. Br. J. Anaesth. 46, 348–357. Flynn, E.P., Auer, R.N., 2002. Eubaric hyperoxemia and experimental cerebral infarction. Ann. Neurol. 52, 566–572. Grabner G., Janke A.L., Budge M.M., Smith D., Pruessner J., Collins D.L., 2006. Symmetric atlasing and model based segmentation: an application to the hippocampus in older adults. Medical image computing and computer-assisted intervention: MICCAI. International Conference on Medical Image Computing and Computer-Assisted Intervention 4191 (Pt 2), 58–66. Hempel, F.G., Jobsis, F.F., LaManna, J.L., Rosenthal, M.R., Saltzman, H.A., 1977. Oxidation of cerebral cytochrome aa3 by oxygen plus carbon dioxide at hyperbaric pressures. J. Appl. Physiol. 43, 873–879. Hossmann, K.A., 2006. Pathophysiology and therapy of experimental stroke. Cell. Mol. Neurobiol. 26, 1057–1083. Iscoe, S., Fisher, J.A., 2005. Hyperoxia-induced hypocapnia: an underappreciated risk. Chest 128, 430–433. Kaanders, J.H., Pop, L.A., Marres, H.A., Liefers, J., van den Hoogen, F.J., van Daal, W.A., van der Kogel, A.J., 1998. Accelerated radiotherapy with carbogen and nicotinamide (ARCON) for laryngeal cancer. Radiother. Oncol. 48, 115–122. Kim, H.Y., Singhal, A.B., Lo, E.H., 2005. Normobaric hyperoxia extends the reperfusion window in focal cerebral ischemia. Ann. Neurol. 57, 571–575.
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Lou, M., Eschenfelder, C.C., Herdegen, T., Brecht, S., Deuschl, G., 2004. Therapeutic window for use of hyperbaric oxygenation in focal transient ischemia in rats. Stroke 35, 578–583. Macey, P.M., Woo, M.A., Harper, R.M., 2007. Hyperoxic brain effects are normalized by addition of CO2. PLoS Med. e173, 4. Mickel, H.S., Vaishnav, Y.N., Kempski, O., von Lubitz, D., Weiss, J.F., Feuerstein, G., 1987. Breathing 100% oxygen after global brain ischemia in Mongolian Gerbils results in increased lipid peroxidation and increased mortality. Stroke 18, 426–430. Nakajima, S., Meyer, J.S., Amano, T., Shaw, T., Okabe, T., Mortel, K.F., 1983. Cerebral vasomotor responsiveness during 100% oxygen inhalation in cerebral ischemia. Arch. Neurol. 40, 271–276. Ohta, S., Meyer, E., Thompson, C.J., Gjedde, A., 1992. Oxygen consumption of the living human brain measured after a single inhalation of positron emitting oxygen. J. Cereb. Blood Flow Metab. 12, 179–192. Ohta, S., Meyer, E., Fujita, H., Reutens, D.C., Evans, A., Gjedde, A., 1996. Cerebral [15O]water clearance in humans determined by PET: I. Theory and normal values. J. Cereb. Blood Flow Metab. 16, 765–780. Petropoulos, I.K., Pournaras, J.A., Munoz, J.L., Pournaras, C.J., 2005. Effect of carbogen breathing and acetazolamide on optic disc PO2. Invest. Ophthalmol. Vis. Sci. 46, 4139–4146. Rostrup, E., Larsson, H.B., Toft, P.B., Garde, K., Henriksen, O., 1995. Signal changes in gradient echo images of human brain induced by hypo- and hyperoxia. NMR Biomed. 8, 41–47. Rusyniak, D.E., Kirk, M.A., May, J.D., Kao, L.W., Brizendine, E.J., Welch, J.L., Cordell, W.H., Alonso, R.J., 2003. Hyperbaric oxygen therapy in acute ischemic stroke: results of the Hyperbaric Oxygen in Acute Ischemic Stroke Trial Pilot Study. Stroke 34, 571–574. Schabitz, W.R., Schade, H., Heiland, S., Kollmar, R., Bardutzky, J., Henninger, N., Muller, H., Carl, U., Toyokuni, S., Sommer, C., Schwab, S., 2004. Neuroprotection by hyperbaric oxygenation after experimental focal cerebral ischemia monitored by MRI. Stroke 35, 1175–1179. Siemann, D.W., 1998. The tumor microenvironment: a double-edged sword. Int. J. Radiat. Oncol. Biol. Phys. 42, 697–699. Singhal, A.B., 2007. A review of oxygen therapy in ischemic stroke. Neurol. Res. 29, 173–183. Singhal, A.B., Wang, X., Sumii, T., Mori, T., Lo, E.H., 2002. Effects of normobaric hyperoxia in a rat model of focal cerebral ischemia–reperfusion. J. Cereb. Blood Flow Metab. 22, 861–868. Watson, N.A., Beards, S.C., Altaf, N., Kassner, A., Jackson, A., 2000. The effect of hyperoxia on cerebral blood flow: a study in healthy volunteers using magnetic resonance phase-contrast angiography. Eur. J. Anaesthesiol. 17, 152–159.
available at www.sciencedirect.com
www.elsevier.com/locate/brainres
Research Report
Carbogen inhalation increases oxygen transport to hypoperfused brain tissue in patients with occlusive carotid artery disease: Increased oxygen transport to hypoperfused brain Mahmoud Ashkanian a,⁎, Albert Gjedde a,b , Kim Mouridsen a , Manouchehr Vafaee b , Kim Vang Hansen b , Leif Østergaard a , Grethe Andersen c a
Center of Functionally Integrative Neuroscience (CFIN), Aarhus University Hospital, Norrebrogade 44, Bygn. 30, 8000 Aarhus C, Denmark PET Center, Aarhus University Hospital, Denmark c Department of Neurology, Aarhus University Hospital, Denmark b
A R T I C LE I N FO
AB S T R A C T
Article history:
Hyperoxic therapy for cerebral ischemia reduces cerebral blood flow (CBF) principally from
Accepted 18 September 2009
the vasoconstrictive effect of oxygen on cerebral arterioles. Based on a recent study in
Available online 25 September 2009
normal volunteers, we now claim that the vasodilatory effect of carbon dioxide predominates when 5% CO2 is added to inhaled oxygen (the mixture known as carbogen).
Keywords:
In the present study, we measured CBF by positron emission tomography (PET) during
Carbogen
inhalation of test gases (O2, carbogen, and atmospheric air) in healthy volunteers (n = 10) and
Oxygen
in patients with occlusive carotid artery disease (n = 6). Statistical comparisons by an
Cerebral hypoperfusion
additive ANOVA model showed that carbogen significantly increased CBF by 7.51 ± 1.62 ml/
Internal carotid artery disease
100 g/min while oxygen tended to reduce it by −3.22 ± 1.62 ml/100 g/min. A separate analysis
CBF
of the hemisphere contralateral to the hypoperfused hemisphere showed that carbogen
PET
significantly increased CBF by 8.90 ± 2.81 ml/100 g/min whereas oxygen inhalation produced no reliable change in CBF (−1.15 ± 2.81 ml/100 g/min). In both patients and controls, carbogen was as efficient as oxygen in increasing SaO2 or PaO2 values. The study demonstrates that concomitant increases of CBF and SaO2 are readily obtained with carbogen, while oxygen increases only SaO2. Thus, carbogen improves oxygen transport to brain tissue more efficiently than oxygen alone. Further studies with more subjects are, however, needed to investigate the applicability of carbogen for long-term inhalation and to assess its therapeutic benefits in acute stroke patients. © 2009 Elsevier B.V. All rights reserved.
1.
Introduction
Hyperoxic therapy (>21% oxygen) is widely used in medical practice, such as short-term administration in emergency
medicine and chronic treatment of obstructive pulmonary disease. Trials have tested the applicability of normobaric (Flynn and Auer, 2002; Singhal et al., 2002; Kim et al., 2005; Singhal, 2007; Flynn and Auer, 2002) and hyperbaric (Schabitz
⁎ Corresponding author. Fax: +0045 8949 4400. E-mail address: [email protected] (M. Ashkanian). 0006-8993/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.brainres.2009.09.076
BR A IN RE S E A RCH 1 3 04 ( 20 0 9 ) 9 0 – 9 5
et al., 2004; Lou et al., 2004; Badr et al., 2001) oxygen therapy in salvaging ischemic brain tissue. Conflicting results reported by these studies have, however, called into question the efficacy of oxygen treatment in patients with cerebral ischemia. Importantly, some studies have shown that pure oxygen inhalation lowers cerebral blood flow (CBF) (Rostrup et al., 1995; Watson et al., 2000). Tissue oxygenation depends not only on the oxygen content of the blood (SaO2) but also on blood flow to the tissue. Previous studies have shown that the gas mixture of 5% CO2 and 95% O2 (i.e., carbogen) raises blood flow to normal brain tissue (Macey et al., 2007) and tumors (Kaanders et al., 1998; Siemann, 1998). Recently, we showed that carbogen differs from oxygen by causing concomitant improvement of CBF and SaO2 in healthy adults (Ashkanian et al., 2008). The purpose of the present study was to determine whether carbogen improves tissue oxygenation also in hemodynamically impaired brain tissue. CBF of normal gray matter (GM) is approximately 60 ml/100 g/ min. Whereas ischemia refers to a restriction in blood supply with resultant neuronal damage or dysfunction, hypoperfusion is used in the literature as a general term covering a wide spectrum of graded reduction of CBF. The most sensitive early sign of hypoperfusion is protein synthesis, which is inhibited by 50% at cortical CBF around 55 ml/100 g/min, corresponding to a CBF reduction by 8%. In our study, tissue hypoperfusion is defined as CBF at least 10% below the average baseline CBF for GM in the control group. Protein synthesis is completely suppressed at blood flows below 35 ml/100 g/min. At lower flow rates, glucose utilization transiently increases before it sharply declines at CBF below 25 ml/100 g/min (Hossmann, 2006). Here, we use positron emission tomography (PET) to measure CBF changes and SaO2 levels in patients with occlusive carotid artery disease as well as in control subjects during
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inhalation of three different gases, namely, carbogen, oxygen, and atmospheric air (baseline).
2.
Results
As expected, inhalation of oxygen caused CBF in GM to decline in patients as well as control subjects, while inhalation of carbogen caused CBF to increase from baseline. Fig. 1 illustrates the CBF changes that occurred in both hemispheres (ipsilateral and contralateral to the hypoperfused side) in response to each of the three different gases in one patient compared to average changes in the control group (n = 10). Average CBF values of the hypoperfused regions (patients) are shown in Fig. 2, with the average CBF values of the healthy controls shown for comparison. No significant interactions were found between group and condition ( P = 0.25), implying similar effects of oxygen and carbogen in the two groups. Comparisons in the additive ANOVA model showed that carbogen significantly increased CBF (7.51 ± 1.62 ml/100 g/min; P < 0.01), while CBF tended to decline following oxygen inhalation (−3.22 ± 1.62 ml/100 g/min; P = 0.06). We found that carbogen raised average CBF in all patients relative to baseline, whereas oxygen caused CBF to decrease in 4 of 6 patients. In the control group, carbogen increased CBF in 9 of 10 subjects, whereas oxygen decreased CBF in 9 of 10 of them. Separate analysis of the hemisphere contralateral to the hypoperfused hemisphere showed that CBF rose by 8.90 ± 2.81 ml/100 g/min (P = 0.01) in the carbogen condition compared to no reliable change in the oxygen condition (−1.15 ± 2.81 ml/100 g/min). As with the ipsilateral hemisphere, carbogen increased average CBF in all cases relative to baseline, while oxygen caused CBF to decrease in 4 of 6 cases.
Fig. 1 – CBF changes in one patient with occlusive carotid artery disease compared to averaged CBF changes in ten healthy controls.
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Fig. 2 – Quantitative changes of CBF during inhalation of carbogen and oxygen relative to atmospheric air (baseline) in patients (ipsilateral and contralateral hemisphere, n = 6) and controls (n = 10). In patients as well as controls, carbogen was as efficient as oxygen to increase SaO2 and PaO2 levels. In the patients, the mean SaO2 rose significantly from 95.2 ± 1.75% at baseline to 99.65 ± 0.19% (P < 0.01) and 99.62 ± 0.19% (P < 0.01) during inhalation of oxygen and carbogen, respectively, and the mean PaO2 increased from 10.0 ± 1.0 kPa at baseline to 65.6 ± 9.9 (P < 0.01) kPa and 71.8 ± 11.0 (P < 0.01) kPa, respectively. In control subjects, the mean SaO2 increased from 97.2 ± 0.65% at baseline to 99.89 ± 0.16% (P < 0.01) and 99.8 ± 0.18% (P < 0.01) during inhalation of oxygen and carbogen, respectively, whereas the mean PaO2 increased from 11.6 ± 0.79 kPa at baseline to 59.3 ±16.7 (P < 0.01) kPa and 61.4 ± 17.1 (P < 0.01) kPa, respectively.
3.
Discussion
The present study shows that carbogen significantly raised CBF, while oxygen inhalation tended to lower CBF in healthy subjects and patients with occlusive carotid artery disease. Since no interactions emerged between groups and conditions, we pooled the results of patients and controls for statistical analysis in an additive model. The effect of carbogen on CBF augmentation was observed more consistently than the usually deleterious effect of oxygen, although oxygen paradoxically raised CBF in 1 of 10 controls and 2 of 6 patients. In patients, inhalation of carbogen and oxygen affected CBF in both hemispheres (the hypoperfused and the contralateral side). While oxygen lowered CBF in 4 of 6 patients, carbogen enhanced CBF in all. Concomitant measurements of arterial blood gases showed that in patients as well as controls, carbogen was as efficient as oxygen in increasing SaO2 and PaO2 levels.
3.1. Effect of oxygen versus carbogen on cerebral oxygenation The amount of oxygen carried by the cerebral circulation depends on oxygen content of the blood as well as CBF.
Therapeutic effects of oxygen therapy in patients with ischemic–hypoxic brain tissue is explained by the relief of negligible mitochondrial oxygen tensions by even minimal increases of arterial oxygen tension that facilitate oxygen diffusion into the intracellular space (Hempel et al., 1977). However, studies on the ability of normobaric (Singhal et al., 2002; Kim et al., 2005; Singhal, 2007; Flynn and Auer, 2002) and hyperbaric (Schabitz et al., 2004; Lou et al., 2004; Badr et al., 2001) oxygen therapy to salvage ischemic brain tissue have had conflicting results that call into question the efficacy of oxygen inhalation for patients with cerebral ischemia. Some researchers have reported that treatment of stroke patients with oxygen supplementation actually worsens clinical outcome (Rusyniak et al., 2003; Mickel et al., 1987; Bromont et al., 1989). It is well-known that administration of pure oxygen decreases CBF in healthy volunteers (Watson et al., 2000). On the other hand, because CO2 acts as a vasodilator, its addition to oxygen may counteract the oxygen-dependent reduction in CBF (Iscoe and Fisher, 2005). In a previous PET study, we showed that carbogen caused concomitant increases of CBF and SaO2 in healthy adults (Ashkanian et al., 2008). It is noteworthy that the ability of carbogen to increase oxygenation of normal brain tissue was recently demonstrated also by fMRI in healthy children (Macey et al., 2007). Likewise, oxygensensitive microelectrodes have shown that carbogen increases optic disc PaO2 significantly more than hyperoxia in minipigs (Petropoulos et al., 2005).
3.2.
Paradoxical vasoreactivity to oxygen versus carbogen
The acute cerebrovascular responses (CVRs) to CO2 and O2 in ischemic brain tissue do not always follow the rules observed in nonischemic tissues. As CVRs may be exhausted in ischemic regions and maintained in nonischemic zones, there is a risk that inhaled CO2 redirects circulating blood away from ischemic tissue, jeopardizing oxygen delivery in affected tissue (i.e., steal phenomenon). On the other hand, inhalation of pure oxygen may, in some cases, result in a paradoxical vasodilation, possibly because of reversed vasomotor reactivity (Nakajima et al., 1983). However, we observed no evidence of stealing in the present study, as carbogen increased CBF of both hemispheres of all patients. In contrast, 2 of 6 patients had a paradoxical reaction to oxygen in the form of elevated CBF, and in the control group, oxygen inhalation increased CBF in one of ten subjects. The observation that oxygen decreased CBF in controls but had a less deleterious effect on CBF in patients is interesting, because it indicates an underlying pathology in mechanisms controlling CBF in this patient population. The physiological mechanisms underlying vascular reactivity to O2 and CO2 are, however, poorly understood.
3.3.
Possible limitations for clinical use of carbogen
The present study involved patients with cerebral hypoperfusion due to severe carotid stenosis, yet with some vascular reserve capacity. The results of this study may therefore not apply to patients with ischemic brain tissue. Future studies of carbogen must determine whether vascular adaptations limit therapeutic effects of long-term administration, since acute CVRs are not always maintained during various types of
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prolonged treatments (Christensen, 1974). In addition, adverse effects or discomfort due to inhaling CO2 may limit the clinical use of carbogen, although we did not observe significant changes in pulse, blood pressure, SaO2, ventilation, alertness/consciousness, or anxiety during inhalation of carbogen, except for one subject who reported mild headache. Accumulation of CO2 due to a higher dose or longer inhalation time would be undesirable in certain clinical situations, such as patients with elevated intracranial pressure. In particular, long-term use of CO2 inhalation may require safety studies. The subjects in our study were exposed to only a few minutes of inhalation of carbogen mixture with 5% CO2. Our results therefore cannot rule out possible side effects of long-term inhalation of carbogen. Further studies with a larger number of subjects are needed to confirm the beneficial effects of carbogen on oxygen content of cerebral circulation and to investigate possible toxic effects of long-term use of carbogen on different organ systems including brain (e.g., intracranial pressure), heart (e.g., heart rate, cardiac output), lungs (e.g., respiratory frequency, tidal volume), and kidney (diuresis, acidity, perfusion).
± 2.45 years), were recruited by Department of Neurology at Aarhus University Hospital. The diagnosis was made by MR and ultrasonic evaluation of carotid arteries. Based on diagnostic as well as clinical findings, patients with suspected misery perfusion (a clinical condition where an increase in oxygen extraction fraction is caused by a decrease in cerebral blood flow, presumably due to a reduction in cerebral perfusion pressure and disturbance of cerebral autoregulation) were recruited to the study. Patient data regarding age, gender, symptoms, imaging, and diagnosis are summarized in Table 1. Exclusion criteria included history of stroke, current cardiovascular, pulmonary or hematological disease, smoking, pregnancy, breastfeeding, and contraindications to MRI (e.g., pacemaker, metal prosthesis, claustrophobia). All subjects underwent medical (including ECG) and neurological examinations before and after participation in the project. National Institutes of Health Stroke Scale (NIHSS) rating was performed.
4.1.2. 3.4.
Summary
Concomitant increases of CBF and SaO2 are readily obtained with carbogen, which is far more effective than oxygen in improving oxygenation of hypoperfused brain tissue. The present results are, however, insufficient to comment on possible therapeutic benefits of carbogen in acute stroke patients. Further studies are needed to (1) verify the present findings, (2) further test the applicability of carbogen in ischemic brain tissue, (3) investigate possible adverse side effects of CO2 during long-term use of carbogen, and (4) assess whether prolonged inhalation of carbogen results in vascular or metabolic adaptations.
4.
Experimental procedures
4.1.
Subjects
4.1.1.
Patients
Six patients with occlusive carotid artery disease (five men and one women), aged between 40 and 80 years (average: 66.5
Healthy subjects
Ten healthy volunteers (5 men and 5 women), aged between 40 and 80 years (mean and SD: 53.4 ± 9.4 years), were recruited by advertisement in a local newspaper. Exclusion criteria included the same conditions as in patients. All subjects underwent medical (including ECG) and neurological examinations before and after participation in the project. National Institutes of Health Stroke Scale (NIHSS) rating was performed, and only subjects with an NIHSS = 0 were included. MRI scans were evaluated by neuroradiologists to exclude the presence of overt brain abnormalities. Written informed consent was obtained from both patients and controls, as approved by the Research Ethics Committee of County Aarhus.
4.2.
PET measurements
PET scans were acquired with a ECAT EXACT HR 47 (CTI/ Siemens) whole-body tomograph, operated in 3D mode, with a transverse resolution of 3.6–7.4 mm and an axial resolution of 4.0–6.7 mm. Images were reconstructed as 128 × 128 matrices of 2 × 2 mm pixels using filtered back-
Table 1 – Patient data describing age, gender, symptoms, and diagnostic imaging findings. Patients: age, gender
Symptoms
(1) 63 years old, male
Episodic left-sided hemiparesis
(2) 58 years old, male
Intermitting blindness (amourosis fugax), right-sided Episodic left-sided hemiparesis Episodic left-sided hemiparesis
(3) 78 years old, male (4) 66 years old, male (5) 67 years old, female (6) 68 years old, male
Episodes of amourosis fugax, right-sided Intermitting amourosis fugax and left-sided hemiparesis
Imaging
Diagnosis
PWI, DWI, and MRA Ultrasound PWI, MRA, Ultrasound
Occluded right ICA and stenotic right MCA
PWI, MRA, Ultrasound DWI, MRA, PET-Diamox test MRA, Ultrasound MRA, Ultrasound
Occluded right ICA, stenotic right ACA Occluded right ICA, stenotic right MCA, Reduced reduced CVR Occluded right ICA and CCA. Occluded right ICA and stenotic left ICA
90% stenosis of right ICA
PWI: perfusion-weighted imaging, DWI: diffusion-weighted imaging, MRA: magnetic resonance angiography, ICA: internal carotid artery, ACA: anterior carotid artery, CCA: common carotid artery, CVR: cerebrovascular reserve capacity.
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projection with a 0.5 cycles−1 ramp filter full width half maximum (FWHM), followed by smoothing with a 6-mm Gaussian filter, resulting in an isotropic resolution of 7 mm. The reconstructed images were corrected for random and scattered events, detector efficiency variations, and dead time. Tissue attenuation correction scans were performed using a rotating 68Ge source. The head of each subject was comfortably immobilized in a customized head holder (Vac-Lock; MED-TECH). Catheters were inserted in the left radial artery for arterial blood sampling and right antecubital vein for tracer injection. The arterial blood radioactivity was measured by an automated blood sampling system and then corrected for external delay and dispersion caused by bolus distortion in the sampling catheter. Dynamic 3-min emission recordings consisting of 21 frames were initiated upon bolus intravenous injection of [15O]-H2O (500 MBq).
4.3.
Study design
In this study, one session of PET measurements consisted of three [15O]-H2O scans; one baseline during which subjects inhaled normal atmospheric air followed by two consecutive measurements in which subjects inspired each of the test gases (100% O2, 5% CO2 + 95% O2) in a random order. Subjects were informed of test procedures and possible side effects and were familiarized with the scan room, personnel, and use of face mask. During the entire experiment, the subjects were breathing via a mask connected to a gas administration unit, which supplied the subjects with inhalation gas at a rate about 7 l/min. PET scans were obtained at intervals of 10 min while subjects rested in a supine position on the scanner bed. Ninety seconds prior to each PET scan, the subjects started breathing the test gas, which was delivered to the mask by the gas application unit. Recordings were obtained with eyes open, and the subjects were instructed to fixate their gaze on a cross-hair during the recordings.
4.4.
Subject monitoring
Throughout the entire experiment, the subjects were monitored by medical staff, who noted reports of possible discomfort (e.g., dyspnea, anxiety, headache) or observations of altered consciousness/alertness, pulse, and SaO2. Standard biochemical parameters were measured prior to the study, and arterial blood gas samples were analyzed at the end of each scan. Blood pressure and pulse were monitored continuously and were recorded throughout the experiment. To compare effects of oxygen and carbogen on oxygen content of the blood, we measured the steady-state oxygen saturation (SaO2) and oxygen tension (PO2) in arterial blood samples during administration of each gas.
4.5.
T1-weighted MR was acquired with a 3. 0-T Signa Excite GE imager using a 3D IR-FSPGR sequence (256 × 256 matrix, TE = min full, TI = 750, slice thickness = 1.2 mm). Standard T2-FLAIR images were acquired to rule out other cerebral pathology.
4.6.
Data analysis
4.6.1.
Coregistration
The T1-weighted MRI images of the brain of each subject were co-registered to an MR template defined by the brain of 85 young adults in Talairach space, using a combination of linear and nonlinear transformations (Grabner et al., 2006). Each summed PET emission recording was then linearly co-registered to the corresponding MR image using automated algorithms by registering the first summed PET recording to MR and then each sequential summed PET recording to the first PET recording. PET images were subsequently re-sampled to standard stereotaxic space.
4.6.2.
Volumes of interest
Using arterial time–activity curves corrected for delay and dispersion, we calculated parametric maps of CBF with the single-step, two-compartment, three-weighted integration method (Ohta et al., 1992, 1996). The corrections apply a global adjustment of the timing difference between the measured arterial curve and the PET scan. We searched the absolute CBF values in the parametric maps. Baseline scans were used to identify hypoperfused cortical regions in the affected hemisphere. Volumes of interest (VOIs) containing hypoperfused lesions were defined as regions with CBF values 10% below the average baseline CBF values for grey matter (GM) in the control group. Multiplying the outlined VOIs with a GM mask produced GM-VOIs, which encompassed the GM portion of the hypoperfused area. All operations were performed in Talairach space. For each test gas, we extracted and averaged CBF values by applying the generated GM-VOIs to the corresponding co-registered parametric CBF maps. Data from a previous PET study (Ashkanian et al., 2008) with identical inhalation and scanning routine in ten healthy volunteers were used as control. We analyzed the effect of oxygen and carbogen on CBF in ischemic and control GM using a mixed-effects model. Subjects were considered as a random factor to model the correlation between measurements performed on the same individual.
Sources of funding This study was supported by The Danish National Research Foundation, The Danish Medical Research Council, The Velux Foundation, and The Toyota Foundation.
MRI measurements
Conflicts on interest disclosures Brain MRI scans of each subject were obtained on a separate day for PET image co-registration. High-resolution
There is no conflict of interest in this study.
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Acknowledgments We wish to thank Michael Geneser for helping with MRI protocol, the technicians and chemists at PET Centre at Aarhus University Hospital for their technical support, and Dr. Donald Smith for his linguistic revision of the manuscript.
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