Question & Answer

Answer (Study-Ready Plan & Mediation Framework)

TL;DR

In a paired, within-patient design, expect blunted chronotropic response and slower HR recovery at RRT/EOS compared with 2–6 weeks post-replacement. Model shows that improved HR dynamics after replacement partly mediate the increase in 6MWD. Use mixed-effects models for the repeated measures and causal mediation with bootstrap CIs for the indirect effect.

1) Design & Timing

• Population: Adults with unicameral RV LPs tested at (a) first RRT/EOS notification and (b) 14–42 days post replacement.
• Within-patient pairing: Each participant serves as their own control to minimize confounding.
• Same protocol both days: corridor, encouragement, time of day, meds taken, and rate-response programming (post-replacement standard) documented.

2) HR Dynamic Metrics

HR_reserve = HR_peak − HR_rest

CI = (HR_peak − HR_rest) / (HR_pred − HR_rest)

HRR1 = HR_peak − HR_1min,  HRR3 = HR_peak − HR_3min

3) Primary Hypotheses (Directionality)

• At RRT/EOS: lower HR_reserve and CI; smaller HRR₁ and HRR₃; shorter 6MWD.
• Post-replacement: higher HR_reserve/CI, faster HRR; longer 6MWD.

4) Analysis Plan

1. Change scores: For each metric compute Δ = Post-replacement − RRT/EOS.
2. Mixed models: Linear mixed-effects with random intercept for subject to test LP status effect on each HR metric and on 6MWD.

   Outcome ~ LP_status (RRT/EOS vs Post) + covariates + (1 | subject)

3. Mediation model: LP_status → (mediator: CI or HR_reserve and HRR) → 6MWD.
   • Use causal mediation (parametric or nonparametric) with within-subject changes.
   • Estimate indirect effect (IE) via bootstrap (e.g., 5,000 resamples); report IE, direct effect (DE), and proportion mediated.
4. Adjustment set (prespecified): age, sex, BMI, β-blocker dose, hemoglobin, eGFR, LVEF, lower-rate limit, rate-response slope/sensitivity, capture threshold.
5. Multiplicity: Control FDR across HR mediators or pick a primary mediator (CI) and relegate others to secondary analyses.

5) Effect Size & Interpretation

• Clinically meaningful 6MWD change: 20–30 m is often cited in chronic cardiopulmonary disease; predefine an MCID relevant to your cohort.
• Expected magnitude: An increase in CI of ~0.10–0.15 may correspond to ~15–40 m longer 6MWD, depending on baseline fitness (interpret as exploratory).
• Proportion mediated: If 20–50% of the LP_status effect on 6MWD is via CI/HRR, that supports chronotropic limitation and recovery as key mechanisms.

6) Practical Protocol

1. Standardize 6MWT (ATS-style corridor, scripted encouragement, 2 cones, timers).
2. Record HR at: pre-walk (seated 2–3 min), stop, +1 min, +3 min.
3. Export device/EGM traces if available to verify capture and any backup/safety behaviors.
4. Keep prediction equations and device programming documentation consistent between visits.
5. Note intercurrent changes (illness, med titration, anemia) that could bias HR dynamics.

7) Reporting Template

RRT/EOS vs Post-Replacement (paired)
Rest HR: ____ vs ____ bpm
Peak HR: ____ vs ____ bpm
HR_reserve: ____ vs ____ bpm
CI: ____ vs ____
HRR1: ____ vs ____ bpm; HRR3: ____ vs ____ bpm
6MWD: ____ m vs ____ m

Mediation (primary mediator: CI)
Indirect effect (IE): ____ m (95% CI ____ to ____)
Direct effect (DE): ____ m (95% CI ____ to ____)
Proportion mediated: ____ %

8) Caveats

• 6MWT is submaximal; CI reflects submax capacity and rate-response behavior, not VO_2max.
• If non-capture/back-up pacing occurs during the walk, analyze those tests separately or adjust with an indicator variable.
• Chronotropic incompetence can be medication-induced; perform sensitivity analyses excluding recent β-blocker changes.

Prepared for clinical research/quality improvement; align with local policy and vendor/device guidance.