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Ann Thorac Surg 1997;63:1026-1033
© 1997 The Society of Thoracic Surgeons

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Original Article: Cardiovascular

Determinants of Longer Duration of Endotracheal Intubation After Adult Cardiac Operations

Section of Cardiothoracic Surgery, Indiana University Medical Center and RL Roudebush Veterans Affairs Medical Center, Indianapolis, Indiana

Accepted for publication October 26, 1996.

	Abstract

Top Footnotes Abstract Introduction Patients and Methods Data Collection Anesthetic Administration and... Data Analysis Results Multivariate Predictors of... Inability of Pulmonary Function... Effects of Cardiac Function... Comment Appendix 1. Definitions References

 
Background. Poor pulmonary reserve is a risk factor that is used to exclude some patients from major operations. However, the value of routine spirometry in patients undergoing cardiac operations has not been widely evaluated.

Methods. The outcomes of 586 consecutive adult patients undergoing cardiac operations were reviewed retrospectively to assess predictors of longer duration of endotracheal intubation.

Results. By univariate analysis, congestive failure (p < 0.001), cardiomegaly (p = 0.002), recent myocardial infarction (p = 0.039), priority of operation (p = 0.005), previous cardiac operation (p < 0.001), and renal insufficiency (p = 0.002) increased the risk of longer endotracheal intubation. Spirometry (forced vital capacity, forced expiratory volume at 1 second, the ratio of forced expiratory volume at 1 second to forced vital capacity) did not correlate with longer endotracheal intubation. Perioperative complications, such as myocardial infarction (p < 0.001), coma, reexploration for bleeding, and reduced cardiac output (p < 0.001 each), correlated with longer duration of intubation. By multiple regression, priority of operation (p = 0.03), congestive failure (p = 0.02), and previous cardiac operation (p = 0.005) among preoperative risks and bleeding, reduced cardiac output, stroke, coma, and MB fraction of creatine kinase released postoperatively (p < 0.001 each) predicted longer duration of endotracheal intubation.

Conclusions. Postoperative cardiac function and the occurrence of complications are more significant than preoperative pulmonary function in determining the duration of endotracheal intubation after cardiac operation. Routine spirometry is probably unnecessary for most adult cardiac patients.

	Introduction

Top Footnotes Abstract Introduction Patients and Methods Data Collection Anesthetic Administration and... Data Analysis Results Multivariate Predictors of... Inability of Pulmonary Function... Effects of Cardiac Function... Comment Appendix 1. Definitions References

 
Pulmonary function tests (PFTs) with spirometry are important parameters for ventilator management after major thoracic operations [1]. However, few studies have assessed risk factors for longer duration of endotracheal intubation after cardiac operation [2–4]. The occurrence of respiratory complications and the duration of endotracheal intubation have been shown to correlate with mortality in patients who have undergone cardiac operations [5, 6]. However, the influence of impaired lung function as indicated by preoperative PFTs on the outcome of cardiac operation remains unclear. To assess the usefulness of routine preoperative pulmonary function testing and the relative significance of other potential risk factors for respiratory compromise, we performed a retrospective cohort study whose first goal was to examine the ability of preoperative PFTs to predict prolonged postoperative endotracheal intubation. The second purpose of this study was to identify specific risk factors beyond baseline lung function that might be associated with longer duration of endotracheal intubation.

	Patients and Methods

Top Footnotes Abstract Introduction Patients and Methods Data Collection Anesthetic Administration and... Data Analysis Results Multivariate Predictors of... Inability of Pulmonary Function... Effects of Cardiac Function... Comment Appendix 1. Definitions References

 
Study Location and Population
This study was conducted at the Indiana University Medical Center and the RL Roudebush Veterans Affairs Medical Center in Indianapolis, between June 1989 and October 1994. A cohort of 586 consecutive adult patients undergoing cardiac operation were included in this study. Patients undergoing either coronary bypass grafting or valve procedures were eligible for inclusion. Patients undergoing operation involving the great vessels or left ventricular aneurysm repair were not included.

During this time frame, all patients had spirometry and diffusion capacity requested as part of their routine preoperative evaluation. No patient who did have PFTs performed was denied operation on the basis of these tests, as the intent was to risk stratify patients and facilitate postoperative management, not to eliminate "high-risk" patients. Operation was not delayed, however, solely to obtain pulmonary function studies. Thus, when PFTs were not available, operation proceeded without obtaining these studies. These patients were included in the study to enhance the ability to identify nonspirometric factors that influenced the duration of endotracheal intubation.

	Data Collection

Top Footnotes Abstract Introduction Patients and Methods Data Collection Anesthetic Administration and... Data Analysis Results Multivariate Predictors of... Inability of Pulmonary Function... Effects of Cardiac Function... Comment Appendix 1. Definitions References

 
The investigators made observations in a concurrent manner during preoperative and intraoperative assessment of the patient or on daily rounds in the cardiac surgery intensive care unit. The information was recorded in a computerized data base. Baseline patient characteristics are given in Table 1. Mean values for the major risk factors that were analyzed are given in Table 2. A complete listing of the clinical parameters and their definitions is given in Appendix 1. Pulmonary function tests included forced expiratory volume at 1 second (FEV₁), forced vital capacity (FVC), the ratio of FEV₁ to FVC, and diffusion capacity. The absolute values and the value of each test expressed as the percentage of the predicted normal value were recorded.

Only 8 patients in the study sample had low cardiac output syndrome as defined by a persistent cardiac index less than 2.0 L • min^-1 • m^-2. To better evaluate the significance of reduced postoperative cardiac function, we identified those patients in whom the cardiac index remained less than 2.5 L • min^{-1 •} m^-2 for 6 or more hours in the first 24 hours after operation (reduced cardiac output).

The duration of endotracheal intubation was calculated as sum of all time spent intubated for ventilation postoperatively. Thus, the time accrued during a second period of endotracheal intubation in those patients who required reintubation was added to the first time period to determine the total intubation time. Reintubation itself was not independently analyzed, as the number of patients involved was too small for an adequate analysis.

The decision to extubate each patient was determined by an independent group of physicians comprised of critical care anesthesiologists at our institution. Patients were candidates for extubation when they were hemodynamically stable, alert, breathing spontaneously, with an oxygen saturation at more than 90% on less than 40% oxygen and with a partial pressure of carbon dioxide of less than 45 mm Hg.

	Anesthetic Administration and Cardiopulmonary Bypass

Top Footnotes Abstract Introduction Patients and Methods Data Collection Anesthetic Administration and... Data Analysis Results Multivariate Predictors of... Inability of Pulmonary Function... Effects of Cardiac Function... Comment Appendix 1. Definitions References

 
The anesthetic technique used for this cohort of patients consisted of a high-dose narcotic (sufentanil, 2 µg/kg loading dose then 1 µg • kg^{-1 •} h^-1 infusion), a muscle relaxant (vecuronium, 20 µg • kg^-1 • h^-1 infusion), and a sedative hypnotic agent (midazolam, 2 to 5 mg load, then 30 µg • kg^{-1 •} h^-1 infusion). Intravenous anesthetics were supplemented if necessary with isoflurane (0.25% to 1.25%) during cardiopulmonary bypass to maintain a mean blood pressure at less than 90 mm Hg. Fast-track anesthetic techniques based on inhalation agents supplemented with low doses of narcotics were not used in this population. Postoperative pain management consisted of 1 to 2 mg of morphine sulfate administered intravenously at 10-minute intervals as needed. This was supplemented with small doses (1 to 2 mg) of midazolam if necessary for patients who became agitated but who had not yet met criteria for extubation.

Cardiopulmonary bypass was conducted under moderate systemic hypothermia (28° to 30°C), using a hollow-fiber oxygenator (Sorin, Sacramento, CA). A roller pump was used and the circuit was primed with 2,400 mL of crystalloid to which 25 g of mannitol and 50 g of albumin were added. Ventilation was stopped, but positive pressure (2.5 to 5 cm H₂O) was maintained within the lungs during cardiopulmonary bypass. Distal anastomoses and valve repair or replacement were done using a single cross-clamp interval and cold blood cardioplegia delivered through a combination of antegrade and retrograde catheters. Proximal coronary graft anastomoses were performed after the removal of the aortic cross-clamp using a partial occluding clamp.

	Data Analysis

Top Footnotes Abstract Introduction Patients and Methods Data Collection Anesthetic Administration and... Data Analysis Results Multivariate Predictors of... Inability of Pulmonary Function... Effects of Cardiac Function... Comment Appendix 1. Definitions References

 
A series of univariate and multivariate analyses were used to define a set of patient-related risk factors containing the maximum predictive power for longer duration of endotracheal intubation in the 586 patients. A two-sample t test was used for parametric analysis of hours of intubation between groups of patients defined by the presence or absence of a single risk factor (Table 3). Linear correlations were computed for each continuous variable versus hours of intubation (Table 4). Variables with a type I error of less than 0.2 on univariate analysis were entered into stepwise linear regression models. All statistical tests were conducted with a commercially available statistical package (CSS Statistica; Statsoft, Tulsa, OK) running on a personal computer.

	Results

Top Footnotes Abstract Introduction Patients and Methods Data Collection Anesthetic Administration and... Data Analysis Results Multivariate Predictors of... Inability of Pulmonary Function... Effects of Cardiac Function... Comment Appendix 1. Definitions References

Among continuous variables, no preoperative pulmonary function study predicted longer postoperative duration of endotracheal intubation when analyzed either as the absolute value or as the percent of predicted. Correlation coefficients of less than 0.06 were generally found for the pulmonary function studies (Table 4).

Preoperative left ventricular wall score, prolonged cardiopulmonary bypass, or cross-clamp time, high postoperative creatine kinase MB fraction (CK-MB) release, and increased chest tube drainage were highly significant (p < 0.001 for each) risk factors for longer duration of endotracheal intubation. Moreover, age (p = 0.03), priority of operation (p = 0.01), and lower final cardiac index after operation (p = 0.03) were associated with longer duration of endotracheal intubation.

The association with age was primarily in the younger patients, and older age was not associated with an increased risk of longer duration of endotracheal intubation in this series. The mean duration of endotracheal intubation in the 122 patients more than 70 years of age was 30.9 hours (versus 29.5 hours in those patients younger than 70, p = 0.797). There were only three octogenarians included in this series, with a mean duration of endotracheal intubation of 23 hours.

	Multivariate Predictors of Longer Duration of Endotracheal Intubation

Top Footnotes Abstract Introduction Patients and Methods Data Collection Anesthetic Administration and... Data Analysis Results Multivariate Predictors of... Inability of Pulmonary Function... Effects of Cardiac Function... Comment Appendix 1. Definitions References

 
Multiple regression analysis (R² = 0.091, p < 0.0001) of the preoperative risk factors revealed that the priority of operation, the presence of congestive heart failure preoperatively, previous cardiac operation, renal insufficiency, and a recent myocardial infarction resulted in longer postoperative endotracheal intubation times (Table 5). When preoperative PFT results were forced into the regression models, they failed to improve the predictive power of the models for longer duration of endotracheal intubation after adult cardiac operation.

	Inability of Pulmonary Function Tests to Predict Longer Duration of Endotracheal Intubation

Top Footnotes Abstract Introduction Patients and Methods Data Collection Anesthetic Administration and... Data Analysis Results Multivariate Predictors of... Inability of Pulmonary Function... Effects of Cardiac Function... Comment Appendix 1. Definitions References

View larger version (21K): [in this window] [in a new window]   Fig 1. . Hours of endotracheal intubation in subsets of patients based on percent of predicted forced expiratory volume at 1 second (FEV1).

Patients who did not undergo routine spirometry had a higher priority of operation (1.9 versus 1.4, p < 0.0001), a lower incidence of dyspnea as a presenting complaint (43% versus 52%, p = 0.039), an average of 16 minutes shorter pump time (145 versus 161 minutes, p = 0.002), and a lower final cardiac index (3.08 versus 3.21 L • min^{-1 •} m^-2, p = 0.013). With the exception of priority of operation, none of these variables were associated with longer duration of endotracheal intubation in the multivariable models. Patients without preoperative PFTs were similar to patients who did receive PFTs with regard to all other risk factors studied.

The higher priority of operation would lead one to expect somewhat longer endotracheal intubation times among patients with no preoperative PFTs. However, the lack of preoperative PFTs did not adversely impact the ability to extubate patients postoperatively. Patients without preoperative pulmonary function testing had a strikingly similar average duration of endotracheal intubation as compared to those patients with known baseline pulmonary function (30.3 versus 30.8 hours, p = 0.91). In addition, the proportion of patients extubated at less than 8 hours, 9 to 18 hours, 19 to 24 hours, and beyond 24 hours was similar in patients with and without preoperative PFTs (Figure 2).

View larger version (23K): [in this window] [in a new window]   Fig 2. . Distribution of extubation times for patients with or without preoperative pulmonary function tests (PTFs).

	Effects of Cardiac Function and Postoperative Complications on Endotracheal Intubation

Top Footnotes Abstract Introduction Patients and Methods Data Collection Anesthetic Administration and... Data Analysis Results Multivariate Predictors of... Inability of Pulmonary Function... Effects of Cardiac Function... Comment Appendix 1. Definitions References

When postoperative complications are taken into account, the predictive power of renal insufficiency and priority of operation declined, suggesting that these were more predictors of postoperative complications, which then affected ventilation rather than primary predictors of respiratory compromise. Only two preoperative predictors remained significant in this model, previous cardiac operation and congestive heart failure. Including postoperative complications, such as bleeding, coma, high CK-MB peak, reduced cardiac output, and cerebrovascular accident significantly improved the predictive power of the multivariate model (R² = 0.316, p < 0.001, Table 6), but still left a large portion of the variability in the duration of endotracheal intubation unaccounted.

	Comment

Top Footnotes Abstract Introduction Patients and Methods Data Collection Anesthetic Administration and... Data Analysis Results Multivariate Predictors of... Inability of Pulmonary Function... Effects of Cardiac Function... Comment Appendix 1. Definitions References

This study demonstrates that perioperative cardiac function and the presence of postoperative complications significantly influence the requirements for endotracheal intubation after adult cardiac operation. In contrast, parameters derived from routine preoperative pulmonary function testing have no significant correlation with postoperative endotracheal intubation requirements. This is true when the individual tests are evaluated by either univariate or multivariate techniques. It remains true when the tests are combined in an attempt to isolate a high risk subset of patients. This suggests that postoperative cardiac function, neurologic events, and other complications outweigh the significance of underlying pulmonary disease for the majority of cardiac patients undergoing operation.

The low correlation coefficient in the multivariable model of preoperative risk factors means that the preoperative parameters used in this study are extremely poor predictors of the duration of endotracheal intubation after cardiac operation. Several of the risk factors identified by univariate analysis may have only an indirect effect on intubation, through their association with an increased risk of complications. Taking into account postoperative complications themselves improves the model, but still does not explain even the majority of the variation in extubation times. This may be a reflection of the fact that the recovery time for a stroke or perioperative ischemic insult varies widely among patients, making it nearly impossible to assess the exact impact of one of these complications on an individual patient.

Other studies of the value of preoperative pulmonary function testing in cardiac operation have generally not found an association with PFTs and longer intubation times. Durand and associates [2], however, in a study reported in 1993, found that both operative mortality and the duration of mechanical ventilation were increased when FEV₁ was less than 1.5 L, ventilatory capacity was less than 2.5 L, or the arterial carbon dioxide tension was less than 8.5 kPa. The mean duration of mechanical ventilation in the Durand study was extremely long (77 hours in patients with moderate pulmonary function, ie, FEV₁ = 1.5 to 2.5 L). Furthermore, the study did not examine perioperative cardiac function or indices of the complexity of operation. Although it is tempting to conclude that poor pulmonary function led to a higher operative mortality, it is also quite possible that differences in baseline cardiac function or postoperative complications led to both high operative mortality and longer ventilation times.

Cain and co-workers [3] studied 106 patients with preoperative and postoperative PFTs including FEV₁, FEF_25–75, FEV₁/FVC, and peak expiratory flow rate. They found a 29% incidence of prolonged cardiac surgery intensive care unit stay (more than 5 days) with lower spirometric values in patients with extended stays. They could not, however, correlate the longer intensive care unit stays with postoperative pulmonary complications and concluded that PFTs were not useful predictors of extended intubation.

In a study that examined more than 45 clinical variables as predictors of prolonged postoperative ventilation in cardiac surgical patients, Ingersoll and Grippi [11] found no correlation between FEV₁, FVC or FEV_1/FVC, and longer duration of endotracheal intubation [11]. In fact, patients with longer duration of endotracheal intubation postoperatively tended to have better pulmonary spirometric values preoperatively, a finding that is consistent with the results of our study.

In a more recent study of the predictive value of preoperative pulmonary function tests, Spivack and colleagues [12] found that pulmonary diagnosis, lung mechanics, and blood gas parameters did not offer useful rules for predicting postoperative respiratory outcome in cardiac operation. The investigators suggested that pulmonary function testing should not be used as exclusion criteria for coronary artery bypass grafting.

One interpretation of these results is that pulmonary function studies obtained immediately before operation do not accurately assess respiratory reserve. Effort-dependent measures may be impaired with a recent severe cardiac event such as myocardial infarction, unstable angina, or congestive heart failure. Another consideration is that changes in extravascular lung water associated with poor ventricular function or valvular heart disease may reduce lung compliance and temporarily alter the results of pulmonary function studies. With diuresis and improved cardiac function postoperatively, the respiratory dysfunction may also resolve. New onset cardiac dysfunction as seen with a perioperative myocardial infarction may result in the opposite sequence of events (good preoperative pulmonary function deteriorating postoperatively). In such circumstances, the time frame of reversal of cardiac dysfunction may be the critical determinant of postoperative respiratory function. This would help to explain the poor predictive value of the multivariable models, as one would be attempting to predict the time frame of recovery of cardiac function in patients with either severe preoperative left ventricular dysfunction or a new perioperative event. This time period is quite variable from one patient to another. It also would help to explain why clinical parameters of chronically impaired heart function, such as congestive heart failure, seem superior to more instantaneous parameters of heart function such as wall score or ejection fraction. A dilated, poorly functioning heart resulting in chronic congestive heart failure (from either severe ischemic cardiomyopathy or valvular heart disease) often takes months to improve after a successful operation. Stunned or hibernating myocardium, on the other hand, might be associated with a poor ejection fraction preoperatively, then show either immediate or delayed improvement after a successful revascularization. The variability in recovery time among patients with low ejection fractions but without clinical congestive heart failure would limit the predictive value of ejection fraction. Other studies have also noted an association with congestive heart failure and longer endotracheal intubation, but rarely have implicated ejection fraction [12–14].

The recent development of strategies for very early extubation and the use of "fast-track" anesthetic protocols raises the question of whether pulmonary function studies might be useful for stratifying patients who are not candidates for early extubation. No study of a fast-track protocol has yet reported the ability to select candidates for early extubation based on preoperative spirometry. The current trend seems to be toward including all or nearly all patients in fast-track protocols and using performance-based criteria with continuous clinical assessment to determine the timing of extubation [15].

In a review of 14 series between 1974 and 1994 using various strategies for early extubation after cardiac operation, Hickey and Carson [16] estimated that up to 80% of all patients undergoing adult cardiac operation might be candidates for early extubation. Only a portion of the studies they reviewed excluded patients from fast-track protocols. Exclusion criteria included "severe functional respiratory disorders" in at least one series, but were more often based on nonpulmonary parameters such as cardiac diagnosis, ventricular impairment, renal dysfunction, or neurologic disease.

Most of the recent reports of success with fast-track protocols leading to early extubation have stressed anesthetic technique, specialized recovery areas with nurses managing ventilator weaning, and standardized ventilator weaning protocols. Chong and colleagues [17] reported a median extubation time of 2 hours postoperatively for 198 patients having a variety of cardiac procedures. The range was 0 to 14 hours and only 15 patients were electively treated off protocol. Engleman and co-workers included all cardiac patients undergoing operation in their fast-track protocol and claimed a reduced level of cardiopulmonary complications with early extubation [18, 19].

An analysis of factors enabling extubation within 8 hours of coronary bypass grafting was recently reported by Habib and colleagues [14]: 513 of 522 patients were extubated within 24 hours of operation (51% before 8 hours). In a multivariable model, the most influential factors for delayed (more than 8 hours) extubation were found to be New York Heart Association class IV, need for an intraaortic balloon pump, and need for banked blood transfusion. In another study of early extubation, Arom and associates [13], found older age, use of preoperative diuretics, and unstable angina to be predictors of late extubation (more than 12 hours postoperatively). Thus, when a relatively short duration of postoperative endotracheal intubation was examined, perioperative cardiac function and complications tended to influence the timing of extubation. Routine spirometric tests were not used to help predict candidates for early extubation.

There is some evidence that the current trend toward early extubation may be the best management of patients with marginal pulmonary function. Several researchers including Engelman and Quasha have reported a lower incidence of cardiopulmonary complications in cardiac patients undergoing operation treated by early removal of the endotracheal tube [18–21]. Other studies have shown improvements in ventricular performance with spontaneous as opposed to positive pressure ventilation [22, 23]. Treating all patients with the intention of achieving early extubation would eliminate the need to distinguish between patients with good versus poor pulmonary function preoperatively.

One final argument in favor of continuing to perform routine pulmonary function testing before cardiac operation is that a "baseline" value is needed to judge when a patient's respiratory function has returned to their normal state before extubation. The remarkably similar distribution of extubation times in patients with or without preoperative pulmonary function studies, suggests that baseline values may not have a great impact on the timing of extubation.

Because the pulmonary function test results used in this study were obtained immediately before operation, this study did not examine the usefulness of PFTs as a measure of chronic pulmonary disease in predicting postoperative complications. Patients with a long-standing history of abnormal pulmonary function tests may well be at higher risk for surgical intervention and logically would have poorer long-term functional results from revascularization or valve replacement. Nor can this study show that there is absolutely no effect of baseline pulmonary function on postoperative complications after cardiac operation. A much larger sample size might well show a small, but statistically significant, effect of pulmonary function. It does show that, compared with other determinants of postoperative endotracheal intubation, routine preoperative spirometry has little predictive value in the average cardiac surgical candidate.

In conclusion, perioperative cardiac function and the occurrence of postoperative cardiac, neurologic, and other complications outweigh the effects of preoperative pulmonary function as measured by spirometry and diffusion capacity in determining the duration of endotracheal intubation after cardiac operation. Predicting the exact timing of extubation based on preoperative parameters is unreliable and decisions on the appropriate time to attempt extubation should probably be made on the basis of concurrent assessment of each patient's postoperative cardiorespiratory function. Routine preoperative pulmonary function tests fail to help predict the timing of extubation or the potential for extremely long endotracheal intubation for the majority of adult patients undergoing cardiac operation.

	Appendix 1. Definitions

Top Footnotes Abstract Introduction Patients and Methods Data Collection Anesthetic Administration and... Data Analysis Results Multivariate Predictors of... Inability of Pulmonary Function... Effects of Cardiac Function... Comment Appendix 1. Definitions References

 

	Footnotes

Top Footnotes Abstract Introduction Patients and Methods Data Collection Anesthetic Administration and... Data Analysis Results Multivariate Predictors of... Inability of Pulmonary Function... Effects of Cardiac Function... Comment Appendix 1. Definitions References

This article has been selected for the open discussion forum on the STS Web site: http://www.sts.org/annals

	References

Top Footnotes Abstract Introduction Patients and Methods Data Collection Anesthetic Administration and... Data Analysis Results Multivariate Predictors of... Inability of Pulmonary Function... Effects of Cardiac Function... Comment Appendix 1. Definitions References

 

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