## 3 Results  
### Study Sample, Demographic Characteristics, and Treatment Status of the Underweight Patients  
As of September 2024, the registry enrolled n = 1420 ADHD diagnosed with any form of HF as defined above. Of these, a total of n = 59 patients (4.2%) were underweight, with 37 (62.7%) classified as mild, 11 (18.6%) moderate, and 11 (18.6%) as severe (Table 1). Among the other 1361 patients, 718 (52.8%) had normal weight, 446 (32.8%) were overweight and 193 (14.2%) obese. The mean age of the entire cohort at the time of evaluation was 31.8 \(\pm\) 11.3 years, ranging from 20 to 67 years. The correlation between age and the level of underweight is presented in Table 2.  
In the underweight group, the mean height was 173 \(\pm\) 13.5 cm (range: 139-200 cm), and the mean body weight was 51.4 \(\pm\) 8.6 kg (range: 35-66 kg), resulting in a body mass index (BMI) of 17.1 \(\pm\) 1.1 kg/m\({}^{2}\) (range: 14.2-18.5 kg/m\({}^{2}\)). Men were significantly taller and heavier than women. Accordingly, the metabolic body weight was significantly lower in women (18.2 \(\pm\) 2.3 (range: 14.4-22.4)) than in men (20.1 \(\pm\) 2.3 (range: 15.0-24.2)) (\(p\) = 0.002) (Table 1). \begin{table}
\begin{tabular}{c c c c} \hline
**Demographics** & **Overall (N = 59)** & **Women (n = 30)** & **Men (n = 29)** & \(p\)**-Value** \\ \hline
**Age [years]** & 31.8 \(\pm\) 11.3 (range: 20–67) & 33.6 \(\pm\) 10.0 (range: 20–61) & 29.9 \(\pm\) 12.5 (range: 20–67) & 0.208 \\ \hline
**Weight [kg]** & 51.4 \(\pm\) 8.6 (range: 35–66) & 48.1 \(\pm\) 7.8 (range: 35–63) & 54.8 \(\pm\) 8.1 (range: 37–66) & **0.002 \({}^{*}\)** \\ \hline
**Metabolic Body Weight [kg]** & 19.1 \(\pm\) 2.5 (range: 14.4–24.2) & 18.2 \(\pm\) 2.3 (range: 14.4–22.4) & 20.1 \(\pm\) 2.3 (range: 15.0–24.2) & **0.002 \({}^{*}\)** \\ \hline
**Height [m]** & 173.0 \(\pm\) 13.5 (range: 139–200) & 167.2 \(\pm\) 13.1 (range: 139–191) & 179.1 \(\pm\) 11.2 (range: 157–200) & **c.001 \({}^{*}\)** \\ \hline
**Dubois Body Surface Area [m\({}^{2}\)]** & 1.61 \(\pm\) 0.20 (range: 1.16–1.99) & 1.52 \(\pm\) 0.19 (range: 1.16–1.88) & 1.69 \(\pm\) 0.18 (range: 1.30–1.99) & **c.001 \({}^{*}\)** \\ \hline
**BMI [kg/m\({}^{2}\)]** & 17.1 \(\pm\) 1.1 (range: 14.2–18.5) & 17.1 \(\pm\) 1.1 (range: 14.2–18.5) & 17.0 \(\pm\) 1.2 (range: 14.7–18.5) & 0.613 \\ \hline _Milit Underweight (BMI 17–18.5)_ & 37 (62.7\%) & 22 (73.3\%) & 15 (51.7\%) & — \\ \hline _Moderate Underweight (BMI 16–16.9)_ & 11 (18.6\%) & 3 (10.0\%) & 8 (27.6\%) & — \\ \hline _Seore Underweight (BMI \(<\) 16)_ & 11 (18.6\%) & 5 (16.7\%) & 6 (20.7\%) & — \\ \hline
**Functional Class (Perloff)** & & & **0.024 \({}^{*}\)** \\ \hline
**1/11** & 49 (83.1\%) & 21 (70.0\%) & 28 (96.6\%) & — \\ \hline
**Ill/IV** & 10 (17.0\%) & 9 (30.0\%) & 1 (3.4\%) & — \\ \hline
**MUC-HF Classification** & & & 0.531 \\ \hline \(B\) & 24 (40.7\%) & 13 (43.3\%) & 11 (37.9\%) & — \\ \hline \(C\) & 34 (57.6\%) & 16 (53.3\%) & 18 (62.1\%) & — \\ \hline \(D\) & 1 (1.7\%) & 1 (3.3\%) & 0 (0.0\%) & — \\ \hline
**Previous Cardiac Surgery** & 42 (71.2\%) & 24 (82.8\%) & 18 (60.0\%) & **0.050 \({}^{*}\)** \\ \hline
**Medication Use** & 39 (66.1\%) & 18 (60.0\%) & 21 (72.4\%) & 0.314 \\ \hline
**Primary Cyanotic CHD** & 25 (42.4\%) & 11 (36.7\%) & 14 (48.3\%) & 0.367 \\ \hline \end{tabular} \({}^{*}\), statistically significant (\(p\) < 0.05); kg, kilograms; m, meters; BMI, body-mass-index; MUC-HF, Munich Heart Failure.  
\end{table} 
Table 1: Demographic parameters, overall and split by sex. Among the underweight patients, no significant gender difference was observed, with 30 (50.8%) being male. The highest prevalence of underweight was observed in 17 patients with complex CHD in the form of conotruncal anomalies (including truncus arteriosus, tetralogy of Fallot, pulmonary with atresia-ventricular septal defect, double outlet right ventricle or transposition of the great arteries), in 16 patients with [PERSON] syndrome, and in 5 with [PERSON]'s anomaly. In 25 cases (43.4%), an underlying primarily CHD was present.