IJCS | Volume 31, Nº4, July / August 2018

434 Borges et al. Oncocardiology and the symptom fatigue Int J Cardiovasc Sci. 2018;31(4)433-442 Review Article The multifactorial nature of fatigue associated with cancer is a crucial point to be considered by professionals dealing with oncological patients. The main causes of fatigue are associated with the cancer effects and treatment on the central nervous system. Other causes include depression and anxiety, anemia, endocrine abnormalities (such as hypothyroidism and diabetes), activation of the immune system, inflammatory mediators, emotional stress, electrolytic disorders, myopathies, pulmonary fibrosis, and heart failure (HF). 6 HF is a complex andmultisystemic syndrome found in elderly patients presenting with the clinical triad fatigue, dyspnea, and edema. The mechanisms associated with fatigue inHF are triggered by inadequate blood perfusion affecting the respiratory and peripheral muscles, leading to decreased oxidative capacity. 7,8 Fatigue affects 50-96% of the patients withHF and is associatedwith a reduction in quality of life, restriction of physical activity, and worse prognosis. 8 AsuccessfulHF treatment depends ona comprehensive assessment of the symptoms and knowledge of the available approaches to alleviating not only the physical aspects of the patient but also his emotional and spiritual suffering. The “person-centered care” strategy, including a partnership between healthcare professionals and patients with HF, decreases the duration of hospitalization. 9 The prescriptions are specific to those patients with HF who present with dyspnea at the end of life, with the objective of symptom relief, in addition to full support of the team specialized in palliative care. 10,11 In clinical practice, healthcare professionals dedicated to oncology and cardiovascular diseases encounter patients, especially elderly ones, presenting with both conditions. Therefore, the identification and evaluation of fatigue by these professionals is fundamental andmust include scientifically validated instruments, as well as a clinical assessment and complementary tests to perform an adequate therapeutic plan. The objective of this review is to discuss the new aspects of fatigue present in oncological patients and emphasize the importance of early detection of HF and monitoring of cardiac function for more appropriate management of patients undergoing chemotherapy and radiotherapy. Defining fatigue in clinical practice There is no consensus in regards to the concept of fatigue. Describing fatigue is a difficult task due to a large number of synonyms associated with this term. Healthcare professionals attribute different terms to fatigue, such as asthenia, lethargy, exhaustion, feeling of weakness, extreme tiredness, and lack of motivation. Patients with cancer refer to fatigue using different terms, such as weakness, exhaustion, fatigue, depletion, slowness, or weight. 11 The scientific literature, in turn, defines fatigue as “a subjective feeling of physical tiredness or exhaustion disproportionate to the level of activity.” Additionally, “fatigue may manifest as difficulty or inability in initiating an activity (perception of general weakness); reduced capacity of maintaining an activity (easy tiredness); and difficult concentration, memory problems, and emotional stability (mental fatigue).” 12 Muscle fatigue, in turn, is regarded by many authors as “an inability to maintain a level of potency or strength during repeated muscle contractions,” 13 “decreased strength in sustained maximal contraction,” 14 and “reduced availability of energy substrate for the skeletal muscle during exercise.” 15 Mechanisms of fatigue Fatigue originates in the cerebral cortex and may extend up to the cross-bridges of the muscle, induced by a reduction in the number of functional motor units involved in the activity or in the frequency of triggering. The mechanisms responsible for fatigue may be central or peripheral and are investigated through kinesthetic sensations (effort and strength) andby electromyography. 15 Electromyographic signs allow the identification of the manifestation of fatigue of a given muscle through a reduction in the amplitude of the electrical impulse registered, indicative of loss of recruitment or synergic activation of multiple muscles. Another method of study of the physiology of fatigue is the addition of a force by supramaximal electrical stimulation during a maximal voluntary contraction, which translates into impaired muscle activation (at a level proximal to the neuromuscular junction). 16 The central mechanism of fatigue occurs due to changes in the neural input arriving at the muscles, i.e. , the recruitment of motor units remains below the ideal one for generation of adequate muscle strength during exercise. 16 Peripheral fatigue stems from homeostatic changes in the skeletal muscle itself and from decreased contractile force. One of the mechanisms inducing muscle fatigue