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# Scottish Out-of-Hospital Cardiac Arrest data linkage project: initial results

**8 Aug 2017**

This report presents the initial results of the Scottish Out-of-Hospital Cardiac Arrest data linkage project (OHCA data linkage project).

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48 page PDF

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### Survival following OHCA

Survival figures for OHCA where resuscitation was attempted are shown in table 2. The percentage of patients alive at 24 hours, 30 days and 1 year is given. In addition, return of spontaneous circulation ( ROSC) is an outcome measure frequently reported in other OHCA registries and publications and therefore we have included it here for comparison. Definitions for ROSC vary. The Scottish Ambulance Service record ROSC if a patient regains a palpable pulse which is sustained until arrival at the Emergency Department, a proxy for the Utstein definition of 'sustained ROSC' (palpable pulse for greater than 20 minutes).

Table 2 shows survival figures after OHCA in Scotland based on two different sets of assumptions in analysis of the data. In column A, we have used only patients where data could be linked (ie around 75%) in the denominator, yielding an overall 30 day survival of 8.4%. For the column B, the assumption has been made that patients who were not part of the linked dataset died shortly after their OHCA, which gives a lower 30-day survival figure of 6.2%. The presupposition here is that patients pronounced dead in the community without being transported to hospital are more likely to have insufficient data recorded for probabilistic data linkage ( see the methodology section for further information). The actual 30-day survival percentage will lie between the two figures, but is probably closer to that reported in column B.

**Table 2: Outcomes after
OHCA**

Outcome | A: Proportion based on linked dataset as denominator (%) | B: Proportion using all arrests as denominator and assumption that insufficient data for linkage = death (%) |
---|---|---|

ROSC | 16.7 | 12.2 |

Alive at 24hrs | 12.7 | 9.3 |

Alive at 30 days | 8.4 | 6.2 |

Alive at 1 year | 8.0 | 5.9 |

In line with other published reports, 30-day survival is used as a proxy for survival to discharge from hospital. Detailed data for hospital discharges will be available for analysis in the future.

In the dataset, just over 800 (8.6%) cases fulfilled criteria
for inclusion in the 'Utstein comparator group' meaning that the
OHCA
cases are witnessed, presented with a shockable presenting heart
rhythm and bystander
CPR was
started. The 30 day survival in this group was 23.2%, substantially
higher than for the whole cohort. The international literature
reports survival to hospital discharge in this group between 18.5%
and 52%
^{
[1],
[33],
[42]}.

**Comparison of Scotland's survival rates with other
registries**

The results of the
EuReCa ONE
project were mentioned earlier - this project demonstrated
significant differences in the incidence of
OHCA,
and also survival rates across Europe. The percentage who had
ROSC
among the 27 included countries ranged from less than 10% to over
40%. The percentage with
ROSC
in Scotland is at the lower end of this spectrum. Similarly, our
30-day survival sitting between 6.2% - 8.4% (see Table 2) puts
Scotland among the countries with the lowest survival rates, lower
than the overall European survival rate of 10.3% reported by
EuReCa ONE.
Substantially higher 30 day survival rates are found in Finland,
the Netherlands and Switzerland
^{
[11],
[43]}. Comparable proportions of people
who survived 30 days were found in Denmark and other parts of the
United Kingdom
^{
[17],
[19]}. It is important to reflect on
these differences in the context of the different population
included in different datasets (eg some studies or registries only
include
OHCA
cases with a cardiac origin, while others include cases with other
aetiologies such as drugs-related cases or cases due to trauma,
some report only witnessed
OHCA
cases, and others also include unwitnessed cases), and the
differences in data collection and data processing (eg handling of
missing data)
^{
[15]
}. The
EuReCa ONE
project collaborators themselves also identified sources of
variability in data processing and collection.

**Using logistic regression to make like-for-like
comparisons**

In addition to overall survival figures we have made a series of comparisons of OHCA survival between different groups using logistic regression techniques. This makes it possible to adjust for factors - for example age, sex and deprivation - that might differ between sub-groups and confound any associations. Logistic regression techniques adjust for these factors enabling us to make like-for-like comparisons.

The results of this type of modelling should be interpreted as relative measures. For example when the survival between males and females is compared using logistic regression, the risk of death for males is used as the reference category and the estimated risk for females is expressed relative to that of males.

*(Technical note: we recognise that odds ratios over-estimate
relative risks when outcomes are common, ie occur in greater than
15% of the study population
^{
[44]
}. For accessibility, the results here are described in terms
of higher and lower risk but are actually calculated as higher and
lower odds. We plan further analysis to compare the results of both
approaches
^{
[45]
}. Using logistic regression models is in line with the
majority of published papers in this research field thus enabling
comparisons.)*

**Effect of age and sex on survival of
OHCA
patients**

The previous section showed that overall, males in the cohort
are more likely than females to be affected by an
OHCA. In
contrast, the percentage of females who survive after an
OHCA is
lower compared to males (see Figure 8, crude analysis). More
detailed analysis of our data have shown that women are more likely
to present with a less favourable heart rhythm (non-shockable). In
the linked dataset, the proportion with shockable presenting heart
rhythm was 17.5% among females and 29.8% among males. The adjusted
analysis presented in Figure 8 show that taking these differences,
and differences in age distribution into account (adjusted
analysis), there is no longer any sex difference in survival. This
finding is in line with other published work
^{
[46]
}.

**Figure 8: Results of logistic regression analysis examining
the effect of sex on risk of death at 30 days after
OHCA.
The diamonds refer to the estimated odds ratios for 30 days
mortality and the bars refer to the 95% confidence intervals. Males
are the reference category. Values above 1 for the lower limit of
the confidence interval suggest a statistically significant higher
risk for 30 days mortality for women in the crude estimates that is
not observed for the adjusted estimates.**

In Figure 8, the crude data shows that a higher proportion of females die within 30 days of their OHCA. However the sex difference is no longer present after adjusting for age and presenting heart rhythm as illustrated by the fact that the 95% confidence intervals include the value of 1.

**Figure 9: Results of logistic regression analysis examining
the effect of age on risk of death at 30 days after
OHCA.
The age group <45 years is the reference category. The diamonds
refer to the estimated odds ratios for 30 days mortality and the
bars refer to the 95% confidence intervals. Values above 1 for the
lower limit of the confidence intervals for the age groups 65-75
and >75 suggest a statistically significant higher risk for 30
days mortality patients older than 65 years compared with patients
younger than 45 years. The adjusted analysis is adjusted for
sex.**

In Figure 9, both the crude and adjusted analysis show that
patients from 65 years and older are more likely to die within 30
days after their
OHCA
compared with younger patients. This finding is consistent with
other published figures
^{
[46]
}.

**Effect of
SIMD
status on survival of
OHCA
patients**

Figure 3 has already shown that people living in more deprived areas are more likely to experience an OHCA. Further analysis show that patients from deprived areas are also less likely to survive following OHCA. Figure 10 shows the likelihood of 30 day survival for each of the first four quintiles of SIMD compared with people from the least deprived areas ( SIMD5). People who lived in the most deprived areas of Scotland ( SIMD1) are 43% more likely to die within 30 days after their OHCA compared with people who lived in the least deprived areas in Scotland ( SIMD5).

**Figure 10: Results of logistic regression analysis
examining the effect of
SIMD
quintiles on risk of death at 30 days after
OHCA.
SIMD5
(least deprived) is the reference category. The diamonds refer to
the estimated odds ratios for 30 days mortality and the bars refer
to the 95% confidence intervals. Values above 1 for the lower limit
of the confidence intervals for
SIMD1
suggest a statistically significant higher risk for 30 days
mortality for patients in
SIMD1
compared with patients from the least deprived areas. The adjusted
analysis is adjusted for sex, age and urban rural
classification.**

Figure 10 shows that in both the crude and adjusted analysis patients who lived in SIMD1 are more likely to die within 30 days after their OHCA compared with patients who lived in the least deprived areas.

The effect of SIMD on survival is most pronounced among males. Furthermore for both males and females the effect is greatest among younger patients (younger than 45 for females and younger than 65 for males). Our data appear to show that the effect of deprivation may be explained by less bystander CPR and higher likelihood of comorbidities among young OHCA patients living in SIMD1 areas compared with people of similar age from less deprived areas (these results are not shown here).

The underlying explanation of these
SIMD-related
differences will be multifactorial, but lifestyle factors such as
smoking are likely to play a significant role
^{
[32],
[47]}.

More in depth analysis focusing on the association between
SIMD and
the incidence and outcomes of
OHCA are
planned.

**Effect of urban and rural environment on survival of
OHCA
patients**

In Table 1, the incidences of
OHCA are
shown for the six urban rural categories
^{
[28]
}. Note, these percentages are based on only the patients in
the linked dataset and are therefore subject to the caveats already
stated. Patients from areas classified as remote accessible towns,
large urban areas and other urban areas have the highest incidences
of
OHCA,
and remote rural areas the lowest.

Given the small numbers in some groups, a dichotomous (two-category) urban-rural measure was created (Table 3) for the following analysis of differences in 30-day survival.

**Table 3: Aggregation of urban and rural
categories**

Six categories |
Two categories |
---|---|

Large urban areas |
Urban |

Other urban areas |
Urban |

Accessible small towns |
Rural |

Remote small towns |
Rural |

Accessible rural |
Rural |

Remote rural |
Rural |

**Figure 11: Results of logistic regression analysis
examining the effect of living in urban or rural areas of Scotland
on risk of death at 30 days after
OHCA.
Urban area is the reference category. The diamonds refer to the
estimated odds ratios for 30 days mortality and the bars refer to
the 95% confidence intervals. Values above 1 for the lower limit of
the confidence intervals for rural category in the adjusted
analysis suggest a statistically significant higher risk for 30
days mortality in patients who lived in rural areas compared with
urban areas. The adjusted analysis is adjusted for sex, age and
SIMD.**

In Figure 11, the adjusted analysis shows that patients living in a rural area are more likely to have died 30 days after OHCA compared with patients living in an urban area.

People living in rural areas are less likely to survive an OHCA. The adjusted analysis shows that this effect persists after taking into account the age, sex and SIMD distributions. Further analysis will investigate the effect of other potential factors such as heart rhythm and bystander CPR.

**Effect of Initial heart rhythm on survival of
OHCA
patients**

As mentioned in the previous section, a shockable initial heart rhythm after OHCA is predictive of increased survival. Taking all patients with a known initial heart rhythm into account (91% of the total dataset), we found that people who present with a shockable initial heart rhythm are 88% more likely to survive until 30 days after the OHCA compared with patients with a non-shockable rhythm. These results are based on a logistic regression model adjusted for age and sex (adjusted odds ratio 0.12 (95% confidence interval 0.10-0.14, further adjustment for SIMD and urban rural category did not change the estimates).

**Effect of bystander
CPR on
survival of
OHCA
patients**

A positive effect of bystander CPR on survival was found in OHCA cases where the first ambulance arrived within eight minutes after one of the Ambulance Control Centres received an OHCA call. In figure 12, the likelihood of survival with and without bystander CPR is compared. This figure shows that on average, cases with bystander CPR are 32% more likely to be alive 30 days after the OHCA.

In Figure 12, both the crude and age, sex, SIMD and urban rural category adjusted analysis show that patients who receive bystander CPR are more likely to survive 30 days after OHCA compared with patients who do not receive bystander CPR.

As mentioned previously, we suspect that the available bystander
CPR data
is underreported. Therefore, it is likely that the true effect of
bystander
CPR on
survival is even greater. Furthermore, not all
CPR is
equally effective. A limitation of the current available data is
the lack of timing of the start of bystander
CPR making
it impossible to calculate how long before the arrival of the first
ambulance a bystander started
CPR. Chest
compressions started earlier are likely to be more effective in
preserving life than those started after a significant delay
^{
[35]
}.

Currently work is underway to solve some data issues for arrival times. Therefore, only cases where the first ambulance arrived within eight minutes are included in the current analysis.

**Figure 12: Results of logistic regression analysis
examining the effect of bystander
CPR on
death at 30 days after
OHCA
within all cases where an ambulance arrived within eight minutes.
The group where bystander
CPR was
unknown is reported as the reference category. The diamonds refer
to the estimated odds ratios for 30 days mortality and the bars
refer to the 95% confidence intervals. Values above 1 for the lower
limit of the confidence intervals for the Yes category in the crude
and adjusted analysis suggest a statistically significant lower
risk for 30 days mortality in patients who received bystander
CPR
compared with the group for which the bystander
CPR status
is missing or unknown. The adjusted analysis is adjusted for age,
sex,
SIMD and
urban rural category.**