Administration of 50% inspired oxygen to the obese patients, a typical concentration, produced a PaO2
of 150 mmHg and an oxygen saturation of 99%. Intraoperative and immediate postoperative subcutaneous tissue oxygenation was nonetheless critically low (approximately 40 mmHg), a value associated with a high risk of infection. In contrast, tissue oxygenation in the non-obese patients was roughly 20 mmHg greater, a value which is considerably less likely to be associated with infection.2
It is thus apparent that subcutaneous tissue is often hypoxic in obese patients undergoing routine anesthetic management. Although we did not evaluate the incidence of infection in this relatively small study, the link between subcutaneous oxygenation and wound infection risk is well established. Inadequate subcutaneous oxygenation is likely to account for the observed increased number of postoperative infections in obese patients.12
An inspired oxygen concentration of 95% was required to increase PaO2
to 300 mmHg in the obese patients. However, this 150-mmHg increase in arterial partial pressure improved subcutaneous oxygenation only 10 mmHg — to a value that remained marginal. Medical oxygen is possibly the least expensive drug, and supplemental intraoperative oxygen is easy to provide. Prolonged administration of oxygen at concentrations near 100% causes pulmonary toxicity.22,23
However, short-term exposure in the perioperative period is non-toxic and may even improve pulmonary resistance to infection24
Perioperative concentrations of oxygen restricted to 80% do not provoke atelectasis or other pulmonary dysfunction.25
Available data thus suggest that it would be prudent to provide obese patients with an inspired oxygen concentration of at least 80% because the risk and cost are small, and doing so somewhat improves tissue oxygenation.
Supplemental oxygen administration is one of many factors influencing subcutaneous tissue oxygen partial pressure. For example, it is well established that hypothermia,7
surgical and postoperative pain,26,27
all reduce tissue oxygen tension. In contrast, administration of supplemental fluid,28
and epidural anesthesia27,30
increase subcutaneous tissue oxygenation. Intraoperative tissue oxygenation was sub-optimal in our obese patients at a PaO2
of 150 mmHg and remained marginal even with an inspired oxygen concentration of 95%. Thus, it is likely that combining supplemental oxygen with other treatments that have a potential of improving tissue oxygenation may prove beneficial in obese patients.
Interestingly, arm and wound tissue oxygen tensions were comparable in obese and non-obese patients on the first postoperative day. Furthermore, supplemental oxygen administration was considerably more effective postoperatively in the obese patients than intraoperatively (≈20 mmHg vs
. ≈10 mmHg) — although delivery of oxygen (via
a face mask) was surely less effective. A potential explanation for this different response is that the obese patients had significantly higher core temperatures. Hyperthermia causes peripheral vasodilation, hyperemia, and increased tissue perfusion.31
Furthermore, it is likely that obese patients are fairly hypercapnic in the postoperative period; mild hypercapnia is known to improve tissue oxygenation.29
Intraoperatively we recorded subcutaneous oxygen partial pressure from a needle-induced surrogate wound in the arm. This is the classical method of evaluating perioperative tissue oxygenation and has been used in numerous previous studies.16,26,29
The primary benefit of this location is convenient access and the fact that measurements can be conducted during surgery. However, the tissue of interest is actually the surgical incision. We therefore simultaneously recorded tissue oxygenation adjacent to the surgical incision postoperatively. The results were encouraging in that average values at each site were virtually identical. Similar arm and wound partial pressures are consistent with the single previous direct clinical comparison by Chang, et al
who reported that measurements in the chest wound are only about 10 mmHg less than in the arm. In that study, though, PsqO2
measurements were performed directly in a mastectomy wound whereas our values were recorded from surrogate wounds adjacent to the surgical incision. While Chang did not measure subcutaneous tissue temperature (Tsq), our PsqO2
values are temperature corrected. This is of importance because Tsq was greater in the wound than in the arm under all conditions, presumably as a result of inflammation and erythema. Taken together, these results suggest that values recorded from the arm are reliable substitutes for less convenient measurements from surgical wounds.
The observed tissue oxygen values in our study were slightly less than in previous studies, even in the non-obese patients.6
This may be explained by the fact that our patients were randomly assigned to two oxygen treatments. Consequently, in 50% of all patients the high oxygen condition was initiated at a point in time during anesthesia and surgery when peripheral tissue perfusion might have already been compromised because of sympatho-adrenergic stimulation and relative hypovolemia.
Naturally, it was impossible to randomly assign patients to the two weight groups. A consequence is that there were more women in the obese than non-obese group. At present, there are no convincing data available that gender per se might be a confounding factor of tissue oxygenation and perfusion. Nevertheless, some endocrinologic influence cannot be excluded. For example, 17ß-estradiol is a vasodilator and increases cardiac output.
The two primary determinants of subcutaneous oxygenation are arterial oxygen partial pressure and tissue perfusion, both of which depend critically on anesthetic and fluid management. Fluid management was thus strictly controlled by protocol, using estimated normal body weight as the basis for management. However, there is no consensus on what constitutes optimal or even comparable fluid management in the obese.33
It is thus possible that this conservative regimen made our obese patients relatively hypovolemic. Nonetheless, hemodynamic parameters, such as mean arterial blood pressure and cardiac index were similar in both groups, as was urine output. These data suggest that our perioperative fluid administration was adequate even in the obese patients.
Surgery lasted significantly longer in the obese than non-obese patients. However, intraoperative tissue oxygen was recorded at similar times, usually during the first two hours of surgery in each weight group. It thus seems unlikely that observed intraoperative differences between the obese and non-obese patients was an artifact of anesthetic or fluid management. There is evidence that duration of surgery is a confounding factor in regards to postoperative tissue oxygen tension,2
we cannot exclude that duration of surgery effected our PsqO2
values in the postoperative period. However, all patients underwent major abdominal procedures with similar surgical stress responses and pathophysiological alterations.32
It seems likely that preoperative tissue oxygenation differed substantially in the obese and non-obese patients. However, we cannot confirm this assertion because our recording began after induction of general anesthesia. The primary reason we delayed data collection was that unanesthetized patients find probe insertion uncomfortable. Furthermore, measurements during baseline conditions in uninjured tissues before initiation of surgical trauma and bacterial contamination are of less clinical impact. Thus, we considered only intra- and post-operative measurements to be of major importance.
In summary, our data indicate that obesity, defined as a BMI ≥30 kg/m2, is a major determinant of perioperative tissue oxygenation. In obese surgical patients, subcutaneous tissue hypoxia was common. Even with supplemental oxygen administration, tissue oxygen tension was reduced to a level that is associated with a substantial increase in infection risk. Tissue oxygenation recorded from a surrogate wound in the arm was similar to that recorded from a probe inserted adjacent to the surgical incision, suggesting that the technically simpler arm measurements can be substituted for more challenging wound recordings.