Background: Intensive care unit (ICU) readmission after cardiac surgery is believed to be associated with higher in-hospital mortality and may predict poor outcomes. ICU readmissions use resources and increase treatment costs.

Aim: To determine reasons for readmission to ICU, evaluate outcomes in these patients, and identify factors predisposing to the need for readmission to ICU.

Methods: We retrospectively investigated a total of 2076 consecutive adult patients who underwent either isolated coronary artery bypass grafting or a valve procedure or combination of both and were discharged from our ICU between January 2008 and December 2010. To identify the factors that increase the risk of readmission to ICU, we used the dominance-based rough set approach (DRSA) which is a methodology of knowledge discovery from data. The knowledge has the form of “if… then…” decision rules relating patient characteristics to the risk of readmission to ICU.

Results: Of 2076 patients discharged from ICU, 56 (2.7%) required a second stay in the ICU (study group) while 2020 patients needed no readmission to ICU (control group). The main causes of readmission were haemodynamic instability (28.6%, n = 16), respiratory failure (23.2%, n = 13), and cardiac tamponade or bleeding (23.2%, n = 13). The mean length of stay (LOS) in the general cardiac ward after primary discharge from ICU until readmission was 3.5 ± 4.2 days. The mean LOS in ICU after readmission was 12.5 ± 21.2 days. Postoperative complications occurred more frequently in readmitted patients (10.2% vs. 48.2%, p < 0.0001). In-hospital mortality was significantly higher in the study group (15 [26.8%] vs. 23 [1.1%] patients, p < 0.0001). As a result of applying the DRSA methodology, the algorithm generated decision rules categorizing patients into high and low ICU readmission risk. Advanced age, non-elective surgery and the length of initial ICU stay after the surgery were the factors of greatest importance for the correct categorisation of patients in the study group.

Conclusions: The most common cause of readmission to ICU is haemodynamic instability. Postoperative complication and in-hospital mortality rates are significantly higher in patients readmitted to ICU. Factors most commonly predisposing to readmission to ICU after cardiac surgery included advanced patient age, non-elective surgery, and longer initial stay in ICU after the surgery.