Special situations in severe COVID-19

Severe COVID-19 | 445

Factors and characteristics to develop one type over the other have been identified: severity of the initial infection, the patient’s immune response, the patient’s physical fitness and comorbidities, the response of the hypoxemia to the ventilation, and the time between first symptoms and hospital admission (Gattinoni 2020). L type patients remain stable before improvement or deterioration. In the latter case the patients develop H type pneumonia (Pfeifer 2020). According to this theory, a ventilation strategy starting with respiratory support with high flow oxygen has been recommended (Gattinoni 2020). To adequately assess oxygenation, the oxygen content (CaO2) in the blood is helpul, as it describes the actual oxygen supply (DO2) better than the oxygen partial pressure (pO2), particularly when combined with the cardiac output (CO): DO2 = CaO2 x CO and CaO2 = Hb x SaO2 x 1.4 With a CaO2 limit of 10 g/100 ml blood, and an appropriate cardiac output, i.e., absence of cardiac failure, a lower O2 saturation (hypoxemia) can be tolerated in the blood before a critical oxygen shortage in the tissue (hypoxia) develops. Therefore, rather than strictly focusing on pO2 values as represented by the oxygenation index PaO2/FiO2 of < 150, it is more reasonable to consider the overall clinical picture while setting individual target values before intubation. Attempting high-flow oxygen and non-invasive ventilation in patients with type L pneumonia is recommended. Intubation should only be performed if there is significant clinical deterioration (Lyons 2020, Pfeifer 2020).

Prone positioning

Prone position (PP) has become a therapeutic option, even in awake, nonintubated patients, during spontaneous and assisted breathing (Telias 2020). In one study, among 50 patients, the median SpO2 at triage was 80%. After supplemental oxygen was given to patients on room air it was 84%. After 5 minutes of proning was added, SpO2 improved to 94% (Caputo 2020). Whether PP prevents intubation is not known yet. In a prospective before-after study in Aix-en-Provence, France among 24 awake, non-intubated, spontaneously breathing patients with COVID-19 and hypoxemic acute respiratory failure requiring oxygen supplementation, the effect of PP was only moderate. 63% were able to tolerate PP for more than 3 hours. Oxygenation increased in only 25% and was not sustained in half of

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those after resupination. However, prone sessions were short, partly because of limited patient tolerance (Elharrar 2020). In a small single-center cohort study, use of the prone position for 25 awake, spontaneously breathing patients with COVID-19 was associated with improved oxygenation. In addition, patients with an SpO2 of 95% or greater after 1 hour of the prone position had a lower rate of intubation. Unfortunately, there was no control group and the sample size was very small. Ongoing clinical trials of prone positioning in non–mechanically ventilated patients (NCT04383613, NCT04359797) will hopefully help clarify the role of this simple, low-cost approach for patients with acute hypoxemic respiratory failure (Thompson 2020).

Extracorporal Membrane Oxygenation (ECMO)

Since the beginning of the pandemic, extracorporeal lung replacement procedures such as ECMO have been recommended with caution and only in selected patients with severe and persistant hypoxemia (PaO2/FiO2 < 80), with minor comorbidities and with full usage of all other measures, such as relaxation and recruiting maneuvers (Smereka 2020). In a single center narrative study regarding ECMO, support for 27 patients with COVID-19 was described (Kon 2020). At the time of the paper submission, survival was 96.3% (one death) in over 350 days of total ECMO support. Thirteen patients (48.1%) remained on ECMO support, while 13 patients (48.1%) were successfully decannulated. Seven patients (25.9%) were discharged from the hospital while six patients (22.2%) remained in the hospital, of which four were on (unmodified) room air. The authors conclude that the judicious use of ECMO support may be clinically beneficial.

Tracheostomy

During the pandemic, an old problem in a new situation arose: When to perform tracheostomy (and how) in COVID-19 patients? In a review of the current evidence and misconceptions that predispose to uncontrolled variation in tracheostomy among COVID-19 patients, the authors conclude that decisions on tracheostomy must be personalized; that some patients may be awake but cannot yet be extubated (favoring tracheostomy); while others may have immediate, severe hypoxemia when lying supine or with any period of apnea (favoring deferral) (Tay 2020, Schultz 2020). Meanwhile, detailed consensus guidance has been published, including on important issues such as timing of tracheostomy (delayed until at least day 10 of mechanical ventilation and considered only when patients are showing signs of clinical im-