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Ann Thorac Surg 2008;86:1883-1887. doi:10.1016/j.athoracsur.2008.08.020
© 2008 The Society of Thoracic Surgeons
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Original Articles: Adult Cardiac

Delirium After Cardiac Surgery and Predictive Validity of a Risk Checklist

Sandra Koster, MANPa,*, Frits G.J. Oosterveld, MS, PhDd, Ab G. Hensens, MSa, Arie Wijma, MSc, Job van der Palen, MS, PhDb

a Department of Thoracic Surgery, Medisch Spectrum Twente, Enschede, the Netherlands
b Department of Epidemiology, Medisch Spectrum Twente, Enschede, the Netherlands
c Department of Psychiatry, Medisch Spectrum Twente, Enschede, the Netherlands
d Saxion University of Applied Sciences, Expertise Center Health, Social Care and Technology, Enschede, the Netherlands

Accepted for publication August 11, 2008.

* Address correspondence to Dr Koster, Haaksbergerstraat 55, Enschede 7500 KA, the Netherlands (Email: s.koster{at}ziekenhuis-mst.nl).


Cardiothoracic anesthesiology: The Annals of Thoracic Surgery CME Program is located online at http://cme.ctsnetjournals.org. To take the CME activity related to this article, you must have either an STS member or an individual non-member subscription to the journal.

 

   Abstract
Top
 Abstract
Introduction
 Patients and Methods
 Results
 Comment
 Acknowledgments
 References
 
Background: Delirium or acute confusion is a temporary mental disorder that occurs frequently among hospitalized elderly patients. Patients who undergo cardiac surgery have an increased risk of delirium, which is associated with many negative consequences. Therefore, prevention or early recognition of delirium is essential.

Methods: In this observational study, a risk checklist for delirium was used during the preoperative outpatient screening in 112 patients who underwent elective cardiac surgery. The Delirium Observation Screening (DOS) scale was used before and after surgery to assess whether delirium had developed in patients. The psychiatrist was consulted to confirm or refute the diagnosis delirium.

Results: The incidence of delirium after cardiac surgery was 21%, and the mean duration of delirium was 2.5 days. The time to discharge was 11 days longer for patients with delirium. The delirium risk checklist could accurately predict postoperative delirium in patients who underwent elective cardiac surgery based on a disturbance in the electrolytes sodium and potassium and on EuroSCORE (European System for Cardiac Operative Risk Evaluation). When using a probability of delirium of 50%, the sensitivity of the risk checklist was 25.0% and specificity was 95.5%. The predictive value of a positive test was 60.0%, and the predictive value of a negative test was 82.4%. The area under the receiver-operating characteristic curve was 0.75.

Conclusions: With the risk checklist for delirium, patients at an increased risk of delirium after elective cardiac surgery can be identified.

Delirium is a common temporary mental disorder among hospitalized elderly patients [1]. In the literature, the incidence and prevalence of delirium vary widely between different study populations. Patients who undergo cardiac surgery have an increased risk of delirium. The incidence of delirium was 13.5% after elective cardiac surgery and increased to 20.0% among patients aged 60 years and older [2]. Generally, an incidence of 25% is assumed among hospitalized elderly patients [1]. As a result of demographic aging, the incidence of delirium is likely to increase in the foreseeable future. Early recognition or prevention of delirium is important, because delirium has been related to a prolonged hospital stay, nursing home placement, and reduced cognitive and functional recovery [3–7].

With the delirium observation screening (DOS) scale by Schuurmans and associates [10], delirium can be recognized at an early stage. The likelihood of delirium increases proportionally with the number of existing risk factors. Some of the greatest predisposing risk factors for delirium are the presence of cognitive impairment, sleep deprivation, immobility, visual and hearing impairment, and dehydration [6]. If the patient has an increased risk for delirium, preventative interventions can be considered [8].

At this time, there is no model available that is able to predict postoperative delirium. We have developed a delirium risk checklist to identify patients with an increased risk of postoperative delirium after elective cardiac surgery. This study will examine the predictive validity of this risk checklist for delirium. If this is successful, in the future, preventive interventions can be taken for patients at high risk for postoperative delirium, such as providing adequate preoperative information or haloperidol prophylaxis [9].


   Patients and Methods
Top
 Abstract
 Introduction
 Patients and Methods
Results
 Comment
 Acknowledgments
 References
 
Between November 2006 and June 2007, our prospective cohort study included 112 consecutive patients, aged 45 years and older, who underwent elective cardiac surgery with and without cardiopulmonary bypass assistance at the department of thoracic surgery. Excluded were patients who did not undergo preoperative outpatient screening and patients with preoperative delirium. Informed consent was obtained in accordance with the hospital's policy.

Procedure
The delirium risk checklist was completed during the preoperative outpatient screening 2 to 6 weeks before the surgery, and it measured all data concerning the potential predictors of delirium and study outcomes. Patients were followed from the time of admission until the time of discharge from the hospital.

The DOS scale was rated by nurses three times a day at the end of every shift, before and after surgery, to assess whether patients had delirium. The DOS scale describes typical behavioral patterns related to delirium in 13 statements or questions, which the observer has to answer with "never" (score = 0) or "sometimes or always" (score = 1) if applicable (Table 1). A DOS score of 3 or more indicates delirium [10]. When the DOS score was 2 or more, the psychiatrist was consulted to confirm or refute the diagnosis delirium. The psychiatrist was not consulted when the DOS score was 1 or less, as the probability of delirium was unlikely [7]. The diagnostic criteria of the psychiatrist to confirm or refute the diagnose delirium were based on the Diagnostic and Statistical Manual of Mental Disorders, Fourth Revision (DSM-IV) criteria for delirium [11] and the nursing documentation. The DSM-IV criteria were based on specific diagnostic criteria to differentiate delirium from other syndromes such as anxiety, depression, and dementia. Table 2 shows the DSM-IV criteria for the diagnosis of delirium.


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  		Table 1 The Working Method of the Delirium Observation Screening Scale
 

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  		Table 2 Diagnostic Criteria of Diagnostic and Statistical Manual of Mental Disorders–IV for the Diagnosis of Delirium
 
Data were collected during the day of admission until the fifth postoperative day. If the patient did not have delirium by then, the DOS scale was not rated anymore. If the patient had delirium during the first 5 postoperative days, the DOS scale was rated until the patient was delirium free for 2 consecutive days.

The Ethics Committee of Medisch Spectrum Twente was asked whether approval was needed. Because this study was observational only, without invasive procedures, it was not deemed necessary.

Measures
Predisposing factors for delirium are older age (≥ 70 years), cognitive impairment, history of delirium, dementia, depression, visual and hearing impairment, functional impairment, preoperative use of alcohol or opiates, preoperative anxiety, smoking, poor nutritional status, severe comorbidity, markedly abnormal renal function, history of cerebrovascular disease, peripheral vascular disease, atrial fibrillation, diabetes mellitus, left ventricular function of 30% or less, preoperative cardiogenic shock, urgent operation, intraoperative hemofiltration, operation time of 3 hours or more, a high perioperative transfusion requirement, and disturbed sodium, potassium, or glucose levels [2, 4, 7, 8, 12]. Beating-heart surgery, compared to cardiac surgery with cardiopulmonary bypass, was identified as having a significant protective effect against postoperative delirium [4].

In the studies mentioned above, the EuroSCORE (European System for Cardiac Operative Risk Evaluation [range, 0% to 100%]) has not been reported as a risk factor for delirium [13]. However, because the EuroSCORE is a combination of many of the risk factors described, it has been incorporated as a separate risk factor in the delirium risk checklist. In total, 25 risk factors were selected (Table 3).


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  		Table 3 Distribution and Univariate Analysis of All Candidate Predictors in the Overall Population and Among Patients With and Without Delirium
 
Validated instruments were used to assess functional impairment (Barthel Index [14, 15]); nutritional status (Short Nutritional Assessment Questionnaire [SNAQ] [16]); cognitive function (Standardized Mini-Mental State Examination [SMMSE] [17]); and hearing and vision (hearing test [18] and visual test [19]).

The DOS scale is a reliable and valid instrument to recognize delirium based on nurses' observations during regular care [20]. The predictive validity of the DOS scale in comparison to the DSM-IV criteria of a geriatrician or psychiatrist was good, with a sensitivity of 94.4% and specificity of 76.6% [3, 10]. In 2007, van Gemert and Schuurmans [21] found a sensitivity of 89% and a specificity of 86% for the DOS scale, at four wards of a university hospital.

Statistical Analysis
The univariate association between each candidate predictor and postoperative delirium was estimated. The independent two-tailed t test was used to compare the means of variables in case of continuous normally distributed data. When data were not distributed normally, Wilcoxon's rank-sum test was utilized. The following risk factors were analyzed as continuous variables: age, EuroSCORE, and operation time. The {chi}2 test was used for the comparison of categorical variables (Table 3). The following cut-off values were used to classify high-risk patients: cognitive impairment, SMMSE less than 23; physical impairment, Barthel index less than 14; visual impairment, Landolt C circles less than 0.5; hearing impairment, mean frequency threshold at 500, 1,000, and 2,000 Hz greater than 25 dB; poor nutritional status, SNAQ of 2 or more; anxiety score of 6 or more; sodium less than 135 or greater than 145 mmol/L; potassium less than 3.5 or greater than 5.0 mmol/L; creatinine, male, greater than 110 micromol/L, and creatinine, female greater than 90 micromol/L; moderate left ventricular function, 30% to 50%; and poor left ventricular function, less than 30%. Hereafter, the factors independently associated with postoperative delirium (p ≤ 0.10) were all entered in a logistic regression analysis. Subsequently, this model was reduced to remove nonsignificant factors (p > 0.05) one by one.

The model's ability to discriminate between patients with and without postoperative delirium was estimated by the area under the receiver-operating characteristic (ROC) curve.


   Results
Top
 Abstract
 Introduction
 Patients and Methods
 Results
Comment
 Acknowledgments
 References
 
Study Population
One hundred and 14 patients were invited to participate in the study, and of those, 2 refused for personal reasons. The majority of patients were male (63.4%). The mean age of the total study population was 70 years (SD 7.3; Table 3). There was a significant difference in age in patients with and without delirium. Sixty-five patients (58.0%) were older than 70 years. The most commonly performed cardiac operations were coronary artery bypass grafting (37.5%) and heart valve surgery (36.6%).

Incidence and Duration of Delirium and Stay in Hospital
The psychiatrist diagnosed postoperative delirium in 24 patients (21%). Of these 24 patients, 13 were male with a mean age of 74 years. Almost 30% of these patients had delirium the first day after cardiac surgery. The mean duration of delirium was 2.5 days (SD 2.1). In the event of delirium, the duration of hospital stay was significantly higher (p < 0.01). For patients with postoperative delirium, the mean hospital stay was 22 days (SD 21.0) versus 11 days (SD 5.9) for patients without postoperative delirium. Four patients (3.6%), of whom 2 had postoperative delirium, died during the postoperative stage.

Validity Risk Checklist for Delirium
In the univariate analysis, type of operation, EuroSCORE, age, use of heart-lung machine, electrolyte disturbance, and diabetes mellitus were associated (p ≤ 0.10) with a higher risk of postoperative delirium after cardiac surgery (Table 3). The overall multivariable model containing all predictors yielded an area under the ROC curve of 0.85 (95% confidence interval [CI]: 0.78-0.93; p < 0.001).

Only two predictors were independently associated with postoperative delirium: disturbance in electrolytes and EuroSCORE (Table 4). There was a disturbance in electrolytes when the sodium or the potassium was disturbed. When the EuroSCORE increased by 1%, the risk of having postoperative delirium was 1.5-fold higher.


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  		Table 4 Predictors From Multivariable Analysis That Independently Contributed to the Prediction of Postoperative Delirium After Cardiac Surgery (n = 112)
 
The sensitivity of this short risk checklist, when using a probability of developing delirium of 50%, amounted to 25.0% and the specificity was 95.5%. The predictive value of a positive test was 60.0% and the predictive value of a negative test was 82.4%. An area under the curve of 0.75 was found (95% CI: 0.66 to 0.85; p < 0.001). To estimate the probability of developing delirium for an individual patient, the prediction formula in Table 4 can be used [22].


   Comment
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
Acknowledgments
 References
 
Two independent predictors of postoperative delirium after cardiac surgery were found: electrolyte disturbances and EuroSCORE. According to the delirium guideline [2], a disturbance in electrolytes (serum concentration of sodium and potassium) is a very plausible precipitating factor for postoperative delirium.

In a recently published study, a EuroSCORE of 5 or more was associated with postoperative delirium [23]. Considering that the EuroSCORE itself contains relevant risk factors for mortality (age, cognitive impairment, and so forth), a positive correlation between the EuroSCORE and postoperative delirium is not too farfetched. That also explains why other predisposing factors for developing delirium such as age 70 years or more, cognitive impairment, and so forth, no longer appear as independent predictors in the final model. Finally, preoperative anxiety, a common problem among patients undergoing cardiac surgery, is a factor that can influence the development of postoperative delirium [2, 3, 24]. However, it was not mentioned as a risk factor in the delirium guideline. Also, it failed to reach significance in our study, possibly because of a too-small sample size.

The incidence of delirium after cardiac surgery was 21%. This figure corresponds well with the incidence of 25% reported by the Inspection of Healthcare according to van Blanken and Robben [1] and the study by van der Mast and coworkers [2], who found a postoperative delirium incidence of 20% among patients aged 60 years and older. The duration of postoperative delirium was 2.5 days, whereas previous studies, and also the delirium guideline [2], have reported delirium duration from several days to a couple of weeks. This discrepancy is possibly due to the use of the DOS scale, which may allow for not only earlier delirium recognition but also earlier resolution of delirium. Furthermore, before the start of the study, nurses were informed about delirium, including risk factors, prevention, and interventions. This informed approach may also have contributed to earlier recognition of delirium and hence to appropriate interventions such as orienting the patient as necessary, correcting fluid balance, providing quiet surroundings, and so forth. In previous studies, it was found that the education of nurses and the initiation of nursing interventions had a positive effect on early delirium recognition [24]. Finally, patients with postoperative delirium had a significantly longer hospital stay, which was also observed in earlier studies [3–7].

If a patient has delirium, mobility decreases, so the revalidation period goes up and the stay in hospital increases. If we exempt the patients who had a much longer stay in hospital, however, the number of patients we kept is too low for doing statistical analysis.

A number of critical considerations pertaining to our study can be made. We have included a relatively small number of patients. That might result in less stable estimates of the independent associations of the predictors in the final model, as well as the model's discriminative ability. However, with 112 patients, a multivariate analysis with six variables is very well possible. A second consideration can be made about the fact that we have derived our model from the same set of data that was used for our test set. A model works always better on the test set than on a separate validation set. In further research, we will test the model on a completely separate second data set.

When patients had postoperative delirium, the psychiatrist prescribed haloperidol, and the nurses created a safe environment and supported the patient mentally and physically (put on their eyeglasses or hearing aid) so patients could orient themselves. We did not review the use of certain drugs after cardiac surgery that could either increase or decrease the incidence of delirium.

The two identified predictors (electrolyte disturbance and EuroSCORE) are relatively easy to measure and can be used to identify patients at greater risk for postoperative delirium after cardiac surgery. In these patients, preventative interventions can be taken, such as providing adequate preoperative information or haloperidol prophylaxis. Maybe we can raise the prophylactic dose of haloperidol, as discussed in the study by Kalisvaart and colleagues [9], to 5 mg/day so we can examine if it is possible to prevent delirium. Early recognition of postoperative delirium can be realized by using the delirium observation screening scale.


   Acknowledgments
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
 Acknowledgments
References
 
The authors wish to thank all patients, nurses, doctors, and medical secretarial staff for their help and cooperation with this study.


   References
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
 Acknowledgments
 References
 

  1. van Blanken G, Robben PBM. Delirium often not recognized Medisch Contact 2005;60:1724-1727.
  2. van der Mast RC, Huyse FJ, Rosier PF. Guideline delirium Ned Tijdschr Geneeskd 2005;149:1027-1032.[Medline]
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  5. Cole MG, McCusker J, Bellavance F, et al. Systematic detection and multidisciplinary care of delirium in older medical inpatients: a randomized trial Can Med Assoc J 2002;167:753-759.[Abstract/Free Full Text]
  6. Inouye SK, Bogardus ST, Charpentier PA. A multicomponent intervention to prevent delirium in hospitalized older patients N Engl J Med 1999;340:669-676.[Abstract/Free Full Text]
  7. Schuurmans MJ, Duursma SA, Shortridge-Baggett LM. Early recognition of delirium: review of the literature J Clin Nurs 2001;10:721-729.[Medline]
  8. Yildizeli B, Ozyurtkan MO, Batirel HF, Kuscu K, Bekiroglu N, Yuksel M. Factors associated with postoperative delirium after thoracic surgery Ann Thorac Surg 2005;79:1004-1009.[Abstract/Free Full Text]
  9. Kalisvaart KJ, de Jonghe JF, Bogaards MJ, et al. Haloperidol prophylaxis for elderly hip-surgery patients at risk for delirium: a randomized placebo-controlled study J Am Geriatr Soc 2005;53:1658-1666.[Medline]
  10. Schuurmans MJ, Shortridge-Baggett LM, Duursma SA. The delirium observation screening scale: a screening instrument for delirium Res Theory Nurs Pract 2003;17:31-50.[Medline]
  11. American Psychiatric Association Diagnostic and statistical manual of mental disorders4th ed.. Washington, DC: American Psychiatric Association; 1994.
  12. Dubois MJ, Bergeron N, Dumont M, Dial S, Skrobik Y. Delirium in an intensive care unit: a study of risk factors Intens Care Med 2001;27:1297-1304.[Medline]
  13. Roques F, Nashef SA, Michel P, et al. Risk factors and outcome in European cardiac surgery: analysis of the EuroSCORE multinational database of 19030 patients Eur J Cardiothorac Surg 1999;15:816-822.[Abstract/Free Full Text]
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