In the 30 years since its inception, coronary artery surgery has been found not only to relieve angina and improve the quality of life in patients with ischaemic heart disease , but also to confer a long-term survival benefit in certain patients . Surgical and anaesthetic techniques have evolved over the last decade to allow 'fast tracking' of patients through high-dependency cardiac recovery areas, where short-term ventilation of the lungs, invasive monitoring and the use of infusions of inotropes is possible [3-6].
While the majority of cardiothoracic surgical patients spend a short period on a cardiothoracic intensive care unit (ICU) or cardiac recovery unit, some require admission to a multidisciplinary ICU for further care. These latter patients form a subgroup with a poor prognosis in which determinants of outcome have been investigated [7-12]. Only a proportion of these investigations have reported mortality following hospital discharge [7-9,11]. The mortality rate in the first few months after hospital discharge is high in these patients with critical illness. The objective of this study was to review clinical factors known at the time of presentation to the ICU that affect medium-term survival.
We reviewed the outcome for adult cardiac surgical patients referred to a multidisciplinary ICU over 6 yr (1991-1997). Factors at the time of their referral to the ICU were examined and the impact of these factors on patient survival assessed.
Study location and population
The study was conducted at the John Radcliffe Hospital, Oxford, UK, which is a tertiary referral centre for cardiac surgery and has a dedicated postoperative cardiac recovery unit as well as a multidisciplinary ICU. The cardiac recovery unit provides high-dependency care, including mechanical ventilation of the lungs for up to 3 days, so that patients who require only short-term ventilatory support or weaning of inotropes are not admitted to the ICU. The median period of ventilatory support for patients admitted to the cardiac recovery unit in Oxford is approximately 90 min . All cardiothoracic surgical patients admitted to the hospital's multidisciplinary ICU between July 1991 and April 1997 were included in the study. These patients were admitted from the operating room, from the cardiac recovery unit or from the cardiothoracic surgical ward postoperatively. Sixty-four patients were admitted routinely to the ICU because of unavailability of beds in the cardiac recovery unit.
Data collection and clinical definitions
Cardiothoracic surgical patient details were obtained by screening an ICU admissions' database. Patient details and hospital admission dates were confirmed using the John Radcliffe Hospital computerized patient administration system. Mortality at hospital discharge was also approximated from the patient administration system, but since some patients were discharged from the John Radcliffe Hospital to their referring hospitals rather than directly to the community, these data were not complete. Six-month mortality data were available from the patient administration system for some patients under the care of General Practitioners in the Oxford district and for patients who had been followed up as out-patients at the John Radcliffe Hospital. In the case of the remaining patients, General Practitioners were contacted by post to determine survival at 6 months after ICU discharge. All clinical and patient characteristic data were obtained retrospectively and were made anonymous in the study.
The types of surgery before ICU admission (Table 1) were coronary artery graft surgery, valve replacement surgery, thoracic surgery, thoracic aorta surgery and surgery for acute ventricular septal defects. Categories not shown in Table 1 were surgery for pulmonary embolism (n = 3), left ventricular assist device (2), left ventricular aneurysm (2), atrial septal defect (1) and atrial myxoma (1).
The reasons for admission to the ICU (Table 2) were classified as any combination of respiratory, renal or cardiac ventricular failure, sepsis, cerebrovascular accident, cardiorespiratory arrest, carcinomatosis, or following laparotomy either for ischaemic bowel or for other reasons (usually gastrointestinal bleeding). Five patients did not fall into these groups: they comprised haemothorax (n = 1), polytrauma (1), pulmonary haemorrhage (1), renal and hepatic failure and gastrointestinal bleeding (1) and haemorrhagic shock (3). Sixty-four patients were routine admissions, with no overt co-morbidity.
Respiratory failure was defined as a requirement for mechanical ventilation of the lungs at admission, renal failure as being sufficiently severe to require haemofiltration, and cardiac failure as a need for intravenous infusion of epinephrine or intra-aortic balloon counter pulsation to support the heart. Sepsis was defined in terms of the American College of Chest Physicians/Society of Critical Care Medicine consensus . Cerebrovascular accident was diagnosed clinically and on computed tomographic scanning.
The diagnostic accuracy and utility of age as a continuous variable was assessed using the receiver operating characteristic (ROC) method . For this analysis, sensitivity was plotted against (1 - specificity) in a ROC plot. The area under the ROC curve was measured to assess diagnostic accuracy in predicting outcome. Statistical analysis of categorical variables was carried out by multivariate logistic regression analysis using SPSS® v.9 (SPSS, Inc, Chicago, IL, USA). A binary data matrix was constructed with 'dead at 6 months' as the dependent variable. Covariates were assessed for inclusion into the model using a backward stepwise technique and a set level of significance of P < 0.05. Results were expressed in terms of the odds ratio of death at 6 months after ICU discharge. Significance was expressed in terms of P. Variables assessed in this way were: (a) surgical type (coronary artery graft surgery, valve replacement, combined coronary artery graft surgery and valve replacement, acute ventricular septal defect, thoracic aorta surgery, and thoracic surgery), (b) clinical condition on admission (ventricular failure, sepsis, renal failure, respiratory failure, cerebrovascular accident, cardiac arrest, laparotomy - ischaemic bowel - laparotomy - gastrointestinal bleeding - carcinoma, routine admissions), and (c) age > 80 yr.
Characteristics of patients admitted to the ICU
Three-hundred-and-one cardiothoracic patients were admitted to the ICU over the study period. Median age was 66 yr (Fig. 1) and the median duration of stay in the ICU was 3 days. Most referrals were from the cardiac recovery unit (56%), followed by the operating room (42%) and then the postoperative ward (2%). The distribution of surgery types is shown in Table 1, the types of clinical 'morbidity' at presentation to the ICU in Table 2.
The overall ICU mortality rate was 34%, and the 6-month mortality rate was 51%. The 64 routine cardiac admissions had an ICU mortality rate of 3.1% and 6-month mortality rate of 7.8%. Six month mortality was stratified with respect to a number of different variables (Tables 1 and 2, Fig. 1). Six month mortality data were not available from General Practitioners for 30 patients (10% of the total) who were alive at discharge from hospital. These 30 patients were assumed to be alive at 6 months, which may cause underestimation of the mortality rate.
Age(Fig. 1). The categorical variable 'age >80 yr' had a striking effect on the 6-month mortality rate (odds ratio of death 9.2, P = 0.034). However, when age was assessed as a continuous variable, it was not diagnostically useful in predicting 6-month mortality rate (area under the ROC curve was 0.56). This may relate to the 'J'-shaped mortality curve (Fig. 1).
Type of surgery(Table 1). Patients referred to the ICU after coronary artery graft surgery exhibited a lower mortality rate than that of the study population as a whole at 6 months (43%). Logistic regression analysis identified significant correlations with survival at 6 months for both coronary artery graft surgery (odds ratio of death 0.28, P = 0.036) and thoracic surgery (odds ratio of death 0.22, P = 0.042). Patients who underwent valve replacement surgery had a relatively higher mortality rate (62%), but this did not achieve statistical significance.
Reasons for referral(Table 2). Patients admitted to the ICU for management of renal or respiratory failure, or both, in the absence of cardiac ventricular failure or sepsis had a lower mortality rate at 6 months (35% overall) than the ICU group as a whole. The groups were assessed individually in the logistic regression analysis and did not achieve significance. Patients with either cardiac ventricular failure or sepsis had the highest mortality rate (79 and 76%, respectively). Logistic regression analysis showed positive correlations with death at 6 months in these two groups (Table 2). Cardiac arrest was associated with non-survival, though this finding was not significant. There was also a trend towards increased survival after laparotomy for reasons other than ischaemic gut (usually gastrointestinal bleeding).
Cardiac surgical patients requiring multidisciplinary intensive care have a poor prognosis. The main end-point in this study was mortality rate at 6 months after ICU discharge in contrast to many of the published studies in this area (Table 3). The mortality rate in the ICU was 34%, but by 6 months after discharge it had risen to 51%. Because of the lack of 6-month mortality rate data for 10% of our patients, the true figure may be higher. In a multiple logistic regression model, isolated coronary artery graft surgery or thoracic surgery correlated with a lower 6-month mortality rate than other types of surgery. This is consistent with the overall relative cardiac surgical mortality. The current hospital mortality rates in this centre are 3% for coronary artery graft surgery, 6% for valve replacement surgery, and 10% for combined coronary artery graft surgery and valve replacement (R. Pillai, personal communication). Ventricular failure or sepsis on admission to the ICU correlated significantly with increased mortality, but patients referred with any combination of respiratory or renal failure exhibited a much lower mortality rate (36% at 6 months). These latter groups, assessed individually in the logistic regression equation, were small and did not achieve statistical significance. Age >80 yr was significantly associated with increased mortality. Even in this group, however, there are reports of a favourable long-term outcome after cardiac surgery [15,16].
In North American patients from the 1980s, prolonged mechanical ventilation of the lungs was associated with a 30-day mortality rate of approximately 25% [17,18]. Respiratory, cardiac or renal illness severity at admission and emergency surgery was associated with a worse outcome . More recently, indices of organ failure in the first 24 h after admission and advanced age predicted hospital mortality in cardiac patients requiring >2 weeks' stay in intensive care . Renal failure was not predictive of hospital mortality, consistent with our findings. In cardiac surgical patients requiring >48 h mechanical ventilation of the lungs , multiorgan dysfunction was the only significant predictor of mortality, again consistent with our data.
One-hundred-and-sixteen cardiac patients referred to a multidisciplinary ICU in Paris, France, had a mortality rate of 47% at 70-93 months . The only predictor of mortality was the preoperative New York Heart Association class. Preoperative health data were not available in our study. In 162 patients requiring >48 h intensive care after cardiac surgery , an acute physiological disturbance algorithm  predicted death in hospital and after discharge poorly. In a retrospective review of 139 cardiac surgical patients whose lungs were ventilated for >7 days, of 43 factors, lung disease, prolonged operation and bypass time were associated with a requirement for prolonged mechanical ventilation . Prolonged use of inotropes, sepsis, cerebrovascular accident and coagulopathy were associated with increased ICU mortality. Worse preoperative cardiorespiratory status, old age and lack of preoperative aspirin therapy were predictive of longer-term (length was not defined) mortality, which was high, at 63%.
Two recent studies from the USA identified factors associated with prolonged ICU stay after cardiac surgery. Higgins and colleagues identified by multiple logistic regression 95 factors that correlated with increased ICU stay in 266 patients . These included advanced age, emergency procedures, coronary artery graft surgery combined with mitral valve surgery, intra-aortic balloon pump, prolonged cardiopulmonary bypass and physiological factors - such as reduced serum albumin, elevated alveolar-arterial oxygen gradient or decreased arterial bicarbonate concentration - at ICU admission. Intensive care outcome was not addressed. At the Cleveland Clinic, Cleveland, OH, USA, multiple factors were associated with failure of tracheal extubation in cardiac surgical patients: advanced age, chronic lung disease, pulmonary hypertension, severe left ventricular dysfunction, elevated blood urea, hypoalbuminaemia, thoracic aorta surgery, massive transfusion or prolonged cardiopulmonary bypass . Extubation failure was associated with a prolonged stay in the ICU but not with increased 30-day mortality rate . However, prolonged stay in the ICU in the 142 patients from the first year of the study was associated with a high mortality rate after hospital discharge and poor functional status in the survivors . This accentuates the need for follow-up after hospital discharge in this type of study.
The conclusions from this study, which examined clinical risk factors for 6-month mortality rate at the point of referral to the ICU, are thus generally consistent with previous work, though a few points deserve emphasis. Both ICU mortality rate (34%) and mortality rate 6 months after ICU discharge (51%) are higher than in many previous studies. This may reflect the modern patient population, or that the majority of patients are managed on a separate cardiac recovery unit and in our institution only those cardiac surgical patients who are very ill are referred to the ICU [3,5]. Differences were identified between patients with different types of organ failure. Those referred with multiorgan system failure - usually including cardiac failure and often complicated by sepsis - generally have a poor prognosis. Extreme old age is also a factor in poor outcome. Patients with potentially reversible single- or double-organ failure, usually respiratory or renal, with clearly defined replacement therapies have a good prognosis and may represent a group that benefits most from the expenditure of multidisciplinary ICU resources. The quality of life of the survivors in this study was not addressed, but it is an important factor in the outcome equation and is frequently reduced in survivors of prolonged intensive care after cardiac surgery [7,9,22,23].
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