Commentary: What Conclusions Can We Draw from Recent Analyses of Anesthesia Provider Model and Patient Outcomes?

Health Services Research, March 2010

In April of 2009, the journal Health Services Research published an article titled “Anesthesia Provider Model, Hospital Resources, and Maternal Outcomes”.  The article was authored by two advanced degree nurses, and funded by the AANA.  To make a long story short, the authors concluded “We find no evidence that, compared with anesthesia models relying exclusively on anesthesiologists, hospitals that use only CRNAs…have systematically higher rates of these complications…”  Even before the article made it into print, the AANA in its March 2009 newsletter proclaimed, “National Study Confirms CRNAs and Anesthesiologists Provide Equally Safe OB Anesthesia”, and the study’s author stated, “Outcomes of this study may serve as a guide to policy makers who use results of studies such as this when considering allocation of resources or regulation.”

Even a superficial reading of the article revealed serious methodological and statistical flaws, so it was very disappointing that respected journal such as Health Services Research would publish such a study.  This month however, the journal published a “Commentary” by Mark Neuman, J. Sanford Schwartz, and Lee Fleisher, of the University of Pennsylvania, which details the most obvious shortcomings of the study, most of which stem from selection bias.  “Ultimately, the potential for inadequate adjustment for selection bias makes it difficult to determine just how much their study tells us about the safety of differing provider models for obstetric anesthesia care.”

By way of summary, the Commentary points out:

• "…selection may play a substantial role in the choice of hospital by women with high-risk pregnancies.’’ Indeed, ACOG recommends selective referral of high-risk obstetric patients based on hospital resources for care.  Because of this system,  “Comparisons of patient outcomes between ‘‘CRNA-only’’ and other types of hospitals are thus likely to reflect institutionalized processes of patient selection based on maternal and fetal risk factors.”
• Neuman and colleagues also point out that unobserved differences may be of relevance.  “While recent studies have estimated diabetes and obesity to occur in 6 and 22 percent of all U.S. pregnancies, these conditions were reported in only 0.54 and 0.59 percent (the study’s) sample. Such large differences in the rates of comorbidities known to increase the risk of adverse maternal outcomes suggest that the study dataset may not accurately reflect the true prevalence of these factors, limiting the authors’ ability to adequately adjust for them.”
• Neuman and colleagues note that some of the data just doesn’t reflect the real world.  “Clinical experience makes it hard to attribute these four- to fivefold reductions in the odds of death to differences in anesthesia provider model; rather, such differences are more likely to point to inadequate adjustment for important unmeasured differences in patient risk between these groups.”
• Given what we know about “triage” of high-risk patients to appropriate facilities, it is possible to look at the same data the study presents, and arrive at the opposite conclusion.  “…the study findings may document effective processes of selection and triage in which high-risk mothers are matched to appropriately skilled providers…through selective referral…of patients to anesthesiologists or CRNAs on the basis of underlying risk. Thus (the findings) are consistent with the argument that current tiered provider privileges may offer benefits to patient safety by supporting effective processes of patient selection.”

Neuman, Schwartz and Fleisher conclude by challenging the study’s attempt to direct national health policy.  “Because of the possible effects of unobserved but important clinical confounders, this study cannot support definitive conclusions about the relative safety of differing anesthesia provider models, nor can it speak meaningfully to the wisdom or folly of ‘‘legislating a specific educational credential’’ for the provision of obstetric anesthesia services.”

Every member of the American Society of Anesthesiologists who spends each day caring for patients and improving patient safety in every circumstance would certainly agree.

Craig M Palmer, MD
Chair, Committee on Obstetrical Anesthesia
American Society of Anesthesiologists