SSI and the anesthesiologist

In this months journal Anesthesiology, a retrospective study from Taiwan was published that looked at the incidence of SSI in patients having either TKA or THA.  It showed that patients who had a regional anesthetic (i.e. Spinal or Epidural) plus or minus GA, had about half the rate of SSI as those who had straight general.  Anesthesiologists, as noted by several studies cited by the authors, have longed had hints that using regional anesthesia might be beneficial in preventing SSI.  In particular, it is known that wound infections are associated w/ SQ partial pressure of oxygen, and as the SQ partial pressure of oxygen increases the chances of SSI decreases (Hopf et al.).  The problem for anesthesiologists, then is determining the most effective method of providing optimal delivery of oxygen to the wound.  Several requirements are necessary.  Fist, the wound must have adequate vascularity.  Certain comorbidities impact on this: DM, PVD, Tobacco use, Obesity, hypovolemia.  Anesthesiologists really can only impact one of these listed parameters, hypovolemia.  Other intraop variables impact on the vasoreactivity: thermoregulation, vasoreactivity etc.  Anesthesiologists play a role in both of these.  We must maintain normothermia. We must also maintain reasonable vasoreactivity by controllling the stress response.  It is known that opioids only do this to a degree.  Furthermore, opioids have been shown to directly impact on immune function.  Pain itself increases vasoreactivity and has been shown to dramatically decrease SQ Oxygen tension (by up to ~30mmHg).  Thoracic epidural anesthesia by itself (independent on its control of pain) can also increase SQ oxygen tension.  Therefore, when you consider the combination of improved pain control with an independent increase in wound or SQ oxygen tension from regional neuraxial local anesthesia, it is not surprising that you might find a decreased incidence of SSI in patients treated with neuraxial anesthesia for THA and TKA.  This is even more remarkable when you see that the incidence of infection at baseline was only 1% to 2%.  It is often difficult to find a clinically significant benefit when the target incidence is already low.  It would be interesting to repeat this retrospective review in patients undergoing open abdominal surgeries with and without neuraxial anesthesia both intraop and post operatively.  Importantly, however, all of these studies should measure wound or at least SQ oxygen tension levels to help elucidate possible mechanisms.