Cephalothin prophylaxis assay during cardiopulmonary bypass

Cephalothin prophylaxis assay during cardiopulmonary bypass Efficient use of prophylactic antibiotics in surgery demands their presence in adequate serum concentration at the time of maximal potential contamination. This cover should extend from the moment of incision until at least the time of removal of large tubes and intravenous cannulas. Critical cardiac contamination may occur during the bypass procedure while the operation within the cardiac chambers is being done. This is a special danger in valve replacement with prostheses. The antibiotic regimen of the Cardiothoracic Unit was studied in 12 consecutive patients and was generally found to provide adequate antibiotic coverage throughout the surgical procedure, including the bypass procedure. In all patients, a reinforcing cephalothin dose on completion of bypass ensured adequate circulating cephalothin levels for the completion of surgery. Clearance of the cephalothin from the blood of patients was found to decrease markedly during cardiopulmonary bypass.

D. J. Williams* and T. W. Steele,** Adelaide, South Australia

IT ostoperative infective endocarditis is such a serious complication of open-heart surgery that most surgical units consider antibiotic prophylaxis essential. Retrospective studies and impressions claim reduced severity and frequency of infection when prophylactic antibiotics are administered, and it appears that proper use of selected antibiotics does lower the incidence of infective endocarditis.8-10 Cephalothin has been used in the Royal Adelaide Hospital Thoracic Unit as the operative and postoperative prophylactic antibiotic. Bacteria colonizing the skin, particularly gram-positive organisms, figure prominently as the etiologic agents of postoperative endocarditis. The majority of such organisms are inhibited in vitro by concentrations of cephalothin greater than 6.0 meg. per milliliter. Endocarditis caused by Staphylococcus aureus may present a therapeutic problem. The sensitivity of these organisms is related to their ability to form /3-lactamases, with the majority of methicillin-sensitive strains being inhibited by 0.1 to 0.5 meg. per milliliter of cephalothin. However, methicillin-resistant strains of Staphylococcus aureus show greater degrees of From the Royal Adelaide Hospital and The Institute of Medical and Veterinary Science, Adelaide, South Australia. Received for publication April 21, 1975. *Resident, Cardiothoracic Unit, Royal Adelaide Hospital. **Director of Clinical Microbiology, The Institute of Medical and Veterinary Science.

resistance to cephalothin, frequently requiring levels of 5 meg. per milliliter or more to inhibit them.2, 6 Infections caused by Staphylococcus epidermidis have been frequently described, and all isolates were inhibited by 1 to 6.3 meg. per milliliter of cephalothin.5, 7 Of the other organisms not infrequently encountered in patients with prosthetic heart valves, Corynebacterum species (diphtheroids)3 are inhibited by very low concentrations of cephalothin, whereas Micrococcus species show great variation in sensitivity. Some of the latter exhibit multiple resistance to a wide variety of antibiotics including cephalosporins (unpublished observations by one of us, T. W. S.). Thus a minimum serum cephalothin level of 7 meg. per milliliter throughout most of an operative procedure was considered desirable to achieve satisfactory prophylactic cover. However, some doubt as to the proper use of prophylactic antibiotics was raised by Benner,1 when he found negligible blood levels of cephalothin during bypass. The routine Benner studied was that of giving the antimicrobial agent intravenously on the night before and then giving a repeat dose early on the morning of surgery. The third dose was given postoperatively, some time after the patient reached the cardiac recovery ward. The present investigation was undertaken to determine if the antibiotic regimen in use in the RAH Cardiothoracic Unit provided adequate antibiotic coverage especially during bypass. 207

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Table I Method

Drug

Dose*

Cephalexin Cephalothin

1 Gm. (500 mg.) 1 Gm. (25 mg./Kg.)

Oral IV

1 Gm. (dose for weight)

IV

Time 12 midnight At or following induction of anesthesia At completion of bypass

*Children's dose in parentheses.

Materials and methods Twelve consecutive patients, age range 6 years to 65 years, over a 2 week period were studied using the antibiotic regimen in Table I. Blood samples were taken before cephalothin was given to ascertain the effect of the preoperative cephalexin. Further samples were taken approximately 15 minutes after the cephalothin administration on induction of anesthesia, just before the start of bypass, 5 minutes after the start of bypass, at 15 minute intervals throughout bypass, and on completion of bypass before any further dose of cephalothin was given. Two further assays were performed on blood samples collected after the postbypass dose of cephalothin was given. Cephalothin was administered via a catheter in the left external jugular vein, inserted immediately before the operation by the anesthetist, and blood was sampled for assay before and after bypass from this catheter or from an inferior vena caval catheter inserted via the long saphenous vein. Sampling during bypass was from the main venous line or the arterialized well of the oxygenator. All patients weighed 10 kilograms or more. A volume of 2.5 L. of blood was used to prime the Melrose pump oxygenator. The patients on bypass were subjected to hypothermia, and Patient 5 (Table II) also had a 17 minute cardiopulmonary standstill during bypass. Immediately at the completion of operation, the blood samples were taken to the laboratory where the microbiological assay was performed by an agar diffusion method. All sera were stored at 4° C. until tested, and most were assayed on the day of receipt. Reassay, if necessary, was performed within 36 hours of the collection of the blood. A stock solution of cephalothin containing 2 mg. per milliliter in distilled water was prepared weekly and stored at 4° C. Standard solutions of cephalothin containing 1, 3, 5, 10, 20, 30, and 50 meg. per milliliter were prepared daily from the stock solution with 25 per cent pooled human serum in distilled water used as the diluent. The pooled human

serum was tested for antibacterial activity before use. Test sera from patients were diluted 1:4 in distilled water before testing. A square plate having an internal width of 304 mm. was constructed from 5 mm. thick plate glass. Seeded agar was prepared by adding 2 ml. of an overnight culture of Staphylococcus aureus N.C.T.C. 6571 to 400 ml. of cooled oxoid-sensitivity agar No. 2, and the plate was poured. When the agar had set, sixty-four wells were cut with a sterile, 6 mm. diameter cork borer. This number of wells permitted the assay of a maximum of nine test sera and seven standards to be performed in quadruplicate each day. A volume of 40 yA of test sera and standards were randomly added to the wells. The plate was covered and incubated at 37° C. overnight. Zone diameters were measured with a Vernier scale caliper, and a standard semilogarithmic graph was constructed each day with a computer being used to obtain the line of best fit. The concentration of cephalothin in the test sera was determined from this graph, and the computer was used to calculate the 95 per cent confidence limits for each value. Results No patient demonstrated adequate cephalosporin activity in response to cephalexin given orally at 12 midnight, approximately 8 hours preoperatively. Data on the weight of individual patients, maximum serum levels of cephalothin (both before and after bypass), and the relevant time intervals are shown in Table II. The average time between the administration of cephalothin and the beginning of cardiac bypass was 52 minutes. Serum cephalothin levels for each patient are plotted against individual sampling times in Figs. 1 to 3. As blood samples were not collected immediately before and for the first 5 minutes after bypass was established, serum levels at the commencement of bypass were calculated. A discontinuous line connects this calculated value with the first subsequent measurement of serum cephalothin in Figs. 1 to 3. Inappropriately high serum levels of cephalothin soon after its administration have not been plotted, nor have those cephalothin levels obtained after the postbypass cephalothin dosage been included on the graphs. The postbypass cephalothin levels, however, demonstrated similar patterns of fall as compared with prebypass levels. Discussion A combination of three factors appreciably influenced the serum levels of cephalothin during bypass. These were (1) the time interval between the admin-

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Cephalothin prophylaxis assay during CPB

Serum levels of cephalothin (mcg/ml) 2,

1401201008060 40

.1 I

4 2

20

-80 -60 prebypass

-40

-20

0

20 40 TIME(mins)

60

80

100

120

140

160

bypass & postbypass

Fig. 1. Variation in serum cephalothin levels in response to time and cardiopulmonary surgery with bypass time of 20 to 40 minutes. J = Intravenous injection of cephalothin. 0 = Commencement of cardiopulmonary bypass. 1 = Completion of cardiopulmonary bypass. 1 to 4 = Patients as numbered in Table II.

Serum levels of cephalothin ( mcg/ml) 140-

120 100 80 60 40 20

n

65

-80 - 6 0 prebypass

-40

-20

0

r-NP-t 20 40 TIME(mins)

60

80

100

120

140 160

bypass & postbypass

Fig. 2. Variation in serum cephalothin levels in response to time and cardiopulmonary surgery with bypass time of 55 to 70 minutes. NP = No perfusion throughout this time. For further legend see Fig. I.

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Serum levels of cephalotftn ( mcg/ml) 140-

120 100

-80

0

-60

prebypass

20 40 TIME ( mins)

60

100

120 140 160 bypass & postbypass

Fig. 3. Variation in serum cephalothin levels in response to time and cardiopulmonary surgery with bypass time of 90 to 140 minutes. For further legend see Fig. I. Table II Serum cephalothin levels (meg./ml.)

Patient

Sex

Body weight (Kg.)

l 2 3 4 5 6 7 8 9 10 ll 12

F M M M F M F F M M M M

41 63 68 72 61 85 15 14 75 88 85 91

Maximum levels

Time I (min.)

Prebypass

Postbypass

Minimum levels

Time 2 (min.)

Operation

47 66 46 74 55 59 31 73 28 58 38 49

94(13)* 148(12) 122(15) 964(15) 122(27) 1,800(15) 49(27) 671(32) 191(10) 89(15) 54(26) 267(15)

157(27)* 74(39) 54(30) 15(60) 253(26) 45(36) 70(34) 112(24) 98(15) 21(65) 63(35) 63(42)

31 21 12 7 33 14 11 5 18 4 7 5

39 33 22 38 70 62 57 62 92 140 85 140

MVR VSD VSD MVR CAVG MVR ASD VSD CAVG CAVG CAVG CAVG

Legend: Time I is interval between administration of cephalothin and the beginning of cardiac bypass. Time 2 is length of cardiac bypass. MVR, Mitral valve replacement. VSD. Repair of ventricular septal defect. ASD. Repair of atrial septal defect. CAVG. Coronary artery saphenous vein bypass graft. *Figures in parentheses refer to time intervals between cephalothin administration and sampling of blood for assay.

istration of cephalothin and the commencement of bypass, (2) the length of the bypass procedure, and (3) the body weight of the patient. Serum cephalothin levels below 10 meg. per milliliter were found in all patients who experienced a long delay between the beginning of the operation and the commencement of the bypass, those who had a prolonged bypass procedure, and those who had a very

high or very low body weight. Two patients (Nos. 4 and 8, Table II) with low levels may, in addition to these factors, have received less cephalothin than was calculated. Their initial blood samples contained abnormally high levels of cephalothin, suggesting that the antibiotic had not been adequately flushed through the catheter. Patient 8 was also subjected to a 17 minute cardiopulmonary standstill under hypothermia, but this

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caused no decrease in the serum cephalothin levels. No effect of body weight alone was evident in this study. In volunteers, intravenous infusion of 1 Gm. of cephalothin given over a 20 minute period resulted in peak serum levels of 70 meg. per milliliter; in these subjects, levels below 3 meg. per milliliter were found 2 hours after infusion. 4 Higher serum levels may be expected if the antibiotic is infused more rapidly. However, in a number of patients, assays of cephalothin levels in samples of blood collected shortly after administration of the drug showed abnormally high levels of antibiotic (Table II). The persistence of small quantities of concentrated cephalothin solution in the sampling catheter could account for these findings. Blood samples collected later in the operation or following a further injection of cephalothin contained levels within the range of 63 to 112 meg. per milliliter in the majority of these patients. Noticeably, once cardiopulmonary bypass was established, serum levels of cephalothin showed little change or declined slowly (Figs. 1 to 3), so that, even 190 to 200 minutes after the administration of cephalothin in Patients 10 and 12, the serum level of cephalothin had not fallen to 3 meg. per milliliter. However, after 120 minutes of cardiopulmonary bypass, serum antibiotic levels of these 2 patients, who weighed 88 and 91 kilograms, respectively, were below 7 meg. per milliliter. Similarly, Patient 9, who was returned to the operating theater for relief of pericardial tamponade after approximately IVz hours in the recovery ward, had a serum cephalothin level of 16.6 meg. per milliliter 1 hour after receiving 1 Gm. of cephalothin in the recovery ward; the serum cephalothin level was 16.8 meg. per milliliter 55 minutes later, there having been no further administration of antibiotic. The reason why the rate of metabolism and excretion of cephalothin is altered in response to cardiac surgery and bypass is unknown. However, this may reflect the effects of hypothermia, the hypotension often associated with perfusion, or the alteration in organ blood-flow patterns caused by perfusion and surgery. No attempt was made to determine the urinary excretion of cephalothin during operation. Conclusions Over-all, three important features were noted from this study: 1. The regimen in use provided adequate antibiotic coverage throughout the surgical procedure, including bypass in most patients. To ensure cover for all patients, it was recommended that all patients weighing more than 10 kilograms should be given the full 1 Gm.

Cephalothin prophylaxis

assay during CPB

21 1

dose of cephalothin initially and be medicated on a dose-for-weight basis at the completion of bypass. Patients whose body weight exceeds 85 kilograms and who are on cardiopulmonary bypass for 120 minutes or more should be given an additional dose of cephalothin after 2 hours on bypass. 2. The practice of giving oral cephalexin more than 4 to 6 hours preoperatively is questionable microbiologically, in view of the absence of adequate antibiotic immediately after the operation. 3. The delayed clearance of cephalothin during cardiopulmonary bypass was a marked feature of this study and may warrant further investigation. We acknowledge the work of Mr. A. Hill, Mrs. N. Glowik, and Mr. R. Ratcliff, Clinical Microbiology Division of The Institute of Medical and Veterinary Science, in assays of cephalothin, and the work of Mrs. H. Lillecrap in the reproduction of figures. We also thank the Surgical, Anesthetic, and Nursing staffs of the Cardio-Thoracic Surgical Unit of the Royal Adelaide Hospital for their cooperation and assistance and Mrs. A. Mounsey for typing of this manuscript. REFERENCES 1 Benner, E. J.: Metabolism of Antibiotics During Cardiopulmonary Bypass for Open-Heart Surgery, Antimicrob. Agents Chemother. 8: 373, 1968. 2 Garrod, L. P., and O'Grady, F.: Antibiotic and Chemotherapy, ed. 3, London and Edinburgh, 1971, E. & S. Livingstone, Ltd. 3 Kaplan, K., and Weinstein, L.: Diphtheroid Infections in Man, Ann. Intern. Med. 70: 919, 1969. 4 Kirby, W. M., and Regamey, C : Pharmacokinetics of Cefazolin Compared With Four Other Cephalosporins, J. Infect. Dis. 128: S341, 1973 (Suppl.). 5 Kjellander, J. O., Klein, J. O., and Finland, M.: In Vitro Activity of Penicillins Against Staphylococcus albus, Proc. Soc. Exp. Biol. Med. 113: 1023, 1963. 6 Sabath, L. D., Barrett, F. F., Wilcox, C , et al.: Methicillin Resistance of Staphylococcus aureus and Staphylococcus epidermidis, Antimicrob. Agents Chemother. 8: 302, 1968. 7 Speller, D. C , and Mitchell, R. G.: Coagulase-Negative Staphylococci Causing Endocarditis After Cardiac Surgery, J. Clin. Pathol. 26: 517, 1973. 8 Manko, M. A., Birkhead, N. C , Bodi, T., et al.: Cephaloridine Prophylaxis in Open Heart Surgery, Curr. Therap. Res. 14: 679, 1972. 9 Amoury, R. A., Bowman, F. O., Jr., and Malm, J. R.: Endocarditis Associated With Intracardiac Prostheses: Diagnosis, Management, and Prophylaxis, J. THORAC. CARDIOVASC. SURG. 51: 36, 1966.

10 Fibor, W. B.: Infection Following Open-Heart Surgery, With Special Reference to the Role of Prophylactic Antibiotics, J. THORAC. CARDIOVASC. SURG. 53: 371,

1967.