We identified 3176 patients who were admitted between March 1 and April 30, 2020 to one of the Northwell Health System hospitals, and who were mechanically ventilated. Of these, 2020 patients were COVID-19 PCR positive and 1554 met inclusion criteria with reliable lung compliance data (Fig. 1). Data for patients who were excluded are presented in the supplement (Table S1). Discharge disposition for index hospitalization was available for all patients except two patients.
Lung Compliance categories
The average lung compliance for the whole cohort was 24·44 mL/cm H2O (SD 11·69). Frequencies per decile of compliance are presented in Fig. 2. Based on clinical observations, the Northwell ARDS Collaborative chose to categorize the cohort into three categories: very low compliance (< 20 mL/cm H2O); low-normal (20–50 mL/cm H2O) and high (> 50 mL/cm H2O) measured by the dynamic compliance over the first 24 hours of intubation in the setting of paralytics or deep sedation. There were 538 (34·6%) patients with very low compliance; 982 (63· 2%) with low-normal compliance, and 34 (2·2%) with high compliance. Given the very small sample size in the higher compliance category, comparators of prevalence and exploratory statistical testing is limited to the very low versus low-normal compliance groups. The average median difference between static and dynamic compliance overall was 6·41 mL/cm H2O (IQR 3·16, 11·42, n = 1053). For the very low compliance group median difference was 4·60 mL/cm H2O (IQR 2·05, 8·10, n = 429); and for the low-normal group 7·89 mL/cm H2O (IQR 4·19, 12·64, n = 610).
Patient demographics are detailed in Table 1. Overall, average age was 65 years, 32% were female, 35% were white, and the average Charlson comorbidity index was 4·9 (SD 3·3) (corresponding to a roughly 52% estimated 1-year survival).8 The MEWS score was also high (4·1, SD 1·9) (corresponding to a roughly 12·7% chance of ICU admission or death within 60 days).9 There was a greater percentage of females in the very low compliance category (43·7%, vs. 24·9%, low-normal; and 29·4%,high), and more were non-white/multi-racial. The most common comorbidity was hypertension (65·1%, n = 1012) and diabetes (43·4%, n = 675). The overall cohort included 16% (n = 251) with BMI indicating extreme obesity (BMI > 40).
Almost all patients (89%, n = 1383) received hydroxychloroquine, 62% (n = 963) received azithromycin, 82% (n = 1278) received steroids, 52% (n = 815) received paralytics, and 49% (n = 769) were proned. IL-1 and IL-6 inhibitors were given to 30% (n = 475) of the patients, while 7% received convalescent plasma (n = 109). During the first 48 hours, 83·5% (n = 449) received at least one vasopressor in the very low compliance category, compared to 77·2% (n = 758) in the low-normal group. (Table 2)
Time to Intubation
On average, COVIDARDS patients were intubated within 50·8 hours (IQR 7·5,123·7) from the time of admission. Patients in the very low compliance group had the longest time between admission and intubation, 107 hours (IQR 26·3, 238·3), compared to 37·9 hours (IQR 4·8, 90·7) in the low-normal compliance group. Prior to intubation, 77% (n = 1196) of patients were receiving oxygen supplementation via non rebreather masks, with 2·1% (n = 33) on HFNC, and 3·3% (n = 51) on NIV, which reflects infection control practices at the time discouraging NIV use. (Table 2)
P/F ratios and blood gas results
The average blood gas pH in the 24 hour period before intubation was 7·29 (SD 0·14), and PaCO2 was 51·4 mmHg (SD 19·31). (Table 3) Patients in the very low lung compliance category had higher levels of PaCO2 and lower mean arterial pH. ABG was not performed in 70·8% cases during the 12 hours prior to intubation. The overall mean derived P/F ratio in the 12 hours prior to intubation was 95 (SD 85), which was lowest for those in the high compliance group (P/F 66, SD 33) (Fig. 5). When including PEEP in the calculation of P/F ratio, the P/FPEEP (PFP)10 also appeared lowest for those in the highest compliance category. There was no correlation between P/F and compliance. (Fig. 2)
In the 12 hours post intubation, the mean ABG P/F ratio was 155·33 (SD 88·47) for the overall group, and similar across groups. (Fig. 2) Those in the very low compliance categories received higher FiO2 for longer periods of time prior to intubation (in the setting of also having longer average time to intubation). Prior to intubation, the group with normal to high compliance were exposed to FiO2 > 60% for 57·53 hours (IQR 6·68, 181·66) compared to 12·82 hours (IQR 0·45, 44·59) in the low-normal category (Table 3).
Duration of intubation
The average duration of intubation was 14·25 days (SD13·69). Among those who survived, median duration was 11·9 days (IQR 4·8, 29·3) and mean was 18·26 (STD 16·91) days. Among those who died, median duration was 8·8 days (IQR 4·0, 17·1) and mean was 12·32 (STD 11·35). It should be noted that the length of intubation for survivors is an underestimation due to the fact that 19.8% of survivors were discharged while still mechanically ventilated.
The general trend of derived P/F ratios paralleled the ABG P/F ratios prior to intubation, although with high degree of variability among the ABG P/F ratios prior to intubation (wide 95% CI, shaded gray), due to many ABGs not being performed. Post-intubation, where many more ABGs were drawn, the two curves diverge for the first 48 hours, and then trend together over time. (Fig. 4)
Oxygenation Index (OI)
The mean OI for the entire cohort in the 24 hours after intubation was 11·12 (5·67), and was slightly worse in the very low compliance group 12·29 (5·70).
Lung Mechanics and Ventilator Settings
Lung compliance for the whole cohort decreased over time, with a steeper trajectory among those who died (Fig. 3). This was seen more clearly in the low-normal compliance group and high compliance groups likely secondary to the ‘floor effect’ (very low compliance numbers starting at a very low value) (Figure S·4). On average, patients received 6·9 cc/kg (SD 1·2) of ideal body weight as the ventilator setting. (Table 4) As expected, the very low lung compliance group had the highest average peak airway pressure, plateau pressure, and resulting driving pressures. The mean driving pressure for the whole cohort was 16·24 (SD 6·37), and 20·54 (SD·77) for the very low compliance group compared to 13·38 (SD 3·88) for the low-normal compliance group.
Proportion of Deaths and Discharge to Home
Table 5 presents the disposition status of patients based on the index hospitalization which was available for all patients (unknown for 2 patients). Overall, of the 1554 patients, 67·5% (n = 1049) died during the index hospital stay. Of the 505 patients who survived to hospital discharge, 100 (19.8%) were discharged while still on a mechanical ventilator. Of those who survived, 44·4% (n = 224) were discharged home and the rest to rehabilitation or longer term care facilities. The very low compliance group had the highest mortality (70·1% versus 66·2%) and fewer survivors were discharged home (41·0% versus 46·0%).