Respiratory Support Strategies For Severe COVID-19
At this moment, over 100,000 people has been diagnosed with COVID-19. The number of confirmed cases outside the Chinese mainland has already reached over 25,000. Countries like Japan, South Korea, Italy and Iran have reported thousands of COVID-19 cases. AS the World Health Organization warned that "the window of opportunity is narrowing" to contain the deadly coronavirus.
Global Cases Reported by WHO
Among patients with COVID-19, approximately 15% to 30% are severe cases with Acute Respiratory Distress Syndrome (ARDS) . This rate can be even higher in some specific areas based on the overall public health landscape. Patients categorized as severe COVID-19 cases have a high risk of death therefore, it is essential to provide appropriate treatment to this group of patients to minimize the overall mortality rate. Respiratory support, also referred to as mechanical ventilation, is one of the primary medical intervention for severe COVID-19 patients. This article will share some insights on the clinical experience of Chinese experts whom been working on COVID-19 patient and the clinical guidance from WHO.
Who Are Categorized as Severe COVID-19 Cases?
Since the difference between severe and critical cases has little influence on the actual treatment offered to the patient, clinicians generally describe them as severe.
When a COVID-19 patient has respiratory dyspnea and the oxygen therapy has failed relief the symptoms, then patient is identified as severe hypoxic respiratory failure. An ineffective oxygen therapy means that the patient is using an oxygen reservoir mask with flow of 10L-15L/min (usually at minimal flow rate to maintain bag inflation with 60%-95% FiO2), but continues to show increasing respiratory rate with hypoxemia. Severe hypoxic respiratory failure often results in intrapulmonary shunt caused by V/Q imbalance (also referred to as ventilation-perfusion mismatch), therefore, different ventilation strategies should be used to adapt to the acuity level of each patient.
Invasive Mechanical Ventilation (IMV)
1. Clinical Consensus
For patients with PaO2/FiO2≤ 150mmHg, IMV should be implemented as soon as possible. According to lung protection strategy, lower tidal volume ventilation is the first choice (6ml/kg PBW to 8 ml/kg PBW). It is also recommended to perform optimal positive end-expiratory pressure (PEEP) titration, appropriate lung recruitment maneuvers [4,5], and other lung protective measures.
For patients with severe ARDS (PaO2/FiO2 < 100 mmHg) who shown little eﬀect to regular ventilation practices, particularly those with unevenly distributed lesions, it is recommended that clinicians give mechanical ventilation with prone positioning to the patient, with a minimum time of 12 hours.
PEEP Titration: Setting the appropriate PEEP is important to maintain oxygenation of ARDS patients and to avoid lung injuries. PEEP titration is necessary to keep the lung open after recruitment. There are several PEEP titration methods commonly used by clinicians including : ARDSnet FIO2-PEEP table, Low ﬂow P-V curve, Best Oxygenation, Stress index, PEEP Decremental, and Transpulmonary pressure.
ARDS net FiO2 - PEEP
2. Solutions for invasive ventilation
Non-Invasive Mechanical Ventilation (NIV)
1. Clinical Consensus
For severe cases of COVID-19, when the patient’s PaO2/FiO2 is between 150 mmHg and 200 mmHg, start with non-invasive ventilation. The initial NIV parameters are to be set as the following:
Inspiratory positive airway pressure (IPAP): 8 cmH2O to 10 cmH2O (1 cmH2O = 0.098 kPa);
Expiratory positive airway pressure (EPAP) : 5 cmH2O to 8 cmH2O;
Observe for 2 hours. During this period, NIV parameters need to be adjusted according to the patient's breathing status, tidal volume (Vt) and SpO2.
if Vt is < 9ml/kg, RR is <30 times/min and PaO2/FiO2 is stable or improved, then continue NIV treatment;
if Vt is between 9 ml/kg - 12 ml/kg, PaO2/FiO2 is stable, then use NIV and observe the patient for 6 hours;
during which if Vt is > 12 ml/kg or PaO2/FiO2 worsens, then immediately stop NIV and change to invasive ventilation (endotracheal intubation).
2. Solutions for NIV
Mindray SV Series ventilators support non-invasive ventilation, equipped with common non-invasive ventilation modes such as PSV-S/T, CPAP/PSV, P-A/C, etc., with the leak compensation up to 65 L/min. The ventilator is used with a dual-limb circuit with a closed non-invasive mask to support NIV. During the ventilation, VTi (insp. tidal volume), VTe(exp. tidal volume), MVleak (leaked volume in a minute) and leak% (percent of leaked tidal volume) can all be closely monitored.
When providing respiratory support for COVID-19 patients, the use of non-invasive ventilation with dual-limb circuits can greatly reduce the amount of gas exhale into the atmosphere (compared with traditional single-limb circuit expiratory valve). At the same time, with additional filter at the expiratory valve can efficiently process the exhaled air, and reduce the risk of aerosol infection to a minimal level.
High-Flow Oxygen Therapy (HFOT)
1. Clinical Consensus
When PaO2/FiO2 is between 200 mmHg and 300 mmHg, it is advised that the patient is supported with high-ﬂow oxygen therapy (HFOT) through nasal cannula (commonly called HFNC or HFNOT). The initial setting of HFNC can be at 40 - 50L/min with 100% FiO2. During the therapy, clinicians should closely observe the patient’s vital signs and oxygenation.
If the oxygenation deteriorates to PaO2/FiO2 < 200 mmHg, or SpO2 falls below 93%, and/or the RR is above 30 times/min, then HFNC is not likely to be eﬀective and NIV may be a better choice in this case.
If the patient has any of the following symptom, invasive ventilation should be used instead of HFOT:
shock (intravenous norepinephrine dosage> 0.1 μg/kgmin);
acute respiratory acidosis (pH < 7.25);
2. Solutions for HFOT
Compared with the standard oxygen therapy, Mindray’s SV Series ventilator’s HFOT can provide 2 - 60 L/min flow of oxygen and up to 100% FiO2. In addition, with humidifier to actively warm and humidify the HFOT’s gas flow delivered to patients, preventing mucociliary damage, sputum buildup, and other complications. Autopsy reports shown that COVID-19 lesions are concentrated in the lungs with a large amount of viscous sputum. Therefore, humidified HFOT has a great significance for patients requiring sputum clearance.
 Critical Care Committee of Chinese Association of Chest Physician, Respiratory and Critical Care Group of Chinese Thoracic Society, Respiratory Care Group of Chinese Thoracic Society. Conventional respiratory support therapy for Severe Acute Respiratory Infections (SARI): Clinical indications and nosocomial infection prevention and control.
 Experts’ Suggestions on Clinical Management of Severe Cases of COVID-19. Chinese Journal of Critical Care & Intensive Care Medicine [e-Journal]. 2020, 06.
 World Health Organization. Clinical management of severe acute respiratory infection when Novel coronavirus (2019-nCoV) infection is suspected: Interim Guidance. 2020 Jan 28.
 Marini JJ. Recruitment maneuvers to achieve an ‘open lung’: whether and how? Crit Care Med 2001; 29:1647–1648.
 Halter JM et al. Positive End-Expiratory Pressure after a Recruitment Maneuver Prevents Both Alveolar Collapse and Recruitment/Derecruitment. Am J Respir Crit Care Med 2003; 167: 1620-1626.
 Chen et al. Implementing a bedside assessment of respiratory mechanics in patients with acute respiratory distress syndrome. Critical Care (2017) 21:84.