Optimal Ventilator Strategies in Covid-19 Acute Respiratory Distress Syndrome (ARDS)

https://www.medscape.com/viewarticle/912758_4

Summary: In mild to moderate ARDS, Non-invasive therapy (NIV) may be utilized judiciously using an Oxygen helmet:

In moderate to severe cases of coronavirus ARDS, the patient is intubated and begun on CMV or continuous (or controlled) mandatory ventilation. This is a mode of mechanical ventilation that takes into account the ability (or lack thereof) of the patient to ventilate (patient's breathing effort). The machine takes over all breathing completely or it assists the patient depending on certain patient variables.* The volume of pure oxygen (or oxygen and air mixture) applied by the machine in each inspiration and expiration depends on the body weight, ideally begun at 6ml/kilogram, or 420 ml in a 70 kg person. The breathing rate is usually initially set at 10 breaths per minute.  Something called PEEP** or positive end expiratory pressure is also applied and Italian physicians recommend the lowest PEEP possible, usually 5cm H2O pressure. The patient is usually paralyzed and sedated., at least, initially. Despite physicians best efforts, 60% of those patient's with severe ARDS will die. When the patient is improving, the patient is weaned from the ventilator either/or by gradual reduction in pressure support or by means of a T-tube:

*The variables are the ability of the patient to ventilate and to tolerate the tube in his or her trachea. In a paralyzed or heavily sedated patient it will take over the breathing completely. In a patient who retains some ability to ventilate (breath in and out) it assists the patient by increasing the pressure of inspired breaths and the negative pressure of expired breaths to a set respiratory rate, usually 10 breaths per minute to start. The tidal volume mentioned previously is multiplied by the respiratory rate to give the minute volume or the volume of inspired and expired oxygen in one minute (initially 10 x 420ml or 4200 ml in a 70 kg person)

*PEEP: "Positive end-expiratory pressure is a therapeutic modality consisting in the active–interventional maintenance of a slightly positive pressure in the tracheobronchial tree during assisted pulmonary ventilation, such that alveoli (basic lung unit) are not allowed to completely collapse between breaths; PEEP is of greatest use in ARDS and generated by attaching an airflow threshold resistance device to the expiratory port of the non-rebreathing valve of a manual or mechanical ventilator, allowing a ↓ of airway pressure to a plateau level Conventional PEEP Pressure is maintained at 5-20 cm H2O; it is indicated where an inhaled oxygen fraction at 0.6 cannot maintain the PaO2 above 60 Torr High PEEP Pressure is maintained at 20-50 cm H2O; it is used for marked hypoxia, as may occur in severe pulmonary edema (not in ARDS)." - McGraw-Hill Concise Dictionary of Modern Medicine. 

The percent of oxygen given to the patient (FIO2) is initially 100% (or 1). It is then, over minutes, hours or days, reduced (as soon as possible) to, at least 50% (0.5). The air we breath has an FIO2 of 21% (0.21). The PaO2 is the amount of oxygen in the arterial blood and is normally 94-99%. A useful way to diagnose the severity of ARDS is the PaO2/FIO2 ratio:

ARDS Severity	PaO2/FiO2	Mortality
Mild	200 – 300	27%
Moderate	100 – 200	32%
Severe	< 100	45%