Study population
In total, this subgroup analysis included 2359 patients from the COVIP study with a documented ADL and CFS (see Fig. 1). Most of the patients did not show any dependence in their daily living prior to hospital admission (80% ADL 6, Fig. 2A), although frailty in CFS was distributed more heterogeneously (Fig. 2B), most patients lived without severe frailty (81% CFS < 5, Fig. 2).
Baseline characteristics of patients with disability compared to patients without disability
Patients without significant impairment in the Activities of Daily Living (ADL 6) were predominantly male (74%, p < 0.001), younger (75 years (IQR 4) vs. 78 years (IQR 5), p < 0.001), less frail (CFS 3 (IQR 1) vs. 5 (IQR 2), p < 0.001) and significantly less affected by comorbidities (Table 2). In contrast, with increasing disability patients were older, more frail and had significantly more comorbidities. SOFA score on admission was significantly lower in patients with high ADL (ADL 6: 5 (IQR 3); ADL < 6: 7 (IQR: 4), p < 0.001) (Fig. 3).
Intensive care treatment and outcome of patients with disability compared to patients without disability
During intensive care treatment, patients with pre-existing disability received significantly less invasive mechanical ventilation (67 vs. 75%, p = 0.001), tracheostomy (12 vs. 20%, p < 0.001), vasoactive drugs (62 vs. 72%, p < 0.001), but more renal replacement therapies (19 vs. 14%, p = 0.006), and non-invasive ventilation (31 vs. 23%, p < 0.001). Limitations of life-sustaining therapy occurred significantly more often in patients without disability. Patients with disability suffered from significantly increased crude ICU- (62 vs. 45%, p < 0.001), 30-day (66 vs. 47%, p < 0.001), and 3-month mortality (71 vs. 53%, p < 0.001, Figs. 4 and 5). Using an ADL of less than 5 as cut-off resulted in similar outcomes (see Fig. 6).
In the mixed-effects Weibull proportional hazard regression, ADL was associated with 3 months mortality as a continuous variable (aHR 0.88 (0.82–0.94, p < 0.001)). This means that with rising ADL (= declining disability), the risk for mortality decreased. As binary variable, an ADL < 6 (“disability”) was associated with an increased 3 months mortality (aHR 1.53 (1.19–1.97, p 0.001), and an ADL of < 5 (aHR 1.57 (1.21–2.03, p 0.001), Table 3).
Baseline characteristics of patients with frailty compared to patients without frailty
Patients with high frailty (CFS ≥ 5) were as well predominantly male (61%, p < 0.001), older (78 years (IQR 5), p < 0.001) and had significantly more comorbidities (Additional file 1: Table S1). Therefore, with decreasing frailty patients were younger and had less comorbidities. Like patients without disability, SOFA score was lower in patients with a low frailty score (CFS < 5: 5 (IQR: 3); CFS ≥ 5: 7 (IQR: 4, p < 0.001). After admission to the ICU, invasive mechanical ventilation (66 vs. 75%, p < 0.001) and tracheostomy (11 vs. 20%, p < 0.001) occurred significantly more often in patients without pre-existing frailty. There was no difference regarding the use of vasoactive drugs. Patients with pre-existing frailty received significantly more non-invasive ventilation (30 vs. 23%, p = 0.005) and more renal replacement therapies (21 vs. 14%, p < 0.001). Frail patients evidenced a significantly increased crude ICU- (65 vs. 45%, p < 0.001), 30-day (71 vs. 46%, p < 0.001), and 3-month mortality (77 vs. 52%, p < 0.001).
Comparison of patients without disability and frailty, with disability or frailty, and of patients with frailty and disability
When dividing into the three groups no disability/frailty, either frailty or disability and frailty and disability, the results were similar: patients with frailty and disability were older (78 years (IQR 5), p < 0.001) and had significantly more comorbidities compared to the former groups (Table 2). Even though there was no difference in SOFA score between patients without disability and frailty and patients with disability and frailty (non-frailty, no disability: 5 (IQR: 3); frailty and disability: 8 (IQR: 4), Table 2), the study shows individually that patients with high independence in daily living evidenced lower scores of organ failure on admission and patients without frailty evidenced lower scores of organ failure on admission.
There were significant differences both in short- and long-term outcome: combining ADL and CFS to three groups (no disability/frailty, either frailty or disability, frailty and disability) resulted into the following.
The two variables frailty and disability as well as either frailty or disability were significantly associated with the 3-month mortality (Table 4): suffering from frailty or disability, was an independent risk factor (aHR 1.88 (1.47–2.40, p < 0.001)) but the highest risk was found for patients with both frailty and disability (aHR 1.94 (1.39–2.71, p < 0.001)). Patients with no frailty and no disability evidenced a significantly lower mortality (ICU-mortality 45%, 30-day mortality 46%, 3-month mortality 52%, p < 0.001), patients with frailty and disability the highest mortality (ICU-mortality 67%, 30-day mortality 72%, 3-month mortality 78%, p < 0.001, Table 3, Figs. 5 and 6). Patients who suffered either from frailty or disability were in-between (ICU-mortality 55%, 30-day mortality 63%, 3-month mortality 68%, p < 0.001). Therefore, with the detection of both ADL and CFS a subset of patients with an almost 80% 3 months mortality can be identified.