Hillman J: Microdialysis for monitoring of the Injured Brain

CLINICAL STUDIES

A MICRODIALYSIS TECHNIQUE FOR ROUTINE

MEASUREMENT OF MACROMOLECULES IN THE

INJURED HUMAN BRAIN

Jan Hillman, M.D., Ph.D.Department of Neurosurgery,University Hospital,Linköping, Sweden

Oscar Åneman, M.D.,Ph.D.Department of Neurosurgery,University Hospital,Linköping, Sweden

Chris Anderson, M.D.,Ph.D.Department of Dermatology,University Hospital,Linköping, Sweden

Florence Sjögren, Ph.D.Department of Dermatology,University Hospital,Linköping, Sweden

Carina Säberg, R.N.Department of Neurosurgery,University Hospital,Linköping, Sweden

Pekka Mellergård, M.D.,Ph.D.Department of Neurosurgery,University Hospital,Linköping, Sweden

Reprint requests:Jan Hillman, M.D., Ph.D.,Neurosurgical Department,University Hospital,S-581 85 Linköping, Sweden.Email: [email protected]

Received, February 17, 2004.

Accepted, January 20, 2005.

OBJECTIVE: To evaluate a new intracerebral microdialysis catheter with a high-cutoffmembrane and its potential for the study of macromolecules in the human brain.METHODS: Paired intracerebral microdialysis catheters were inserted in 10 patientswho became comatose after subarachnoid hemorrhage or traumatic brain injury andwere then treated in our neurosurgical unit. The only differences from the routine useof microdialysis in our clinic were the length (20 mm) and cutoff properties of thecatheter membranes (100 kD) and the perfusion fluids used (standard perfusion fluid,3.5% albumin, or Ringer-dextran 60). Samples were weighed (for net fluid fluxes) andanalyzed at bedside (for routine metabolites) and later in the laboratory (for totalprotein and interleukin-6). The in vitro recovery of glucose, glutamate, and glycerolwere also investigated under different conditions.RESULTS: Even brief perfusion with standard perfusion fluid resulted in a significantloss of volume from the microdialysis system. For albumin and Ringer-dextran 60 fluid,recovery was comparable to standard settings. Interleukin-6 (highest value close to25,000 pg/ml) was sampled from all catheters, and total protein was analyzed fromcatheters perfused with Ringer-dextran 60 (average concentration, 234 �g protein/ml).There were detectable patterns of variations in the concentration of interleukin-6,seemingly related to concomitant variations in intracerebral conditions. In the presentstudy, no direct comparison was made with the standard CMA 70 catheter (CMAMicrodialysis, Stockholm, Sweden), but in vivo, the measured mean concentrations ofglucose, glycerol, lactate, and pyruvate were comparable to those previously reportedfrom standard catheters. In vitro, the recovery of metabolites was better when usingRinger-dextran 60 compared with albumin.CONCLUSION: Microdialysis catheters with high-cutoff membranes can be used inroutine clinical practice, allowing for sampling and analysis of cytokines and othermacromolecules.

KEY WORDS: Cerebral microdialysis, Head trauma, Intensive care, Interleukin-6, Monitoring, Protein,Subarachnoid hemorrhage

Neurosurgery 56:1264-1270, 2005 DOI: 10.1227/01.NEU.0000159711.93592.8D www.neurosurgery-online.com

Microdialysis has become a major toolin the study of brain biochemistry inboth experimental settings and clin-

ical practice (2, 8, 20). Until recently, only cath-eters with membrane cutoff properties (20 kD)permitting recovery of small tissue moleculeswere commercially available for use in hu-mans (9, 12). Even so, such catheters haveprofoundly influenced the study of ischemicand other injuries to the human brain throughsequential analysis of neurotransmitters and

metabolites and, in addition, have been ofclinical use in the neurointensive care unit(NICU) for patient monitoring (4, 10, 19, 21).

Proteins act as important regulators of cel-lular responses to injury. It seems likely thatcerebral microdialysis in humans, with stud-ies of macromolecules, such as cytokines, neu-rotrophic factors, or enzymes, would add anew and important dimension to the under-standing of brain injury and subsequent re-parative processes (11, 15, 22). Microdialysis

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membranes with high cutoff properties, allowing sampling ofmacromolecules, have been used successfully in animal exper-iments and in human dermis (6, 17). We are aware of only oneprevious report using such technology for studies of proteinsin the human brain (23). Because the method presented thereseems to have limitations for routine clinical use, the aim ofthe present study was to investigate the potential for a simplermethod. With minimal changes of the routines used for mi-crodialysis in our NICU for several years, we investigatedproperties of a new microdialysis catheter with a high-cutoffmembrane in a series of seriously injured patients.

PATIENTS AND METHODS

Intracerebral microdialysis using a modified type of cathe-ter (see below) was performed in 10 patients treated in ourNICU. In this unit, microdialysis with conventional cathetershas been used routinely for several years as one of severalmethods in a multimodality monitoring system designed tomeet the therapeutic challenge of each individual patient.Ethical approval was granted by the Ethical Committee of theUniversity Hospital of Linköping.

Catheter Placement and Microdialysis Setup

Insertion of the catheters was performed according to ournormal routines, except that for this study, pairs of catheters(instead of single catheters) were used. The catheters wereinserted via burr holes or through a craniotomy at the end ofopen surgery. The positioning of catheters followed our pre-viously established routines for microdialysis monitoring. Ac-cording to these routines, catheters are most often placed inthe frontal lobe, 1 to 2 cm anterior to the coronary suture,although in some patients, catheters are placed in tissuesconsidered “at risk,” e.g., in the proximity of a contusion.Irrespective of locality, the two catheters were always posi-tioned in close proximity to each other to allow for compari-son of their properties in areas with comparable biochemistry.

All catheters were provided by CMA Microdialysis, Stock-holm, Sweden, and were identical to the CMA 70 catheterroutinely used in our clinic, except that the membrane lengthwas 20 mm (instead of 10 mm) and had cutoff properties atmolecular weights of 100,000 (instead of 20,000). Perfusion ofthe microdialysis system was standardized at 0.3 �l/min,using a commercially available perfusion pump (CMA 106;CMA Microdialysis). The outflow hydrostatic pressure of theperfusion system was set at the zero midcranial reference levelby attaching the perfusate collecting vials to the bandage onthe patient’s head. In a few early patients, the microdialysissystem was initially perfused with standard perfusion fluid(CMA Microdialysis) for a few hours. Subsequently, all cath-eters were perfused with either Ringer-dextran 60 (RD60)(Braun Medical AB, Stockholm, Sweden) or human albuminsolution (Albumin Immuno, 35 mg/ml; Baxter Medical AB,Stockholm, Sweden), one perfusate fluid in each of the pairedcatheters.

In Vivo Assays

Samples were analyzed at bedside for glucose, glutamate,glycerol, lactate, pyruvate, and urea using the CMA 600 ana-lyzer (CMA Microdialysis). The perfusates collected duringthe initial 2 to 3 hours were discarded, and thereafter, consec-utive 4- to 6-hour samples were collected continuously andwere weighed at a 0.1-�g precision level to estimate net trans-membrane fluid fluxes. The same samples were then frozen(�70 C°) for later analysis in the laboratory for total proteincontent and the cytokine interleukin-6 (IL-6). Protein was mea-sured with a DC Protein Assay kit from Bio-Rad (Stockholm,Sweden). This kit is based on the Bradford method, whichmeasures basic and aromatic amino acids. A sample volume of5 �l was used for analyzing the total protein concentration.IL-6 was determined by use of enzyme-linked immunosorbentassay Quantiglo kits, a chemiluminescence-based method. TheIL-6 kit allows detection in the range of 0.3 to 3000 pg/ml.(Samples with concentration values falling outside this rangeat the first analysis were diluted and reanalyzed.) The assayswere performed according to the manufacturer’s instructions.Standards and samples (when possible) were analyzed induplicate.

In Vitro Assays

Microdialysis catheters were placed in standard solutions ofglucose, glycerol, and glutamate provided by CMA. RD60 orhuman albumin solution was perfused through the cathetersat a flow rate of 1 and 0.3 �l/min. Samples were assayed onthe CMA 600 analyzer (see above).

RESULTS

The 10 patients investigated had experienced either a trau-matic brain injury or severe aneurysmal subarachnoid hem-orrhage. On admission and during the monitoring, they werecomatose, with a Glasgow Coma Scale score of 9 or less.Microdialysis continued from 1 to 10 days. In 1 patient, per-fusion with 3.5% albumin was accompanied by strong netfluid absorption. This was most likely because of a membranefailure, related to technical problems during insertion of thecatheter. Microdialysis was discontinued, and the data fromthis patient were not included in the material, which thereforerefers to the observations in 9 patients.

Pilot observations made it clear that even a brief perfusion(4–8 h) with standard perfusion fluid resulted in a significantloss of volume from the system, which had been suspectedalready by visual inspection of the collecting microvial. Theidea of using standard perfusion fluid for comparison withother types of perfusates was therefore abandoned right at thebeginning of the study.

For the two other types of perfusate, fluid recovery wascomparable to what is observed during standard settings ofclinical microdialysis (18–20). Data on transmembrane fluidbalance for the different perfusion solutions are given in Table1. For RD60, the average fluid recovery was 94.0%, with little

HUMAN INTRACEREBRAL MICRODIALYSIS OF MACROMOLECULES

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variation between different patients. For 3.5% albumin, theaverage fluid recovery was 100.1%, with the spreading of databetween different patients being somewhat larger comparedwith RD60.

Total protein content was not analyzed regularly, and onlyin samples collected from catheters perfused with RD60. Insamples in which analyses were made, protein concentrationvaried between 50 and 1520 �g protein/ml, the average con-centration being 234 �g protein/ml (213–254 �g protein/ml;95% confidence interval). Figure 1 illustrates variations in totalprotein over time in two patients (Patients A and B).

During the course of monitoring, IL-6, which was selectedas a model molecule of measurable regulatory proteins in thehuman brain, was sampled from all patients. IL-6 was alsodetected in all the catheters perfused with albumin. The IL-6concentration varied between patients and over time in anygiven patient. Initial values were seldom below 1000 pg/ml.The highest value for IL-6 was close to 25,000 pg/ml.

Two examples of the variation of IL-6 concentration over time(and between different patients) are given in Figures 2 and 3.Figure 2A illustrates the temporal profile of IL-6 in extractedextracellular fluid in a patient (Patient C) who had experienced asevere traumatic brain injury. The microdialysis catheters werepositioned in an area close to a major contusion soon after injury.As shown in Figure 2B, high levels of glycerol and a high lactate-

to-pyruvate ratio were ob-served, indicating threateningischemic conditions. IL-6 in-creased rapidly and thengradually declined (Fig. 2A),parallel to reduction of theischemic stress to the tissue(Fig. 2B).

Figure 3A illustrates the IL-6profile in a patient (Patient D)with subarachnoid hemor-rhage. At the end of the firstweek of observation, this pa-tient developed clinical signsof vasospasm. TranscranialDoppler showed increasedflow velocities, with a com-puted tomographic scan de-picting areas of lowered den-sity. Focal ischemia was alsoindicated by increases in ex-tracellular glycerol and in thelactate-to-pyruvate ratio (Fig.3B). As illustrated in Figure3A, IL-6 concentrationshowed a sharp increase asischemia developed.

The measured mean me-tabolite concentrations ofsome important metabolites(glucose, glutamate, glyc-erol, lactate, and pyruvate) were comparable to what wouldbe expected from measurements with standard catheters (e.g.,CMA 70), as summarized in Table 2. Clearly, the spread ofthese values depends on the types of patients surveyed. Forour present purpose, it was more important to compare therecovery rates of catheters perfused with different types offluids (albumin and RD60, respectively). Thus, the relation-ship (quota) between the concentration of each metabolite inthe two different types of perfusion fluid was measured ateach sampling time. In Table 2, “Q” is the average value for allsuch paired comparisons. When comparing the measurementsof metabolites in this way, the overall ratio of recovery be-tween albumin- and RD60-perfused catheters was 1.06. Asalso shown in Table 2, glutamate and glucose showed higherrecovery in albumin, whereas the recovery of lactate andpyruvate was slightly lower in albumin.

The results from the in vitro recovery studies of some me-tabolites are presented in Table 3. In vitro, the recovery for themetabolites was better when using RD60 than when usingalbumin. As expected, increasing the perfusion flow rate from0.3 to 1.0 �l/min was accompanied by a lower degree ofrecovery. Albumin showed a low recovery percentage regard-less of the flow rate used. The highest recovery was seen whenRD60 was used as the perfusion fluid at 0.3 �l/min.

FIGURE 2. Graphs showing tem-poral profile of intracerebral IL-6(A) and the corresponding concen-tration of glycerol and the lactate/pyruvate (L/P) ratio (B) in a patient(Patient C) who had experiencedsevere traumatic head injury.Threatening ischemia was accompa-nied by rapidly increasing IL-6, fol-lowed by a gradual decline (seeResults).

TABLE 1. Fluid recovery from microdialysis catheters perfusedwith Ringer-dextran 60 or 3.5% albumin (paired observationsin nine patients, n � 114)a

Ringer-dextran 60 Albumin 3.5%

Mean 94% 100.1%

95% CI 92.5–95.5% 90.7–109.2%

a CI, confidence interval.

FIGURE 1. Graph showing temporal profiles of total protein concentra-tion in microdialysis fluid sampled from two patients (Patients A and B)with subarachnoid hemorrhage.

HILLMAN ET AL.


DISCUSSION

The present study showsthat microdialysis catheterswith high-cutoff membranes,allowing for sampling of cyto-kines and other macromole-cules, can be used in routineclinical practice. The in vitrorecovery data and the fact thata significant amount of pro-teins could be recovered fromthe extracellular fluid in neu-rosurgical patients clearlypoint to the potential of suchprobes for extending the ex-ploration and monitoring ofhuman brain biochemistry.

With a standard mem-brane, only very small net fil-tration of standard perfusionfluid occurs to the brain atthe perfusion rate of 0.3 �l/min (2, 8, 20). However, withthe larger pores of the mem-brane used in the presentstudy, there is an excessiveloss of standard perfusionfluid to the surrounding tis-sue, as was also observed inour pilot experience (see above). This kind of “leakage” hasbeen a major obstacle in attempts to develop a clinical routinemicrodialysis technique with high-cutoff membrane cathetersfor the human brain (9, 23).

The transmembrane fluid balance mechanism in a microdi-alysis catheter is comparable to that of a single capillary.Resistance to fluid flux through a single pore is inversely

related to the fourth power of its radius, and the overallhydrodynamic properties of the membrane are described bythe filtration coefficient. The intraluminal and tissue hydro-static pressures are balanced against each other, and the col-loid osmotic pressure of the perfusate is balanced against thecolloid osmotic pressure of the surrounding extracellular fluid(2, 20). Several approaches to balancing transmembrane fluidfluxes in microdialysis catheters with large pore membraneshave been described for various tissues (1, 17, 23). However,these methods have shortcomings entailing limitations fortheir application in neurosurgical patients, particularly if thegoal is to use microdialysis as a clinical routine in an NICUsetting.

In the present approach, we chose to use widely availableequipment favored by many neurosurgical clinics as part oftheir patient monitoring equipment. The outflow pressure ofthe microdialysis system was kept at the level of the intrace-rebral catheter, i.e., close to the zero level, thus avoiding theproblems of a “push and pull” technique. Instead, the focuswas on the choice of perfusion fluid and, in particular, thecolloid osmotic pressure of the perfusate. For routine applica-tion, a commercially available pharmaceutical product wouldbe favored as perfusate. It is therefore encouraging that ourdata show that both 3.5% human albumin solution and RD60effectively counterbalance the outward hydrostatic force overthe membrane segment, thereby minimizing net fluid loss.

For both albumin and RD60, fluid recovery was comparableto that observed with standard perfusion fluid, with less in-terpatient variation experienced with RD60. In the presentstudy, no direct comparison was made with the standardcatheter (e.g., CMA 70), but the recovery of metabolitesseemed to be comparable to what has been reported previ-ously for standard catheters (18, 19).

An important finding of the study was that significantamounts of cytokines and other proteins could be sampledfrom every patient. With the possible exception of the S-100protein marker for brain injury (13), at present, proteins have

no defined role in clinicalmultimodality monitoring ofthe injured brain. However,it may well prove that somemacromolecular mediators ofearly tissue injury or somemediators of early reparativeprocesses in injured tissuecan become sensitive indica-tors of the progression of in-jury or the effects of therapy.Among such macromole-cules are chemokines and cy-tokines (with powerfulproinflammatory and anti-inflammatory actions); en-zymes, e.g., cathepsins (likelyto play a role in apoptosis);and a host of different neuro-

FIGURE 3. Graphs showing tem-poral profile of intracerebral IL-6(A) and the corresponding concen-tration of glycerol and the lactate/pyruvate (L/P) ratio (B) in a patient(Patient D) with subarachnoid hem-orrhage (SAH). At the end of thefirst week, the patient developedclinical and Doppler-verified vaso-spasm, with an accompanyingincrease in IL-6 concentration.

TABLE 2. Measurements of five different metabolites and comparison of recovery rates duringmicrodialysis in nine neurointensive care unit patients with paired catheters, perfused with Ringer-dextran 60 or 3.5% albumina

Metabolite Q (alb/RD60) 3.5% albumin RD60

Glucose (mmol/L) 1.32 (1.04–1.61) 1.9 (1.7–2.2) 1.7 (1.5–2.0)

Glutamate (�mol/L) 1.53 (1.13–1.92) 149 (74–224) 142 (69–216)

Glycerol (�mol/L) 0.99 (0.88–1.10) 216 (155–279) 236 (180–292)

Lactate (mmol/L) 0.91 (0.85–1.01) 6.6 (5.9–7.2) 7.8 (7.0–8.6)

Pyruvate (�mol/L) 0.90 (0.85–0.95) 230 (209–251) 275 (246–304)

a The relationship (quota) between concentrations of each metabolite in the respective perfusion fluid was measured ateach sampling interval, Q being the average value for all such paired comparisons (values given are mean and 95%confidence interval; n � 114). alb, albumin; RD60, Ringer-dextran 60.



trophic factors likely to be instrumental in posttraumatic brainrepair processes (5, 15, 16, 22). New techniques for analysis,e.g., proteomics, are likely to add considerably to this list (7).

A potentially useful clinical marker, the cytokine IL-6, wassampled from all patients. The normal extracellular concen-tration of IL-6 in the extracellular space of the human brain isnot known. The highest concentration measured in the presentstudy approached 25,000 pg/ml, but in most measurements,the concentration was considerably lower, not seldom beingless than 1000 pg/ml. More interesting, however, was theobservation illustrated in Figures 2, A and B, and 3, A and B,namely, the indications that a detectable pattern of variationsin IL-6 related to different intracerebral conditions may exist.

CONCLUSION

To analyze the protein content, it is currently necessary tobring the sampled fluid to a laboratory. This definitely limits theusefulness of bedside sampling of macromolecules. However, inthe near future, one can expect development of new technologies(e.g., laboratory chip-based) to bring effective methods for thebedside analysis of proteins and other macromolecules into theclinical setting (3, 14). It may prove possible to find alternativeindicators of impending threats to the cerebral tissue that can beused on-line to select therapy tailored to the individual patient’sneeds. The present study clearly indicates that microdialysiscould play a role in such development.

REFERENCES

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7. Gozal E, Gozal D, Pierce WM, Thongboonkerd V, Schein JA, Brittain KR,Guo SZ, Klein J: Proteomic analysis of CA1 and CA3 regions of hippocam-pus and differential susceptibility to intermittent hypoxia. J Neurochem83:331–345, 2002.

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9. Hillered L, Persson L: Neurometabolic monitoring of the acutely injuredhuman brain. Scand J Clin Lab Invest Suppl 229:9–18, 1999.

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11. Holmin SS, Biberfeld P, Mathiesen T: Intracerebral inflammation after hu-man brain contusion. Neurosurgery 42:291–298, 1998.

12. Hutchinson PJ, O’Connell MT, Al-Rawi PG, Maskell LB, Kett-White R, Gupta AK,Richards HK, Hutchinson DB, Kirkpatrick PJ, Pickard JD: Clinical cerebralmicrodialysis: A methodological study. J Neurosurg 93:37–43, 2000.

13. Ingebrigtsen T, Romner B: Biochemical serum markers of traumatic braininjury. J Trauma 52:798–808, 2002.

14. McGlennen R: Miniaturization technologies for molecular diagnostics. ClinChem 47:393–404, 2001.

15. Morganti-Kossman MC, Rancan M, Stahel PF, Kossmann T: Inflammatoryresponse in acute traumatic brain injury: A double-edged sword. Curr OpinCrit Care 8:101–105, 2002.

16. Rothwell NJ, Hopkins SJ: Cytokines and the nervous system: Part II—Actions and mechanisms of action. Trends Neurosci 18:130–136, 1995.

17. Sjögren FS, Andersson C: Technical prerequisites for in vivo microdialysis deter-mination of interleukin-6 in human dermis. Br J Dermatol 146:375–382, 2002.

18. Ståhl NM, Hallström Å, Ungerstedt U, Nordström CH: Intracerebral microdialysisin clinical practice: Baseline values for chemical markers during wakefulness,anesthesia and neurosurgery. Neurosurgery 47:701–710, 2000.

19. Ståhl NU, Ungerstedt U, Nordström CH: Brain energy metabolism duringcontrolled reduction of cerebral perfusion pressure in severe head injuries.Intensive Care Med 27:1215–1223, 2001.

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21. Unterberg AW, Sakowitz OW, Sarrafzadeh AS, Benndorf G, Lannksch R: Role ofbedside microdialysis in the diagnosis of cerebral vasospasm following aneurysmalsubarachnoid hemorrhage. J Neurosurg 94:740–749, 2001.

22. Wilcockson DC, Campbell SJ, Anthony DC, Perry VH: The systematic andlocal acute phase response following acute brain injury. J Cereb Blood FlowMetab 22:318–326, 2002.

23. Winter CD, Iannotti F, Pringle AK, Trikkas C, Clough GF, Church MK: Amicrodialysis method for the recovery of IL-1�, IL-6 and nerve growth factorfrom human brain in vivo. J Neurosci Methods 119:45–50, 2002.

AcknowledgmentsWe thank CMA Microdialysis, Stockholm, Sweden, for providing the cathe-

ters used in this study. We have no financial or other affiliation with anymanufacturer of the products used in the study.

COMMENTS

When a neurosurgeon thinks of cerebral microdialysis, heor she most likely envisions patients who suffer from

severe traumatic brain injury or aneurysmal subarachnoid

TABLE 3. In vitro recovery of glucose, glutamate,and glycerola

SubstancePerfusion flow

(�l/min)

% Recovery

RD60 No.Albumin(3.5%)

No.

Glucose 0.3 99.6 2 84.2 2

1.0 73.4 5 48.5 7

Glutamate 0.3 90.6 2 80.0 2

1.0 71.8 5 42.5 7

Glycerol 0.3 97.7 2 83.8 2

1.0 83.8 5 53.3 7

a RD60, Ringer-dextran 60. Both RD60 and 3.5% albumin were used asperfusion buffers at both flow rates of 0.3 and 1.0 �l/min.

HILLMAN ET AL.


hemorrhage. Part of the reason why champions of microdi-alysis have gravitated toward such conditions relates to thenature of the substances that can be measured with this tech-nique. Limitations on the size of the molecules that can beassayed have contributed to an emphasis on such moleculessuch as glucose, lactate, pyruvate, and certain neurotransmit-ters. Alterations in concentrations of these substances are as-sociated with ischemia and other injuries to the brain.

In this article, Hillman et al. describe a simple yet importantstudy of the effectiveness of a microdialysis membrane with alarger pore size. These catheters enable the analysis of mole-cules up to 100 kD, as opposed to the cutoff of 20 kD forstandard catheters. By replacing the standard perfusion fluidwith either albumin or a Ringer’s-dextran solution, the in-creased loss of fluid through the larger pores is offset by thehigher oncotic pressure of the perfusate.

This demonstration of the technical feasibility of analyzingproteins and other larger molecules may help to extend thehorizons of cerebral microdialysis. As the authors discuss intheir conclusion, new technologies may facilitate bedside anal-ysis of proteins and other macromolecules that have beencollected with the catheters described here. Further advancesin microdialysis methodology by the authors and by othergroups may improve our ability to care for our most criticallyill patients.

Alex B. ValadkaHouston, Texas

The development of microdialysis has contributed signifi-cantly to our understanding of brain pathophysiology.

Because of the cut-off properties of the catheters, however, ithas only been possible to recover relatively small molecules.

Here, Hillman et al. present an elegant study of 100 kDcut-off catheters. The investigation was done in 10 patientswho were experiencing coma after subarachnoidal hemor-rhage or traumatic brain injury. One 100 kD cut-off catheterwas placed parallel to a control catheter in each patient.

The authors show that even brief perfusion with the stan-dard perfusion fluid caused significant loss of volume,whereas fluid recovery with albumin and RD-60 was satisfy-ing. Extracellular proteins (including interleukin-6 [IL-6])were sampled from all catheters, and the authors noted pat-terns of variations in the concentration of IL-6, apparentlyrelated to pathophysiological changes. Concentrations of glu-cose, glycerol, lactate, and pyruvate were comparable to thoserecorded by standard catheters. The new catheters will allowrecovery of an expanded fraction of the large molecules thatwe know from studies in preclinical models are important forneuronal death or recovery.

Iver A. LangmoenStockholm, Sweden

The authors report the use of a microdialysis catheter thatallows sampling of macromolecules from human brain

tissue. They used IL-6 as the test macromolecule and deter-

mined that the behavior of the assay seemed adequate to trackchanges in this molecule in response to threatened ischemia inpatients with head injury or vasospasm owing to subarach-noid hemorrhage. The authors further demonstrated that thecatheter under study was able to collect small molecules ofinterest with reasonable recovery rates. A major differencebetween the standard catheter and the test catheter techniqueswas the use of either an albumin or Ringer’s-dextran perfusatesolution. These were better able to counterbalance the fluidloss owing to larger pore size and hydrostatic effects. Metab-olite recovery was better at lower perfusion rates.

This article is well written and the information is of value tothose using microdialysis techniques in humans. The ability toevaluate changes in a variety of proteins in the human brain inclinical settings is a major advance that will open new avenuesof understanding of pathophysiology and normal brain func-tion. This is a valuable contribution to our literature.

Charles J. Hodge, Jr.Syracuse, New York

This is an important article because, to my knowledge, itrepresents the first published experience with the new

CMA 100 microdialysis catheter (CMA Microdialysis, Stock-holm, Sweden), which allows time-dependent, relatively non-invasive, in vivo measurement of small molecular weightproteins in the living human brain. This has never before beenpossible.

This newly designed catheter has not yet been approved forhuman use in the United States. This article is a demonstrationthat in nine patients and over about 114 microdialysis mea-surements, this technique seemed to be safe. Hopefully, asmore experience is obtained with these catheters, they willreplace the conventional ones and allow parallel estimation ofproteins and peptides, as well as 3-carbon based substrates.There is a revolution of interest in proteomics in the brain aftera variety of brain insults, and this new catheter may make itpossible to serially measure proteins in different parts of theliving human brain. The authors have shown that the cyto-kine, IL-6, seems to fluctuate in broad agreement with theother microdialysis analytes. The theoretical possibilities withthese techniques are enormous. For example, it is not knownwhen the signal for apoptotic cell death is expressed in thehuman brain after traumatic brain injury, although immuno-histochemical studies on excised human contusion materialhas shown that this process is prominent. With this newmicrodialysis technique, it would be possible to measure pro-apoptotic cytokines and allow early pre-treatment strategieswith, for example, IL-1 receptor antagonists, such as anakinra.

The authors have convincingly shown that the best dialy-sate perfusion fluid for these studies is probably Ringer’s-dextran 60. Normal saline or artificial cerebrospinal fluid isnot a suitable perfusion fluid with these probes, because, asthe authors have shown, much of the fluid migrates across themembrane to the brain.



Unfortunately, these experiments were quite badly designed. Al-though the authors implanted a standard CMA-20 microdialysisprobe and the new CMA 100 probe together, they did not makedirect cross comparisons for all the analytes of interest. The reasonsfor this are not clear. Similarly, the authors do not make direct crosscomparisons using this 2-probe design with the three types of per-

fusion fluid that they have evaluated. Nevertheless, this data is stillan important first start using a new technique, which may havegreat potential for the future.

M. Ross BullockRichmond, Virginia

HILLMAN ET AL.


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Hillman J: Microdialysis for monitoring of the Injured Brain