This research was characterized as a cross-sectional observational study, following the recommendations of the STROBE instrument. Therefore, it was conducted in the Intensive Care Unit of Otávio de Freitas Hospital (HOF) in Recife/PE, with patients over 18 years old who had a clinical diagnosis of COVID-19, using two methods of oxygen therapy (Nasal Oxygen Therapy) and (non-rebreather mask). Consequently, clinical evaluations were performed regarding the disease, severity of COVID-19, perception of respiratory effort, and electromyography of respiratory muscles.

1. Leading Question: How does the recruitment pattern of diaphragmatic and extradiaphragmatic muscles in patients with acute hypoxemia caused by COVID-19 behave when assessed through EMGs, considering the SpO2/FiO2 ratio as the analysis parameter?
2. Leading Question: Is there a relationship between the respiratory work estimated by electromyographic activity of diaphragmatic and extradiaphragmatic muscles in patients with acute hypoxemia due to COVID-19 and the parameters of respiratory frequency and levels of acute hypoxemia measured by the SpO2/FiO2 ratio?

The rationale for this study is that the COVID-19 pandemic has caused significant impacts on global public health due to the high volume of respiratory complications in the face of a limited supply of healthcare resources, consequently leading to high mortality. Complications are initially manifested by acute hypoxemia associated with COVID-19 infection, sometimes silent upon visual inspection or using less sensitive methods. This complicates the management of compensatory respiratory work for hypoxemia, even with oxygen therapy support. Respiratory muscle recruitment involves aspects of inspiratory effort and ventilatory mechanics. Recognizing the presence and recruitment pattern of muscles involved in a timely manner can contribute to the clinical management success rate of individuals affected by respiratory dysfunction associated with COVID-19, especially at different levels of acute hypoxemia. Surface electromyography is a non-invasive, low-risk tool compatible with the analysis of respiratory muscle recruitment patterns. However, there are no studies describing this pattern in COVID-19 patients, serving as a basis for personalized therapeutic strategies.

1. GENERAL OBJECTIVE To analyze the clinical aspects of surface electromyography of respiratory muscles in COVID-19 patients under different levels of hypoxemia.
2. SPECIFIC OBJECTIVES To analyze the recruitment pattern of diaphragmatic and extradiaphragmatic muscles (sternocleidomastoid, scalene, and rectus abdominis) under different levels of acute hypoxemia in COVID-19 patients using the SpO2/FiO2 ratio as an analysis parameter and their relationships with clinical outcomes.
3. STUDY SITE AND PERIOD This study was conducted in the Intensive Care Unit in the Severe Acute Respiratory Syndrome (SARS) sector of Otávio de Freitas Hospital (HOF), located in the city of Recife, Pernambuco - Brazil.
4. SAMPLE The sample for this study consisted of spontaneously breathing patients admitted to the ICU diagnosed with COVID-19, who required low-flow oxygen therapy, using a nasal oxygen catheter or non-rebreathing mask.
5. CLINICAL EVALUATION At the beginning, a comprehensive clinical assessment was carried out which included the following parameters: name, assessment date, date of birth, address, telephone contact, age (years), sex (male/female), weight (kg), height (cm ), Body Mass Index (BMI - weight, kg / height², m²), Heart Rate (HR - bpm), Respiratory Rate (RR - rpm) assessed through chest expansibility (assessed visually for 1 minute), Systolic Blood Pressure (SBP - in mmHg), Diastolic Blood Pressure (DBP - in mmHg), Mean Arterial Pressure (MAP), Peripheral Oxygen Saturation (SpO2), method of administration in oxygen therapy, medications in use, other diagnoses, length of stay in the ICU , as well as outcomes of tracheal intubation and death were analyzed prospectively.
6. ASSESSMENT INSTRUMENTS Assessment of the severity of COVID-19 using the National Early Warning Score (NEWS2) and the Simplified Acute Physiological Score (SAPS3); Stratification of oxygenation status based on the relationship between blood oxygen saturation (SpO2) and the fraction of inspired oxygen (FiO2) SpO2/FiO2, divided into three categories: normal (> 315), mild to moderate (314 - 235) and severe (< 234); Assessment of the perception of respiratory effort using the modified Borg scale and assessment of respiratory work specific to COVID-19, which has been predicting the need for intubation; Evaluation of surface electromyography in the respiratory muscles, electrodes positioned in such a way that they aligned with the muscle fibers of the sternocleidomastoid, scalenes, diaphragm and rectus abdominis muscles.
7. RESULTS Independently of the severity of hypoxemia, the observed pattern of muscle recruitment remained similar between the groups, with higher RMS values for the extradiaphragmatic muscles (scalene and sternocleidomastoid), followed by the diaphragm and rectus abdominis muscles, respectively. When comparing the SpO2/FiO2 groups, significant differences were observed in the scalene and sternocleidomastoid muscles. The scalene muscles showed lower activation in mild-moderate and severe cases compared to the normal group, while the sternocleidomastoid also exhibited reduced activation in mild-moderate cases compared to the normal group. However, no differences were identified in the recruitment level of the diaphragm and rectus abdominis muscles among the SpO2/FiO2 groups.
8. ETHICAL ASPECTS This research project was approved by the Human Research Ethics Committee of the Federal University of Pernambuco (UFPE), where it complied with the postulates of the Declaration of Helsinki, following the terms recommended by the National Health Council (Resolution No. 466/ 2012) for research involving human subjects (approval number 4,560,383)

Inclusion criteria:

• Both genders
• Age ≥ 18 years
• Confirmed diagnosis for COVID-19 considering its positivity through the RT-PCR test
• Patients who required low-flow oxygen therapy (nasal oxygen catheter or non-rebreathing mask)
• Hemodynamic stability (heart rate between 60 - 149 bpm, systolic blood pressure ≤149/89 mmHg or diastolic ≥ 90/60 mmHg at the time of collection)
• Body Mass Index ≤ 30 kg/m².

Exclusion criteria:

• Anxiety and psychomotor agitation
• Postural deformities (hyperlordosis, hyperkyphosis, lateral inclinations and antalgic postures)
• Patients with chronic lung disease or diaphragmatic disorders
• Undergoing surgical procedures on the spine
• Neurological diseases that affect myoelectric conduction
• Trauma or musculoskeletal injuries to the rib cage or respiratory muscles
• Use of tracheostomy tubes
• History of use of invasive mechanical ventilation during hospitalization (diagnosed and previously described in the electronic medical record)
• Those who were using sedatives and bronchodilators, as well as a reduction in SpO2 during signal acquisition at the time of collection.

The sample of this study consisted of patients admitted to the ICU with spontaneous breathing diagnosed with COVID-19, who required low-flow oxygen therapy, using a nasal oxygen catheter or non-rebreathing mask.