Left Ventricular Hypertrophy is Prevalent in Sprague–Dawley Rats

Dear Editor:

A recent article in Comparative Medicine by McAdams and colleagues⁶ claimed to find a high prevalence of left ventricular hypertrophy in Sprague–Dawley rats originating from both Charles River and from Harlan Sprague–Dawley. The authors further theorized that this cardiac abnormality was responsible for high mortality in rats within their laboratory that underwent anesthesia and middle cerebral artery occlusion, even though similar mortality was observed in Lewis rats which they concluded had a much lower incidence of ventricular hypertrophy. They found no evidence of cardiomyocyte proliferation or myocardial fibrosis. The apparent discovery of a prevalent and consequential anomaly in a major organ in 2 long-separated lines (sources) of outbred rats, used by the millions in research, including studies of experimentally-induced cardiac hypertrophy that use Sprague–Dawley rats as controls,^3,5 should require substantial evidence. We sympathize with the difficulties experienced by the authors during their studies, but disagree with their conclusion that the rats had left ventricular hypertrophy.

The findings of McAdams and colleagues result from inadequate methodology. McAdams and colleagues base their claim on their measurement of the thickness of the left ventricular wall, cross-sectional area of cardiomyocytes, and decreased lumen area of the left ventricle, as measured in dead rats. However, these parameters are not sufficient for a diagnosis of cardiac hypertrophy and can all be produced by normal postmortem constriction of the myocardium. As recently explained by Diwan and Dorn,⁴ “Hypertrophy [from the Greek hyper (over) and trophy (growth)] of the human heart is a morphological clinical diagnosis defined by increased myocardial mass. Premortem, the diagnosis is usually based on calculated echocardiographic or magnetic resonance imaging estimates of left ventricular mass. Postmortem, the pathologic diagnosis is based on direct measurements of gravimetric heart weight. Notably, there is no functional aspect to the diagnosis of cardiac hypertrophy, and it can occur in hearts with normal, supernormal, or depressed cardiac performance.” McAdams and colleagues did not weigh the hearts, nor did they assess ventricular mass in vivo by other methods such as echocardiography that have been used in various mammalian species, including humans, rats and mice,^1,2 and others.

The failure of McAdams and colleagues to weigh the hearts is problematic because postmortem muscle contraction, rigor mortis, causes contraction of the left ventricle. In fact, normal postmortem contraction of the left ventricle should expel essentially all blood, similar to the “hypertrophic” heart shown in Figures 1 and 2 in the paper by McAdams and colleagues. To further emphasize the normality of this process, that is, postmortem contraction of the left ventricle sufficient to expel most blood, if a blood clot is found remaining in the left ventricle it might even be considered as some evidence of a pathologic process.⁷ Thus, the apparently smaller left ventricular lumen should not be considered evidence of ventricular hypertrophy in the absence of corroborating data and in this case is likely a normal postmortem finding. Furthermore, the increased cross-sectional area of the cardiomyocytes is merely a consequence of contraction; contracted myocytes are thicker than stretched myocytes. Naturally, this ventricular constriction results in the wall appearing thicker than in a relaxed heart, even though the total mass of the ventricular muscle is not increased. This is why heart weights are necessary and why ventricular thickness measurements considered without the context of heart weights can be misleading. In safety assessment studies of novel compounds, where any evidence of cardiac disease is an important finding, weighing hearts is standard, and the size of the heart is often expressed as organ-to-body weight ratios. Organ-to-body weight ratios normalize for differences in body size. For example, differences in body size, Sprague–Dawley rats are larger than Lewis rats, may at least partly explain why McAdams and colleagues found that the Sprague–Dawley rats had thicker ventricular walls than the Lewis rats.

Postmortem contraction of the myocardium thus could have caused all of the evidence of myocardial thickening reported by McAdams and colleagues. Postmortem contraction of the myocardium could be why rats that died during or after anesthesia, where the time until necropsy was perhaps longer than rats euthanized specifically for necropsy, more often had hearts which appeared thick than did those collected at euthanasia.

We conclude that McAdams and colleagues have presented no evidence to support a claim of left ventricular hypertrophy in Sprague–Dawley rats.

In addition, the mortality noted by McAdams and colleagues during anesthesia is not typical. For example, Charles River conducts surgical procedures on rats as a paid service for clients. Anesthetic-related deaths are not tracked separately, but failures for all reasons during surgery and recovery combined, including anesthesia, discovery of anatomic variations, and iatrogenic deaths, such as death during induction of myocardial infarctions, accounted for only 2% of the last 100,000 surgical procedures in rats. Thus, not only do McAdams and colleagues fail to demonstrate ventricular hypertrophy, but our experience does not support the possibility of any type of cardiac anomaly that could increase anesthetic-related deaths.

Sincerely,

Charles B Clifford, DVM, PhD, DACVP
Director, Pathology and Technical Services Charles River Research Models and Services
Kathleen Pritchett-Corning, DVM, DACLAM, MRCVS
Director, Research and Professional Services Charles River Research Models and Services
Guy B Mulder, DVM, MS, DACLAM
Director, Professional Services Charles River Research Models and Services