A Fifteen-Year Wound Surveillance Study After Coronary Artery Bypass Mark S. Slaughter, MD, Mary M. Olson, RN, BSN, James T. Lee, Jr, MD, PhD, and Herbert B. Ward, MD, PhD Department of Surgery, Minneapolis Veterans Affairs Medical Center and University of Minnesota, Minneapolis, Minnesota

Wound infections after coronary artery bypass opera- tions have been continuously monitored at the Minneap- olis Veterans Affairs Hospital for 15 years. All patients were followed up for 30 days. From 1977 to 1991, 2,402 coronary artery bypass operations were performed, and wound infections developed in 125 (5%) patients. There were 71 (3%) chest infections of which 33 (1.4%) were major and 38 (1.6%) superficial. Greater than 94% of these grew only a single organism, of which 74% were StuphyZococcus species. There were 63 (2.6%) leg wound infections. More than 50% of these grew multiple organ- isms, of which 68% were enteric in origin. Nine (0.4%) patients had simultaneous chest and leg infections.

espite increased awareness of risk factors for postop- D erative complications, wound infections continue to be a problem after coronary artery bypass (CAB) opera- tions. Besides the obvious increase in morbidity and mortality, postoperative wound infections frequently re- sult in an increased length of hospitalization, increased overall cost, and a net financial loss to the hospital [l]. In elderly patients undergoing coronary revascularization, a subgroup that is increasing in numbers, a postoperative wound infection incurs an even greater monetary cost when compared with wound infections in younger pa- tients [2]. Current financial considerations are causing a trend toward same-day admission and early discharge, even in patients undergoing heart operations. Therefore, our usual practice of preoperative and postoperative ob- servation must be reevaluated and adjusted to provide adequate care for our patients. It is our concern that shorter hospital stays and fewer office visits may lead to a delay in the diagnosis of wound infections, causing an increase in overall morbidity, hospital stays, and cost. We reviewed the results of our ongoing wound surveillance program after CAB operations to look for changing trends in wound infections that might be adjusted to improve patient care.

Presented in part at the Poster Session of the Twenty-ninth Annual Meeting of The Society of Thoracic Surgeons, San Antonio, TX, Jan 25-27, 1993.

Address reprint requests to Dr Ward, Surgical Service (112), Veterans Affairs Medical Center, One Veterans Dr, Minneapolis, MN 55417.

Wound infections were diagnosed an average of 15.3 zk 6.7 (range, 4 to 30) days postoperatively, with 50% occur- ring after discharge from the hospital. Of 14 variables evaluated by multivariate logistic regression analysis, only steroids ( p = 0.005) and diabetes ( p = 0.003) were identified as independent risk factors for wound infec- tions. Patients taking steroids or with diabetes tended to have chest infections, whereas obese patients tended to have more leg infections (p = 0.08). During an interval in the surveillance program, a trend toward increasing infections was identified and successfully reversed.

(Ann Thoruc Surg 1993;56:1063-8)

Material and Methods

In 1977, criteria for a wound infection after CAB or CAB plus valve replacement were established. Potential risk factors were identified and monitored on a daily basis by a single nurse epidemiologist from 1977 through the present time. Variables that were continuously monitored included age, redo operation, renal insufficiency, chronic obstructive pulmonary disease, reoperation for bleeding, no preoperative antibiotic, obesity, number of bypass grafts, preoperative days in the hospital, cardiopulmo- nary bypass time, total operating room time, diabetes, steroid use, and blood loss. Although the results are not presented in this article, during the past 5 years data on mammary artery use and smoking were also collected.

Antibiotic prophylaxis consisted of 1 or 2 g of cefazolin administered preoperatively, intraoperatively every 2 to 4 hours, and postoperatively every 8 hours for 48 hours. Patients with a penicillin allergy were given vancomycin preoperatively, once during the operation, and every 12 hours for 48 hours.

A wound was defined as being infected if (1) purulent material was discharged from the wound, with or without a positive culture, or (2) the responsible surgeon deemed a wound was infected based on clinical judgment. In no case could the nurse epidemiologist's diagnosis of infec- tion be overruled by a surgeon. For chest wounds, a superficial infection included those infections limited to the subcutaneous tissues. A major chest wound infection included all cases where tissues were opened down to the sternal wires or beyond (including mediastinitis). Leg wound infections were not stratified based on depth of infectious involvement.

0 1993 by The Society of Thoracic Surgeons 0003-4975/93/$6.00

1064 SLAUGHTER ET AL WOUND INFECTIONS AFTER CAB

Fig 1 . Yearly rates of corona y arte y bypass and corona y a r t e y bypass + valve replacement procedures.

NO. OP 3 0 0

2 5 0 1 Ann Thorac Surg

1993;56 1063-8

2 0 0

1 5 0

1 0 0

5 0

0

1 9 7 7 1 9 7 8 1 9 7 9 1 9 8 0 1981 1982 1 9 8 3 1 9 8 4 1985 1 9 8 6 1 9 8 7 1 9 8 8 1 9 8 9 1 9 9 0 1 9 9 1

Surveillance of wounds was performed prospectively by the nurse epidemiologist using a combination of tech- niques. The methods used included (1) daily visits to the surgical floor for direct inspection of any suspicious wounds, (2) daily review of postoperative wound cultures at the microbiology laboratory, and (3) continuous contact by the nurse epidemiologist with ward and clinic nurses for their input regarding any patient believed to be developing clinical signs of incisional infection. All pa- tients received postoperative follow-up care at our clinic and were routinely seen 4 weeks after discharge from the hospital. Therefore, all wounds were subject to postdis- charge surveillance with a minimum of 30 days of follow- UP.

All patients with wound infections were randomly matched with patients without a wound infection for type of operation and month and year of the procedure. Logistic regression was used to determine which factor or factors were associated with or even predictive for wound infection. The BMDP software for logistic regression (BMDP Statistical Software, Los Angeles, CA) was ap- plied. The 2 test was applied where appropriate. A p value less than 0.05 was considered to be significant. Data are presented as mean -I- standard deviation.

Results From 1977 through 1991, 2,226 CAB procedures and 176 CAB plus valve replacement procedures were performed at the Minneapolis Veterans Affairs Hospital. Figure 1 shows the number of CAB procedures and CAB plus valve replacement procedures that were done per year between 1977 and 1991. All procedures used greater saphenous vein, therefore, all cases involved both a chest and leg incision. Of the 2,402 CAB procedures, there were a total of 125 patients (5%) with wound infections. There were 33 (1.4%) major and 38 (1.6%) superficial chest wound infections. Infections in the leg incision occurred

in 63 patients (2.6%). Simultaneous infections (leg and chest) occurred in 9 patients (0.4%).

Yearly wound infection rates are depicted in Figure 2. The highest percentage of infections occurred between 1983 and 1988. This adverse trend was thoroughly inves- tigated. In 1983 the overall infection rate had clearly increased. The increase was due to an increase in the leg wound infections. It was believed that this rise was not a major problem and could be improved by simply increas- ing the awareness of infectious complications. The leg wound infection rate did slowly decline between 1983 and 1986. Concomitantly, there was an increase in the super- ficial chest wound infections, resulting in a slight increase in the total wound infection rate between 1983 and 1986. At this point no significant changes had been made as the major chest wound infection rate had remained less than 2%. However, in 1987 the leg wound infection rate increased as did the percent of major chest wound infec- tions. Therefore, we reviewed our preoperative and op- erative protocol and had an independent evaluator ob- serve each step for possible violations of technique. It was determined that (1) preoperative scrubs were being per- formed in a haphazard, nonuniform fashion; (2) the preparation in the operative room was variable (occasion- ally less than 5 minutes for scrubbing and painting); (3) instruments were handed between the leg and chest wounds; and (4) there was excessive movement of equip- ment in and out of the operating room. Changes were implemented to improve each of these issues. Figure 3 demonstrates that there was a rapid improvement in the wound infection rate concurrent with the changes insti- tuted. In 1988 two physician assistants were hired specif- ically to harvest vein. Before this, the vein was procured by second- or third-year residents who rotated every 2 months. The leg infection rate decreased after this change (see Fig 2).

Figure 4 shows that wound infections were frequently diagnosed after discharge from the hospital. Major chest

Ann Thorac Surg 1993;561063-8

SLAUGHTER ET AL 1065 WOUND INFECTIONS AFTER CAB

YO

9 -

a --

7 --

6 --

5 --

4 --

I HTOTAL LEG INF a SUPERFICIAL CHEST MAJOR CHEST I

1977 1978 1 9 7 9 1980 1981 1982 1983 1984 1985 1986 1 9 8 7 1988 1989 1990 1991

Fig 2. Yearly wound infection rates and distribution.

and leg wound infections were diagnosed as early as 4 days and as late as 30 days postoperatively. Of 125 wound infections, 50% were diagnosed after discharge from the hospital. The average number of days until diagnosis of the wound infection, before and after discharge, are shown in Figure 5.

Figure 6 shows the 73 pathogens cultured from 71 patients with chest wound infections. All but 4 patients grew a single organism (94%), and 2 patients grew no

organism. Of the 73 pathogens identified, 54 (74%) were Staphylococcus species. The incidence of gram-negative organisms in chest infections was 22% (8/36) between 1977 and 1987, and 8% (3/35) between 1988 and 1991 ( p = not significant by 2 test).

Figure 7 shows the 104 pathogens cultured from 63 patients with leg wound infections. Of these, 36 patients (57%) grew more than one organism, and 3 patients grew no organisms. In contrast to the chest infections, only 24 leg wound infections (38%) were the result of Staphylococ- cus species.

2 0

i a 1 6

1 4

1 2

1 0

a 6

4

2

0 JAN-MAR APRIL-JUNE JULY-SEPT OCT-DEC

Fin 3. Percentage of infections occurring in each 3-month aeriod in

60%

50%

40%

30%

20%

10%

0% MINOR CHEST MAJOR CHEST LEG TOTAL

Fig 4. Percentage of infections diagnosed after discharge from the hos- - , , 1&8 after instituting new patient care a k operating procedire changes. pital.

1066 SLAUGHTER ET AL WOUND INFECTIONS AFTER CAB

Ann Thorac Surg 1993;56:1063-8

Fig 5. Average number of days postoperatively that Pre-discharge Post-discharge wound infection was diagnosed (mean 2 standard deviation). 28

26

24

2 ! 20 - T

Of the 14 variables that were continuously monitored, only steroids and diabetes were identified as being inde- pendent risk factors for wound infections (Table 1). Bac- teremia occurred in 22 patients (18%) with wound infec- tions. In 16 (73%) of these patients, the blood culture grew the same pathogen as the wound. The bacteremia oc- curred in 18 patients (82%) before or within 24 hours of the diagnosis of wound infection.

The 14 potential risk factors were also evaluated for an

PS AERUG N=l ORGANISMS HISTOPLASMID PROTEUS ANAEROBIC GPR ACID FAST BACILLI

Fig 6 . Organisms cultured from chest wounds including both major and superficial chest wound infections.

T

Minor Chest Major Chest Leg Total

association with the location of the wound infection. Diabetes and steroids tended to be associated with chest infections and obesity with leg infections (Table 2), how- ever, this trend did not reach statistical significance (p = 0.08 by 2 test).

The incidence of diabetes, steroids, and obesity before the rise in the infection rate was compared with the peak year and subsequent decline in infections (Table 3). The increase in the number of patients with diabetes, steroids,

/ I i' SERRATIA ,,=-

PSEUWMONAS

N.10

STAPH AUREUS

N=18

ENTEROCOCCUS

ACINETOBACTER N=l ORGANISMS CITROBACTER HAEMOPHILUS HISTOPLASMID CLOSTRIDIUM PERF PEPTOSTREPTOCOCCUS

Fig 7. Organisms cultured from leg wound infections.

Ann Thorac Surg 1993:561063-8

SLAUGHTER ET AL 1067 WOUND INFECTIONS AFTER CAB

or obesity implies that we are operating on sicker patients. This trend toward sicker patients has continued even through the period of improvement in our infection rate.

Comment Our data indicate that a comprehensive wound surveil- lance program can identify adverse trends in surgical wound infections, which can then be manipulated to improve overall patient care. By tracking our wound infections on a continuous basis, we detected a rise in the overall rate of infections. As shown in Figure 2, the percent of wound infections began to increase in 1983 and peaked in 1988. Before 1987, no substantive changes in techniques were implemented because the major chest infection rate had remained less than 2%. However, when the major chest infection rate began to increase (early 1987), we closely evaluated our preoperative, operative, and postoperative protocols. Floor nurses were educated as to the importance of and the appropriate techniques for the preoperative scrub. Operating room nurses were taught a more consistent and meticulous scrub technique.

Table 1. Risk Factors for Wound lnfection

Risk Factor Infection Control Value P

Diabetes Steroids Redo operation No. of bypass grafts

1-3 2 4

COPD Reop for bleeding Blood loss (mL)

0-1,000 21,001

OR time (h) 0-5 >5

Renal insufficiency Pump time (min)

0-106 107-125 126150 2151

Yes No

0-6 2 7

40-64 65-84

Obesity

Preop antibiotics

Preop days in hospital

Age (Y)

40 8

14 125 71 54 23 11

125 66 59

125 58 67 10

125 25 32 36 32

125 112 13

125 63 62

125 60 65 48

19 0 7

125 81 44 15 6

125 75 50

125 66 59 6

125 33 35 30 27

125 108 17

125 68 57

125 64 61 45

0.003 0.005 0.17 0.2

0.22 0.31 0.31

0.37

0.44 0.53

0.56

0.61

0.72

0.79

COPD = chronic obstructive pulmonary disease; room; Preop = preoperative; Reop = reoperative.

OR = operating

Table 2. Risk Factors and Distribution of Wound lnfections (Observed Frewencv Table) Location Steroids Diabetes Obesity

Chest 5 25 17 Leg 3 13 24

Total 8 38 41

Surgeons' gloves were changed if there was any indica- tion of puncture, when changing operative fields, and after the sternal wires had been placed. The cardiopulmo- nary bypass machine, cell saver, and blood gas machine were brought into the room before preparing and draping the patient, and they remained in the room until the case was completed. This significantly reduced the amount of traffic into and out of the room.

In contrast to chest infections, leg wound infections had a greater preponderance of enteric pathogens, and 57% were polymicrobial. We believe this diversity of patho- gens is due to the proximity of the leg incision to the perianal region. This would be particularly true when the incision begins or is extended up into the thigh. It is important, therefore, that the leg and chest wounds be considered as separate fields. Instruments should not be shared, and personnel should change gloves when mov- ing from one field to the other. In our series, instruments were frequently shared and gloves were not routinely changed when moving from one field to the other before 1987. Beginning in April 1987, the two fields were consid- ered separate, and changing gloves became mandatory practice. After this change was instituted, we saw a trend toward a decrease in the number of enteric organisms causing chest infections (from 22% to 8%; p = not signif- icant). In addition, harvesting the saphenous vein is now performed or supervised by the same two physician assistants. We believe this has also contributed to our decreased leg infection rate, as it was the only change we instituted in 1988.

The changes described resulted in a decrease in our wound infection rate to less than 2% for all wounds and less than 1% for major chest wounds. This rapid improve- ment demonstrates the effectiveness of both recognition of a problem and prompt efforts at eradicating it. As part of this program, we believe that it is important that each surgeon be made aware of his or her own infection rate as well as that of the subspecialty as a group. By minimizing postoperative infectious complications, we should pro- vide an overall cost reduction for the hospital and im- proved patient care [3].

Postoperative wound infections result in longer hospi- tal stays, increased overall cost, and even net loss for the hospital [I, 21. Our data indicate that a large proportion of postoperative wound infections (50%) are diagnosed after discharge from the hospital. Of the 66 wound infections diagnosed after discharge, 14 were major chest, 24 super- ficial chest, and 28 leg infections. With emphasis on shorter hospital stays, the number of wound infections diagnosed after discharge will probably increase. The

1068 SLAUGHTER ET AL WOUND INFECTIONS AFTER CAB

Ann Thorac Surg 1993;561063-8

Table 3 . lncidence of Diabetes, Steroids, and Obesity Before and After 1983

p Value Variable 1977-1982 1983-1991 (2 Test)

Total infections 13 112 Diabetes 2 (15.4%) 38 (33.9%) NS Steroids 1(7.7%) 7 (6.3%) NS Obesity 3 (23.1%) 45 (40.2%) NS

NS = not significant.

effect of early discharge on cost savings, patient rehabili- tation, and possible delay in diagnosis of postoperative complications is unknown and will require further inves- tigation.

Our major chest infection rate of 1.4% is similar to reports by other centers [ M I . Staphylococcus species was responsible for a majority of our chest infections, and this is consistent with other series [4, 81. Because of the preponderance of gram-positive organisms, prophylactic antibiotics should be chosen accordingly. For the patient without a penicillin allergy, a cephalosporin should be adequate. The choice of a cephalosporin is not made clear by reports in the literature [%12], but certainly individual hospital susceptibility and administrative costs should be taken into consideration. First-generation cephalosporins are inexpensive and cover the majority of Staphylococcus species and a significant proportion of aerobic enteric organisms. For these reasons, they remain our antibiotic of choice. Administration of prophylactic antibiotics be- yond 48 hours does not decrease the incidence of wound infections. There is evidence, however, that prolonged antibiotic administration may reduce the number of post- operative urinary tract infections [13].

Several series have reported a correlation between bacteremias and wound infections [8, 141. Kohman and colleagues [14] noted that a positive blood culture was often the first sign of a wound infection, and that the sternal wound was the most common source of the bacteremia. In our series, 73% of patients with bactere- mias had the same pathogen in the blood as in the wound, and the chest incision was the most common source.

Using multivariate analysis, several series have identi- fied risk factors for wound infection including prolonged ventilation, female sex, male sex, age, obesity, chronic obstructive pulmonary disease, blood transfusion, reop- eration, duration of cardiopulmonary bypass, bilateral internal mammary artery grafts, internal mammary artery grafts in diabetics, and prolonged operative time [ P S I . We believe our series is relatively unique in that all operations were performed at one hospital, the proce- dures were performed or supervised by the same two surgeons, and all of the cases were essentially elective.

This allowed optimizing of the patients preoperatively by vigorously treating chronic obstructive pulmonary disease exacerbations, controlling diabetes, initiating weight loss programs, and delaying surgery in patients with upper respiratory tract infections. Preoperative hospital stays were also limited to prevent nosocomial infections.

In conclusion, we have a cohort of patients with 30 days of close observation. Wound infections were frequently diagnosed after discharge from the hospital. Diabetes and steroids were the only independent risk factors for wound infection. Whereas diabetes and steroids were more com- monly associated with chest infections, obesity was closely associated with leg infections.

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