Cardiopulmonary exercise testing after cardiac resynchronization therapy: unit harmonization and effect-size methods require clearer reporting

This correspondence refers to ‘The effect of cardiac resynchronization therapy on functional capacity based on cardiopulmonary exercise testing: a systematic review and meta-analysis’, by J. Solano et al. https://doi.org/10.1093/ehjopen/oeaf176.

The authors’ response is available at https://doi.org/10.1093/ehjopen/oeag035.

We read with great interest the systematic review and meta-analysis by Solano and colleagues assessing the effect of cardiac resynchronization therapy (CRT) on functional capacity using cardiopulmonary exercise testing (CPET).¹ The authors included 14 studies (n = 858) and reported pooled improvements in peak VO₂, anaerobic threshold, and ventilatory efficiency, alongside substantial heterogeneity, and evidence suggestive of publication bias for peak VO₂.¹

Several reporting and analytical issues may materially influence interpretation of the pooled estimates.

First, peak VO₂ unit harmonization appears inconsistent. Table 1 labels mean baseline pVO₂ as mL/kg/min, yet at least two entries are marked with an asterisk and appear to be reported in mL/min (absolute VO₂).¹ As absolute and weight-normalized VO₂ are not interchangeable without explicit conversion,² mixing metrics may contribute to heterogeneity and reduce clinical interpretability. Separate analyses for absolute vs. weight-normalized VO₂ (or clear conversion where feasible), together with a sensitivity analysis excluding non-mL/kg/min studies, would strengthen confidence in the pooled peak VO₂ estimate.¹

Second, the methods describe meta-analyses comparing baseline vs. follow-up means within the same cohorts.¹ For pre–post (paired) designs, the sampling variance depends on the within-participant correlation between baseline and follow-up measurements; if this correlation is not incorporated, study weights and confidence intervals may be misestimated.^3,4 Clear reporting of the exact pre–post effect-size method used, any assumed correlation, and sensitivity analyses across plausible correlation values would improve transparency.^3,4

Third, the review includes two randomized controlled trials (RCTs) in addition to cohort studies.¹ Where available, controlled RCT contrasts (between-group difference in change) are generally preferable to within-arm pre–post changes alone, as they better isolate intervention effects from time trends and regression to the mean.⁴ Presenting controlled RCT estimates and an RCT-only synthesis (where data permit) would improve causal interpretability.⁴

Finally, heterogeneity is very high for key outcomes (for example, I² reported as 94% for peak VO₂ and 96% for anaerobic threshold).¹ With such variability, additional robustness checks, including leave-one-out analyses, subgrouping by CPET modality and follow-up timing, and prediction intervals, would help readers understand how effects may vary across settings.¹ The reported funnel plot asymmetry and significant Egger’s test for peak VO₂ (P = 0.014) further support cautious interpretation of pooled effects.¹

This review addresses an important clinical question and provides a helpful synthesis of CPET changes after CRT.¹ Clarifying unit handling and effect-size methods, and adding targeted sensitivity analyses, would strengthen the reliability and clinical interpretability of the conclusions.

Author contributions

Shoaib Majeed (Visualization [lead], Writing—original draft [lead], Writing—review & editing [lead]) and Jie Tu (Supervision [equal], Visualization [equal])

Funding

None declared.

Data availability

No new data were generated or analysed in this study.