148
Added Hospital Stay Due to Wound Infections Following Cardiac Surgery I. Kapps tein. G. Schulqen", G. Fraedrich"; E Schlosse r" , 1\1. Schumacher". and F. /J. Deschner Dep ar tment of Ilospita l Epidemi ology .. Departm ent of Medica l Biometry a nd Statistics ** Dep ar tment of Cardiovasc ular Surge ry Univers ity Hospital ofF re iburg , Germ an y
To determine the pro lon gation of hospi tal stay du e to post operative wound infection s followi ng ca rdiac s urge ry, a p rospective cohort study was per form ed by ma tchi ng multiple control patients without infection to ea ch infected patient (= ease l. Out of 22 ca ses , no pa tient d ied. reo ca se had to be exclude d from the matching pro cess beca use of a lack of suita ble contr ol patients. The maximum nu mb er of co nt rols per case wa s to . The mea n add ed stay wa s 12.2 days constituting a cons id erable pr olong atio n of stay d ue to wou nd infection in cardiac s urge ry. Key word s Prolongation - Hos pital stay - Costs - Surgical wound infecti on - Ca rd iac su rge ry
Introdu ct ion Due to th e prol onged and often severe morhidity result ing in an increased duration of hospitalization, nosocomial infection s cons titute a su bsta ntial med ical and socioeconomic prohlem , ap ar t from the individual consequences for th e affected patient. Postop erative wound infections are the second most common nos ocomial infections contributing most to the prolongati on of stay and considerably to the added hospital costs (12J. The influen ce of nosocomial infections on the durati on of hosp italization a nd res ulting costs ha ve been discussed extensively in the liter ature (I , 2, 4, 5, S- J:J, 16). However, th e methods employed in ord er to estimate th e adde d hospital stay are quite differ ent. As a result, the re are conside rably varying findin gs de pen dent upon the particular meth od used and , in addition, th e types of pa tient included in th e studies . One such method is the physician s ' estimate technique: a physician not involved in the treatment of the patients retrosp ectively rev iews th e medical record s and oth er clinical inform ations cons ide red important an d judges as to whet he r contin ued hospital stay sho uld be attribute d to nosocom ial infection , thereby estima ting the ad de d stay due to nosocomia l infection . Obvious ly, this method conside rably depends upon th e subject ive as sess ment of the physi-
Thora e. ca rdiovasc . Surgeon 40 (t 992 ) 148 t 51 © Georg Th ieme Ve rlag Stuttga rt · New York
Verla nger ung des Krankenhau sau fenth altes a ufgru nd wu udinfekt lonen naeh He rznpe rutlone n In eincr prosp ektiven Kohorten -Studie wurd e d ie VerHi. ngerun g der Kran kenhau sverweildaue r. bedingt du rch postoperat ive Wund infc ktione n nach herzchir ur gischen Eingr iffen . un ters ucht. Daz u wu rd c ein Ma tching-Vcrfa h rcn a nge wc ndot. bel dem jedern Pat ien ten mit postoperativ er Wund infektion 1= Fall) ci n odcr mc hrere Kontroll-Paticn te n ohne Wu ndi nfe ktion zugeord net wu rd en. Kei ner der 22 Full-Patie nten sta rb. und kei n Fallmuf3te vern xtatchtng-t'rozca a usgcschlossen worde n. well ct n gee ignete r Kon troll-Patle nt nicht gefunden worden kon nte . Die maxim ale Za hl von Kontr olJen pro Fall war 10 . Die mittlere zus iitzliche vc rwotlda ucr bctr ug 12.2 Ta ge. Die Ergebni sse zcrgen eine n er heblich ver lengcncn Kran kvnha usaufe nth alt . bedlngt durc h pos topera tive Wundinfekt ione n bel her zchirurgischen Patientcn.
clan, and ther e is also agree me nt that it results in a n un derest imation of the extra hospital da ys IS, 10, 11, 13). Direct comparison of the tota l hospital stay of all infected and non -infected patients is anothe r method leading to an overestimation of the added hospit alization becau se of ignoring the duration of sta y of the infected patient prior to the onset of his/h er nosocomial infection (1, 2, 9- 11, 13): frequ entl y, risk factor s for nosocom ial infections are also the cause for a longer hosp italizati on . The re fore, excluding thos e cons ide rati ons would lead to groups of patient s not dir ectly compa ra ble. To redu ce th e effects of potentially confounding factors such as und erlying diseases or age it is necessary to match infected pati en ts with as similar as possible control patien ts who did not acquire a nosocomial infection du ring th eir hospitalizati on. There are already man y dat a in the literature concerning th e effect of postoperative wound infections on th e length of hospitalization, but most of these stud ies were performed with pati en ts of different surgical se rvices (1, 2, 9, 10, 13). There is only one stud y considering exclusively ca rdiac surgery pati ents as we did in our pr esent study (I) . However , compared to all other st udies we employed a meth odologically imp roved matchin g pro cedure using mu ltiple control subjects for each of the infected pa tients by conside ring the time spent in the hospital prior to onset of infection as a matching criterion.
Received for Pul ication: December 2. 1991
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Summar)'
Material and Methods From Novemher 1988 to Septe mber 1989, 569 pati ents und ergoing open hea rt surge ry (hea rt valve replacement, corona ry arte ry bypass graft ) from a large un ivers ity hospital were pro spectively included into the study. Pati ents were mon itored daily du rin g tbe entire hospital stay by a ph ysician who was not involved in the trea tment of the pati ents. The following data were also recorded: time of admission to th e hospital , tim e of surgery, time of onset of woun d infection , an d time of dischar ge from the hospita l or time of death. The diagnosis oj postoperative wound injection was bas ed up on the definitions for nosocomial infections by the Centers for Disease Control (7). Accordingly, woun d infection had to meet th e following criteria: 1. lncision al surgica l wo un d infectio n: infection occurs at incision site and involves skin, subcutaneo us tissue, or bone and any of the following: a) purulent dra inage from incision or subcutaneous drain, b) organism isolated from culture of fluid from wound closed primarily, c) surgeon deliber ately opens the wound , unless wound is culture -nega tive, d) surg eon's or att ending physician 's diagnosis of infection . 2. Deep surgical wou nd infection : infection occurs at operati ve site and infection appears related to surger y and infection involves deep tissues or spaces and an y of the following: a) purulen t drain age from deep drain, b) wou nd spontaneo usly dehisces or is delib erately opened by sur geon when pati ent has fever (> 38 ' C) and /o r localized pai n or tend er ness, un less wound is culture negat ive. c) an absc ess or other evidence of infection seen on direct exa mina tion. du ring surgery, or by histopath ologic exa minatio n, d) surgeon's diagnosis of infection , The prolongatio n oj hospital stay was determined using a procedure to match eac h of the infected pati ents ( ~ cases) with a var ia ble num ber of multiple correspo nding contro l patient s who did not acquire wound infection (15, 17). Each cas e was matched to one or more control subjects on the basis of th e following var ia bles considered to be poten tially confoun ding factors : 1) Surgical pr ocedure case and controls had to und ergo th e same. 2)Age, it was matched within ten yea rs of the case and controls. 3) Duration of hospital stay, it had to be at least as long as the time interval un til infection of th e case patien t. It did not seem reasonable to cons ider further matching criteria since a greater similarity betw een cases and contro ls would have resulted in a significant selective effect. Pat ient s who died ha d to be excluded from the ana lysis. So far , it is a meth odologically unresolved problem to conside r death adequa tely since it does not see m suitab le to use th e tim e of dea th as the time of dischar ge, for one thereb y puts patients who died in the sa me category as the pat ients dischar ged . Given that nosocomial infection increases the risk of mort ality an d, furtherm ore, death occur s shortly afte r th e onse t of nosocomial infection one would conclude th at nosocomial infections shorten the dur ation of hospital stay. At lea st, however, the excess days would be und er estima ted , To utilize the pool of appropr iate controls as far as possible, a matching procedure with a variable numher of mu ltiple control patient s per ea ch infected patient was employed, First, a pool of suita ble comparison subjects was
Thom e. eardiol'asc. Surgeon 40 (J 992)
established according to the aforementioned matching criteria for th e "most difficult" case [i. e.. that for which it is hardest to find a ma tch becau se of a relatively late onset of wound infection). Subsequently, th e patient with the least differ ence of age was selected as control from th e pool of possible contro l subjects and the re ma ining cont rols were retu rn ed into the pool. For a case who rema ined without contro l in this first matching procedure a suita ble contro l could not be identified and, thereby, the added length of stay could not be dete rmined . Altogether, the pr ocedure was repeated ten times so tha t up to ten contro ls could have been ass igned to each case . The procedu re was carried out aut omatically by means of a compute r pr ogram writte n in SAS (Statistical Analysis System) by one of the authors (G, 5.). The added length of stay was calculated by subtract ing the mean sta y ofall cont rols for a given case from th e stay of th at case. The resulting differ ence in length of hospitalizati on was the added stay associa ted with postop er ative wound infection for the given case. In order to estimate th e additional costs due to the prolongatio n of stay, the usual daily rate per patient cared for in a norm al wa rd was used because a more exact calculation of costs by considering everv mea sur e actua llv employed in the care of infected pati ents would have been too difficult to realize. Ther efore, the average numb er of extra days of all cases was multiplied by the cur rent daily ward cha rges at the University Hospital of Freiburg. Res ults Overall , 569 consecutive pati ents were includ ed in the study, The baseline data of the study patient s are sh own in Tahle 1. Twenty-two (3.9 %) patien ts acquired postoper ative sternal wound infection , none of th ese patient s died and none were cases of reoperation , Of the 547 patien ts without wound infection , 9 (1.7 %) died and , therefore, were excluded from the ana lysis. From th e remaining 536 uninfected patien ts, 132 (24.6 %) cont rol pati ents could be selected. The maximum numher of contro ls per infected patient was 10. In Table 2 some characteristics of the 22 case s and their controls are summa rized. Discussion There is a lot of informati on in the liter atur e on the effect of nosocomial infections on the length of hospitalization and inh erent costs (I, 2, 4, 5, 8-13, 16). However , th e differ ent meth ods used estimating the prolongati on of hospit al stay in Table 1 Baselinedataand underlying diseases of the studypatients Patient characteristics
Number (%) of patients
All patients Male Female Age (yea rs) (average ± SO l)
569(100) 418 (73.5) 151(26.5) 59.6 ± 9.1
Malignancy Diabetesmellitus Leukocytosis Hypoproteinemia Anemia Infection at admission Clean surgery Clean-contaminated surgery Preoperative antibiotic prophylaxis' ]
4 ( 0.7) 83 (14.6) 24 ( 4.21 11 ( 1.9) 15 ( 2.6) 5 ( 0.9) 565(99.3) 4 ( 0.7) 567 (99.6)
I SD =
standarddeviation; 2 single shotwith a 2ndgeneration cephalosporin
149
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Added lI ospital S tay Due to I1bund Infe ctions Following Cardiac Su rgery
l. Kappstein. G. Schu/gen. G. Fraedrich. V. Schlosser. M. Schumacher. and F. D. Daschner
Thorae. cardiovasc . Surgeon 40 (199 2) Case no.
Diagnosis'
Age (years)
Type of
Onset
Postop.
operation'
of WI'
stay(days) of controls
Number
postap.
stay(days)'
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
CAD CAD CAD CAD CAD CAD CAD CAD CAD AVD CAD CAD AVD CAD CAD CAD CAD CAD CAD CAD CAD CAD
56 75 42 62 61 59 67 58 64 53 46 67 48 63 71 61 54 45 69 45 76 59
CABG CABG CABG CABG CABG CABG CABG CABG CABG AVR CABG CABG AVR CABG CABG CABG CABG CABG CABG CABG CABG CABG
14 14 12 14 10 6 18 14 21 9 11 6 10 25 20 19 15 5 7 24 14 13
18 30 15 27 23 35 36 42 48 17 26 27 29 54 36 40 27 14 17 73 21 25
5 3 5 5 10 10 3 5 3 10 6 10 8 2 4 3 5 10 10 2 3 10
Table 2
Controls
18.8 ± 2.8 23.7 ± 5.7 15.4 ± 1.1 16.8 ± 2.5 11.8 ± 1.3 8.4 ± 1.6 21.0 ± 0.0 16.0 ± 1.7 22.7 ± 0.6 10.6 ± 0.8 13.4 ± 1.6 9.3 ± 1.6 12.5 ± 1.2 39.5 ± 17.7 33.0 ± 14.5 21.0 ± 1.0 19.2 ± 1.8 7.1 ± 1.0 9.8 ± 2.2 48.5 ± 7.8 184 ± 2.9 14.6 ± 1.6
age (Years)'
54.6 ± 2.8 77.0 ± 4.4 44.6 ± 6.8 64.0 ± 3.7 60.6 ± 2.1 59.0 ± 0.0 68.0 ± 1.7 59.4 ± 5.5 63.4 ± 4.9 53.6 ± 1.2 48.4 ± 5.9 67.0 ± 0.5 52.8 ± 4.3 68.0 ± 0.0 68.3 ± 3.6 61.4 ± 2.3 58.2 ± 5.4 45.0 ± 0.7 69.1 ± 0.3 48.5 ± 0.7 73.7 ± 6.8 61.6 ± 3.7
1 CAD _ coronary artery disease, AVD "" aortal valve defect; 2 CABG - coronary artery bypass graft, AVR replacement; 3 number of days postoperatively; 4 average ± SD
various pati ent groups lead to considerably different results. In our study. a matchin g procedure with a variable numb er of multiple control patients per each infected pati ent was employed. In addition. since we considered the time of onset of wound infection. an overes timation of the added length of stay should have been avoided . The re are severa l stud ies about the influence of post opera tive wound infection on the durati on of hospital stay 11 . 2. 9. 10. 13), however . a matc hing procedure compa rable with ours was never used. The greatest methodological pr oblem of most studies using a matching procedure is the num ber of patients definitively remaini ng for statisti cal ana lysis. In our study . all 22 pat ients with surgical woun d infection entered the mat ching analysis . Although the number of cas es availabl e was quite limited. the validity of ou r resu lts is imp roved by an increased numb er of control patients (up to 10) per case while other investigat ors used only one control pati ent. The main criticism of matching procedures is the potential confounding ofthe results by the exclusion of cases who could not be matched with appropriate controls thus allowing the possibility of selection bias (6). A similar prob lem is the exclusion of pati ents with fatal outcom e. In our study. no patient with surgica l wound infection had to be excluded beca use of a lack of suitable contro ls or because of dea th. In conclusio n. a cons iderable prolongation of hospital stay (+ 12.2 days) an d significant additional costs (on average 5 9 10 Germa n mark s per patient) caused by postoper ative wound infection could be confirmed in our study. Ther e are sufficient data in the liter ature supporting that the costs necessaryforinfection control are more thanreimbursed by the redu ction of nosocomial infections and their inher ent costs (3, 12. 14. 18). It is a well known fact that a substa ntial proporti on of about up to 70 of all nosocomial infections cons tituting an irr educible minimum cannot be pr event ed
Characteristics of the
22 cases with postoperative wound infection. (W3)
=
aortal valve
even by optimal hygienic measures. However. to redu ce the number ofinfe ctions as far as possib le. established infection control measures must be strictly followed. Referen ces Boyce. J. M.. G. Pouer-Bunoe. and L. Dziobek: Hospital Reimbursement Patterns among Patients with Surgical Wound Infections following Open Heart Surgery. Infect. Control. Hasp. Epidemiol. 11 (990) 89- 93 2 Cruse. P. J. E.. and R. Foard: The Epidemiology of Wound Infections. Surg. Clio. N. Am . 60 (980) 27- 40 a Deschn er. E : Cost-effectiveness in Hospital Infection Control-Lesso ns for the 19905 . J. Hasp. Infect. 13 (1989 ) 325-336 .. Freeman. 1.. B. A. Rosner. and}. E. McGowan. Jr.: Adverse Effectsof Nosoco mial Infections. J. Inf. Dis. 140 (1979) 73 2-740 5 Freeman. J.. and J. E. McGowan. Jr.: Methodologic Issues in Hospital Epidemiology. III. Investigating the Modifying Effects of Time and Severity of Underlying Illness on Estimates of Costs of Nosocomial lnfection. Rev. Inf. Dis. 6 (1984) 285- 300 6 Freeman. J.. D. A. Goldman. and J. E. McGowan. Jr.: Methologic Issues in Hospital Epidemiology. IV. Risk Ratios. Confounding. Effect of Modification. and Analysis of Multiple Variables. Rev. Inf. Dis. 6 ( 988) 1118-1141 1 Garner. J. 5.. W. R. Jarvis. T. G. Emort; T. C. Horan. and J. M. Hughes: CDC Definitions for Nosocomial Infections . 1988. Am . J. Infect. Control. 16 (1988) 128-140 8 Green. M. S.. E. Rubinst ein. and P. Amit: Estimating the Effects of Nosocomial Infections on the Length of Hospitalization. J. lnf Dis. 145 (1982) 667-672 9 Green. J. w.. and R. P. Wenzel: Postoperative Wound Infections: A Controlled Study of the Increased Duration of Hospital Stay and Direct Cost of Hospitalization . Ann. Surg. 185 (1977) 264-268 10 Haley. R. W. D. R. Schaberg. S. D. von Allm en. and J. E. McGowan. Jr.: Estimating the Extra Charges and Prolongation of Hospitalization due to Nosocomial Infections: A Comparison of Methods. J. Inf. Dis. 141 (1980) 248-25 7 II Haley. R. W . D. R. Schaberg. K. B. Crossley. S. D. vonAllmen. and J. E. McGowan. Jr.: Extra Charges and Prolongation of Stay Attributable to Nosocomial Infections:A Prospective InterhospitalComparison. Am. J. Med. 70 (1981) 51-58 I
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150
Added Hospital S lay Due to Wound Injections Following Cardiac Su rgery
13
14
15 If>
17
18
Haley, R. W : Man aging Hospital lnfection Control for Cost-effectiveness. Chicago. Ame rica n Hosp ital Pub lish ing Inc. 1986 McGowan. 1. E.: The Cost of Hosp ital-acquired Infection . In: Sabri. 5.. and J. R. Titten sor . ed .: Proceedings of the First Middle East Symposiu m on Hospi tal Infection a nd its Control, Kuwait 1981. Richmond . Barker Publications Limited 1982. pp 27-30 Miller. P. J.• B. M. Farr. a nd J. M. Gwaltn ey. Jr.: Economic Benefits of a n Effective Infection Control Program : Case Study a nd Proposal. Rev. lnr. Dis. 11 (1989) 284- 288 Sc hlesselman. J.: Case-control Studies: Design. Conduct and Analy sis. New York. Oxford Univers ity Press 1982 Spengler. R. F.. a nd W B. Greenough.lll: Hospital Costs and Mortal ity Attri buted to Nosoco mia l Bactere mias. J. Am. Med . Ass. 240 11978 1 2455- 245R Ury. H. K.: Efficiency of Case -contro l Studies with Multiple Contro ls per Case: Contin uous or Dichotomous Data. Biometrics 31 (1975) 643 -649 Wenzel, R. P.: Nosocomial Infections. Diagnosis-re lated Groups . a nd Study on th e Effica cy of Nosocomial Infection control. Am. J. Med. 78 , (1985) Suppl . 6B, 3-7
151
Ines Kappstein, MD Department of Hosp ital Epidemiology University Hospital of Freiburg Hugstetter StraBe 55 D-7800 Freiburg Germa ny
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11
Thom c. cardiovasc. S urgeon 40(1 992)
Added Hospital Stay Due to Wound Infections Following Cardiac Surgery I. Kapps tein. G. Schulqen", G. Fraedrich"; E Schlosse r" , 1\1. Schumacher". and F. /J. Deschner Dep ar tment of Ilospita l Epidemi ology .. Departm ent of Medica l Biometry a nd Statistics ** Dep ar tment of Cardiovasc ular Surge ry Univers ity Hospital ofF re iburg , Germ an y
To determine the pro lon gation of hospi tal stay du e to post operative wound infection s followi ng ca rdiac s urge ry, a p rospective cohort study was per form ed by ma tchi ng multiple control patients without infection to ea ch infected patient (= ease l. Out of 22 ca ses , no pa tient d ied. reo ca se had to be exclude d from the matching pro cess beca use of a lack of suita ble contr ol patients. The maximum nu mb er of co nt rols per case wa s to . The mea n add ed stay wa s 12.2 days constituting a cons id erable pr olong atio n of stay d ue to wou nd infection in cardiac s urge ry. Key word s Prolongation - Hos pital stay - Costs - Surgical wound infecti on - Ca rd iac su rge ry
Introdu ct ion Due to th e prol onged and often severe morhidity result ing in an increased duration of hospitalization, nosocomial infection s cons titute a su bsta ntial med ical and socioeconomic prohlem , ap ar t from the individual consequences for th e affected patient. Postop erative wound infections are the second most common nos ocomial infections contributing most to the prolongati on of stay and considerably to the added hospital costs (12J. The influen ce of nosocomial infections on the durati on of hosp italization a nd res ulting costs ha ve been discussed extensively in the liter ature (I , 2, 4, 5, S- J:J, 16). However, th e methods employed in ord er to estimate th e adde d hospital stay are quite differ ent. As a result, the re are conside rably varying findin gs de pen dent upon the particular meth od used and , in addition, th e types of pa tient included in th e studies . One such method is the physician s ' estimate technique: a physician not involved in the treatment of the patients retrosp ectively rev iews th e medical record s and oth er clinical inform ations cons ide red important an d judges as to whet he r contin ued hospital stay sho uld be attribute d to nosocom ial infection , thereby estima ting the ad de d stay due to nosocomia l infection . Obvious ly, this method conside rably depends upon th e subject ive as sess ment of the physi-
Thora e. ca rdiovasc . Surgeon 40 (t 992 ) 148 t 51 © Georg Th ieme Ve rlag Stuttga rt · New York
Verla nger ung des Krankenhau sau fenth altes a ufgru nd wu udinfekt lonen naeh He rznpe rutlone n In eincr prosp ektiven Kohorten -Studie wurd e d ie VerHi. ngerun g der Kran kenhau sverweildaue r. bedingt du rch postoperat ive Wund infc ktione n nach herzchir ur gischen Eingr iffen . un ters ucht. Daz u wu rd c ein Ma tching-Vcrfa h rcn a nge wc ndot. bel dem jedern Pat ien ten mit postoperativ er Wund infektion 1= Fall) ci n odcr mc hrere Kontroll-Paticn te n ohne Wu ndi nfe ktion zugeord net wu rd en. Kei ner der 22 Full-Patie nten sta rb. und kei n Fallmuf3te vern xtatchtng-t'rozca a usgcschlossen worde n. well ct n gee ignete r Kon troll-Patle nt nicht gefunden worden kon nte . Die maxim ale Za hl von Kontr olJen pro Fall war 10 . Die mittlere zus iitzliche vc rwotlda ucr bctr ug 12.2 Ta ge. Die Ergebni sse zcrgen eine n er heblich ver lengcncn Kran kvnha usaufe nth alt . bedlngt durc h pos topera tive Wundinfekt ione n bel her zchirurgischen Patientcn.
clan, and ther e is also agree me nt that it results in a n un derest imation of the extra hospital da ys IS, 10, 11, 13). Direct comparison of the tota l hospital stay of all infected and non -infected patients is anothe r method leading to an overestimation of the added hospit alization becau se of ignoring the duration of sta y of the infected patient prior to the onset of his/h er nosocomial infection (1, 2, 9- 11, 13): frequ entl y, risk factor s for nosocom ial infections are also the cause for a longer hosp italizati on . The re fore, excluding thos e cons ide rati ons would lead to groups of patient s not dir ectly compa ra ble. To redu ce th e effects of potentially confounding factors such as und erlying diseases or age it is necessary to match infected pati en ts with as similar as possible control patien ts who did not acquire a nosocomial infection du ring th eir hospitalizati on. There are already man y dat a in the literature concerning th e effect of postoperative wound infections on th e length of hospitalization, but most of these stud ies were performed with pati en ts of different surgical se rvices (1, 2, 9, 10, 13). There is only one stud y considering exclusively ca rdiac surgery pati ents as we did in our pr esent study (I) . However , compared to all other st udies we employed a meth odologically imp roved matchin g pro cedure using mu ltiple control subjects for each of the infected pa tients by conside ring the time spent in the hospital prior to onset of infection as a matching criterion.
Received for Pul ication: December 2. 1991
Downloaded by: Universite Laval. Copyrighted material.
Summar)'
Material and Methods From Novemher 1988 to Septe mber 1989, 569 pati ents und ergoing open hea rt surge ry (hea rt valve replacement, corona ry arte ry bypass graft ) from a large un ivers ity hospital were pro spectively included into the study. Pati ents were mon itored daily du rin g tbe entire hospital stay by a ph ysician who was not involved in the trea tment of the pati ents. The following data were also recorded: time of admission to th e hospital , tim e of surgery, time of onset of woun d infection , an d time of dischar ge from the hospita l or time of death. The diagnosis oj postoperative wound injection was bas ed up on the definitions for nosocomial infections by the Centers for Disease Control (7). Accordingly, woun d infection had to meet th e following criteria: 1. lncision al surgica l wo un d infectio n: infection occurs at incision site and involves skin, subcutaneo us tissue, or bone and any of the following: a) purulent dra inage from incision or subcutaneous drain, b) organism isolated from culture of fluid from wound closed primarily, c) surgeon deliber ately opens the wound , unless wound is culture -nega tive, d) surg eon's or att ending physician 's diagnosis of infection . 2. Deep surgical wou nd infection : infection occurs at operati ve site and infection appears related to surger y and infection involves deep tissues or spaces and an y of the following: a) purulen t drain age from deep drain, b) wou nd spontaneo usly dehisces or is delib erately opened by sur geon when pati ent has fever (> 38 ' C) and /o r localized pai n or tend er ness, un less wound is culture negat ive. c) an absc ess or other evidence of infection seen on direct exa mina tion. du ring surgery, or by histopath ologic exa minatio n, d) surgeon's diagnosis of infection , The prolongatio n oj hospital stay was determined using a procedure to match eac h of the infected pati ents ( ~ cases) with a var ia ble num ber of multiple correspo nding contro l patient s who did not acquire wound infection (15, 17). Each cas e was matched to one or more control subjects on the basis of th e following var ia bles considered to be poten tially confoun ding factors : 1) Surgical pr ocedure case and controls had to und ergo th e same. 2)Age, it was matched within ten yea rs of the case and controls. 3) Duration of hospital stay, it had to be at least as long as the time interval un til infection of th e case patien t. It did not seem reasonable to cons ider further matching criteria since a greater similarity betw een cases and contro ls would have resulted in a significant selective effect. Pat ient s who died ha d to be excluded from the ana lysis. So far , it is a meth odologically unresolved problem to conside r death adequa tely since it does not see m suitab le to use th e tim e of dea th as the time of dischar ge, for one thereb y puts patients who died in the sa me category as the pat ients dischar ged . Given that nosocomial infection increases the risk of mort ality an d, furtherm ore, death occur s shortly afte r th e onse t of nosocomial infection one would conclude th at nosocomial infections shorten the dur ation of hospital stay. At lea st, however, the excess days would be und er estima ted , To utilize the pool of appropr iate controls as far as possible, a matching procedure with a variable numher of mu ltiple control patient s per ea ch infected patient was employed, First, a pool of suita ble comparison subjects was
Thom e. eardiol'asc. Surgeon 40 (J 992)
established according to the aforementioned matching criteria for th e "most difficult" case [i. e.. that for which it is hardest to find a ma tch becau se of a relatively late onset of wound infection). Subsequently, th e patient with the least differ ence of age was selected as control from th e pool of possible contro l subjects and the re ma ining cont rols were retu rn ed into the pool. For a case who rema ined without contro l in this first matching procedure a suita ble contro l could not be identified and, thereby, the added length of stay could not be dete rmined . Altogether, the pr ocedure was repeated ten times so tha t up to ten contro ls could have been ass igned to each case . The procedu re was carried out aut omatically by means of a compute r pr ogram writte n in SAS (Statistical Analysis System) by one of the authors (G, 5.). The added length of stay was calculated by subtract ing the mean sta y ofall cont rols for a given case from th e stay of th at case. The resulting differ ence in length of hospitalizati on was the added stay associa ted with postop er ative wound infection for the given case. In order to estimate th e additional costs due to the prolongatio n of stay, the usual daily rate per patient cared for in a norm al wa rd was used because a more exact calculation of costs by considering everv mea sur e actua llv employed in the care of infected pati ents would have been too difficult to realize. Ther efore, the average numb er of extra days of all cases was multiplied by the cur rent daily ward cha rges at the University Hospital of Freiburg. Res ults Overall , 569 consecutive pati ents were includ ed in the study, The baseline data of the study patient s are sh own in Tahle 1. Twenty-two (3.9 %) patien ts acquired postoper ative sternal wound infection , none of th ese patient s died and none were cases of reoperation , Of the 547 patien ts without wound infection , 9 (1.7 %) died and , therefore, were excluded from the ana lysis. From th e remaining 536 uninfected patien ts, 132 (24.6 %) cont rol pati ents could be selected. The maximum numher of contro ls per infected patient was 10. In Table 2 some characteristics of the 22 case s and their controls are summa rized. Discussion There is a lot of informati on in the liter atur e on the effect of nosocomial infections on the length of hospitalization and inh erent costs (I, 2, 4, 5, 8-13, 16). However , th e differ ent meth ods used estimating the prolongati on of hospit al stay in Table 1 Baselinedataand underlying diseases of the studypatients Patient characteristics
Number (%) of patients
All patients Male Female Age (yea rs) (average ± SO l)
569(100) 418 (73.5) 151(26.5) 59.6 ± 9.1
Malignancy Diabetesmellitus Leukocytosis Hypoproteinemia Anemia Infection at admission Clean surgery Clean-contaminated surgery Preoperative antibiotic prophylaxis' ]
4 ( 0.7) 83 (14.6) 24 ( 4.21 11 ( 1.9) 15 ( 2.6) 5 ( 0.9) 565(99.3) 4 ( 0.7) 567 (99.6)
I SD =
standarddeviation; 2 single shotwith a 2ndgeneration cephalosporin
149
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Added lI ospital S tay Due to I1bund Infe ctions Following Cardiac Su rgery
l. Kappstein. G. Schu/gen. G. Fraedrich. V. Schlosser. M. Schumacher. and F. D. Daschner
Thorae. cardiovasc . Surgeon 40 (199 2) Case no.
Diagnosis'
Age (years)
Type of
Onset
Postop.
operation'
of WI'
stay(days) of controls
Number
postap.
stay(days)'
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
CAD CAD CAD CAD CAD CAD CAD CAD CAD AVD CAD CAD AVD CAD CAD CAD CAD CAD CAD CAD CAD CAD
56 75 42 62 61 59 67 58 64 53 46 67 48 63 71 61 54 45 69 45 76 59
CABG CABG CABG CABG CABG CABG CABG CABG CABG AVR CABG CABG AVR CABG CABG CABG CABG CABG CABG CABG CABG CABG
14 14 12 14 10 6 18 14 21 9 11 6 10 25 20 19 15 5 7 24 14 13
18 30 15 27 23 35 36 42 48 17 26 27 29 54 36 40 27 14 17 73 21 25
5 3 5 5 10 10 3 5 3 10 6 10 8 2 4 3 5 10 10 2 3 10
Table 2
Controls
18.8 ± 2.8 23.7 ± 5.7 15.4 ± 1.1 16.8 ± 2.5 11.8 ± 1.3 8.4 ± 1.6 21.0 ± 0.0 16.0 ± 1.7 22.7 ± 0.6 10.6 ± 0.8 13.4 ± 1.6 9.3 ± 1.6 12.5 ± 1.2 39.5 ± 17.7 33.0 ± 14.5 21.0 ± 1.0 19.2 ± 1.8 7.1 ± 1.0 9.8 ± 2.2 48.5 ± 7.8 184 ± 2.9 14.6 ± 1.6
age (Years)'
54.6 ± 2.8 77.0 ± 4.4 44.6 ± 6.8 64.0 ± 3.7 60.6 ± 2.1 59.0 ± 0.0 68.0 ± 1.7 59.4 ± 5.5 63.4 ± 4.9 53.6 ± 1.2 48.4 ± 5.9 67.0 ± 0.5 52.8 ± 4.3 68.0 ± 0.0 68.3 ± 3.6 61.4 ± 2.3 58.2 ± 5.4 45.0 ± 0.7 69.1 ± 0.3 48.5 ± 0.7 73.7 ± 6.8 61.6 ± 3.7
1 CAD _ coronary artery disease, AVD "" aortal valve defect; 2 CABG - coronary artery bypass graft, AVR replacement; 3 number of days postoperatively; 4 average ± SD
various pati ent groups lead to considerably different results. In our study. a matchin g procedure with a variable numb er of multiple control patients per each infected pati ent was employed. In addition. since we considered the time of onset of wound infection. an overes timation of the added length of stay should have been avoided . The re are severa l stud ies about the influence of post opera tive wound infection on the durati on of hospital stay 11 . 2. 9. 10. 13), however . a matc hing procedure compa rable with ours was never used. The greatest methodological pr oblem of most studies using a matching procedure is the num ber of patients definitively remaini ng for statisti cal ana lysis. In our study . all 22 pat ients with surgical woun d infection entered the mat ching analysis . Although the number of cas es availabl e was quite limited. the validity of ou r resu lts is imp roved by an increased numb er of control patients (up to 10) per case while other investigat ors used only one control pati ent. The main criticism of matching procedures is the potential confounding ofthe results by the exclusion of cases who could not be matched with appropriate controls thus allowing the possibility of selection bias (6). A similar prob lem is the exclusion of pati ents with fatal outcom e. In our study. no patient with surgica l wound infection had to be excluded beca use of a lack of suitable contro ls or because of dea th. In conclusio n. a cons iderable prolongation of hospital stay (+ 12.2 days) an d significant additional costs (on average 5 9 10 Germa n mark s per patient) caused by postoper ative wound infection could be confirmed in our study. Ther e are sufficient data in the liter ature supporting that the costs necessaryforinfection control are more thanreimbursed by the redu ction of nosocomial infections and their inher ent costs (3, 12. 14. 18). It is a well known fact that a substa ntial proporti on of about up to 70 of all nosocomial infections cons tituting an irr educible minimum cannot be pr event ed
Characteristics of the
22 cases with postoperative wound infection. (W3)
=
aortal valve
even by optimal hygienic measures. However. to redu ce the number ofinfe ctions as far as possib le. established infection control measures must be strictly followed. Referen ces Boyce. J. M.. G. Pouer-Bunoe. and L. Dziobek: Hospital Reimbursement Patterns among Patients with Surgical Wound Infections following Open Heart Surgery. Infect. Control. Hasp. Epidemiol. 11 (990) 89- 93 2 Cruse. P. J. E.. and R. Foard: The Epidemiology of Wound Infections. Surg. Clio. N. Am . 60 (980) 27- 40 a Deschn er. E : Cost-effectiveness in Hospital Infection Control-Lesso ns for the 19905 . J. Hasp. Infect. 13 (1989 ) 325-336 .. Freeman. 1.. B. A. Rosner. and}. E. McGowan. Jr.: Adverse Effectsof Nosoco mial Infections. J. Inf. Dis. 140 (1979) 73 2-740 5 Freeman. J.. and J. E. McGowan. Jr.: Methodologic Issues in Hospital Epidemiology. III. Investigating the Modifying Effects of Time and Severity of Underlying Illness on Estimates of Costs of Nosocomial lnfection. Rev. Inf. Dis. 6 (1984) 285- 300 6 Freeman. J.. D. A. Goldman. and J. E. McGowan. Jr.: Methologic Issues in Hospital Epidemiology. IV. Risk Ratios. Confounding. Effect of Modification. and Analysis of Multiple Variables. Rev. Inf. Dis. 6 ( 988) 1118-1141 1 Garner. J. 5.. W. R. Jarvis. T. G. Emort; T. C. Horan. and J. M. Hughes: CDC Definitions for Nosocomial Infections . 1988. Am . J. Infect. Control. 16 (1988) 128-140 8 Green. M. S.. E. Rubinst ein. and P. Amit: Estimating the Effects of Nosocomial Infections on the Length of Hospitalization. J. lnf Dis. 145 (1982) 667-672 9 Green. J. w.. and R. P. Wenzel: Postoperative Wound Infections: A Controlled Study of the Increased Duration of Hospital Stay and Direct Cost of Hospitalization . Ann. Surg. 185 (1977) 264-268 10 Haley. R. W. D. R. Schaberg. S. D. von Allm en. and J. E. McGowan. Jr.: Estimating the Extra Charges and Prolongation of Hospitalization due to Nosocomial Infections: A Comparison of Methods. J. Inf. Dis. 141 (1980) 248-25 7 II Haley. R. W . D. R. Schaberg. K. B. Crossley. S. D. vonAllmen. and J. E. McGowan. Jr.: Extra Charges and Prolongation of Stay Attributable to Nosocomial Infections:A Prospective InterhospitalComparison. Am. J. Med. 70 (1981) 51-58 I
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Added Hospital S lay Due to Wound Injections Following Cardiac Su rgery
13
14
15 If>
17
18
Haley, R. W : Man aging Hospital lnfection Control for Cost-effectiveness. Chicago. Ame rica n Hosp ital Pub lish ing Inc. 1986 McGowan. 1. E.: The Cost of Hosp ital-acquired Infection . In: Sabri. 5.. and J. R. Titten sor . ed .: Proceedings of the First Middle East Symposiu m on Hospi tal Infection a nd its Control, Kuwait 1981. Richmond . Barker Publications Limited 1982. pp 27-30 Miller. P. J.• B. M. Farr. a nd J. M. Gwaltn ey. Jr.: Economic Benefits of a n Effective Infection Control Program : Case Study a nd Proposal. Rev. lnr. Dis. 11 (1989) 284- 288 Sc hlesselman. J.: Case-control Studies: Design. Conduct and Analy sis. New York. Oxford Univers ity Press 1982 Spengler. R. F.. a nd W B. Greenough.lll: Hospital Costs and Mortal ity Attri buted to Nosoco mia l Bactere mias. J. Am. Med . Ass. 240 11978 1 2455- 245R Ury. H. K.: Efficiency of Case -contro l Studies with Multiple Contro ls per Case: Contin uous or Dichotomous Data. Biometrics 31 (1975) 643 -649 Wenzel, R. P.: Nosocomial Infections. Diagnosis-re lated Groups . a nd Study on th e Effica cy of Nosocomial Infection control. Am. J. Med. 78 , (1985) Suppl . 6B, 3-7
151
Ines Kappstein, MD Department of Hosp ital Epidemiology University Hospital of Freiburg Hugstetter StraBe 55 D-7800 Freiburg Germa ny
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Thom c. cardiovasc. S urgeon 40(1 992)
