Anesthesia in the News
  • Blood Pressure Management by Nurse Anesthetists

    A study recently published in the journal Anesthesia & Analgesia sought to evaluate CRNAs’ BP management as a metric of clinician skill based on data from thousands of cases extrapolated from electronic medical records.

    The study evaluated 25,702 adult cases of noncardiac surgery at the Cleveland Clinic Main Campus in Cleveland, Ohio, between January 2005 and September 2015. This represented 110,391 hours of anesthesia by 99 nurse anesthetists. All patients had general or neuraxial anesthesia lasting at least one hour.

    Predicting hypotension

    The investigators first developed a multivariable model to predict the amount of hypotension, which was defined as minutes of mean arterial pressure (MAP) <65 mm Hg, for noncardiac surgical cases, using baseline patient characteristics such as medical history and type of surgery but ignoring the indicator for certified registered nurse anesthetist (CRNA) provider.

    They then compared observed and predicted amounts of hypotension for each case and summarized “excess” amounts across providers.

    This was followed by estimation of the extent to which hypotension on an individual case level was independently associated with a composite of serious complications.

    Minutes of MAP

    The fourth and final step in the protocol was an assessment of the range of actual and excess minutes of MAP <65 mm Hg on a provider level and the extent to which these pressure exposures were linked to complications.

    A MAP threshold of 65 mm Hg was chosen because the investigators had previously shown that exposure below this degree of hypotension is linked to myocardial and kidney injury compared to patients whose pressure never went below MAP of 65 mm Hg. Thus, the duration of MAP <65 mm Hg was the primary exposure variable.

    CRNA inclusion criteria

    Analysis was restricted to cases in which a particular CRNA was assigned throughout anesthesia, minus meals and short breaks. A single CRNA was also required to log in for at least 80% of the total anesthesia time. In addition, only CRNAs who performed at least 200 anesthesia procedures over the study period were included.

    CRNAs were selected for analysis because “they are ‘hands-on’ providers, and because their skill level over time is relatively stable compared to residents,” wrote the authors.

    Predictive value

    A total of 69% cases had at least 1 minute of MAP <65 mm Hg, with a median (quartiles) of 4 (0 to 15) minutes on the case level. However, the investigators were unable to explain much variance of intraoperative hypotension from baseline patient characteristics.

    Nonetheless, cases in the highest two quartiles (>10 and >24 minutes/case more than predicted) were an estimated 27% and 31%, respectively, more likely to experience complications compared to those with 0 excess minutes (both P < 0.001).

    But there was little variation of the average excess minutes <65 mm Hg across the nurse anesthetists, with median (quartiles) of 1.6 (1.2 to 1.9) minutes/hour.

    The investigators also found no connection in confounder-adjusted models on the nurse anesthetist level between average excess hypotension and complications, either for continuous exposure (P = 0.09) or as quintiles (P = 0.30).

    Further, the authors expressed surprise that patient baseline characteristics explained only a modest fraction of the observed variance in minutes of MAP <65 mm Hg across several different statistical models.

    ‘Blood pressure is a complex signal’

    One corollary to the study, they note, in being unable to accurately predict hypotension in individual patients from baseline variables is that clinicians cannot reliably know in advance which patients might most benefit from invasive or noninvasive continuous blood pressure monitoring, guided fluid management, or a priori vasopressor infusions.

    The authors added that there are situations for which the type of surgery contributes to hypotension, citing blood loss, vena caval pressure, abdominal insufflation, and patient positioning. Also, in some cases, surgeons may request hypotension in order to reduce bleeding.

    Anesthetic drugs, perhaps combined with inadequate vascular volume, likely caused most intraoperative hypotension, the authors postulated. Thus, while hypotension cannot be reliably predicted, much of it can probably be prevented or minimized by skilled anesthetic management, making it reasonable to consider hypotension as a performance metric.

    In conclusion, although avoiding hypotension is a worthy clinical goal, it does not appear to be a valuable metric of performance because “the range of average amounts per clinician is not meaningfully associated with patient outcomes,” wrote the authors, at least among CRNAs at the study’s single tertiary center.

    Read the complete study, Sessler DI, Khan MZ, Maheshwari K, Liu L, Adegboye J, Saugel B, Mascha EJ. Blood Pressure Management by Anesthesia Professionals: Evaluating Clinician Skill From Electronic Medical Records. Anesth Analg. 2021 Apr 1;132(4):946-956 at