To the Editor:—
With interest I read the recent article by Karpati et al. 1in the January issue of Anesthesiology. I have several questions, mostly regarding the study design, and hope the authors will comment.
It was the defined goal of the study to prospectively evaluate the incidence of myocardial ischemia in women with postpartum bleeding (introduction). In a prospective trial, one would expect to learn about admission criteria, predefined group selection criteria, and outcome measures. This did not seem to be the case in this study. The authors did not mention inclusion or exclusion criteria and, for the most part, seemed to use laboratory data from patient admission as outcome measures. In the Discussion, the authors assert, “myocardial ischemia–induced injury associated with hemorrhagic shock is likely to increase the incidence of [postpartum hemorrhage–associated] cardiac complications.” This statement implies that the authors observed cardiac complications. How did the authors define or measure this variable?
The title of this study describes the incidence of ischemia, which implies that the authors identified new cases and the population at risk. The study cohort exhibited severe postpartum hemorrhage. Most subjects were troponin positive or negative at the beginning of the study. There was a good possibility of an admission rate bias (Berkson fallacy).
There were 21 comparisons between troponin-positive and -negative groups, but there was no mention of an adjustment for multiple comparisons. With reference to the multiple logistic regression used, it is a rule of thumb to have approximately 10 outcomes for every independent variable in the model to achieve adequate power. This would call for a total sample size of at least 210 patients in this study. To illustrate this point, how can the authors assure that being catecholamine dependent, a cell with zero outcomes, is not predictive of ischemia?
The authors should comment on their definition of design, direction (prospective vs. cross-sectional), and study factors. They describe a cohort of poorly resuscitated female patients in “severe hemorrhagic shock” with an apparent high prevalence of being cardiac troponin positive. Did the authors find that surprising?
University of Alabama, Birmingham, Alabama. email@example.com