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Ann Thorac Surg 1999;67:661-665
© 1999 The Society of Thoracic Surgeons

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Original Articles

Variables predicting reintubation after cardiac surgical procedures

^a Department of Anesthesiology, Internal Medicine, Saint Vincent Mercy Medical Center, Toledo, Ohio, USA
^b Department of Research, Saint Vincent Mercy Medical Center, Toledo, Ohio, USA
^c Department of Cardiovascular and Thoracic Surgery, Saint Vincent Mercy Medical Center, Toledo, Ohio, USA

Accepted for publication August 5, 1998.

Address reprint requests to Dr Engoren, Department of Anesthesiology, Saint Vincent Mercy Medical Center, 2213 Cherry St, Toledo, OH 43608
e-mail: engoren{at}pol.net

	Abstract

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Background. This study sought to determine patient characteristics, processes of care, and intermediate outcomes as predictors of reintubation after cardiac surgical procedures.

Methods. We performed a retrospective case-control study that included all patients undergoing cardiac surgical intervention who required reintubation and an equal number of control patients not requiring reintubation. Putative risk factors were analyzed univariately by ², Fisher exact, Student’s t, or Mann-Whitney tests. A logistic regression model was developed using data from patients requiring reintubation for cardiorespiratory reasons.

Results. Of the 1,000 consecutive patients reviewed, 41 (4.1%) required reintubation (30 [3%] for cardiorespiratory reasons and 11 [1.1] for unplanned operations). Univariate predictors of reintubation (p < 0.05) were older age, chronic obstructive pulmonary disease, New York Heart Association functional class IV, preoperative renal failure, lower arterial oxygen tension, insertion of intraaortic balloon pump, longer time in the operating room, longer duration of cardiopulmonary bypass times, positive fluid balance, postoperative renal failure, and worse pulmonary mechanics. Patients requiring reintubation also required a longer initial period of mechanical ventilation (median, 16.3 versus 6.0 hours; p < 0.05). Excellent prediction was found with a model consisting of four variables: operating room time, respiratory rate, vital capacity, and chronic obstructive pulmonary disease.

Conclusions. Patients who required reintubation were sicker and had worse respiratory function and more comorbidity. Prompt extubation did not contribute to reintubation. Patients identified as having a high risk for reintubation should be followed up closely, and interventions should be directed to treating the problems leading to reintubation.

	Introduction

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Cardiac surgical intervention has become a common method of treating coronary artery disease and valvular dysfunction. More than 550,000 adult cardiac surgical procedures are performed annually in the United States [1]. Weaning from mechanical ventilation and endotracheal extubation usually proceeds straightforwardly. Failure of the patient to tolerate extubation may reflect premature extubation or may be a marker of a sicker patient. Few studies have looked at the incidence of reintubation (not just within 24 hours after extubation), the factors associated with reintubation, and patient outcome.

Premature extubation may lead to hypercarbia, hypoxemia, pulmonary hypertension, right heart failure, and myocardial ischemia [2]. Additionally, it subjects the patient to the physical risks of reintubation, including esophageal intubation, laryngeal trauma, and pulmonary aspiration [3]. The purpose of the present study was to determine the incidence of reintubation, the variables associated with reintubation, and patient outcome.

	Material and methods

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Patients
After approval by the institutional review board of Saint Vincent Mercy Medical Center (November 13, 1995), 1,000 consecutive patients undergoing cardiac surgical procedures with cardiopulmonary bypass between January 1, 1994, and June 18, 1995, were retrospectively studied. The computerized database located 41 patients who had undergone reintubation. Control subjects not requiring reintubation were selected from the same sample of 1,000 patients. Selection was conducted using a computerized random number generator. Control patients and the 1,000 patients had the same characteristics: age, sex, times of mechanical ventilation and operation, and type of operation (p > 0.10).

All patients had undergone general anesthesia with diazepam (2.5 to 10 mg), fentanyl (10 to 25 µg/kg), pancuronium (0.1 mg/kg for induction, then 0.01 mg/kg as required), thiopental (0 to 250 mg for induction), and isoflurane for maintenance. They were monitored with electrocardiography, arterial catheter, pulse oximetry, capnography, and central venous and left atrial pressure catheters. During cardiopulmonary bypass, the heart was cooled with topical ice slush, but, the patient was actively heated to maintain urinary bladder temperature at 37°C.

After transfer to the cardiovascular intensive care unit, patients were weaned from bypass by our standard protocol [4]. Ketorolac and morphine were used for analgesia. Diazepam was used for anxiolysis.

When patients were awake and hemodynamically stable, they were considered for extubation. They then underwent extubation if they met standard criteria: respiratory rate of 10 to 28 breaths/min, tidal volume greater than 5 mL/kg, vital capacity greater than 10 mL/kg, negative inspiratory force greater than -25 cm H₂O, arterial carbon dioxide tension of 45 mm Hg or higher, and the ability to maintain adequate oxygenation with supplemental oxygen. If these criteria were not met, patients underwent extubation if they remained comfortable after a trial of continuous positive airway pressure without any pressure support.

Charts were abstracted for preoperative patient characteristics, processes of care, and intermediate outcome. The presence or absence of chronic obstructive pulmonary disease (COPD), diabetes mellitus, and insulin-dependent diabetes mellitus was defined from clinical history as noted in the preoperative evaluation. Preoperative renal failure was defined as a serum creatinine concentration greater than 1.5 mg/dL and postoperative renal failure as a rise in creatinine concentration of 1.0 mg/dL or greater above baseline or the continued presence of preoperative renal failure.

Reasons for reintubation were obtained from the clinical notes. Patients were classified into three groups: Group RE-All included all 41 patients requiring reintubation for any reason. Group RE-Resp, a subset of RE-All, included 30 patients who required reintubation to support cardiorespiratory function. The remaining 11 patients in Group RE-All required reintubation solely for unplanned operations. The third group included 41 control patients.

Statistical analyses
Group RE-Resp and the control group were univariately compared with ² and Fisher’s exact tests for categoric variables, and the Student’s t test or Mann-Whitney test for continuous variables. A stepwise forward selection procedure was then used to develop a model predictive of reintubation to support cardiorespiratory function. Variables that were univariately significant at the 0.10 level were considered for inclusion in the model. A p value less than 0.05 was required for variables to enter and stay in the model. Improvement of models was judged by the Akaike information criterion, the Schwartz criterion, percent of pairs of observation with predicted probabilities concordant with outcome, and percent of observations correctly predicted by the model (SAS software, SAS Institute Inc, Cary, NC). Goodness of fit was assessed using the Hosmer-Lemeshow goodness-of-fit test [5–8].

	Results

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Of 1,000 consecutive patients undergoing cardiac surgical procedures, 982 underwent extubation. Of the 18 patients who never achieved extubation, 12 died, and 6 underwent tracheostomy (3 of whom later died). A total of 41 patients required reintubation, 30 of whom (group RE-Resp) required reintubation for the following cardiorespiratory reasons (Fig 1):

Respiratory fatigue/failure 7 Secretions 4 Hypoxemia 3 Congestive heart failure 3 Cardiorespiratory arrest 3 Bronchospasm 1 Pulmonary emboli 1 Arrhythmia 1 Seizures 1 Gastrointestinal hemorrhage 1 Pneumonia 1 Airway obstruction 1 Stroke 1 Adult respiratory distress syndrome 1 Severe agitation 1

View larger version (20K): [in this window] [in a new window]   Fig 1. Time to extubation and reintubation for patients in Group RE-Resp. The lower point of each vertical line is the time in hours of extubation for each patient; the higher point is the time in hours of reintubation for each, so that the length of each line is duration of extubation before reintubation. Four patients intubated for the following reasons eventually died: patient 3 of pulmonary emboli, patient 10 of gastrointestinal hemorrhage, patient 19 of cardiac arrest, and patient 29 of increased work of breathing.

For patients requiring reintubation for cardiorespiratory reasons, univariate patient characteristic predictors (p < 0.05) of reintubation were history of COPD, higher New York Heart Association functional class, preoperative renal failure, and lower preoperative room air arterial oxygen tension (Tables 1, 2). Processes of care that univariately predicted (p < 0.05) reintubation were insertion of intraaortic balloon pump, longer operating room time, longer cardiopulmonary bypass time, and greater intraoperative fluid balance (Table 3). Postoperatively, intermediate outcomes that univariately predicted (p < 0.05) reintubation were higher initial fraction of inspired oxygen, worse respiratory mechanics, postoperative renal failure, and longer mechanical ventilation time (Table 4).

View larger version (18K): [in this window] [in a new window]   Fig 2. Receiver operating characteristic curve shows that at a probability of reintubation of 0.69, the model has a sensitivity of 0.76 (95% confidence interval, 0.55 to 0.91) and a specificity of 1.00 (95% confidence interval, 0.91 to 1.00). The area under the curve is 0.94.

	Comment

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

The goal of the present study was to identify variables that could predict reintubation at the time of extubation. We identified four variables by multivariate analysis that predicted reintubation. By univariate analysis, patients who required longer periods of mechanical ventilation were more likely to require reintubation; however, after we adjusted for the effects of operating room time, respiratory rate, vital capacity, and COPD, time on mechanical ventilation was not significant. The area under the receiver operating characteristic curve of our multivariable model was 0.94, indicating the model has a high probability of correctly identifying patients as needing or not needing reintubation (Fig 2). An area of 1 indicates perfect prediction ability; an area of 0.5 indicates that prediction with the model is no better than chance.

Of these four variables only vital capacity is considered a standard criterion for extubation. Operating room time is physician related but probably reflects sicker patients or patients with more complications and was highly correlated with cardiopulmonary bypass time (r = 0.81; p < 0.001) but not with fluid balance (r = 0.24; p = 0.03). Although fluid balance is a marker for prolonged mechanical ventilation [18] and is a univariate predictor of reintubation, it was not a multivariate predictor. Cardiopulmonary bypass has been shown to damage pulmonary vascular endothelium and to lead to increased lung water, which may not correlate with total body water [19, 20]. Increased lung water can decrease pulmonary compliance and hence lower vital capacity and raise respiratory rate [21]. Some evidence suggests that ß₂-agonists may decrease excessive lung water [22], and a controlled study of ß₂-agonists to evaluate their effectiveness may be worthwhile. Preextubation respiratory rate and vital capacity reflect both the patients’ underlying medical conditions and perioperative events. Left phrenic nerve dysfunction from topical ice slush is common after cardiac surgical procedures and may decrease vital capacity [23]. Although phrenic nerve function was not measured in the present study, efforts to protect it from frostbite may help preserve vital capacity. A history of COPD reflects the patient’s preoperative condition and is associated with a 13.5% rate of reintubation after coronary artery bypass grafting [13]. A limitation of our study is that the diagnosis of COPD was taken from the admitting records rather than by preoperative pulmonary function testing, which was not done. This choice might limit the generalizability of our findings to other institutions. Nevertheless, the admitting diagnosis of COPD was a strong predictor of reintubation.

Twenty three of the 30 patients who required reintubation for cardiorespiratory reasons had been extubated for more than 24 hours and were initially doing well before deteriorating and requiring reintubation; yet the model was predictive for these patients. This finding suggests that the model identifies sicker patients, those with less reserve, and those more likely to have cardiorespiratory problems when they have a postoperative complication.

Four (9.8%) of our 41 patients who required reintubation died. Other studies looking at reintubation in patients undergoing cardiac surgical procedures did not report mortality rates [9, 12]. In a general surgical intensive care unit, the overall mortality rate was 31% in patients requiring reintubation, which may be related more to the different patient populations than to any other factor [13].

Previous studies evaluating reintubation have either looked at the specific risk factors of COPD or stroke or merely determined the incidence without determining predictors. Our study found that four predictors can easily and accurately be used to predict reintubation. We are not suggesting that these patients not undergo extubation: Although the prediction model had high sensitivity and specificity, the incidence of reintubation was low. Thus, many patients would be maintained by mechanical ventilation for an unnecessarily long period, but it might not prevent the complications for which they required reintubation. We suggest that extubation be performed in these patients but that they be monitored closely. Future research should be directed toward determining whether interventions such as left phrenic nerve protection, ß₂-agonists and ipratropium aerosols, diuresis, and more intensive chest physiotherapy would prevent reintubation by providing more respiratory reserve when complications develop.

	Acknowledgments

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
This study was sponsored by the F. M. Douglass Foundation (grant 96-162) and by the Saint Vincent Mercy Medical Center. We gratefully acknowledge Don White, PhD, for statistical assistance.

	References

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 

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