• cardiopulmonary exercise test
• diabetes
• effort intolerance
• exercise physiology
• heart failure

Background and aim. Patients with type 2 diabetes (T2D) early develop reduced oxygen uptake (VO2) and thus effort intolerance (EI), a marker of increased risk of developing heart failure (HF). Nevertheless, EI in T2D remains an understudied issue, and its cardiopulmonary characterization and the physiological determinants are currently unknown. The aim of this dissertation is to characterize cardiopulmonary correlates of EI in uncomplicated T2D and to identify its pathophysiological determinants.

Methods. Cardiopulmonary exercise test (CPET) is the gold standard for the quantification of VO2; when paired with exercise echocardiography it grants the distinction of central (i.e. cardiac, respiratory) from peripheral (i.e. metabolic, muscular) determinants of exercise physiology. The addition of indirect calorimetry may provide additional information regarding substrate oxidation during exercise, which we aimed to validate against the reference method (steady state). Secondary, we performed several case-control studies and one intervention trial to identify the impact of T2D on cardiopulmonary performance, the main pathophysiological mechanisms, the defects associated with EI, and the effect of sodium-glucose cotransporter 2 (SGLT2i) inhibitors, which have proved to reduce the incidence of new-onset decompensated HF in T2D.

Results.
1. Cardiopulmonary characterization of EI in T2D. 88 adults with well-controlled and uncomplicated T2D underwent a maximal echo-CPET. EI (defined as VO2peak <80%VO2max) was present in 45 patients (55%), reaching a lower VO2peak (16.5±3.2 mL/min/kg, vs 21.7±5.4 mL/min/kg, p<0.0001). They showed reduced peak peripheral oxygen extraction (AVO2diff, 11.3±3.1 vs 12.7±3.3 mL/dL, p=0.002), lower VO2/work slope (9.9±1.2 vs 11.2±1.4, p<0.0001), impaired left ventricular systolic reserve (peak S’ 13.5±2.8 vs 15.2±3.0, p=0.009), lower global longitudinal strain (ΔGLS 1.7±1.5 vs 2.5±1.8, p=0.03) than subjects without EI. Determinants of AVO2diff, which explained the reduced VO2, were systolic reserve, female sex, and hemoglobin values. In conclusion, EI is severe and highly prevalent in uncomplicated, otherwise asymptomatic T2D, being the result of a major defect in AVO2diff and subtle myocardial systolic dysfunction.
2. Impact of epicardial adipose tissue (EAT). We measured EAT by echocardiography in 72 patients with uncomplicated T2D. When divided EAT thickness above vs below the median (5 mm), higher EAT thickness was associated with higher fat mass, smaller indexed left ventricular dimensions, and marginally reduced diastolic function at rest. Higher EAT thickness was associated with lower peak oxygen uptake (VO2peak 17.1±3.6 vs 21.0±5.7 mL/min/kg, p=0.001), reduced systolic reserve (ΔS’ 4.6±1.6 vs 5.8±2.5 m/sec, p=0.02), higher natriuretic peptides (NT-proBNP 64 [29–165] vs 31 [26–139] pg/mL, p=0.04), as well as chronotropic insufficiency and impaired heart rate recovery.
3. Independent effect of T2D and obesity. 109 patients with uncomplicated T2D and 97 controls underwent echo-CPET. The effects of T2D and obesity were estimated by multivariable models accounting for known/potential confounders and the major pathophysiological determinants of VO2peak normalized for fat-free mass (FFM). The two groups had similar demographic and anthropometric characteristics except for higher body mass index (BMI) in T2D (28.6±4.6 vs 26.3±4.4 kg/m2, p=0.0003) but comparable FFM. Patients with T2D achieved lower VO2peak than controls (18.5±4.4 vs 21.7±8.3 mL/min/kgFFM, p=0.0006). Subclinical cardiovascular dysfunctions were observed in T2D: concentric left ventricular remodeling, autonomic dysfunction, systolic dysfunction, and reduced systolic reserve. After accounting for confounders and major determinants of VO2peak, T2D still displayed reduced VO2peak by 1.0 [-1.7/-0.3] mL/min/kgFFM, p=0.0089, while the effect of BMI (-0.2 [-0.3/0.1] mL/min/kgFFM, p=0.06 per unit increase), was largely explained by a combination of chronotropic incompetence, reduced peripheral oxygen extraction, impaired systolic reserve, and ventilatory (in)efficiency. In conclusion, T2D is an independent negative determinant of VO2peak whose effect is additive to other pathophysiological determinants of oxygen uptake, including obesity.
4. Effect of SGTL2-inhibitors. We analyzed data from the EMPA-HEART trial, aimed at verifying whether empagliflozin can improve myocardial contractility (GLS) and/or VO2peak in subjects with T2D without heart disease. Patients with T2D, normal LV systolic function (2D-Echo EF>50%), and no heart disease were randomized to either empagliflozin 10 mg or sitagliptin 100 mg for 6 months and underwent repeated echo-CPET. Analysis was performed on 44 patients, 22 per arm. Despite comparable glycaemic control, modest reductions in body weight and plasma uric acid as well as an increase in haemoglobin were evident with empagliflozin. No difference was detectable in either GLS at 1 month (empagliflozin vs sitagliptin: +0.44 [-0.10/+0.98]%, p=0.11) and 6 months of therapy (+0.53 [-0.56/+1.62]%), or in VO2peak (+0.43 [-1.4/+2.3] ml/min/kg, p=0.65). With empagliflozin, the subgroup with baseline LV-GLS below the median experienced a greater increase (time*drug p<0.05) in LV-GLS at 1 month (+1.22 [+0.31/+2.13]%) and 6 months (+2.05 [+1.14/+2.96]%), while sitagliptin induced a modest improvement in LV-GLS only at 6 months (+0.92 [+0.21/+0.62]%). In conclusion, empagliflozin has neutral impact on both GLS and VO2peak. However, in patients with subclinical dysfunction (LV-GLS <16.5%) it produces a rapid and sustained amelioration of LV contractility.
5. Applying indirect calorimetry to cardiopulmonary exercise test. Sixteen recreationally active, healthy volunteers performed two echo-CPET with different protocols (step graded of 35W/4min vs individualized ramp) on separate days, in randomized order. Comparable values and kinetics of hemodynamic and ventilatory parameters were observed for matching values of exercise intensity (%VO2max), while a 12% higher workload was achieved with ramp (p<0.05). The anaerobic threshold zone, the VO2/Work constant during the whole test, and VCO2/Work constant below the anaerobic threshold were identical between the two tests with high level of agreement and strong correlations, with a downward shift with ramp (-0.130.09 L•min-1•W-1 for VO2 and -0.180.09 L•min-1•W-1 for CO2). The ramp yielded comparable values of lipid oxidation values (0.01±0.01 g•min-1) and kinetics with respect to steady-state, granting a more accurate work-related profiling of the oxidation curve, thus increasing the potential to correctly identify MFO.

Conclusions. EI is severe and highly prevalent among patients with uncomplicated T2D, is associated with reduced muscle oxygen uptake and subclinical systolic dysfunction. Increased EAT thickness is associated with worse cardiopulmonary performance. T2D is a negative determinant of VO2 independently from known pathophysiological determinants. SGLT2-inhibitors have a neutral effect on cardiopulmonary function but increase myocardial contractility in those with subclinical systolic dysfunction.