Neuromuscular Fatigue and Exercise Capacity in Patients With Type 2 Diabetes Mellitus and HFpEF

An important feature of patients with HFpEF is impaired exercise tolerance, resulting in worsening and reduced quality of life. Studies in the literature on patients with HFpEF suggest that the limited transport of oxygen to the muscles can be one factor leading to the early development of fatigue during physical activity and reduced effort tolerance. A recent study also shows that patients with HFpEF have an increased susceptibility to both central and peripheral fatigue, suggesting that neuromuscular fatigue may be one of the main mechanisms limiting exercise in this population.

Type 2 diabetes mellitus (T2DM), which affects 90-95% of diabetic patients, is a comorbidity of particular interest in heart failure (HF). In T2DM, as in HF, some observed an altered energy metabolism of the muscle and a shift in the type of muscle fibers. Hyperglycemia influences neuromuscular function and appeared to be one of the major causes of oxidative stress by affecting the intrinsic properties of the muscle (mitochondrial activity and function, myofilaments) related to the expression of force. The impact of diabetes on neuromuscular function is also linked to long-term complications such as diabetic peripheral neuropathy involving impairment of motor nerve conduction and vascular complications. This opens up a rather complex picture suggesting that T2DM in patients with HF could contribute to a further decline in muscle strength by further reducing the aerobic capacity of these patients.

It seems, there are currently no studies in the literature evaluating how much the coexistence of T2DM impacts neuromuscular fatigue and strength in patients with HF. Thus, the primary aim of this study will be to evaluate the differences in central and peripheral neuromuscular fatigue - determined by a submaximal exercise protocol with intermittent isometric contractions - in two groups of patients with heart failure with preserved ejection fraction with or without type 2 diabetes mellitus. Secondary outcomes will be related to the investigation of the differences in NO-mediated vascular function induced by a single passive movement of the leg, in the energy cost of walking, and in muscle oxygenation between the two groups.

The progressive aging of the population of industrialized countries is accompanied by a dramatic increase in the prevalence of chronic diseases. Heart Failure (HF) is one of the major public health problems, with a high incidence of health care costs. In the general population, HF has a prevalence of approximately 1-2% of adults and rises to 10% in the population over 70 years of age.

Heart failure is defined as "the inability of the heart to supply blood adequately to meet the needs of the body, or the ability to do so only at the cost of increased ventricular pressures" Heart failure with preserved ejection fraction (HFpEF) is the most common condition in older adults and its prevalence is increasing as the population ages.

An important feature of patients with HFpEF is impaired exercise tolerance, resulting in worsening and reduced quality of life. Although the attention of the researchers has mostly focused on the definition of central pathophysiological and hemodynamic mechanisms, it has been seen how physical training can improve functional capacity through peripheral adaptive mechanisms that have not been yet clarified. Studies in the literature carried out on this type of patient suggest that the limited transport of oxygen to the muscles is one factor that can lead to the early development of fatigue during physical activity and reduced effort tolerance. A recent study also shows that patients with HFpEF have an increased susceptibility to both central and peripheral fatigue, suggesting that neuromuscular fatigue may be one of the main mechanisms limiting exercise in this population.

Type 2 diabetes mellitus (T2DM), which affects 90-95% of diabetic patients, is a co-condition of particular interest in heart failure, considering the large and ongoing increase in risk associated with heart failure among patients with T2DM. In T2DM, as in HF, it is observed an altered energy metabolism of the muscle and a shift in the type of muscle fibers. Furthermore, hyperglycemia influences neuromuscular function and appears to be one of the major causes of oxidative stress by affecting the intrinsic properties of the muscle (mitochondrial activity and function, myofilaments) related to the expression of force. The impact of diabetes on neuromuscular function is also linked to long-term complications such as diabetic peripheral neuropathy involving impairment of motor nerve conduction and vascular complications. This opens up a rather complex picture that suggests that T2DM in patients with HF could contribute to a further decline in muscle strength by further reducing the aerobic capacity of these patients.

Objective of the study

The primary aim of this study will be to evaluate the differences in central and peripheral neuromuscular fatigue - determined by a submaximal exercise protocol with intermittent isometric contractions - in two groups of patients with heart failure with preserved ejection fraction with or without type 2 diabetes mellitus.

Secondary outcomes: to investigate the differences in NO-mediated vascular function induced by a single passive movement of the leg, in the energy cost of walking, and in muscle oxygenation. These assessments will have the purpose of integrating the assessment of fatigue, to identify the underlying causes.

Measures

The cardiologist will evaluate eligibility to the inclusion/exclusion criteria of patients attending the divisional cardiological visit at Istituti Clinici Scientifici Maugeri (Institute of Lumezzane, Italy) and will request the patient the possibility to join the study, proceeding to collect the informed consent and the following clinical and functional measures present in the patient's clinical documentation: circumferences, weight, height, blood tests, 6-minute walking test, echocardiography. These tests are usually in the patient's possession (preparatory to the cardiological check-up). In the absence of blood exams, tests will be prescribed and scheduled before the start of the study protocol. In any case, glycated hemoglobin will be prescribed for non-diabetic patients.

Regarding therapy and in particular, the drug dapagliflozin, which has recently been introduced into the clinical practice of patients with HFrEF with effects on the reduction of cardiovascular events, there are currently no indications for its use in HFpEF.

Enrolled patients will also be provided with a portable metabolimeter (Dynaport MM, McRoberts), which they will be asked to wear for at least 4 consecutive days before entering the laboratory for testing, in order to monitor their home physical activity profile (main outcome measure: daily steps).

Within one month after the visit, the patient will be invited by a dedicated researcher to go to the gym for the study protocol at the Institute of Lumezzane (BS). There, the patient will perform a series of tests.

Sample size

The sample size was calculated using neuromuscular fatigue as the primary outcome by considering the percentage of change in quadriceps MVC after the fatigue task (percentage of change in maximal isometric force expressed in Newtons). Using data from Weavil et al. (2021) who found a pre-post fatigue MVC reduction of 29 ± 12% in the HFpEF (reference group) and in agreement with Senefeld et al. (2020) who evaluated a reduction in MVC in T2DM of 46.1 ± 11.3%. Assuming an alpha significance level of 0.05 (2-sides), a beta error of 80%, and a 1:1 study-to-control ratio, a minimum total sample size of 20 participants (10 per group) is sufficient to detect significant differences.

Statistical analysis

Data will be analyzed using STATA.12 software. The normality of the distribution for continuous physiological variables will be evaluated with the Shapiro-Wilk test. Data will be expressed as mean ± standard deviation for normally distributed measures or as median (interquartile range) for non-normally distributed measures. For normally distributed data, it will be applied the unpaired or paired t-test for between-group and within-group comparisons, respectively. For data that will not follow a normal distribution, and it will be used the Mann-Whitney U test or Wilcoxon signed-rank test for between-group and within-group comparisons, respectively. The χ2 test will be also applied to compare between-group differences in categorical and binary variables. A P-value \<0.05 will be considered significant.

Eligibility Criteria:
Inclusion Criteria:

1. Have a diagnosis of heart failure with preserved ejection fraction (\>50%, NYHA class II - III)
2. Have an age between 65 and 80 years
3. Be males
4. Have at least one hospitalization for heart failure during the previous 10 years
5. Have a diagnosis of diabetes for no more than 10 years at the time of cardiology examination

Exclusion Criteria:

* 1. Unstable diabetes documented by HbA1c ≥ 9%
* 2. Significant additional valvular heart diseases
* 3. Unstable heart failure
* 4. Presence of a pacemaker or implanted defibrillator (AICD)
* 5. Changes in drug therapy in the previous three months because of clinical instability
* 6. Body mass index (BMI) \> 35 and \< 20 kg/m2
* 7. Orthopedic limitations that prevent the exercise
* 8. Presence of diagnosis and signs and symptoms of diabetic neuropathy (intensified perception of pain, burning or cold sensation, tingling, pins, and needles, hypo-hypersensitivity to touch)
* 9. Severe deconditioning (patient is confined to home) or vigorous physical activity (sports or similar activity, estimated as more than two hours/day of vigorous exercise)