Table 1. Study characteristics; all studies were single centre. ARDS – acute respiratory distress syndrome; NR − not reported; P/F ratio − PaO₂/FiO₂, where PaO₂ is the arterial oxygen partial pressure and FiO₂ is the fraction of inhaled oxygen); SpO₂ – peripheral oxygen saturation .

Study	Study design	Inclusion criteria	Exclusions	Intervention
Chen[25] China	Case series, n = 5	Progressive hypoxaemia, moderate-severe ARDS, laboratory confirmed COVID-19	NR	Multiplace 203 kPa for one patient, 162 kPa for four patients, 60−90 min. Mean five HBOT sessions (range 3−8)
Gorenstein[30] USA	Cohort study with controls chosen from patients treated in the same hospital and time period using propensity score matching, n = 80 (20 intervention; 60 control)	Age ≥ 18, laboratory confirmed COVID-19, SaO2 < 93% on air	Pregnancy, pneumothorax, positive troponin	Monoplace 203 kPa, 90 min per day, once daily for five days
Guo[29] China	Case reports, n = 2	Confirmed COVID-19, and one of: shortness of breath; respiratory rate ≥ 30 breaths·min-1; SpO2 ≤ 93% at rest; and P/F ratio ≤ 300 mmHg	Pneumothorax, pulmonary bullae	Monoplace, 152 kPa, 60 min, once daily for seven days
Qian[26] China	Prospective case series, n=4	Progressive dyspnoea, lung CT lesion area > 30%, SpO2 < 90% on air, clear consciousness, able to communicate in words, minimal education to junior high	Pneumothorax, pulmonary bullae	Portable monoplace chamber; 152 kPa, 90 min, once daily for seven days
Thibodeaux[27] USA	Retrospective case series, n=5	Impending respiratory failure, imminent intubation	NR	Type of chamber not reported. 203 kPa, 90 min. Mean five HBOT sessions (range 1−6)
Zhong [28] China	Case report, n = 1	Critically ill with pneumonia and tracheal intubation, confirmed COVID-19 from tracheal aspirate	NR	Multiplace chamber, 162−182 kPa for 70–100 min. Four HBOT sessions