Methods

This is a prospective study with follow-up until 90 days after cardiac surgery. The protocol for this study has been published. (8)

Study population and data sources

Patients having undergone cardiac surgery from April 2013 to 2014, aged 18 or above were eligible for inclusion. Information were obtained through national Danish registers for the total cohort: the Danish National Patient Register (DNPR) (cardiac surgery procedures, length of hospitalisation) (9), the Danish Civil Registration System (date of birth, sex, cohabitation status, migration, vital status) (10), Danish Education Registers (educational level) (11) and the Danish Register on Personal Income (disposable income) (12). Information on socio-economic factors included educational level, income and cohabitation status. Educational level was categorised as basic school (≤10 years), upper secondary or vocational education, and higher education. Income was categorised in three groups according to median; ≤50%, >50%-150%, and >150%. Cohabitation was defined as being married or living with a partner. Living alone included singles, divorced and widowed.
Furthermore, analyses were done for a nested subsample of patients that participated in the national cross-sectional survey DenHeart (13). All patients discharged from a Danish Heart Centre were asked to fill out a questionnaire at hospital discharge to evaluate patient reported outcomes, reporting information on health-related quality of life (HRQoL), emotional and cognitive functioning, as well as questions about health behaviour. (13).
Information on EuroSCORE and length of ICU stay was received from two clinical databases (14).

Patient-reported outcomes (DenHeart)

HRQoL was measured using the 12-Item Short-Form Health Survey (SF12) and the HeartQoL questionnaires.
The (SF-12) is a generic measure of self-rated health constituting a measure of mental (MCS) and physical (PCS) health. Higher scores (0-100) indicate better perceived health. (15) As recommended the cut-off was set as the mean minus one standard deviation, using the Danish normed score (16).
The HeartQol is a disease-specific tool, scored from 0 (poor) to 3 (best) (17). Scores are summarized in a global, a physical, and an emotional subscale score. For this study HeartQol quantities were converted to binary quantities based on the median score. Both the SF-12 and the HeartQoL questionnaires have a 4 week recall period.
Emotional and cognitive perceptions were measured by the Brief Illness Perception Questionnaire (B-IPQ). Higher scores (0-10) indicates stronger perceptions. (18). No clear cut-offs for screening have been determined for B-IPQ. To reduce the degrees of freedom only the summary score was included for the main analyses in three categories based on the 25th and 75th quartile in the studied sample.
Loneliness was assessed by two ancillary questions, which have previously been used and tested in the Danish National Health Survey (19). One question concerned whether patients experienced having someone to talk to if they needed support or were having problems, and the second question if they were alone, though preferring to be with others.
For health behaviour, patients reported status of current or previous smoking behaviour and alcohol intake during a typical week, as well as, current height and weight.

Outcomes

Mortality is a reliable and clinically important outcome in cardiac surgery; however, duration of hospitalisation and stay in the ICU are common endpoints in cardiac surgical studies. The ICU stay is a standard component of the treatment and provides an indication of the patient’s recovery profile and is in effect a composite measure of the entire perioperative process. (20). Readmission is frequent, why it is an outcome with significant health and economic implications. Readmission rates are about 15% at 30 days after discharge (21,22), but varies greatly after 30 days from 19 to 56% (22,23). Thus, four outcomes were included, 1) death within 90 days of cardiac surgery, 2) prolonged stay in the ICU (≥ 72 hours), 3) prolonged hospital admission (≥ 10 days) and 4) readmission within 90 days from the time of cardiac surgery. Each outcome was evaluated in separate models.
Death
‬From the Danish Civil Registration System information on all-cause mortality within 90 days from cardiac surgery was obtained.
Prolonged length of stay
Length of stay was included as number of days in the ICU (LOS-ICU), as well as total length of hospital stay (LOS-HOSP). Length of hospital stay, and ICU stay were dichotomised to designate normal and prolonged length of stay. There is no consensus on the definition of prolonged length of stay in hospital following cardiac surgery. Previous studies have adopted the 75th percentile of the length of stay distribution, while others have defined prolonged length of stay as hospitalisation for 10 or more days following cardiac surgery (24,25), which was used in this study.
In previous studies, prolonged length of stay in the ICU has been defined as from >24 to as much as >96 hours (24,26–29). For the present study, based on the existing literature and clinical framework, prolonged length of stay in the ICU was defined as >72 hours.
Readmission
Information on rehospitalisation was obtained from the DNPR and was included as a dichotomous outcome of readmission within 90 days following cardiac surgery.

Statistical analysis

Baseline characteristics at time of admission were described using means and standard deviations (SD) for continuous measures and percentages for categorical measures.
Initially, logistic regression analyses were conducted to investigate the association between each candidate predictor variable and outcomes for both the total and DenHeart population. Using logistic regression models, we estimated odds ratio (OR) for death, readmission, LOS-ICU and LOS-HOSP adjusting for (1) age (10 years intervals) and sex, and (2) EuroSCORE I.
The number of missing values in the register-based data was low for educational level (n = 110 (3%)) and income (n = 28 (<1%)), however, to determine the best model based on variable selection, data were imputed, by assigning missing for educational level to basic education and missing for income to the median value. For the DenHeart population of 982 patients, 456 patients had missing data in one or more variables. Thus, single mean imputation for each item was conducted for continuous variables whilst for categorical variables (smoking and loneliness), imputations were done by assigning missing to the category most frequently occurring, since missingness was <5% (see Supplementary Table 1).
To determine the incremental value of each candidate predictor variable, each of the predictor variables were excluded separately in a multiple regression model by using an automated backwards selection procedure with a set liberal significance level of 0.10. EuroSCORE was maintained in the models. The Receiver Operating Characteristic (ROC) curve including Area Under the Curve (AUC) and Brier score were used to determine discrimination and calibration, respectively (30,31).
All analyses were conducted using SAS version 9.4.