| Summary: | Objective—To determine radiographic reference limits for cardiac width (CW) in peregrine falcons. Design—Retrospective cross-sectional study. Animals—100 peregrine falcons. Procedures—CW, thoracic width (TW), and sternal width (SW) were measured at the level of maximum CW on ventrodorsal radiographs of 64 falcons. Ratios of CW:TW and CW:SW were compared between positive pressure insufflation (PPI) radiographs and standard radiographs in 6 falcons. A hypothesis for the most accurate predictor of CW was formulated and validated in a new set of 36 falcons. Results—PPI radiographs had a significantly lower CW:TW ratio than did standard radiographs. The CW:SW ratio was unaffected. The 95% confidence interval for a regression-based predicted CW was 19% larger for any given TW, compared with SW. The difference between predicted and measured CW was significantly smaller with SW, compared with TW. Validation of these findings in a new data set of 36 falcons confirmed that prediction of CW when SW was used as the independent variable in a regression equation was superior to use of the CW:SW ratio or the CW:TW ratio and was superior to prediction of CW when TW was used as the independent variable in a regression equation. Conclusions and Clinical Relevance—Of the variables examined, SW seemed to be the most precise and most accurate predictor for CW in peregrine falcons. Thoracic width seemed inferior as a predictor for CW because TW was influenced by respiratory movements. The following formula can be used to predict the 95% confidence interval for CW in peregrine falcons: CW = 0.83SW + 0.37 ± 0.16.
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