Our principal finding is that the use of RA reduces the risk of blood transfusion in hip but not knee arthroplasty. The effect size is large. Hip arthroplasty subjects receiving RA had an OR of transfusion of 0.646, compared with those receiving only general anesthesia, after we adjusted for other factors that are known to affect the transfusion rate.
This finding is biologically plausible. RA is thought to reduce bleeding in orthopedic surgery by lowering pressure on both the arterial and venous sides of the circulation. In a study of 38 subjects undergoing total hip arthroplasty, Modig and Karlstrom32
showed that the use of epidural anesthesia, compared with general anesthesia, was associated with reductions in mean arterial pressure, pulmonary artery pressure and right atrial pressure of 13%, 21% and 44%, respectively. They also transduced the pressure wave from a peripheral vein within the surgical incision and recorded values close to 20 mm Hg during the procedure in the epidural group, compared with 30 mm Hg in those given general anesthesia with mechanical ventilation. In hip arthroplasty, the hemodynamic effect of single-shot spinal anesthetic with bupivacaine coincides with the intraoperative period, where most of the bleeding takes place. However, in knee arthroplasty, which is performed under tourniquet in our centre, most of the bleeding occurs after the period of anesthetic effect, into surgical drains. A further possibility is that RA does actually reduce blood loss in knee arthroplasty but that our analysis did not detect it. Since we analyzed almost 1000 knee procedures, we contend that, if such an effect exists, it is probably small.
Direct comparisons between the present study and previous investigations are made difficult by several factors. Most trials have compared continuous epidural anesthesia delivered via an indwelling catheter with general anesthesia. This design permits the epidural infusion to continue for 24 hours or more, in contrast to the general anesthetic, which wears off much more quickly. General anesthesia has also changed since the earlier studies were performed. Many of these used opioid/butyrophenone neuroleptanalgesia or nitrous oxide/opioid/muscle relaxant combinations in the general anesthetic arm — techniques that are rarely used today. Some studies re-corded actual blood loss in the perioperative period, whereas others recorded the volume or number of blood transfusions given. In some studies, blood loss was automatically replaced by allogeneic transfusion once the loss had reached a certain volume (e.g., 500 mL); in other studies, transfusions were given according to clinical criteria or for unstated reasons. Thus transfusion rates vary across these studies from 0% to 100%.
The overall effect of autologous blood donation in our series was negligible because only 57 of our 1877 subjects underwent it.
One limitation of our study is that our data were collected retrospectively from sources not specifically intended for research. The data on blood product use, however, were drawn from the computerized blood product inventory management system and were confirmed by manually reviewing the patients' records. We did not specifically record the use of intraoperative cell salvage and reinfusion, because this technique is rarely used in our region.
Statistical modelling methods cannot correct for confounding factors that are not recorded in the data set. The most relevant example of this shortcoming in our current study relates to the use of medications that affect blood coagulation, such as ASA and warfarin. In our region, the clinical practice is for these medications to be discontinued prior to surgery and for coagulation parameters to be documented to have returned to normal (in the case of warfarin) before proceeding with the case, but we did not record whether this had in fact been done. A possible source of bias is that, in subjects with known or suspected imperfect coagulation, RA was avoided. However, preliminary data from a separate investigation (currently being conducted in our region) indicate that this may not be a significant concern: of 197 TJA subjects in that series, 31 were taking ASA when surgery was scheduled, but 24 of them went on to receive spinal anesthesia. An additional 5 were taking warfarin at scheduling, but 4 received spinal anesthesia.
It may seem surprising to those who perform the procedure that revision surgery does not appear as an independent predictor of transfusion risk in the final model. This is because revision procedures took much longer than did the primary cases, and the model was better able to capture the difference in blood transfusion rates by using the duration of surgery, which is a continuous variable expressed in minutes, rather than revision status, which is a binary variable.
We divided subjects into 2 groups by severity of comorbid illness, using the ASA grade. The ability of this simple clinical score to discriminate well between those at high and those at low risk of perioperative complications has been clearly documented.33
The decision to transfuse in any given subject was made by the individual clinician, but there was a high degree of uniformity in transfusion practice nonetheless: Each of the 28 surgeons in our study individually initiated postoperative transfusion at a mean hemoglobin in the range 76–86 g/dl (the mean for the entire series was 81 g/dl).
An RCT can answer clinical practice questions and would be the best way of determining the overall effect of RA on outcomes such as mortality and major morbidity, because the randomization process would divide known and unknown confounding factors evenly between the groups. The value of an observational study such as this in an area that has already been the subject of many RCTs may therefore be questioned. When all relevant trials are examined together, it is impossible to definitively answer whether RA protects against transfusion in TJA or not. Although a meta-analysis of the existing trials might overcome some of the limitations of these individual RCTs, this is not currently available. Our analysis, based on a large unselected consecutive series of nonexperimental cases, may better reflect the clinical reality.
RA in the form of single-shot spinal anesthesia is strongly protective against allogeneic transfusion in total hip arthroplasty.