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Extrapolating beyond the Data

Fisher, Dennis M. M.D.

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To the Editor:—
In my years as an editor of Anesthesiology and a reviewer for numerous journals, I frequently encountered submissions in which figures contained fitted curves (linear regression or other nonlinear functions) extrapolated beyond the range of measured X values. Authors were often resistant to changing these figures to limit their predictions to the observed range of X values. The folly of extrapolating beyond the observed range is seen in figure 4 in a recent review article in Anesthesiology 1. The fitted function indicates that a decrease in blood volume increases cardiac output, certainly an unexpected and unphysiologic finding (and not supported by the data). Fortunately, the authors limited their extrapolation; had they extended their curve further, it would suggest that profound hypovolemia was desirable, certainly not the authors’ intention.
Dennis M. Fisher, M.D.
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1. Liu S, McDonald SB: Current issues in spinal anesthesia. A nesthesiology 2001; 94: 888–906

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