Letter to the Editor: “Accurate diagnosis of ischemic heart disease without exposure to radiation using non-stress unshielded magnetocardiography (MCG)”

We read with great interest the study by Tolstruf et al., “Accurate diagnosis of ischemic heart disease without exposure to radiation using non-stress unshielded magnetocardiography (MCG).” (1) The authors deserve applause for their innovative work, which highlights a novel and non-invasive approach to detecting ischemia, offering potential advantages over current standard methods. While the findings are highly promising, we would like to share a few observations that could help refine and guide future studies in this area.

The study includes patients both with and without ischemic heart disease, consisting of 99 patients presenting with acute chest pain, evaluated at a large hospital in the US using a 9-channel MCG device, and 34 patients with stable angina, studied at a university hospital in Italy using a 36-channel device. The distinct patient populations, acute chest pain in the US versus stable angina in Italy, may introduce regional or clinical differences, further limiting the ability to generalize the findings across diverse patient groups or settings.

Another limitation of the study is related to the group of healthy controls. Obviously, confirmatory tests were not performed to exclude significant coronary artery disease (CAD) in presumed healthy controls as they were assumed free of disease. If this assumption is incorrect, the negative predictive value (NPV) of the MCG system could be overestimated. In an ideal world, controls should have been validated through negative functional testing to strengthen the conclusions, but an alternative could be to have a higher sample size in which case cohort with non-ischemic chest pain could serve this purpose.

Common arrhythmias, such as ST-elevation myocardial infarction (MI), atrial flutter, left bundle branch block (LBBB), and third-degree atrioventricular block, were excluded from the study without any explanation. If the authors are aware of evidence suggesting that these conditions interfere with MCG findings, this should have been clarified. Otherwise, excluding these arrhythmias limits the clinical relevance of the results, as they are frequently encountered when evaluating ischemic chest pain.

The definition of ischemic chest pain, based on positive functional testing, coronary angiography, or elevated troponin I, raises concerns. While perfusion defects and significant luminal stenosis (>70 %) are diagnostic, type II NSTEMI and other causes of elevated troponin could introduce bias. A standardized definition of ischemic chest pain would improve clarity and reliability.

The abstract mentions that all patients were “chest pain-free” at the time of scanning. However, this seems contradictory to the study's objective of evaluating MCG for ischemic chest pain. Clarification is needed on whether being chest pain-free was a predefined criterion and how this aligns with the study's goals. Additionally, it should be clarified whether having chest pain at the time of the study would have been grounds for exclusion from the study.

The MCG system uses seven parameters, based on magnetic vectors during the T-wave of the heart's electrical cycle, to detect ischemia. These parameters were defined using data from 197 patients who underwent tests like troponin, stress tests, and coronary angiography. If any of the parameters fall outside the normal range, the patient is classified as ischemic. (2) However, a major concern is that these criteria have not been validated in larger patient populations. To improve reliability, the next step should be to validate these criteria in a larger cohort using gold standard tests, such as coronary angiography, similar to how ST elevation in the right context on an ECG is validated.

Another issue is the equal weighting of all seven parameters. Treating them all equally may not reflect their true clinical importance, as some parameters might be more strongly associated with ischemia than others. This could be further studied in larger cohorts, which would also help refine the diagnostic performance.

The study reports a sensitivity of 76.4 %, specificity of 74.3 %, positive predictive value (PPV) of 70 %, and negative predictive value (NPV) of 80 % for detecting ischemia. While the MCG performed better than the 12‑lead ECG (accuracy: 75.2 % vs. 61.6 %), there is still room for improvement. This performance gap may be due to the oversimplification of the parameters, which have not been thoroughly studied yet. This suggests that further research and validation are necessary to enhance the MCG system's accuracy.

Including presumed healthy controls increased the specificity and negative predictive value (NPV) to 89 % and 94 %, respectively. However, as mentioned earlier, without validating these controls through functional testing, this approach may overestimate diagnostic accuracy. The study found that MCG achieved 100 % sensitivity and NPV in low-to-intermediate risk patients but performed less effectively in high-risk patients. This raises the question of whether the criteria for defining ischemia via MCG need to be re-examined and refined.

The performance of MCG in patients with prior IHD (sensitivity 86 %, specificity 90 %, PPV 94 %, NPV 77 %) also warrants further exploration. It remains unclear how myocardial scarring, common in patients with prior MI, may have influenced the results. Given MCG's previous role in assessing myocardial viability, (3) distinguishing between scarring and acute ischemia could improve its clinical utility. Future studies should address this gap by delineating criteria to differentiate chronic scars from acute ischemic changes.

The study presents an exciting development in MCG as a non-invasive tool for detecting ischemia. While the findings are promising, further large-scale studies are needed to refine the accuracy and standardize the diagnostic criteria. This is an excellent study, and we look forward to continued research to optimize and validate this innovative technology.

CRediT authorship contribution statement

Muhammad Ahmad Qureshi: Conceptualization, Data curation, Writing – original draft, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.