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October 17, 2009
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Hospital Stay and Mortality Are Increased by a “Triple Low” of Blood Pressure, BIS & Anesthetic Level
  *  Leif Saager, M.D., Scott D. Greenwald, Ph.D., Scott D. Kelley, M.D., Armin Schubert, M.D., Daniel I. Sessler, M.D.
Anesthesiology Institute, Cleveland Clinic, Cleveland, Ohio

Prior work indicates that low arterial pressure and duration at low Bispectral Index (BIS) values are independent risk factors for one-year postoperative mortality [1-3]. We now evaluate the interaction of these factors, and of anesthetic concentration, on postoperative hospital length-of-stay and mortality.


With IRB approval, BIS, mean arterial pressure (MAP), and end-tidal volatile anesthetic concentrations in MAC-equivalents (MAC) were extracted from our perioperative registry. Average MAC, MAP, and BIS were calculated for each adult general surgical patient given volatile anesthesia. We defined a reference state consisting of patients whose average BIS, MAP and MAC values were within ∼20% of the population means. The remaining patients were classified to a clinical state characterized by whether their average BIS, MAP, and MAC values were higher or lower relative to the reference state. Cox Proportional Hazards Regression was used to create prediction models for postoperative hospital length-of-stay, 30-day mortality, and one-year mortality by identifying significant predictors (using forward conditional selection) from MAC, MAP, BIS, demographic predictors (age, gender, race, BMI, ASA Physical Status), and a composite risk stratification derived from ICD-9 codes. To compensate for multiple comparisons, p<0.005 was considered statistically significant.


Data from 18,035 non-cardiac procedures were available for analysis. The average MAC, MAP, and BIS of the High, Reference, and Low groups were, respectively, MAC: 0.72, 0.57, 0.39; MAP: 96, 86, 78 mmHg; and BIS: 52, 45, 38. The relative risk of mortality was significantly greater in patients who appear most sensitive to anesthesia as indicated by the combination of low MAC and low MAP; the risk of mortality was further increased in patients who also had low BIS. Patients given low MAC who maintained high MAP and BIS were likely to be discharged soonest (Table1).[figure1]Conclusions:

Patients who are sensitive to anesthesia do poorly. The combination of low MAC and low MAP is a strong and highly statistically significant predictor for mortality. When combined with low BIS, mortality is even greater. The combination of low MAC, low MAP, and low BIS is thus a “Triple Low” which is associated with a nearly tripled risk of mortality at 30 days, and nearly doubled risk of mortality at one year. This result is especially concerning since the average low values for each state were well within the range that many anesthesiologists tolerate routinely.


[1] Anesthesia & Analgesia 2005; 100: 4-10.

[2] Anesthesiology 2008; 109:A1.

[3] Anesthesia & Analgesia 2009; 108: 508-512.

From Proceedings of the 2009 Annual Meeting of the American Society Anesthesiologists.

Some of the co-authors of this abstract are employees of Aspect Medical, Inc, which also provided financial support for the work.
Figure 1