Häggström J, Andersson Å O, Falk T, et al. Effect of Body Weight on Echocardiographic Measurements in 19,866 Pure-Bred Cats with or without Heart Disease. J Vet Intern Med. 2016 Aug 30.
Echocardiography is potentially the most important diagnostic tool for the investigation and categorization of heart disease in companion animals. Measurement of the relative dimensions of the hearts chambers and walls is a key part of an echocardiographic exam. In canine patients, allometric scaling is commonly used to determine the appropriate references ranges for chamber diameters and wall thickness. In cats, however, static values have traditionally been used (for example, 5.5 to 6mm for the diastolic left ventricular wall thickness). While these are adequate for the average feline patient, it will likely misdiagnose disease in smaller or larger cats.
The purpose of this study was to investigate the effect of body weight on heart rate, Aortic Diameter (Ao), Left Atrium (LA), and Left Ventricle (LV) linear dimensions in healthy adult pure bred cats. Fractional Shortening (FS%) and LA:Ao ratio were calculated from these values.
Data was collected from echocardiograms preformed from 1999 to 2014 in Europe, Australia, New Zealand and North America using the PawPeds screening program. Cats were screened from beneath in right lateral recumbency and standard parasternal long and short axis views acquired. Cats were classified as normal, abnormal, or equivocal based on echocardiogram results.
Data from 19866 cats was included in the study. Younger cats, female cats, and intact cats were over represented. All cats were purebred, with the majority being Maine Coons (5274), Norwegian Forest Cats (3301), and British Shorthairs (2663). Echocardiograms were normal in 18460 cats, equivocal in 529, consistent with HCM in 686, and other cardiac disease in 191.
Various statistical methods were used to generate dot-plots of relevant data and apply linear regression and allometric scaling in order to calculate mean and 95% confidence interval values for each of the measurements in question. For example, IVSd scaled as 2.83 x BW0.204
The results of this very large study suggest that body weight has a clinically relevant effect on echocardiographic parameters, with the exception of LA:Ao, FS%, and heart rate. While there was a statistically significant trend for larger cats to have slower heart rates, this was not considered to be clinically relevant. Allometric scaling provided a superior prediction of dimension than linear regression. This allowed the authors to calculate a table of values for expected normal ranges in cats of various weights.
Surprisingly, the vales that were determined were in many cases significantly smaller than previously utilized ranges. This suggests that, for many cats (ie those in the 4-4.5kg range), upper limits for left ventricular wall thicknesses should be closer to 5mm rather than 5.5mm. While this may seem like a small change, it does carry clinical relevance in many cases of borderline hypertrophy.
Some limitations to this study do exist. While the large sample size ensured a robust ability to predict measurements, echocardiographic parameters in cats often vary by fractions of a millimeter, lower than the resolution of many modern machines. There is also a relatively small weight range of cats, limiting mathematical precision. There also existed significant potential for machine and operator variation.
Despite these potential shortcomings, this paper provides valuable information on normal echocardiographic parameters in cats and establishes a new set of reference ranges that may supersede previous values. (MRK)
Drourr L, Lefbom BK, Rosenthal SL, et al. Measurement of M-mode echocardiographic parameters in healthy adult Maine Coon cats. J Am Vet Med Assoc 2005;226:734–737.