Study findings from scientific literature published after 1994 (not excluding sentinel articles published prior to 1994) were aggregated and reported in summary form by evidence category, as described later. All literature (e.g., randomized controlled trials, observational studies, and case reports) relevant to each topic was considered when evaluating the findings. For reporting purposes in this document, only the highest level of evidence (i.e., levels 1, 2, or 3 identified below) within each category (i.e., A, B, or C) is included in the summary.
Category A: Supportive Literature
Randomized controlled trials report statistically significant (P<0.01) differences among clinical interventions for a specified clinical outcome.
Level 1: The literature contains multiple randomized controlled trials, and the aggregated findings are supported by meta-analysis.*
Level 2: The literature contains multiple randomized controlled trials, but there is an insufficient number of studies to conduct a viable meta-analysis for the purpose of these Guidelines.
Level 3: The literature contains a single randomized controlled trial.
*All meta-analyses are conducted by the American Society of Anesthesiologists (ASA) methodology group. Meta-analyses from other sources are reviewed but not included as evidence in this document.
Category B: Suggestive Literature
Information from observational studies permits inference of beneficial or harmful relations among clinical interventions and clinical outcomes.
Level 1: The literature contains observational comparisons (e.g., cohort, case-control research designs) of two or more clinical interventions or conditions and indicates statistically significant differences between clinical interventions for a specified clinical outcome.
Level 2: The literature contains noncomparative observational studies with associative (e.g., relative risk, correlation) or descriptive statistics.
Level 3: The literature contains case reports.
Category C: Equivocal Literature
The literature cannot determine whether there are beneficial or harmful relations among clinical interventions and clinical outcomes.
Level 1: Meta-analysis did not find significant differences among groups or conditions.
Level 2: There is an insufficient number of studies to conduct meta-analysis and (1) randomized controlled trials have not found significant differences among groups or conditions or (2) randomized controlled trials report inconsistent findings.
Level 3: Observational studies report inconsistent findings or do not permit inference of beneficial or harmful relations.
Category D: Insufficient Evidence from the Literature
The lack of scientific evidence in the literature is described by the following conditions.
- No identified studies address the specified relationships among interventions and outcomes.
- The available literature cannot be used to assess relationships among clinical interventions and clinical outcomes. The literature either does not meet the criteria for content as defined in the "Focus" of the Guidelines or it does not permit a clear interpretation of findings due to methodologic concerns (e.g., confounding in study design or implementation).
All opinion-based evidence relevant to each topic (e.g., survey data, open-forum testimony, Internet-based comments, letters, and editorials) was considered in the development of these Guidelines. However, only the findings obtained from formal surveys are reported.
Opinion surveys were developed by the Task Force to address each clinical intervention identified in the document. Identical surveys were distributed to two groups of respondents: expert consultants and ASA members.
Category A: Expert Opinion
Survey responses from Task Force–appointed expert consultants are reported in summary form in the text. A complete listing of consultant survey responses is reported in Table 2 in the original guideline document.
Category B: Membership Opinion
Survey responses from a sample of members of the ASA are reported in summary form in the text. A complete listing of ASA member survey responses is reported in Table 3 in the original guideline document.
Expert consultant and ASA membership survey responses are recorded using a 5-point scale and summarized based on median values.**
Strongly Agree: Median score of 5 (at least 50% of the responses are 5)
Agree: Median score of 4 (at least 50% of the responses are 4 or 4 and 5)
Equivocal: Median score of 3 (at least 50% of the responses are 3, or no other response category or combination of similar categories contains at least 50% of the responses)
Disagree: Median score of 2 (at least 50% of responses are 2 or 1 and 2)
Strongly Disagree: Median score of 1 (at least 50% of responses are 1)
Category C: Informal Opinion
Open-forum testimony, Internet-based comments, letters, and editorials are all informally evaluated and discussed during the development of Guideline recommendations. When warranted, the Task Force may add educational information or cautionary notes based on this information.
**When an equal number of responses are obtained, the median value is determined by calculating the arithmetic mean of the two middle values. Ties are calculated by a predetermined formula.