Supplemental Perioperative Oxygen and the Risk of Surgical Wound Infection
Abstract & Commentary
By Dean R. Hess, PhD, RRT, Assistant Director, Respiratory Care, Massachusetts General Hospital, Department of Anesthesiology, Harvard Medical School, is Associate Editor for Critical Care Alert.
Assistant Director, Respiratory Care, Massachusetts General Hospital, Department of Anesthesiology, Harvard Medical School
Dr. Hess reports no financial relationship relating to this field of study.
Synopsis: Breathing 80% oxygen during and for 6 hours after major colorectal surgery reduced postoperative wound infection risk by roughly a factor of two.
Source: Belda FJ, et al. Supplemental perioperative oxygen and the risk of surgical wound infection: a randomized controlled trial. JAMA. 2005;294:2035-2042.
The objective of this study was to test the hypothesis that supplemental oxygen reduces infection risk in patients following colorectal surgery. It was a double-blind, randomized, controlled trial of 300 patients aged 18 to 80 years who underwent elective colorectal surgery in 14 Spanish hospitals from March 1, 2003, to October 31, 2004. Wound infections were diagnosed by blinded investigators using criteria established by the Centers for Disease Control and Prevention. Baseline patient characteristics, anesthetic treatment, and potential confounding factors were recorded. Patients were randomly assigned to breathe either 30% or 80% oxygen intraoperatively and for 6 hrs after surgery. Anesthetic treatment and antibiotic administration were standardized. The main outcome measure was surgical site infection (SSI). Secondary outcomes included return of bowel function and ability to tolerate solid food, ambulation, suture removal, and duration of hospitalization. A total of 143 patients breathed 30% oxygen perioperatively and 148 breathed 80% oxygen perioperatively.
Surgical site infection occurred in 35 patients (24.4%) who breathed 30% oxygen and in 22 patients (14.9%) who breathed 80% oxygen (P = 0.04). The risk of SSI was 39% lower in the 80% oxygen group (relative risk [RR], 0.61; 95% confidence interval [CI], 0.38-0.98) vs the 30% oxygen group. After adjustment for important covariates, the RR of infection in patients administered supplemental oxygen was 0.46 (95% CI, 0.22-0.95; P = 0.04). None of the secondary outcomes varied significantly between the groups. Belda and colleagues concluded that patients receiving supplemental inspired oxygen had a significant reduction in the risk of wound infection. Supplemental oxygen appears to be an effective intervention to reduce SSI in patients undergoing colon or rectal surgery.
Commentary
It was reported several years ago (and commented on in Critical Care Alert) that providing 80% oxygen throughout surgery and for 2 postoperative hours decreased infection risk by half compared with patients who were administered 30% oxygen (5% vs 11%).1,2 However, another study concluded that the risk of infection in a general surgical population doubled in patients who were administered supplemental oxygen during surgery (25% vs 11%).3 This study was conducted to clarify this disparity of results.
Greif et al1 randomly assigned 500 patients undergoing colorectal surgery to breathe 30% or 80% oxygen during the operation and for 2 hrs afterward. With use of a double-blind protocol, wounds were evaluated daily until the patient was discharged and then at a clinic visit 2 weeks after surgery. They considered wounds with culture-positive pus to be infected. This study differs from the current study of Belda et al in several respects. First, oxygen was administered for 2 hrs post-operatively by Greif et al rather than 6 hrs post-operatively. Second, Greif et al used microbiological criteria for infection whereas Belda et al used a clinical diagnosis. This likely explains the lower infection rate reported in the earlier study by Greif et al.
Pryor et al3 randomly assigned 165 patients undergoing major intra-abdominal surgery to breathe 35% or 80% oxygen during surgery and for 5 hrs post-operatively. Like Belda et al and Greif et al, it was a double-blind placebo-controlled trial. However, there are several methodologic problems with this study. First, it may have been under-powered with only 165 patients. Second, and perhaps more important, the presence of infection was determined retrospectively by chart review.
Of interest is the mechanism whereby high inspired oxygen may reduce the risk of surgical wound infection. With healthy lungs, the blood is nearly saturated with oxygen when breathing room air. Given that oxygen has a low solubility in blood, raising the PaO2 above normal will add little to oxygen content. However, the higher PaO2 may increase the driving pressure for diffusion of oxygen into tissue. Bactericidal activity of neutrophils is mediated by oxidative killing, a critical defense against surgical pathogens.4,5 Oxidative killing is dependent on the production of bactericidal superoxide radicals from molecular oxygen. The rate of this reaction is dependent on the PO2 in the tissue. Surgical wounds disrupt the local vascular supply as a result of vessel injury and thrombosis, causing wounds to be hypoxic compared to normal tissue. Resistance to infection depends on the PO2 in the wound, which may be improved by increasing PaO2 beyond that required to saturate blood.
Both the studies by Belda et al and Greif et al evaluated wound infections following colorectal surgery. Whether similar results might occur after other surgeries (eg, cardiothoracic, vascular, penetrating trauma) is unknown. The precise time of oxygen exposure is also unknown. For example, Belda et al continued oxygen therapy for 6 hrs following surgery, whereas Greif et al continued therapy for only 2 hrs following surgery. The dose is also not known. Although both the Belda et al and Greif et al studies showed benefit for 80% oxygen (compared to 30% oxygen), it is not know if a similar benefit would be observed with a lower concentration.
The study of Belda et al, confirming the results of Greif et al, is thought-provoking. Traditional teaching has stated that raising PaO2 to supra-normal levels has little benefit outside a few indications (carbon monoxide poisoning, extravascular free air such as pneumocephalus). Perhaps prevention of post-operative wound infections following colorectal surgery should be added to that list.
References
- Greif R, et al. Supplemental perioperative oxygen to reduce the incidence of surgical-wound infection. Outcomes Research Group. N Engl J Med. 2000;342:161-167.
- Pierson D. Perioperative oxygen administration reduces postoperative wound infections. Critical Care Alert. 2000;8:25-26.
- Pryor KO, et al. Surgical site infection and the routine use of perioperative hyperoxia in a general surgical population: a randomized controlled trial. JAMA. 2004;291:79-87.
- Hopf HW, et al. Wound tissue oxygen tension predicts the risk of wound infection in surgical patients. Arch Surg. 1997;132:997-1004.
- Allen DB, et al. Wound hypoxia and acidosis limit neutrophil bacterial killing mechanisms. Arch Surg. 1997;132:991-996.
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