in the previous issue of critical care , van herpe and colleagues describe a new method of blood glucose reporting for hospitalized patients , the glycemic penalty index ( gpi ) . a stepped scale weights the clinical significance of differences of blood glucose ( bg ) from target .
the gpi scale is appropriately accordioned on the hypoglycemic range and expanded on the hyperglycemic range .
the authors recommend that use of the index should be combined with counting of episodes of severe hypoglycemia .
the index , capturing both overall hyperglycemia and hypoglycemia , could permit analysis of the ability of an algorithm to control between - patient glycemic variability . in the evaluation of glycemic control , the measures that are simplest to ascertain are the average and standard deviation , using the bg as the unit of observation .
in the leuven , belgium surgical intensive care unit , the standard deviations in intensively and conventionally treated groups was 19 and 33 mg / dl , respectively .
it could have been asked whether the greater bg variability in the conventionally treated group reflected the contribution of a hyperglycemic subgroup . until a second report was issued referring to control at the patient level , within the conventionally treated group even the relationship between hyperglycemia and mortality rate was unclear . in order to evaluate patient outcomes , there is no substitute for reporting on the patient as the unit of observation .
a key question is whether patient outcomes relate to overall hyperglycemia during a critical timeframe or to specific episodes of severe hyperglycemia .
the answer , predicated upon the mechanism of harm , could depend upon what outcome is being studied .
single episodes of severe hyperglycemia , such as the in - hospital development of diabetic ketoacidosis or hyperglycemia - associated dehydration , may result in specific consequences , such as dialysis fistula thrombosis , readmission to a critical care unit , or treatment - related pulmonary edema .
the association of single episodes with outcomes was recognized by stagnaro - green and colleagues .
furnary and colleagues , capitalizing upon a critical window of time in the postoperative interval following heart surgery , have reported on outcomes in relation to the " 3-day bg , " each value representing a patient 's 3-day average of postoperative bg measurements .
recent literature supports the importance of overall prevention of hyperglycemia , at least during critical windows of time , with respect to survival and morbidities such as sepsis , renal failure , duration of ventilator dependency , or transfusion requirement .
unfortunately , we have no simple measure comparable to the a1c by which the short - term inpatient overall glycemic control of an individual may be described . although hypoglycemia might simply reflect severity of comorbidities , the correlation between hypoglycemia and mortality of hospitalized patients is well known .
evidence from a mixed intensive care unit suggests that hypoglycemia is an independent predictor of mortality . among groups of patients having myocardial infarction or congestive heart failure
, observational patient - level data suggest that when mortality is considered as a function of overall bg concentration , there may be a j - shaped curve , such that patients having the lowest and the highest averages experience outcomes worse than those having intermediate range bg control .
is there a plausible mechanism by which modest overall reduction of average bg might cause harm , and if so , what specific harm is caused ?
alternatively , is harm a consequence of isolated episodes of severe hypoglycemia , to which the population having lower average bg is more vulnerable ? in the literature concerning strict glycemic control , serious or fatal consequences of hypoglycemia occasionally are reported . however , even when severe hypoglycemia is reported , sublethal permanent neurological injury seldom is described .
if a patient - level metric relied only upon bg averaging methods , isolated episodes of severe hypoglycemia could be overlooked that had resulted in altered function with respect to the activities of daily living or reduction of intellectual capacity .
application of the gpi index is cumbersome , such that other centers may have difficulty in adopting the method ; therefore , the measure may remain unfamiliar .
perhaps overall hyperglycemia and discrete episodes of hypoglycemia ought not to be captured by the same metric .
an extra mental step is needed to quantify the contribution of hypoglycemia and hyperglycemia as a fraction of the total value of the index .
multiple small episodes of hypoglycemia , none having clinical impact , might be weighted equally to one severe life - changing episode .
analysis of episodes of severe hypoglycemia should complement the use of the gpi , as the authors acknowledge . with that caveat , can a single measure sum up patient risk for a variety of outcomes that might be related to glycemic control ?
we await evidence that the gpi will improve upon our ability to define glycemic targets and predict clinical outcomes .
 the article by van herpe and colleagues in the previous issue of critical care describes the glycemic penalty index ( gpi ) , which weights both hyperglycemic and hypoglycemic blood glucose measurements commensurate to their clinically significant difference from target . 
 although certain adverse consequences result from isolated severe hyperglycemic episodes , several specific outcomes depend upon overall hyperglycemia . 
 in contrast , although mortality has been related epidemiologically to overall low blood glucose , specific negative outcomes may depend upon isolated episodes . capturing both hypoglycemia and hyperglycemia in a single index 
 will be shown to be useful if the gpi enables us to better define insulin strategies , outcomes , and targets . 
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