Supplemental Postoperative Oxygen & Wound Infection in Morbidly Obese Patients

Wound infection is a common and serious consequence of anesthesia and surgery. Infection is associated with major complications including intra-abdominal abscess, peritonitis without leakage, and sepsis. The average wound infection thus prolongs hospitalization by a week, doubles the need for ICU care, and doubles mortality.

The first few hours following bacterial contamination constitute a decisive period during which infection is established. Techniques aimed at improving resistance to surgical wound infections are thus most effective when implemented during the decisive period - even though infections are not detected clinically until five-to-ten days after surgery.

The primary defense against surgical pathogens is oxidative killing by neutrophils. This reaction critically depends on tissue oxygen tension. It is therefore unsurprising that subcutaneous tissue oxygen tension (PsqO2) is inversely correlated with the risk of surgical wound infection. As might be expected, supplemental perioperative oxygen administration increases tissue oxygen partial pressure (compared with an FIO2 of 30%) and decreases wound infection risk. While 100% oxygen is associated with atelectasis and pulmonary inflammation, we and others have shown that 80% oxygen is not.

Inadequate healing is also common. Scar formation starts with transcription of pro-collagen, which undergoes post-translational hydroxylation of the abundant proline and lysine residues. This promotes the cross-linking within and between collagen strands that provides tensile strength which begins the healing process. The prolyl and lysyl hydroxylases that catalyze this reaction depend on the substrate oxygen. The Km (substrate concentration at which the velocity of the reaction is half-maximal) for O2 of prolyl hydroxylase has been variously estimated at 20, 25, and 100 mmHg. Even using the most conservative estimate, these data suggest that proline hydroxylation of collagen (e.g., scar formation) will be PO2 dependent through the range of 0 to ≈200 mmHg, with 90% of the effect occurring by 90 mmHg. In vitro and in vivo animal evidence supports this theory. Inadequate healing is associated with anastomotic leakage, wound dehiscence, and bleeding.

In morbidly obese patients, the incidence of infection and major complications related to either infection or inadequate healing is 15-20% after gastric bypass based on our preliminary data and a review of records at Washington University. Roughly 30% of the American population is now obese with a body mass index (BMI) > 30 kg/m2; about 5% of the population is morbidly obese, with a BMI exceeding 40 kg/m2. These patients are at extraordinarily high risk of wound infection, and infections in these patients are especially severe.