Appendix
Table 1: Mechanically ventilated admissions (n=104,844) between 2008 and 2010 by critical care unit quartile for annual volume
Quartile 1
Quartile 2
Quartile 3
Quartile 4
(Lower volume)
(Higher volume)
Total number of admissions
13,201
19,014
29,161
43,468
Total number of units
49
48
48
48
Number of mechanically ventilated admissions per year
Median (iqr)
141 (117, 168)
199 (186, 210) 267 (253, 287)
480 (362, 599)
minimum, maximum
33, 175
175, 219
220, 316
317, 809
iqr, interquartile range
Table 2:Baseline characteristics and outcomes for mechanically ventilated admissions in outlier units versus whole cohort, highest
volume quartile and highest volume quartile with outliers removed
Characteristic
Whole cohort
Quartile 4
Quartile 4
Outlier 1
Outlier 2
(without
outliers)
Total number of admissions
104,844
43,468
41,090
471
1,907
Total number of units
193
48
46
1
1
Number of mechanically ventilated
admissions per year
Median (iqr)
220 (175, 320) 480 (362, 599) 434 (353, 528)
775
809
minimum, maximum
33, 809
317, 809
317, 718
Demographics
Age, mean (sd)
60.3 (17.9)
58.7 (18.0)
59.1 (17.9)
61.0 (18.7)
50.5 (19.5)
Male sex (%)
60,422 (57.6)
25,837 (59.4)
24,196 (58.9)
286 (60.7)
1,355 (71.1)
White ethnicity (%)
94,056 (90.0)
37,727 (87.2)
36,095 (88.3)
376 (79.8)
1,256 (65.9)
Acute severity of illness, median (iqr)
APACHE II score
17 (13,22)
17 (12,22)
17 (12,22)
15 (11, 19)
15 (11, 21)
ICNARC Physiology Score
21 (15,29)
20 (15,28)
21 (15,28)
17 (12, 23)
19 (14, 27)
Medical historyΩ
Severe comorbidities (%)
Any prior illness
15,970 (15.2)
7,254 (16.7)
6,973 (17.0)
75 (15.9)
206 (10.8)
Severe cardiovascular disease
1,692 (1.6)
591 (1.4)
586 (1.4)
1 (0.2)
4 (0.2)
Severe respiratory disease
3,456 (3.3)
1,303 (3.0)
1,255 (3.1)
11 (2.3)
37 (1.9)
Renal disease
1,445 (1.4)
852 (2.0)
770 (1.9)
13 (2.8)
69 (3.6)
Chronic liver disease
3,627 (3.5)
1,983 (4.6)
1,926 (4.7)
18 (3.8)
39 (2.0)
Hematologic malignancy
1,649 (1.6)
697 (1.6)
684 (1.7)
3 (0.6)
10 (0.5)
Metastatic disease
1,897 (1.8)
803 (1.9)
772 (1.9)
9 (1.9)
22 (1.2)
Immunological dysfunction
5,049 (4.8)
2,247 (5.2)
2,174 (5.3)
24 (5.1)
49 (2.6)
Activities of daily living (%)
No assistance
79,817 (77.2)
33,267 (77.9)
31,326 (77.6)
394 (85.1)
1,547 (82.7)
Partial assistance
22,678 (21.9)
9,048 (22.0)
8,673 (21.5)
65 (14.0)
310 (16.6)
Total assistance
961 (0.9)
383 (0.9)
366 (0.9)
4 (0.9)
13 (0.7)
CPR (%)
12,584 (12.0)
4,458 (10.3)
4,217 (10.3)
63 (13.4)
178 (9.3)
Admission typeβ (%)
non surgical
66,988 (63.9)
25,648 (59.0)
24,101 (58.7)
361 (76.6)
1180 (61.9)
Surgical
37,856 (36.1)
17,820 (41.0)
16,983 (41.3)
110 (23.4)
727 (38.1)
Location prior to admission (%)
Emergency/urgent surgery
25,942 (24.8)
11,751 (27.0)
11,136 (27.1)
74 (15.7)
541 (28.4)
Elective/scheduled surgery
11,914 (11.4)
6,069 (14.0)
5,847 (14.2)
36 (7.6)
186 (9.8)
Hospital ward
30,646 (29.2)
11,029 (25.4)
10,627 (25.9)
125 (26.5)
277 (14.5)
High dependency unit
4,329 (4.1)
2,211 (5.1)
2,024 (4.9)
61 (13.0)
126 (6.6)
1
Emergency department
Hospital type (%)
University or university affiliated (%)
Length of stay, median (iqr) days
Critical care unit
Acute hospital
Mortality (%)
Critical care unit mortality
Ultimate acute hospital mortality
31,992 (30.5)
12,402 (28.5)
11,450 (27.9)
175 (37.2)
777 (40.7)
52,060 (49.7)
34,152 (78.6)
31,774 (77.3)
471 (100)
1,907 (100)
3 (1,8)
13 (5,29)
3 (1,8)
14 (6,31)
3 (1,8)
14 (6,31)
5 (2, 11)
16 (5, 35)
3 (1, 7)
16 (6, 33)
29,074 (27.7)
38,056 (36.7)
10,659 (24.5)
14,196 (33.0)
10,119 (24.6)
13,478 (33.1)
120 (25.5)
155 (33.3)
420 (22.0)
563 (30.2)
Figure 1: Relationship between annual volume of mechanically ventilated admissions and
ultimate acute hospital mortality in non-surgical admissions compared with
surgical admissions
Volume modelled using fractional polynomials (degree 2) relative to mean volume of 332
admissions per year. Model adjusted for age (fractional polynomials degree 2), sex, ICNARC
Physiology Score (fractional polynomials degree 2), comorbidities, activities of daily living,
CPR prior to admission, location prior to admission, and hospital type. The p- value is testing for
a global interaction between annual volume of mechanically ventilated admissions and
admission type. The dashed lines represent 95% confidence intervals.
Non-surgical
1.8
1.5
1.2
1.0
0.8
P<0.001
Surgical
1.8
1.5
1.2
1.0
0.8
0
200
400
600
800
Volume of mechanically ventilated admissions
Figure 2: Relationship between annual volume of mechanically ventilated admissions and
ultimate acute hospital mortality by lowest PaO2/FiO2 ratio during the first 24
hours of critical care admission
2
Volume modelled using fractional polynomials (degree 2) relative to mean volume of 332
admissions per year. Model adjusted for age (fractional polynomials degree 2), sex, ICNARC
Physiology Score (fractional polynomials degree 2), comorbidities, activities of daily living,
CPR prior to admission, location prior to admission, and hospital type. The p- value is testing
for a global interaction between annual volume of mechanically ventilated admissions and
PaO2/FiO2 ratio. The dashed lines represent 95% confidence intervals.
PaO2/FiO2 = 50
PaO2/FiO2 = 100
1.5
1.2
1.0
0.8
0.6
PaO2/FiO2 = 200
P=0.12
PaO2/FiO2 = 300
1.5
1.2
1.0
0.8
0.6
0
200
400
600
800
0
200
400
600
800
Volume of mechanically ventilated admissions
Figure 3: Relationship between annual volume of mechanically ventilated admissions and
ultimate acute hospital mortality, after excluding outlier critical care units, in
non-surgical admissions compared with surgical admissions
Volume modelled using fractional polynomials (degree 2) relative to mean volume of 321
admissions per year. Model adjusted for age (fractional polynomials degree 2), sex, ICNARC
Physiology Score (fractional polynomials degree 2), comorbidities, activities of daily living,
CPR prior to admission, location prior to admission, and hospital type.
3
Non-surgical
1.8
1.5
P=0.02
1.2
1.0
0.8
Surgical
1.8
1.5
1.2
1.0
0.8
0
200
400
600
Volume of mechanically ventilated admissions
4
800
Figure 4: Relationship between annual volume of mechanically ventilated admissions and
ultimate acute hospital mortality, after excluding outlier critical care units, in
admissions by lowest PaO2/FiO2 ratio
Volume modelled using fractional polynomials (degree 2) relative to mean volume of 321
admissions per year. Model adjusted for age (fractional polynomials degree 2), sex, ICNARC
Physiology Score (fractional polynomials degree 2), comorbidities, activities of daily living,
CPR prior to admission, location prior to admission, and hospital type.
PaO2/FiO2 = 50
1.5
PaO2/FiO2 = 100
P=0.27
1.2
1.0
0.8
0.6
PaO2/FiO2 = 200
PaO2/FiO2 = 300
1.5
1.2
1.0
0.8
0.6
0
200
400
600
800
0
200
400
Volume of mechanically ventilated admissions
5
600
800
Figure 5: Relationship between annual volume of mechanically ventilated admissions and
ultimate acute hospital mortality using restricted cubic splines
Model adjusted for age (fractional polynomials degree 2), sex, ICNARC Physiology Score
(fractional polynomials degree 2), comorbidities, activities of daily living, CPR prior to
admission, location prior to admission, and hospital type.
1.2
Odds ratio (95% CI)
1.1
1.0
0.9
0.8
0
200
400
600
Volume of mechanically ventilated admissions
Fractional polynomial
800
Restricted cubic splines
6
Figure 6: Relationship between annual volume of mechanically ventilated admissions and
ultimate acute hospital mortality after excluding outlier critical care units and
using restricted cubic splines
Model adjusted for age (fractional polynomials degree 2), sex, ICNARC Physiology Score
(fractional polynomials degree 2), comorbidities, activities of daily living, CPR prior to
admission, location prior to admission, and hospital type.
1.2
1.1
1.0
0.9
0.8
0
600
400
200
Volume of mechanically ventilated admissions
Restricted cubic splines
Fractional polynomial
7
800
Figure 7: Frequency distribution of critical care unit’s annual volume of
mechanically ventilated admissions
25
20
15
10
5
0
0
200
400
600
Volume of mechanically ventilated admissions
8
800