Our study comparing single-slice skeletal muscle evaluation at the widely used L3 level on abdominopelvic CT with immediately above the aortic arch, T8, and T12 on chest CT demonstrates that SMI (muscle quantity) at T12 has the strongest correlation with L3. This was also true for IMAT% (muscle quality); however, the average IMAT% among the L3 SMI tertiles was only significantly different at L3, making it potentially less valuable at the other levels. Thus, based on these results, T12 may be a good target for opportunistic muscle evaluation in patients with HF without CT of the abdomen/pelvis to predict outcomes or diagnose sarcopenia or malnutrition. The cutoffs based on the lowest sex-stratified L3 SMI tertile were 26.3 cm2/m2 for females and 31.1 cm2/m2 for males at the T12 level, while the cutoffs based on the frequently used cancer L3 SMI cutoffs were 27.5 cm2/m2 for females and 41.0 cm2/m2 for males at the T12 level.

Early sarcopenia identification is critical for intervention and mitigation of poor outcomes, but a formal diagnosis can be complex, given the need for muscle strength testing [11]. Opportunistic evaluation of skeletal muscles on imaging alone to identify myopenia is useful for outcomes prediction and early recognition of muscle wasting [19]. However, the widely used abdominopelvic muscle measurements at L3 [11] are of low opportunistic use in cardiology, where most imaging is done of the chest.

Unfortunately, there is significant heterogeneity in muscle measurements of the chest used in the literature, with most data derived from oncology and pulmonology populations [20]. A notable 2017 study compared SMI at L3 to T12 and T7 on preoperative CT of the aorta in patients undergoing transcatheter aortic valve replacement (TAVR) [21]. They found a higher correlation at T12 (r = 0.709, p < 0.001), which agrees with our study, indicating T12 as the most likely candidate for opportunistic use in cardiology patients. The close correlation of muscle area at the third or fourth lumbar vertebral levels with whole-body muscle measurements has been postulated to be due to the psoas, paraspinal, and abdominal muscles at these levels being minimally influenced by activity, unlike the appendicular muscles [22]. Thus, the close correlation seen at T12 may be due to its proximity to the lumbar vertebrae with the presence of abdominal muscles at the level; although T12 does not contain psoas, muscles seen at this landmark include rectus abdominis, diaphragm, external oblique, intercostals, latissimus dorse, and erector spinae.

A unique aspect of thoracic imaging in cardiology, particularly HF, is the presence of CIEDs, which may impede the assessment of adjacent structures due to metal artifacts in CT studies. Prior studies have addressed this limitation by making unilateral measurements opposite the device implantation site [23], a technique we also utilized in our study. Despite this, 26 patients were excluded from the original sample due to significant artifacts extending to both sides. To ensure that the presence of devices did not alter the correlation of upper thoracic measurements with L3, we performed a subgroup analysis of patients with versus without CIEDs. Both groups showed good correlation without a significant difference between them, indicating that the contralateral artifact, not extending beyond the midline subjectively, did not alter HUs, and thus measurements, in the upper chest.

Given the widespread use of L3 SMI, various cutoff values have been proposed. The first and most used cutoffs were established in 2008 in an obese Canadian population with respiratory or gastrointestinal cancers [17]. Sex-specific cutoffs of 52.4 cm²/m² in men and 38.5 cm²/m² in women were found based on association with mortality. Despite this, most studies derive their own cutoff values from morbidity and mortality or sex-specific lowest tertile, quartile, or fifth percentile of subjects, particularly outside of oncology [22]. Given the lack of cutoffs in cardiology populations, the latter definitions are likely more appropriate.

The previously mentioned 2017 study in patients undergoing TAVR utilized the frequently used L3 SMI cancer cutoff established in 2008 and proposed T12 cutoffs of 42.6 cm2/m2 in men and 30.6 cm2/m2 in women [21]. We focused on using the lowest sex-stratified L3 SMI tertile for low muscle mass evaluation with T12 SMI cutoffs of 31.1 cm²/m² in men and 26.3 cm²/m² in women. In addition, we also provided cutoffs at T12 of 41.0 cm²/m² in men and 27.5 cm²/m² in women based on those L3 SMI cancer cutoffs. The cancer cutoffs identified a much higher number of patients as having low muscle mass than sex-stratified tertiles (71.0% vs. 32.5%). However, an outcomes-based evaluation of these cutoffs in larger samples is required to compare their utility in cardiology patients.

Among the grouped participants based on sex-stratified L3 SMI tertiles, a significant difference was seen in NT-proBNP and albumin levels. The median NT-proBNP among patients in the lowest tertile was more than double that of the other groups. This agrees with prior studies showing a strong inverse relationship between SMA and NT-proBNP [24]. Although this difference could also be attributed to the higher BMI in patients without low muscle mass, multiple studies have demonstrated lean rather than fat mass to be responsible for the association between higher BMI and lower NT-proBNP [24,25,26]. Although the mechanism is unclear, the influence of sex steroid hormones has been postulated [27]. The association between low muscle mass and hypoalbuminemia is also not surprising, given the close relationship between myopenia and malnutrition, both contributing to frailty and worse outcomes [28].

The association between low muscle mass and lower statin use in our study is less clear. Statins have well-known benefits for primary and secondary prevention of cardiovascular disease, but they can induce adverse effects on the muscles. Although statin use post-endovascular aortic repair has shown lower long-term mortality without predisposition to muscle wasting based on successive SMA measurements [29], statin-induced myopathy shares some proposed mechanisms with myopenia and sarcopenia in HF, such as mitochondrial dysfunction [30] and ubiquitin-proteasome system upregulation [31, 32]. This leads us to believe that the lower statin use in patients with myopenia may be due to intolerance from worsened myopathy instead of statin use being protective against muscle wasting.

There are several limitations to our study. Although the study sample was more than most myopenia or sarcopenia studies, it was still small without power or sample size calculations, making type II error possible. Several patients also had to be excluded, primarily because of issues with opportunistic CT windows; this may be addressed by incorporating standardized institutional scanning protocols on routine imaging to ensure adequate windows containing all tissues. Measurements were done using the update to ABACS, ABACS+, for which there is limited validation, particularly on vertebral levels other than L3. Measurements AbvAoAr also had to be made unilaterally, given the presence of unilateral artifacts in most patients. The sample was obtained from patients hospitalized for ADHF, which may alter study variables; however, data such as weight and creatinine were obtained from the last values before discharge to ensure euvolemia and homeostasis. Finally, the diagnosis of sarcopenia was not made formally, given the lack of muscle strength testing.

In conclusion, frequently obtained imaging studies, such as computed tomography, are of opportunistic use for body composition analysis in chronic diseases like heart failure. However, such evaluation has been difficult in cardiac patients given the current consensus of using third or fourth lumbar vertebrae on abdominopelvic imaging while most imaging in cardiology is of the chest. This study adds to the limited literature supporting using the twelfth thoracic vertebra as a landmark for skeletal muscle evaluation. This allows us to further expand our knowledge on myopenia and sarcopenia in patients with heart failure and work towards a consensus for skeletal muscle evaluation in this population.