Participants
There were 1045 observations found for 153 episodes in 152 unique patients. The episodes comprised 71 admissions of women and 82 admissions of men.
The median age was 60, with a range of 24–87, and an interquartile range of 53–68. The age distribution is shown more completely in the histogram in Fig. 1.
A total of 58 episodes ended in death, giving a crude mortality rate of 37.9% (95% confidence interval 30.3% − 46.1%) per admission.
The median length of stay in hospital was 11.8 days, with a range of 0.2–115 days and interquartile range of 8–18.5 days.
The mean age of those who survived was 55.7, whereas those who died had a mean age of 65.1 (p < 0.001 by Mann-Whitney). Survivors were therefore around 10 years younger than non-survivors on average. The relative age distribution of survivors and non-survivors is depicted in Fig. 2.
Association between age and mortality
Age had a significant association with mortality. Logistic regression (Fig. 3) showed that the odds of death approximately doubled for every extra 8.0 years of age, with the survival odds being 1 (i.e. 50% probability of survival) at an age of 66.5 years.
Association between sex and mortality
31 out of the 71 women died (43.7%) versus 27 of the 82 men (32.9%). This difference was non-significant by Chi Square test (p = 0.231).
A comparison of the Kaplan Meier survival curves for men and women is shown in Fig. 4, and does appear to show a difference, with the hazard ratio for men being 0.606, but the effect size just misses statistical significance (CI 0.352–1.044, p = 0.071). This effect does not appear to be attributable to age, as there was less than 3 months’ difference between the mean age of men and women in the cohort.
ICU admission and mortality
Of the patients who died after admission to cHDU, 27 of these died in ICU. The other 31 died either in cHDU or in a step down ward.
Use of CPAP and HFNO
142 patients had either continuous positive airway pressure (CPAP) applied via a facemask, or had high flow nasal oxygen (HFNO) for at least part of their time in cHDU.
The predominant modality was CPAP, with 95 (62.1%) patients receiving CPAP only as their main modality. Only 17 (11.1%) patients had HFNO only, with 30 (19.6%) switching between modalities for various reasons, and 11 (7.2%) not receiving either.
Of the 11 patients who did not receive an AGP, four patients were Covid positive with complex HDU needs other than respiratory support. Four patients had been stepped down from ICU, two were thought to require an AGP but stabilized quickly on admission and never required one. There was also one patient who turned out not to have Covid at all.
These findings are summarised in Table 2.
Table 2
Modes of respiratory support used
CPAP only (+/- HFNO for breaks)
|
HFNO only
|
Switched between CPAP and HFNO as primary device
|
Had neither HFNO nor CPAP
|
95
|
17
|
30
|
11
|
Association between level of respiratory support and survival
Of the 125 patients who received CPAP at some point during their stay, there was a strong correlation between the need for maximum supplemental oxygen (30 litres per minute) and subsequent mortality. Only six out of 41 patients who required 30 litres per minute of supplemental oxygen with CPAP survived, a survival rate of 14.6%. Of those who survived after reaching 30 litres, there were two patients who did not receive treatment in ICU. These both reached 30 litres for relatively short periods.
Conversely, patients who required less than 30 litres per minute had an overall survival rate of 81.0%, though only nine patients reached a peak of between 20 and 30 litres per minute and only five (55.6%) of them survived. The distribution of peak oxygen requirements, and their associated outcomes, is shown in Fig. 5.
This can be shown in percentage terms to get a better feel for the survival rate according to peak oxygen requirement (Fig. 6).
Again, we can perform a regression of peak supplemental oxygen with survival to show this correlation. The result is shown in Fig. 7.
We can see that a peak oxygen requirement of above 21 litres per minute makes death the most likely outcome. The log odds of death increase by 0.16 per litre of oxygen required, amounting to a doubling of the odds of death for every extra 4.2 litres of oxygen required per minute (p < 0.001).
A similar pattern was seen with patients on HFNO. Those with a peak FiO2 below 0.9 had a survival rate of 72.7%, whereas above 0.9 the survival rate fell to 41.7% (Fig. 8).
If we consider high respiratory support as being an FiO2 of 0.9 or more or CPAP with more than 15 litres of supplemental oxygen, then we can further examine the outcomes based on the number of days that a patient spent on high respiratory support (Fig. 9), where we can see that those with the greatest time on high respiratory support had the worst outcomes.
ICU transfers and outcomes
44 patients (28.8%, CI 21.9% − 36.7%) went to ICU. Of these, 17 (38.6%) survived.
Of these 17 ICU survivors, in two cases, the Covid swab was negative and the final diagnosis was not Covid pneumonitis. A further three patients had radiological evidence of bacterial pneumonia in addition to Covid at the time of their admission to ICU.
This means that 12 patients out of the 39 (30.8%) patients with uncomplicated severe Covid pneumonitis survived their ICU admission. Even among this group, there is significant residual morbidity. One patient went home on domicillary oxygen; another with a Zimmer frame for mobility. One patient has since been readmitted with right heart failure due to post-Covid pulmonary fibrosis. Most have community rehab needs and some require ongoing community mental health input.
Total ICU admissions during the study period and ICU avoidance
There were 57 admissions in total to our ICU during the study period. These comprised 53 new admissions and four re-admissions. Our ICU were heavily pressured during the majority of this period, with full occupancy plus expansion beds opened elsewhere in the hospital. During this time, there were 41 patients treated in cHDU with levels of respiratory support exceeding the threshold that would normally mandate a transfer to ICU for IMV - that is, being on CPAP plus more than 15 litres per minute of oxygen, or being on HFNO with greater than 90% FiO2. These are patients who typically could not be oxygenated on a ward without NIV or IMV. Although some of these patients may not have been deemed appropriate for IMV in ICU, this sets an upper limit of a 42% reduction in ICU admissions during the study period as a result of the facilities available in cHDU.
Although these patients are likely to have had a shorter length of stay in ICU than those who were actually taken for IMV, this still represents a large reduction in the burden placed on ICU resources.
ICU admissions and maximal CPAP
None of the 22 patients who were on the maximal 30 litres per minute of oxygen for more than 24 hours survived without going to ICU. Of the 14 patients on maximum CPAP for more than 24 hours who were taken to ICU, none who were on maximum CPAP for more than 3 days survived. There is likely an element of selection bias here, and the numbers are small, but in our cohort it seems that if a patient was on maximum CPAP for more than 24 hours, the prognosis was grim.
Complications
Six of the patients treated with CPAP (5%) developed a pneumomediastinum, and one of these cases was associated with a small pneumothorax. Although two of these patients subsequently died, the pneumomediastinum was not thought to be a contributory cause. However, when pneumomediastinum or pneumothorax was identified in any of these patients, CPAP was discontinued and HNFO was used as an alternative. The increased incidence of pneumomediastinum in mechanically ventilated patients has already been discussed in the literature, and there are case reports of patients with COVID-19 pneumonitis developing pneumomediastinum even without IMV or NIV, so it is not clear that CPAP was the direct cause of these complications [16, 17].