A novel method optimizing the normalization of cardiac parameters in small animal models: the importance of dimensional indexing

Abstract

For indexing cardiac measures in small animal models, tibia length (TL) is a recommended surrogate for body weight (BW) that aims to avoid biases because of disease-induced BW changes. However, we question if indexing by TL is mathematically correct. This study aimed to investigate the relation between TL and BW, heart weight, ventricular weights, and left ventricular diameter to optimize the current common practice of indexing cardiac parameters in small animal models. In 29 healthy Wistar rats (age 5–34 wk) and 116 healthy Black 6 mice (age 3–17 wk), BW appeared to scale nonlinearly to TL1 but linearly to TL3. Formulas for indexing cardiac weights were derived. To illustrate the effects of indexing, cardiac weights between the 50% with highest BW and the 50% with lowest BW were compared. The nonindexed cardiac weights differed significantly between groups, as could be expected ( P < 0.001). However, after indexing by TL1, indexed cardiac weights remained significantly different between groups ( P < 0.001). With the derived formulas for indexing, indexed cardiac weights were similar between groups. In healthy rats and mice, BW and heart weights scale linearly to TL3. This indicates that not TL1 but TL3 is the optimal surrogate for BW. New formulas for indexing heart weight and isolated ventricular weights are provided, and we propose a concept in which cardiac parameters should not all be indexed to the same measure but one-dimensional measures to BW1/3 or TL1, two-dimensional measures to BW2/3 or TL2, and three-dimensional measures to BW or TL3. NEW & NOTEWORTHY In healthy rats and mice, body weight (BW) scales linearly to tibia length (TL) to the power of three (TL3). This indicates that for indexing cardiac parameters, not TL1 but TL3 is the optimal surrogate for BW. New formulas for indexing heart weight and isolated ventricular weights are provided, and we propose a concept of dimensionally consistent indexing. This concept is proposed to be widely applied in small animal experiments.

IGF1 does not overcome sexual dimorphism of body and muscle size in Mstn−/− mice

“…The length of the tibia was measured and used to normalise the mean masses of skeletal muscles (White et al 2016) and the heart was normalised to body mass (Hagdorn et al 2019). The gonadal fat pads were excised and weighed and were expressed as a percent of body mass.…”

“…Normalising the mass of the heart to body mass, or to tibia length raised to the power of 3, has now been shown to be a better method than to tibia length alone (Hagdorn et al 2019) and we used this method here. However, the mass of the heart in Mstn -/mice has been reported to be unchanged from WT mice up to 100 days of age (Rodgers et al 2009).…”

Intrinsic Exercise Capacity and Mitochondrial DNA Lead to Opposing Vascular-Associated Risks

“…It has been shown that body weight correlated nonlinearly (cubically) with tibia length (TL), but linearly with TL 3 (cubed). 19 The linear relation with left ventricle (LV) and TL 3 showed that LV weight crossed the x -axis at TL 3 −26.73 (95% confidence interval (CI) −29.8 3 ; −23.4 3 ). Therefore, TL and LV weight were indexed by dividing the weights by 26.7 3 + TL 3 .…”

Volume Load-Induced Right Ventricular Failure in Rats Is Not Associated With Myocardial Fibrosis

“…LV ejection fraction (EF) was assessed using M-mode in the short axis, using Teichholz's formula. RVIDd, LVIDd, and TAPSE were indexed by dividing by tibia length (TL), and LV CO was indexed by dividing by tibia length to the power of three (TL 3 ), as described previously (Hagdorn et al, 2019).…”

“…Cardiac weights (LV, RV, both atria) and TL were measured. Cardiac weights were indexed as described previously (Hagdorn et al, 2019). The heart was partially snap-frozen for gene and protein analyses, and partially formalin fixed and embedded in paraffin for histological analyses.…”