Abstract Body (Do not enter title and authors here): Background
Cardiac rehabilitation (CR) patients demonstrate a high burden of chronic conditions (CC). Both multimorbidity (MM; ≥2 coexisting CC) and aging negatively impact functional capacity, whereas CR improves performance. Age-related difference in cardiorespiratory fitness (CRF, peak oxygen uptake (VO₂)) in CR patients with MM has not been studied.

Hypothesis
We hypothesized improvement in CRF will be attenuated by MM with increasing age in CR patients.

Aims
We aimed to identify age-related differences in CRF improvement in CR patients with MM.

Methods
Patients ≥18 yrs old who attended ≥1 CR sessions from 1999-2017 and completed cardiopulmonary exercise stress test before and after CR were included. The prevalence of CC was assessed using Rochester Epidemiology Project records-linkage system. Age categories included: Younger (18-49 yrs), Midlife (50-64 yrs) and Older (≥65 yrs). CRF categories included: <0%, 0-8% and >8%. Analysis included Kruskal-Wallis and Chi-squared.

Results
Of 622 patients, 75.4% were male. Mean age: 62.9±11.2 yrs; Younger (n=70) age: 42.7±6.8 yrs, 67.1% male; Midlife (n=283) age: 58.3±4.3 yrs, 78.8% male; Older (n=269) age: 73.0±5.2 yrs, 74.0% male. BMI was 29.9±5.4 kg/m² (Younger: 30.0±6.7, Midlife: 30.4±5.6, Older: 29.3±4.7; p<.05). Median (Q1, Q3) no. of CR sessions attended was 33(22, 35), with lower attendance in the Younger group: 25(16,34) vs Midlife: 33(20,35) and Older: 34 (29,35), p<.05. Overall, percent change in peak VO₂ was 11.5±21.6% (Younger: 18.8±22.8%, Midlife: 11.6±19.5%, Older: 9.4±23.1%; p<.01). The median number of CC was 7(5,8) with significant differences across groups: Younger: 5(3,7), Midlife: 6(5,8), Older: 7(6,9), p<.01. Number of CC increased with age across all CRF categories (p<.01, Fig. 1). However, there was no difference in number of CC within age groups across categories of CRF, Table 1.

Conclusions
Our results suggest that MM increases with age in patients attending CR; however, within a given age group, improvement in CRF does not appear dependent on MM. The impact of age appears to have a greater impact than MM on CRF improvement. Individualized treatment plans should focus on the impact of age-related disparities on CRF improvement among CR patients with MM.