Mericka P. ¹, Hosek F.²

¹Tissue Centre, Medical School Hospital Hradec Kralove
²Department of War Surgery, Military Medical Academy J. E. P., Hradec Kralove, Czech Republic

SUMMARY. This study provides supportive evidence of a possible role played by planning supplies of biological covers needed in fire disaster, based on the experience of the authors. The major steps to be taken are these: 1. Providing a technically and technologically adequate base for collection and long-term storage of cells and tissues ready for use in case of catastrophe. 2. Developing a method for estimating the amount of reserved tissue grafts. 3. Solution of logistical problems associated with providing supplies for operating theatres treating disaster casualties. 4. Organisation of national and international network of graft exchange capable of supporting local skin banks in times of need. In contrast to the situation in the 1970s and 1980s, nowadays the Czech Republic can deliver any required amount of biological covers without having to face technological difficulties. The idea of collaborating in the management of a fire disaster emerged from experience gained by the authors during the Bashkir disaster in 1989, relating particularly to an inadequate reserve stock of skin grafts and the impossibility of increasing their production. Intensified demands for the safety of grafts and recent experience from the US emphasise the need for an immediate conceptual solution concerning production of reserves of biological covers that should be ready for transportation to wherever needed. Another urgent necessity is the establishment of conditions enabling effective international collaboration at a disaster event.

ZUSAMMENFASSUNG

Ist die Reservebildung der biologischen Hautdeckungen wichtig fur Sicherstellung der Verbranntenbetreuung im Falle unerwarteter Umstande?

Mencka P., Hosek F.

Die Autore rekapitulieren eigene Erfahrungen mit der Planung der Verwendung von biologischen Hautdeckungen im Falle massenhaftes Aufkommen von Verbrannten. Die Hauptsaufgaben solcher Planung sind: 1. Die Bildung einer adaquaten technischen and technologischen Grundlage fur die Abnahme and langfristige Bewahrung der Zellen and Gewebe, die im Falle einer Katastrophe benutzt werden konnen. 2. Entwicklung einer Methode zur Abschatzung der Reserven von Gewebepropfen. 3. Die 1_osung logistischer Probleme, vornehmlich der Versorgung von chirurgischen Arbeitsstatte, die an der Folgenbeseitigung einer Katastrophe teilhaben. 4. Organisierung eines nationalen and internationalen Austauschnetzes der Propfen, die die lokale Gewebebanke im Falle-einer Katastrophe unterstiitzen konnen. Die Sicherstellung der zureichenden Reserven der biologischen Hautdeckungen stellt in der Tschechischen Republik zur Zeit kein technologisches Problem dar. Eine Idee von Eingliederung eigener Arbeitsstatte in die LSsung einer katastrophenartigen Situation ging von Erfahrungen der Autore wahrend der Katastrophe in Bashkir im 1998 aus. In Hinsicht auf hohere Forderungen nach der Sicherheit der Hautdeckungen and gleichzeitige Erkenntnisse aus der USA, ein Konzept fur Bildung der Reserven von Hautdeckungen ist von groBer Bedeutung. Andere wichtige Aufgabe ist eine effektive Internationale Kooperation in den Katastrophenfalle.

Key words: fire disaster, reserves of biological skin covers, allotransplantation of skin, xenotransplantation of skin, skin bank

The availability of an adequate store of temporary skin covers to meet the needs of a mass burn disaster is of utmost importance (Gunn, 1992, Masellis et Gunn, 1995). Moreover, the store should contain a good supply of both allogenic and xenogenic biological covers (Masellis et Gunn, 1995, Klein, 2000). For example, TosinskaOkroj et al. (1994-1995) described the use of glycerolized allogenic dermo-epidermal grafts from the Euroskin Bank at Beverwijk (Holland) (van Barre1994) in treating mass bum casualties injured at a concert in the area of the Gdansk shipyard (320 casualties).

Recently, similar grafts have been used in treating victims of the terrorist attack on the World Trade Center in New York on September 11, 2001. These grafts (hypothermal or deep freeze conservation) were obtained from the American Association of Tissue Banks (AATB, 2001).

We witnessed the use of both lyophilized xenotransplants (preparation Suiderm, Bioveta, Terezin, Czech Republic) and chemically preserved, radiation-sterilised xenotransplants prepared by a local skin bank (Laboratory for a production of transplants, Alloplast, Ufa, SSR) (Mericka et a1.,1990, 1995) during the Bashkir train gas-pipeline disaster (gas explosion with 700 victims) (Herndon, 1990, Kulyapin et al., 1990). Despite considering the ' use of either allogenic dermo-epidermal grafts or cultured keratinocytes, neither were used at this occasion.

All these facts justify contemporary programmes emphasising the necessity of setting up stores of tissue grafts - which in fact existed in the past in the Czech Republic. Within the framework of these programmes financed by the former Federal Ministry of National Defense, numerous methods were developed, e.g. cryo-conservation of allogenic dermoepidermal grafts (Bohm et DvoMk, 1981), xenogenic dermo-epidermal grafts (Praus, Bohm, Dvordk, 1980, MoserovA et Houskova, 1989), lyophilisation of xenogenic dermo-epidermal grafts (preparation Suiderm produced by Bioveta, Terezin), lyophilisation of allogenic and chorioamniotic grafts (Klen et Skalska, 1976). After the break-up of the Czechoslovak Federation, financing of these programmes by the army was stopped completely, and at present all services of tissue banks are paid for by health insurance companies. This system is not interested in supporting the idea of storing biological material; on the contrary, it restricts it.

According to the available literature, a similar situation seems to occur all over the world except in Israel, where skin banks store supplies of deep-frozen allotransplants with an expiration time of 5 years (assumed duration of graft viability). At the end of this time grafts are given to the clinic for use, and stocks are replenished regularly.

OUR APPROACH TO A SOLUTION OF THE PROBLEM

Since the late 1980s systematic attention has been given to the role of tissue banks in a fire disaster. Our ideas and experience have repeatedly been discussed at various occasions (Congress of European Burn Association, Prague 1989, Second International Conference: The Management of Burns and Fire Disasters: Perspectives 2000, Palermo, 1992) and also published (Mericka et al. (1990), Mericka et al. (1995)).

A textbook for military physicians called „Preparation and Use of Biological Covers in the Treatment of Burns in War Surgery and Disaster Medicine" was published by Klein, Mericka et al. (2000) at the Military Medical Academy of Jan

Evangelista Purkyne in Hradec Krdlove. Our objectives were to familiarise the wider, professional public with these specific problems. The textbook deals only with classical biological covers and not with cultured skin substitutes (keratinocytes).

In the early 1990s we formulated 4 conditions as guidelines for tissue banks in the event of fire disaster (Meiicka et al., 1995):

1. establishing an adequate technical and technological base for collection and long-term storage of cells and tissues to be ready for use in case of disaster
2. designing methods capable of assessing volume of stored grafts
3. solving logistic problems associated with supplies for operating theatres in disaster areas
4. organising exchange of grafts on a national and international scale in support of local skin banks exposed to disastrous conditions

METHODS OF COLLECTING AND PROCESSING SKIN GRAFTS

In contrast to technologies used in the 1970s in processing xenogenic and allogenic dermo-epidermal grafts (storage in liquid phase or nitrogen vapours) (Praus et a1.,1980, Bohm et Dvorak, 1981), our present practice includes sampling with a dermatome, deposition in plastic covers and dry-freeze storage at a maximum temperature of -80 °C (Mericka et al, 1995, 2001, 2002).

Using a dermatome enables standardisation of samples, the use of a uniform lyophilisation technology for both xeno- and allogenic dermoepidermal grafts and subsequent sterilisation with gamma radiation (Mericka et al., 2000, 2001). Recently sterilisation with gamma radiation has been replaced by sterilisation with ethylene oxide or the so-called plasma (hydrogen peroxide).

The method of sampling with a Humby knife (blade) is justified under field conditions. We used it in preparing xenogenic dermo-epidermal grafts at the Ufa laboratory after the disaster in 1989, using an emergency set of sterilised tools which we always carry with us during a disastrous situation (Mericka et a1.,1995) (Figs. l,la). Grafts were processed aseptically in a laminar box and preserved in Petri dishes at +4° C. They served to complete the amount of grafts produced by the Ufa laboratory on a Czech-made machine identical to that used in producing Suiderm. Chemical conservation and sterilisation was followed by gamma radiation. These grafts were used mainly at the burn casualty centre, Ufa. Total production of biological cover material was 30 m2. We learned from this catastrophe that providing there is an effective organisation of work, production of grafts may be increased to meet demands for temporary covers of necrotomies even under exceptional conditions.

METHOD OF CALCULATING THE VOLUME OF RESERVES

In assessing anticipated amount of stored grafts and necessary quantity of stored material there are two issues to be resolved:

1. identifying average consumption of covers per casualty depending on degree of injury and pertinent confidential intervals. Our own values (Mericka, Klein et al, 1995, Mericka et al. 2000) indicated that we needed approximately between 6 and 11 m2 of xenotransplants for 10 seriously injured adult patients.
2. identifying the current and security reserves of a tissue bank. Here we may well use our method designed for identifying pharmacy stocks (Mericka, Klein et al., 1995, Mericka et al. 2000). We distinguished between current stock, changing at regular intervals (e.g., by one graft collection of pig butt a week), and reserve stock, consisting of grafts deposited permanently in the bank and issued only in case of emergency or close to expiration time (see e.g. Israel tissue banks). The volume of reserve stock can be calculated with equation S, = K.s whereby S = standard deviation of demands, K differs in agreement with chosen deficit of probability e.g., K = 3 stands for probability deficit 0.001. Equation 2 may be used in calculating current stock: Cb=C . t : T whereby C = number of days within supply cycle, t = turn-over within period under consideration T = number of days covered by pertinent period.

Establishment of reserves may clearly be influenced by individual technologies. In contrast to the situation in the 1990s, when grafts were conserved in Petri dishes and stored in mechanical freezing equipment, and the volume of emergency stores was 2.5 m2 at maximum, our present technologies (using space-saving plastic bags for storage in deep-freezer) have almost doubled the volume of stored material (4 m2).

Collaboration between Skin Banks on both National and International Level

Consideration should be given to the establishment of a national or international network of skin banks for graft exchange capable of supporting local skin banks or burn casualty centres under conditions of fire disaster.

Data from report of committee for collection and transplantation of tissue, Cesky transplant and yearbook of Bioimplant Service, Leiden, Holland 1997.

Table 1 presents information about the size of biological covers issued for clinical use in the Czech Republic in 1996 (Czech transplant 1996, Mericka and Klein, 2000). Data is missing from the Ostrava centre, which did not have an independent skin bank at that time and is focussing its activities on preparing allogenic dermo-epidermal grafts, using for conservation a method designed by the Euroskin Bank (conservation in concentrated glycerol at +4 °C). The size of issued grafts compares well with Euroskin Bank production in Beverwijk, Holland (Bioimplant, Service 1997). The table shows the number of skin covers delivered to clinics. It does not present information about the reaction of individual teams to demands for increased production of skin covers under exceptional circumstances.

Table 2 presents information about both the number of burn casualties supplied with skin covers from our own reserves and an increase in production. As can be seen coping with up to 10 heavy burn casualties is not difficult. We were able to prepare simultaneously grafts for 5 heavy casualties. On the other hand, coping simultaneously with 20 heavy cases may just be beyond our capacity.

TRANSPORTATION OF BIOLOGICAL SKIN COVERS

Exceptional situations may require collaboration with all Czech tissue banks in preparing skin covers for transportation to individual burns centres. Hypothermally-preserved grafts may be transported in polystyrene or other boxes used for transporting blood, using simple ice or ice in plastic boxes as a cooling medium. Hypothermal conditions are suitable for transporting both allogenic and xenogenic dermo-epidermal grafts. Transportation of deep frozen grafts is more demanding, because the temperature inside the transport box should not rise much above -80 °C. This requires perfect heat-insulating cover with either solid carbon dioxide or liquid nitrogen as a cooling medium. Fig 2 (2, 2a) shows the type of box suitable for transporting dermo-epidermal grafts on dry ice. We use either cars or the services of Czech Railways for transportation.

If we were to request international help in case of a catastrophe, we should be prepared to return it. This involves securing long distance transportation of grafts. For civil transportation abroad, we employ authorized removal companies which are also licensed for air transport by IATA. We should make sure that the company has a license for carrying dry ice to the country of destination. Generally, very few companies possess such a license to countries outside the EU. In this case, a classic cooling medium such as frozen saline solution would have to be used - which is what we did last year to transport tissue grafts to the Ukraine (Figs 3, 3a). A great disadvantage of this cooling medium is its considerable weight. Another aspect to be remembered is the system of transportation. DHL places different consignments in the same transport, while World Courier guarantees individual transport. In both cases, the routes are generally far from direct, i.e. from door to door. For instance, DHL transports a consignment from Hradec Kralove to Bratislava by way of Prague, Brussels and Cologne. Some companies take full responsibility for temperature conditions inside the consignment by using data loggers. Table 3 surveys transportation times. It may be advisable to complete each graft with serum from its donor, owing to rules observed by several countries which require repeated serological tests at their own laboratories. Frozen sera may be transported in cryovials.

Figs 2a, 2b. Polystyrene box with dry ice for air transport of allogenic dermo-epidermal grafts to Berlin. Cooling medium - 6 kg dry ice

Fluid nitrogen has to be transported by air in „dry shippers", which are Dewar vessels filled with porous material that soaks up liquid nitrogen. Owing to their bottle-necked shape, this type of vessel is suitable for transporting smaller, cylindrically shaped samples.

The easiest material for transportation are lyophilised grafts at room temperature. Therefore, they ought to be made part of the equipment carried by a rescue team.

DISCUSSION AND CONCLUSIONS

Our description of using biological covers in case of disaster demonstrates differences between various countries. While allotransplants conserved either hypothermically or by deep-freeze are in common use in the States (Hansbrough, 1992), western European countries conserve allotransplants in concentrated glycerol (van Baare, 1994), and the Czech Republic uses either heator cryoconserved xenotransplants (Moserova et Houskova, 1989, Konigova, 1989, Klein, 1989, 1995). Although a small quantity of allotransplants is stored in skin banks of Ostrava, Brno, Hradec Kralove, this amount would hardly be enough to cover the requirements of afire disaster, and therefore production would have to be increased considerably. Experience from the Bashkir disaster also indicates the possibility of using chemically conserved and lyophilized xenotransplants. When comparing the amount of xenotransplants issued in the Czech Republic with that of allotransplants prepared by the Euroskin Bank (Table 1), it has to be remembered that in contrast to allotransplants, xenotransplants generally require repeated application. The amount of allotransplants prepared by the Euroskin Bank is substantially lower than that prepared in the US and Canada over an approximately identical period. Kagan (1998) reports that production of 44 skin banks existini in these countries in 1995, issued 8 732 880 cm of hypothermically or cryo-conserved allogenic dermo-epidermal grafts. However, just 20 of these skin banks were accredited at that time to the American Association of Skin Banks.

Figs 3a, 3b. Large polystyrene box (60 x 40 x 50 cm) plastic bricks filled with concentrated saline solution pre-cooled to 80 degree C, used for transporting autologous bone graft for replantation (reconstruction of calva after neuro-surgery in Hradec Kralove) Ukrainian patient. Weight of consignment more than 20 kg, size of inside wrapping of graft 20x32x3cm.

Calculation of consumption of biological covers per 10 patients depends naturally on methods and procedures employed in practice (these may be either radical or conservative). Under circumstances of an extensive catastrophe, conservative procedures will evidently be preferred although this means a potential increase in demands for biological covers.

All published data including those concerning the attack on September 11 (Tosinska Okroj, 1994-1995, Meiicka et al., 1990, 1995, AATB, 2001) indicate that efforts to produce reserves or increase production of skin grafts in skin banks are focused on types of covers that are currently used in practice.

Our idea about participating in the management of an exceptional situation was based on the fact that we possessed a small reserve of cryoconserved xenotransplants and were ready to increase production of biological covers whenever needed (Mericka et a1.,1995, Mericka et Klein, 2000). We are not certain that our approach is still justified and whether it would not be more appropriate to try to increase reserves of biological materials by involving in their production several institutions of the Czech Republic, so as to be ready for increased demand in the event of a fire disaster. An increase in transplant production at the time of an outbreak of disaster may be impeded by the availability of auxiliary services at, for instance, sterilisation plants or control laboratories - which are available solely to clinical institutions in case of an extensive disaster. Another argument would favour stricter safety measures and subsequently a protraction of the technological cycle of graft production (Mericka et al., 2002). In addition, we cannot be certain that all grafts produced will answer to the criteria of microbiological control.

Apart from increasing reserve stocks for which technical conditions are available, an exceptional situation requires absolutely perfect collaboration among tissue banks. A good example is the reaction of American tissue banks to the September disaster (AATB, 2001). Following the attack, the AATB immediately set up a special service calling upon all accredited banks to announce the amount of grafts they had available for dispatch. The first call was answered immediately by 20 banks offering a total of 1,500 square feet of allotransplants (approx. 1,383,545 CM2 roughly equalling the annual production of the Euroskin Bank in 1997) (Table 1). The New York Center received biological covers from skin banks in geographically remote areas (Ohio, Virginia, Texas) indicating a considerable mobility in dispatching reserve stocks. It is this mobility that should be given utmost attention in designing plans for coping with disastrous situations. In our opinion, air transport of grafts on dry ice is an easier alternative than in vapours of liquid nitrogen (dry shippers).

Figs 4a, 4b. Lyophilised xenogenic dermo-epidermal grafts sterilised with gamma radiation. Prototype of Suiderm, Bioveta, Terezin, Czech Republic (Fig. 4a). Graft prepared in 1998 by Tissue Centre, Medical School Hospital, Hradec Kralove (Fig. 4b).

The least suitable alternative appears to be transportation in pre-cooled bricks filled with concentrated saline solution. However, if there was no other alternative and the distance to be covered is great, temperature conditions inside the bricks should always be validated in advance in order to avoid an increase in temperature above the permitted limit.

When considering setting up a reserve stock of skin covers, we should not forget to include lyophilised grafts as a possibility worth considering. Although production of Suiderm was stopped some time ago, we are familiar with the knowhow involved (Figs. 4, 4a). Their transportation is very easy, but the clinic has certain reservations, such as inadequate adherence to excised skin, a worse barrier function than that of vital grafts. In addition, they are ineffective in preventing microbial growth (Pruitt et Levine, 1984, Kagan, 1998).

Recent experience in the States has shown that safety rules of transplants are strictly observed even under the most exceptional conditions. This was demonstrated by the fact that only accredited skin banks were asked for help (these offered more skin covers than requestedAATB, 2001). This example underlines the importance of a uniform system of accreditation under exceptional conditions. The situation in Europe is quite different. Transportation of biological covers from one country to another is impeded by differences in legislation betwen individual countries or even an absence of individual rules ensuring safety of tissue transplants.

A change may be brought about by introducing and accepting uniform EU directives concerned with high-quality standards and safety measures to be observed in collecting,testing, preparing, storing and distributing human cells and tissues for clinical transplantation.

Negotiations are under way, and among various suggestions is one for the establishment of a network of skin banks accredited throughout Europe.

REFERENCES

1. American Association of Tissue Banks: Association Responds to Attacks on New York City and Washington, D. C. AATB Information Alert XI, No. 6, 2001, p. 4.
2. Bohm, F., Dvorak, R. Deep freezing of dermo-epidermal graft. In: Bohm, F., Dvoiak, R. (eds.): International Meeting of the Society for Low Temperature Biology. Prague, May 12-16, 1980. Institute of Experimental Medicine, Czechoslovak Academy of Sciences, Prague, 1981, p. 128-129.
3. Grossman, N. Multidisciplinary approaches and services in skin banking. Pre Congress Interest Group - Skin Banking, 10th Congress of the International Society for Burn Injuries, Jerusalem, 1.-5.11.1998.
4. Gunn, SWA. The scientific basis of disaster medicine. In Masellis, M., Gunn, SWA. (eds.): The Management of Mass Burn Casulties and Fire Disasters. Dordrecht, Boston, London, Kluwer : Academic Publishers, 1992, p. 13-18.
5. Hansbrough, JS. Wound covera~e with biological dressings and cultured skirt substitutes. is ed., Austin : RG. Landes, 1992, p. 152.
6. Herndon, DN. A survey of the primary aid response to the Bashkir train-gas pipeline disaster. Burns, 16, 1990, p. 323-324.
7. Kagan, RJ. Human skirt banking: Past, present and future. In Phillips, GO., Strong, DM., von Versen, R., Nather, A. (eds.): Advances in Tissue Banking, Vol. 2., Singapore, New Jersey, London, Hong Kong : World Scientific, 1998, p. 297321.
8. Klein, L., Meiicka, P., Pres, J. Pinter, L, Talabova, Z. Xenotransplantace v lecbe popalenin. Voj. zdrav. Listy, 58, 1989, p. 196-198.
9. Klein, L., Mericka, P., Hosek, F. Zajisteni pece o termicke urazy na nasem pracovisti. Voj. zdrav. Listy, 64, 1995, p. 147-149.
10. Klein, L. Popczleniny a vcilecna chirurgie. In Klein, L., Meiicka, P. Piiprava a vyuziti biologickych krytu v lecbe popalenych z pohledu valecne chirurgie a mediciny katastrof Ucebni texty VLA JEP v Hradci Kralove, Vol. 326, 2000, p. 716.
11. Klein, L., Mericka, P. et al. Priprava a vyuziti biologickyeh krytu v lecbe popalenych z pohledu valecne chirurgie a mediciny katastrof. Ucebni texty VLA JEP v Hradci Kralove, Vol. 326, 2000, p. 83.
12. Klein, R., Skalska H. A comparison of dermoepidermal and chorionamniotic grafts in the treatment of burns. Acta Chir. Plast., 18, 1976, p. 225-232.
13. Komise 6eskeho transplantu pro odbery a transplantace tkani: Zprava o cinnosti tkanovych bank a odberech tkani v 6eske republice v roce 1996. Cesky transplant, 1996.
14. Kulyapin, AV., Sakhautdinov, VG., Temerbulatov, VM., Becker, WK., Waymack, JP. Bashkiria train-gas pipeline disaster: a history ofthe U.S.S.RJU.S.A. collaboration. Burns, 16, 1990, p. 339-342.
15. Masellis, M, Gunn, SWA. (eds.). The Management of Burns and Fire Disasters: Perspectives 2000. Dordrecht, Boston, London, Kluwer : Academic Publishers, 1995, p. 603.
16. Meiieka, P., Klein, L., Strakova, H., Sorma, M., Pint6r, L., Talabova, Z., Vavra L. Biologicke kryty pro lecbu popalenych - vlastni zkusenosti. Voj. zdrav. listy, LXIV, 1995, p. 150-153.
17. Meficka, P., Klein, L., Preis, J., Ettlerova, E. The role of the tissue bank in disaster planning. In Masellis, M., Gunn, SWA. (eds.) The Management of Burns. and Fire Disasters: Perspectives 2000. M. Dordrecht, Boston, Eondon,.HIuwer : Academic Publishers, 1995, p. 73-83.
18. Meiicka, P., Strakova, H. Cermak, P., Spepanova, V., Hradecky, Z., Drahosova, M. New safety assurance for biological skin covers. Acta Chir. Plast., 42, 2000, p. 23-29.
19. Meiicka, P., Klein, L., Moznosti pouNti biologickych krytu pii hromadnem vyskytu popalenych. In Klein, L., Meiicka, P. PHprava a vyuziti biologickych krytu v lecbe popdlenych z pohledu vdlecne chirurgie a mediciny katastrof. Ucebni texty VLA JEP v Hradci Kralove, Vol.326, 2000, p. 17-44.
20. Moserova., J.; Hougkova, E. The healing and treatment of skin defects. Basel, Miinchen, Paris, London, New York, New Delhi, Singapore, Tokyo, Sydney, Karger, 1989, p. 163.
21. Praus, R., B6hm, F., Dvorak, R. et al. Skin cryopreservation 1: Incorporation of radioactive sulphate as a criterion of pigskin graft viability after freezing to -196 °C in presence of cryoprotectants. Cryobiology, 17, 1980, p. 130-134.
22. Pruitt, BA., Levine, NS. Characteristics and uses of biologic dressings and skin substitutes. Arch. Surg., 119, 1984, p.312-322.
23. Tosinska-Okroj, H., Grobelny, L, Renkielska, A., Miron-Klein, E., Honory, M., Jankau, J., Muraszko-Kuzma, M. Analysis of procedures, trauma severity and methods of treatment in the mass burn in the entertainment of the Gdansk shipyard. Roczniki Oparzen - Ann. Burns, 5-6, 1994-1995, p. 103-110 (in Polish).
24. Van Baare, J., Buttenwefr, J., Hoeckstra MJ., Dupont, JS. Virucidal effect of glycerol as used in donor skin preservation. Burns, 20, 1994, p. 77-80.