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J Thorac Cardiovasc Surg 2007;134:484-490
© 2007 The American Association for Thoracic Surgery

Surgery for Acquired Cardiovascular Disease

Ventilatory dependency after cardiovascular surgery

Sudish C. Murthy, MD, PhDa,*, Alejandro C. Arroliga, MDb, Peter A. Walts, MDa, Jingyuan Feng, MSc, Jean-Pierre Yared, MDd, Bruce W. Lytle, MDa, Eugene H. Blackstone, MDa,c

a Department of Thoracic and Cardiovascular Surgery, Cleveland Clinic, Cleveland, Ohio
b Department of Pulmonary, Allergy, and Critical Care Medicine, Cleveland Clinic, Cleveland, Ohio
c Department of Quantitative Health Sciences, Cleveland Clinic, Cleveland, Ohio
d Department of Cardiothoracic Anesthesia, Cleveland Clinic, Cleveland, Ohio.

Received for publication November 28, 2006; revisions received February 23, 2007; accepted for publication March 8, 2007.

* Address for reprints: Sudish C. Murthy, MD, PhD, Department of Thoracic and Cardiovascular Surgery, Cleveland Clinic, 9500 Euclid Avenue/Desk F24, Cleveland, OH 44195. (Email: murthys1{at}ccf.org).


   Abstract
Top
 Abstract
Introduction
 Patients and Methods
 Results
 Discussion
 Conclusions
 Appendix E1
 Appendix E2
 References
 
Objectives: Ventilatory dependency is a widely recognized complication of cardiovascular surgery, often leading to tracheostomy. Some risk factors for its occurrence have been documented. Less well characterized are short- and long-term outcomes. Therefore, objectives were to identify risk factors for ventilatory dependency, assess its short- and long-term outcomes, and determine impact of tracheostomy.

Methods: From January 1998 to September 2001, 12,777 patients underwent cardiovascular surgery and survived at least 72 hours. Of these patients, 704 (5.5%) developed ventilatory dependency (cumulative intubation >72 hours); 185 (26%) underwent tracheostomy. Preoperative, intraoperative, and intensive care unit admission data were used sequentially to understand predictors of ventilatory dependency. Outcomes were analyzed by time-related methods, and impact of tracheostomy was assessed using competing-risks analysis.

Results: Hemodynamic status on intensive care unit admission (low cardiac output, vasopressor use, pulmonary hypertension; P < .0001) and early postoperative events (stroke, bacteremia; P < .0001) were more important than preoperative and intraoperative variables in predicting ventilatory dependency. Survival at 30 days, 1 year, and 5 years thereafter was 76%, 49%, and 33% and was strongly associated with favorable hemodynamic status. By 28 days, 24% of patients received tracheostomy; survival at 30 days and 2 years thereafter was 74% and 26%, considerably below anticipated survivals of 84% and 58%.

Conclusions: Improved operative and postoperative strategies to preserve myocardial function and restore hemodynamics should decrease the prevalence of ventilatory dependency. Unfortunately, preoperative models of ventilatory dependency are too insensitive for clinical use. Tracheostomy and its outcome are also poorly predicted, highlighting the complex interaction of events altering patients’ conditions before and after tracheostomy.

Abbreviations and Acronyms CL = confidence limit; CTA = Cardiothoracic Anesthesia registry; CVIR = Cardiovascular Information Registry; ICU = intensive care unit; NYHA = New York Heart Association; STS = The Society of Thoracic Surgeons


   Introduction
Top
 Abstract
 Introduction
Patients and Methods
 Results
 Discussion
 Conclusions
 Appendix E1
 Appendix E2
 References
 
Ventilatory dependency is a widely recognized complication of cardiovascular surgery, occurring in up to a fifth of patients and, in some, leading to tracheostomy.1-7Go Recently, we have examined outcomes after tracheostomy in such patients, demonstrating that only a third of patients were long-term survivors, with most dying of multisystem organ failure.8Go Given these findings, we were curious about predictors not only of tracheostomy, but also of ventilatory dependency in this population. Although some risk factors for ventilatory dependency have been documented,4,5,7,9,10Go less characterized are short- and long-term outcomes.4,5,7Go Therefore, the intent of this companion study was to focus on (1) factors associated with ventilatory dependency after cardiovascular surgery, (2) time-related outcomes of ventilatory dependency, and (3) predictors and impact of tracheostomy after ventilatory dependency.


   Patients and Methods
Top
 Abstract
 Introduction
 Patients and Methods
Results
 Discussion
 Conclusions
 Appendix E1
 Appendix E2
 References
 
Patients
Between January 1998 and September 2001, 12,836 patients underwent cardiovascular surgery at Cleveland Clinic, exclusive of heart transplantation and insertion of ventricular assist devices. Fifty-nine who died within 72 hours were excluded from the study, leaving 12,777 patients. Of these, 704 (5.5%) experienced ventilatory dependency (defined in text that follows), of whom 185 (26%) underwent tracheostomy.

Patient characteristics and operative variables were extracted from the Cardiovascular Information Registry (CVIR); respiratory and hemodynamic condition on intensive care unit (ICU) admission and medications administered within 24 hours thereafter (Tables E1-A and E1-B) Go were extracted from the Cardiothoracic Anesthesia (CTA) registry. Data are abstracted for both registries concurrently with patient care by experienced technicians and nurses and subjected to quality control. Both databases have been approved for use in research by the Institutional Review Board of the Cleveland Clinic, with patient consent waived.


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  		TABLE E1-A Characteristics of patients, details of procedures, ICU admission data, and interim events (categorical variables)
 

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  		TABLE E1-B Characteristics of patients, details of procedures, and ICU admission data (continuous variables)
 
Ventilatory Dependency
All operative and ICU intubation and extubation times were recorded in the CTA database. Ventilatory dependency was defined as 72 hours or more of postoperative endotracheal intubation, starting from completion of the index operation. Of the 704 patients experiencing ventilatory dependency according to this definition, 2 populations were represented: those with early and persistent ventilatory dependency who were not extubated within the initial 72 hours (n = 395, 56%) and those who had 1 or more unsuccessful extubation attempts (n = 309, 44%) and ultimately accumulated at least 72 hours of endotracheal intubation. For the latter, interval between initial extubation and reintubation was calculated and used for some analyses.

Time zero for patients experiencing ventilatory dependency was the point at which they had accumulated 72 hours of endotracheal intubation. For all others, it was 72 hours after completion of their index operation.

Primary responsibility for ventilatory management, including timing of extubation and reintubation, was assumed by a dedicated group of cardiothoracic anesthesiologists and respiratory therapists under the direction of the same individual (J.P.Y.) throughout the time course of the study. Algorithms for extubation and respiratory care were standardized and followed throughout the study time frame.

End Points
Primary end points were (1) all-cause mortality and (2) tracheostomy. Vital status was obtained from the Social Security Death Index on December 6, 2005, and was available for 11,666 patients (91%).11Go Reliable information was considered available 6 months earlier, so a common closing date of June 6, 2005, was used for analyses. Among surviving patients, mean follow-up was 5.0 ± 1.9 years; 57,603 patient-years of information were available for analyses.

Secondary end points after time zero included in-hospital stroke, renal failure, arrhythmia, sepsis, mediastinitis, and reoperation for bleeding, as defined by The Society of Thoracic Surgeons (STS; see http://www.ctsnet.org/file/rptDataSpecifications252_1_ForVendorsPGS.pdf).

Data Analysis
Risk factors for ventilatory dependency
A sequence of logistic regression analyses was performed to identify risk factors for ventilatory dependency, based first on preoperative factors, including intended operation, then on these variables and (1) additional operative details, (2) ICU and entry variables, and (3) interim events (Appendix E1). Variable selection used bootstrap aggregation ("bagging").12,13Go In brief, 100 data sets were obtained by random sampling with replacement, automated stepwise regression was performed, and variables with P < .05 were identified. Analyses included exploration of transformations of continuous variables. After aggregation of all analyses, variables appearing in 50% or more of them were selected as reliable associations.

Outcome of ventilatory dependency
Impact of ventilatory dependency was assessed by in-hospital morbidity developing after ventilatory dependency and by time-related survival. Survival was estimated nonparametrically by the Kaplan–Meier method and parametrically by multiphase hazard decomposition.14Go

Risk factors for death after ventilatory dependency were identified by multivariable multiphase hazard decomposition.14Go Variables considered in risk factor identification are listed in Appendix E1. Bagging was used for variable selection, based on 1000 bootstrap samples, conducted as described under "Risk Factors for Ventilatory Dependency."

Impact of tracheostomy
Time of occurrence of tracheostomy in the course of ventilatory dependency was estimated nonparametrically and parametrically. Variables considered in risk factor identification are listed in Appendix E1. Bagging was used for variable selection.

To explore the interrelation of mortality and tracheostomy, we performed a competing-risks analysis for (1) death before tracheostomy, (2) recovery from ventilatory dependency, and (3) tracheostomy. Nonparametric estimates were obtained by the method of Andersen and colleagues15Go and parametric estimates by numerical integration. Predicted survival after tracheostomy was estimated by calculating parametric survival curves for each patient conditional on survival to tracheostomy using the analysis of death before tracheostomy (Appendix E2 and Table E2). The average of these survival curves was compared with observed survival.


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  		TABLE E2 Incremental risk factors for death before tracheostomy in patients with ventilatory dependency
 
Data Presentation
Survival curves are presented using both parametric and nonparametric estimates. Confidence limits (CL) are asymmetric and equivalent to ±1 standard error (68%). Tabular presentation of multivariable models is in terms of regression coefficients and their standard errors rather than odds and hazard ratios. This is in part because most continuous variables required transformation of scale, making these ratios difficult to interpret, and also because the models of survival are inherently ones of nonproportional hazards.


   Results
Top
 Abstract
 Introduction
 Patients and Methods
 Results
Discussion
 Conclusions
 Appendix E1
 Appendix E2
 References
 
Risk Factors for Ventilatory Dependency
Preoperative prediction model
Ventilatory dependency gradually declined in frequency over the study period (Figure E1, A; P < .0001). Preoperative factors that predisposed patients to postoperative ventilatory dependency included higher body mass index, higher New York Heart Association (NYHA) class, chronic obstructive pulmonary disease, and any aortic procedure (Table E3). To illustrate the impact of these factors on postoperative ventilatory dependency, for a typical patient undergoing elective primary isolated coronary artery bypass grafting in 2001 (body mass index 27 kg · m–2, NYHA class II, blood urea nitrogen 18 mg · dL–1, hematocrit 39%, no previous myocardial infarction, no peripheral arterial disease, no preoperative heart failure, no chronic obstructive pulmonary disease), risk of ventilatory dependency is predicted to be 0.89% (CL 0.81%-0.97%). In contrast, for a patient undergoing elective reoperative double valve replacement with otherwise similar characteristics (except NYHA class III or IV, hematocrit of 30%, and tricuspid valve regurgitation), risk of ventilatory dependency is predicted to be 11.4% (CL 11.1%-11.7%).


Figure 5
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  		Figure E1. Relationship of various factors to postoperative ventilatory dependency. Closed circles represent summary data, and solid lines are trend lines enclosed within 68% confidence limits (±1 standard error). A, Date of operation. B, Preoperative hematocrit. C, Cardiopulmonary bypass (CPB) time.

 

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  		TABLE E3 Risk factors for ventilatory dependency: Preoperative model
 
Postoperative prediction model
When operative details, measurements made on ICU admission (including medications for the first 24 hours), and events occurring before ventilatory dependency developed were considered, almost all preoperative factors were displaced from the prediction model (Table 1). Consequently, only higher NYHA class, higher blood urea nitrogen, lower hematocrit (Figure E1, B), emergency operation, and operation performed earlier in the series remained in the postoperative prediction model.


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  		TABLE 1 Incremental risk factors for development of ventilatory dependency, based on preoperative, intraoperative, and postoperative variables
 
At induction of anesthesia, patients ultimately experiencing ventilatory dependency were tachycardic (Figure 1, A) and had higher central venous pressure. The only surgical risk factor was longer cardiopulmonary bypass time, particularly beyond 2 hours (Figure E1, C). Postoperative risk factors on entry to the ICU were pulmonary hypertension and low cardiac index (Figure 1, B and C). Not surprisingly, patients with ventilatory dependency required inotropic and vasoactive medications. They also were more likely to have experienced early postoperative events (before ventilatory dependency or within the first 72 hours), including bleeding necessitating reoperation, stroke, myocardial infarction, and bacteremia and sepsis.


Figure 1
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  		Figure 1. Relationship of various factors to postoperative ventilatory dependency. Closed circles represent summary data, and solid lines are trend lines enclosed within 68% confidence limits (±1 standard error). A, Heart rate at anesthesia induction. B, Pulmonary artery (PA) diastolic pressure at intensive care unit (ICU) admission. C, Cardiac index at ICU admission.

 
Outcomes of Ventilatory Dependency
Patients experiencing ventilatory dependency had high hospital morbidity (bacteremia, 159/704, 23%; renal failure, 104/704, 15%; stroke, 45/704, 6.4%; myocardial infarction 4/704, 0.52%). Hospital mortality was 30% (209/704). At 30 days, 6 months, and 5 years, survival was 76%, 53%, and 33%, respectively (Figure 2).


Figure 2
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  		Figure 2. Survival of patients experiencing ventilatory dependency. Symbols represent deaths, vertical lines are 68% CLsequivalent to ±1 standard error, and numbers in parentheses are patients remaining at risk. Solid lines enclosed within dashed CLs are parametric estimates. Time zero is after 72 cumulative hours of intubation after cardiovascular surgery.

 
Risk factors for early mortality after ventilatory dependency were dominated by those relating to cardiac, rather than pulmonary, dysfunction (Table 2). These included low cardiac index (Figure E2, A), hypotension, metabolic acidosis (Figure E2, B), and use of vasopressin at initial ICU admission. Although overall survival of patients with early and persistent ventilatory dependency was similar to that of those who had 1 or more unsuccessful extubation attempts (P[log–rank] = .8), the longer the interval before reintubation, the higher the risk of death. Older age, chronic renal failure, and early carbon dioxide retention dominated the late hazard phase.


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  		TABLE 2 Incremental risk factors for all-cause mortality after development of ventilatory dependency
 

Figure 6
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  		Figure E2. Univariable trends of risk factors with 2-year survival. A, Cardiac index at intensive care unit (ICU) admission. B, Acidosis at ICU admission.

 
Impact of Tracheostomy
Use of tracheostomy peaked about 9 days after the onset of ventilatory dependency and declined rapidly thereafter, with 24% of all patients in ventilatory dependency receiving tracheostomy by 28 days (Figure 3). Risk factors for tracheostomy included older age (Figure E3), chronic obstructive pulmonary disease, pre-existing renal insufficiency, and longer cardiopulmonary bypass time (Table 3).


Figure 3
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  		Figure 3. Timing of tracheostomy after onset of ventilatory dependency.

 

Figure 7
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  		Figure E3. Relationship of age and timing of tracheostomy.

 

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  		TABLE 3 Factors predictive of time to tracheostomy in patients with ventilatory dependency
 
Observed survival after tracheostomy was 74% at 30 days and 26% at 2 years.8Go In contrast, predicted survival for these patients based on a competing-risks model that included only preoperative, operative, and ICU admission variables (Appendix E2) was 84% at 30 days and 58% at 2 years (Figure 4).


Figure 4
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  		Figure 4. Observed (open circles) versus predicted (solid line enclosed within dashed 68% CLs) survival after tracheostomy.

 

   Discussion
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Discussion
Conclusions
 Appendix E1
 Appendix E2
 References
 
Principal Findings
Ventilatory dependency, when present, complicates recovery from cardiovascular surgery and is associated with high early mortality. Risk factors representing the dynamic evolution of the patient’s condition become increasingly more sensitive to this event as it approaches. Specifically, interim events and the condition of the patient in the immediate postoperative period are more important than operative procedure and preoperative patient characteristics.

For most of these patients, ventilatory dependency occurs in the presence of a systemic syndrome, of which heart dysfunction appears to be a central component. We speculate that as myocardial function improves and hemodynamics stabilize, early ventilatory dependency abates, leading to patient survival. However, this is not a universal occurrence; ability to withstand the initial insult after surgery is not the same among patients, because factors such as age, chronic renal insufficiency, and preoperative pulmonary dysfunction also interact and affect survival after ventilatory dependency has developed. In this context, tracheostomy is not a lifesaving intervention; rather, it appears to be a marker for patients less likely to recover from their ventilatory dependency.8Go

Prevalence
Prevalence of ventilatory dependency after cardiovascular surgery in this series was within the range reported by others,7,16,17Go varying from 3% to 22%. This wide variability is in large part attributable to the discrepant definitions of ventilatory dependency in the literature. The 72 hours of cumulative intubation16Go used in this study to define ventilatory dependency was believed to provide ample time for expected convalescence, even after complex operations involving systemic hypothermia and circulatory arrest. Application of STS guidelines (48 hours of intubation) would have led to a considerably larger and more heterogeneous patient population.

Decline in prevalence of ventilatory dependency in this study is a continuation of a trend apparent from previous studies from our institution.18-20Go It is surprising that this decline has occurred in the face of increasing complexity and acuity of illness.21-23Go Because early cardiac function appears to be of critical importance, perhaps strategies developed to preserve myocardial function and minimize hemodynamic instability help explain this. Intraoperative myocardial protection has become increasingly more sophisticated, routine intraoperative echocardiography has reduced air embolism and arterioembolism, and cardiopulmonary bypass has been refined. In addition, collaborative management of heart failure has developed to include judicious use of intra-aortic balloon pumps, newer pharmacologic agents, and adherence to multidisciplinary management algorithms.

Risk factors
A strategy was developed to identify risk factors sequentially for ventilatory dependency based temporally on preoperative, operative, and early postoperative conditions, plus interim events, to find the most sensitive predictors and determine whether preoperative prediction was accurate. The rapidly changing condition of the patient (both worsening and improving) made preoperative modeling much less reliable than models generated from variables representing condition closer to the actual event of ventilatory dependency. This is not surprising in light of our companion study of risk factors for survival after tracheostomy, in which data pertaining to the events leading up to tracheostomy were the most predictive of outcome.8Go

Most risk factors for ventilatory dependency reflect early primary myocardial dysfunction and hemodynamic instability, both preoperatively and, more important, postoperatively. This is similar to the findings of others and previous reports from this institution.8,19,20Go Jubran and colleagues24Go have identified an association between myocardial function and successful weaning from mechanical ventilatory support, demonstrating that failure to wean was less related to gas exchange than to the heart’s inability to meet increased demands accompanying spontaneous respiration.

Outcomes
Not surprisingly, mortality was high in patients experiencing ventilatory dependency.19Go This may reflect our ability today to palliate ultimately unsalvageable patients beyond 72 hours, whereas previously, these same patients once died of acute cardiac failure shortly after their index operation. This is supported by our finding that risk factors for death, similarly to risk factors for ventilatory dependency, principally encompass variables related to myocardial dysfunction and hemodynamic instability.

It appears as though ventilatory dependency precipitates, follows, or is concurrent with a cascade of morbid events, ultimately resulting in multisystem organ failure and death. For patients surviving the early postoperative period, noncardiac comorbidities eventually become more important.

Impact of tracheostomy
About a fifth of our patients with ventilatory dependency underwent tracheostomy. They appeared to have weathered the initial operative insult and demonstrated sufficient hemodynamic stability to be considered for tracheostomy. Nevertheless, a number of events between operation and tracheostomy interceded.8Go The cumulative effect of ventilatory dependency and these additional complications appeared to reduce survival after tracheostomy compared with that predicted without knowledge of these interim events occurring after the index operation. Because the postoperative course before tracheostomy so dramatically influences outcome thereafter, algorithms to guide tracheostomy as a salvage intervention have been difficult to generate. Unfortunately, decision for tracheostomy cannot currently be made on the basis of a potential survival advantage, but rather must be made for other indications, such as airway preservation, improved pulmonary toilet, ease of nursing care, and facilitation of patient mobilization.8Go

Limitations
This is a single-institution study. However, in contrast to other studies, it provides information about long-term survival of patients experiencing ventilatory dependency and, by means of competing risks analysis, the impact of tracheostomy. For some early postoperative events, it is difficult to say whether they accompanied ventilatory dependency or contributed to it. Use of inotropic and vasoactive agents is confounded by protocols specific to this institution. It is institutional policy to minimize use of these agents, and this may magnify their association with ventilatory dependency. We appreciate that these protocols may vary in other settings.


   Conclusions
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Discussion
 Conclusions
Appendix E1
 Appendix E2
 References
 
Continued improvement in operative and postoperative strategies to preserve myocardial function and stabilize hemodynamics after cardiovascular surgery should decrease the prevalence of ventilatory dependency. Preoperative models of ventilatory dependency are currently too insensitive for clinical use. Tracheostomy for ventilatory dependency and its outcome are poorly predicted, highlighting the complex interaction of events altering the patient’s condition before and after tracheostomy.


   Appendix E1
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Discussion
 Conclusions
 Appendix E1
Appendix E2
 References
 
Variables Available for Analysis

Preoperative
Demography
Sex, age at operation (y), height (cm), weight (kg), body surface area (m2), body mass index (kg · m–2).

Clinical condition
NYHA functional class (I–IV), Canadian angina class (0-4), emergency operation.

Cardiac status
Left ventricular dysfunction (grade), left ventricular ejection fraction (%), electrocardiogram infarction, previous myocardial infarction.

Cardiac comorbidity
Pulmonary hypertension; number of previous cardiovascular operations; number of coronary artery systems diseased (≥50% stenosis); ≥50% and ≥70% stenoses of left main coronary artery, left anterior descending coronary artery, circumflex coronary artery, right coronary artery; atrial fibrillation/flutter; complete heart block/pacer; ventricular arrhythmia; endocarditis; previous cardiac operation.

Noncardiac comorbidity
Serum albumin (g · dL–1), blood urea nitrogen (mg · dL–1), creatinine (mg · dL–1), creatinine clearance (mL · min–1), glomerular filtration rate (mL · min–1), bilirubin (mg · dL–1), hematocrit (%), chronic obstructive pulmonary disease, history of heart failure, hypertension, history of smoking, stroke, diabetes (diet controlled, oral hypoglycemic treated, insulin treated), dysrhythmia, peripheral arterial disease, carotid disease, popliteal disease, renal disease.

Intraoperative
Resting hemodynamics at anesthesia induction
Heart rate (beats · min–1), mean arterial pressure (mm Hg), pulmonary artery diastolic pressure (mm Hg), pulmonary artery systolic pressure (mm Hg), cardiac output (L · min–1), cardiac index (L · min–1 · m–2).

Procedure
Coronary artery bypass grafting, aortic valve replacement, mitral valve repair, mitral valve replacement, thoracic aortic surgery, cardiopulmonary bypass time (min), aortic clamp time (min).

Experience
Date of operation (years since January 1, 1998).

On Admission to ICU
Cardiac status
Cardiac output (L · min–1), cardiac index (L · min–1 · m–2), central venous pressure (mm Hg), heart rate (beats · min–1), pulmonary artery diastolic pressure (mm Hg), pulmonary artery systolic pressure (mm Hg), mean systemic arterial pressure (mm Hg).

Respiratory status
Ventilatory rate (breaths · min–1), ventilatory support (mode), positive end-expiratory pressure (cm H2O), tidal volume (L), minute volume (L · min–1), FIO 2, core temperature (°C), pH, PaCO 2, PaO 2, HCO3–, positive end-expiratory pressure (cm H2O), pH.

Medications given in ICU (first 24 hours)
Amiodarone, dobutamine, epinephrine, lidocaine, milrinone, norepinephrine, phenylephrine, vasopressin.

Interim events between operation and ventilatory dependency
Reoperation for bleeding, stroke, myocardial infarction, septicemia/bacteremia, sepsis, renal failure.


   Appendix E2
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Discussion
 Conclusions
 Appendix E1
 Appendix E2
References
 
Competing Risks of Death and Tracheostomy

Because death removes patients from risk for tracheostomy, true prevalence of tracheostomy is not the same as probability of receiving a tracheostomy. Thus, the 2 driving forces, hazard functions for death and tracheostomy, were allowed to act simultaneously from onset of ventilatory dependency to determine the proportions of patients receiving tracheostomy and dead as a function of time (Figure E4, A). The proportion of patients receiving tracheostomy was approximately 20%, about balanced early after onset of pulmonary failure by death before tracheostomy; however, thereafter there were continuing deaths, such that by 2 years, more than a third of patients were dead (Figure E4, B).


Figure 8
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  		Figure E4. Instantaneous risk of events. A, Competing risks of death before tracheostomy and of undergoing tracheostomy among patients with ventilatory dependency. Time zero is 72 cumulative hours of intubation after cardiovascular surgery. B, Result of simultaneous risk of competing events on prevalence of each state. All patients start in the state "event-free survival" and migrate at rates shown in panel A into "death before tracheostomy" or "tracheostomy."

 
These figures, however, are only averages. Figure E5, A, depicts a low-risk patient and Figure E5, B, a high-risk patient to illustrate the interplay between death and tracheostomy.


Figure 9
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  		Figure E5. Predicted survival after developing ventilatory dependency, in the competing-risks format of Figure E4, B. A, Low-risk patient (see table). B, High-risk patient (see table).
Low risk High risk
50 Age (y) 70
80 PaO 2 (mm Hg) 80
15 PPA, diastolic (mm Hg) 20
90 Aortic clamp time (min) 90
110 Total CPB time (min) 200
72 Interval from end of operation to ventilatory dependency (h) 200
24 HCO3– (mmol · L–1) 23
7,200 Minute volume (L · min–1) 5,000
3 Cardiac index (L · min–1 · m–2) 2.2
16 BUN (mg · dL–1) 37
No Insulin-treated diabetes Yes
No Vasopressin Yes
No Hypertension Yes
No COPD Yes

Values for variables used to simulate low- and high-risk patients. These values are used in multivariable equations, represented by Tables 2 and 3, for all-cause mortality after development of ventilatory dependency and for time to tracheostomy. BUN, blood urea nitrogen; COPD, chronic obstructive pulmonary disease; CPB, cardiopulmonary bypass.

 


   Acknowledgments
 
We thank Marvin Leventhal and Angela York for data management, Songhua Lin for statistical programming, and Lucinda Mitchin and Tess Parry for editorial assistance.


   References
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Discussion
 Conclusions
 Appendix E1
 Appendix E2
 References
 

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