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From the Department of Medicine, Vanderbilt University and Saint Thomas Hospital, and the Division of Cardiothoracic Surgery, Saint Thomas Hospital and the Saint Thomas Heart Institute, Nashville, TN.
A complete list of participants is located in the Appendix.
Correspondence to: Richard W. Light, MD, FCCP, Director of Pulmonary Disease Program, Saint Thomas Hospital, PO Box 380, 4220 Harding Rd, Nashville, TN 37202; e-mail: RLIGHT98{at}yahoo.com
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Abstract |
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 TOP Abstract IntroductionMaterials and MethodsResultsDiscussionConclusionAppendix 1References |
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Design: Retrospective study of 4,863 consecutive patients using univariate and multivariate survival analysis to identify independent risk factors.
Setting: Saint Thomas Hospital, Nashville, TN, a 575-bed, academically affiliated, regional referral hospital specializing in cardiovascular diseases.
Patients: All patients undergoing CABG in our hospital from January 1, 1996, to December 31, 1997.
Interventions: None.
Measurements and results: Duration of mechanical ventilation and mortality were measured. More than 94% of the patients were extubated in the first 3 days following surgery, 4% more were extubated from postoperative days 4 to 14, and almost 2% were receiving ventilation for > 14 days. Those risk factors that reflect preoperative medical instability, especially cardiac or respiratory insufficiency, were associated with the highest incidence of prolonged postoperative mechanical ventilation and for operative mortality. The Society of Thoracic Surgeons-predicted mortality estimate was the best single independent predictor for prolonged postoperative ventilation.
Conclusions: Typically, patients can be expected to be extubated within 3 days after CABG. Certain preoperative comorbidities, especially preoperative cardiac or respiratory instability, are predictive of prolonged postoperative mechanical ventilation.
Key Words: coronary artery bypass surgery • mechanical ventilation • ventilator weaning
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Introduction |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionConclusionAppendix 1References |
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The purpose of this study was to identify the typical postoperative duration of mechanical ventilation in a large cohort of CABG recipients and to identify factors in the patient’s history, physical examination, or preoperative treatment that were associated with prolonged mechanical ventilation.We hypothesized that failure to wean from the ventilator in this population would be predominantly related to postoperative medical instability, the risk of which would be increased by significant preoperative comorbid conditions.
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Materials and Methods |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionConclusionAppendix 1References |
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Patient Selection
All patients undergoing CABG (whether CABG alone or CABG
combined with valve surgery) at Saint Thomas Hospital in the 2-year
period from January 1, 1996, through December 31, 1997, were included
in the study. The cardiac surgeons at our hospital do not routinely
exclude patients with poor preoperative pulmonary function from
surgery, but rather counsel them as to the increased risk of operative
complications. This study was approved by the Institutional Review
Board of Saint Thomas Hospital.
Database
The medical records of all patients undergoing CABG at Saint
Thomas Hospital during the study period were reviewed after discharge
or death by a trained nurse abstractor according to the guidelines set
forth by the American College of Cardiology and the STS.3
The abstractor completed a standard database form comprised of patient
variables as defined by the STS.3
For quality assurance
purposes, the records of 10% of the patients were audited a second
time by a separate reviewer. Unless otherwise noted, the data were
available for each variable in > 99.5% of the patients. This
information was reported to the STS national database and kept in
spreadsheet form for internal use. Variables included in the database
include details of the patient’s history, physical findings,
laboratory data, cardiac catheterization data, intraoperative and
perioperative data, and postoperative complications, as documented in
the medical record. Additionally, a separate computerized record was
maintained of all services provided by the respiratory therapy
department, including total time receiving mechanical ventilation.
Variables
Duration of Mechanical Ventilation:
This is the number of
calendar days from the time of surgery to extubation. Fractions of a
day are rounded up, so that the hypothetical patient who is extubated
6 h after surgery would be recorded as extubated on postoperative
day 1. For the purpose of calculating days receiving ventilation,
patients reintubated within 24 h after extubation are considered
to have an uninterrupted period of ventilation.
Demographic Factors: These include age, gender, race (white or nonwhite), and body mass index (BMI; weight in kilograms divided by square of the height in meters).
Chronic Medical Problems: These include medical problems that have been present, or occurred > 30 days prior to the surgery, including the following: renal failure (as documented by the patient’s physician) or dialysis-dependent renal failure; prior aortic or mitral valve dysfunction; congestive heart failure (based on symptoms of paroxysmal nocturnal dyspnea or dyspnea on exertion due to heart failure, or chest radiograph showing pulmonary congestion) or cardiomyopathy (diagnosed clinically by a cardiologist or diagnosed by imaging study); history of diabetes, hypertension, myocardial infarction (MI), stroke, transient ischemic attack, hyperlipidemia, or COPD; and positive family history of early (< 55 years old) coronary artery disease (CAD). Also included is history of tobacco use, including current smokers (including any cigarette, cigar, pipe or chewing tobacco in the 30 days prior to surgery) and reformed smokers (those who have abstained from tobacco products for > 30 days prior to surgery).
Previous Cardiac Interventions: These include any history of angioplasty (including percutaneous atherectomy and stent placement), placement of a permanent pacemaker or implantable cardioverter/defibrillator, and any previous CABG or valve surgeries occurring > 30 days preoperatively.
Acute Medical Problems: These include medical problems that have occurred during the immediate preoperative hospitalization, including the following: respiratory failure requiring mechanical ventilation; cardiac arrest requiring cardiopulmonary resuscitation within 1 h prior to surgery; the use of preoperative vasopressors or inotropes; the use of an intra-aortic balloon pump prior to surgery; the use of IV heparin or IV nitroglycerin; MI < 30 days preoperatively; use of thrombolytics for recent MI; and preoperative arrhythmia (atrial fibrillation, atrioventricular block, or ventricular tachycardia).
Type of Operation: This specifies whether the surgery consists of CABG alone, CABG with valve, or CABG performed using minimally invasive approach (MIDCAB).
Operative Urgency: This indicates whether the operating surgeon considers the planned procedure to be elective (could be deferred without increased risk of poor cardiac outcome), urgent (required during same hospitalization to avoid clinical deterioration), or emergent (immediate operation indicated for refractory cardiac ischemia, shock, or other major clinical compromise).
STS score: This is a predicted risk of mortality (given as a percentage) that is derived from the STS national database, based on a proprietary logistic regression equation using preoperative variables.3 Centers that subscribe to this service purchase software that converts the values of the preoperative variables into the predicted mortality estimate without revealing the exact equation.
Mortality: Operative mortality includes all patients who died during the surgical procedure or any time during the postoperative hospitalization.
Statistical Analysis
Variables were analyzed using survival analysis techniques.
Kaplan-Meier curves were drawn using days after surgery on the x axis
and the probability of receiving ventilation on the y axis. A
"survival" curve corresponding to daily probability of continued
mechanical ventilation was drawn for the population of patients with
the variable present and for the population with the variable absent.
Variables identified as significant (p < 0.001) by the log-rank test
were included in a Cox proportional hazards regression model. Deaths
were censored in the analyses. The proportional hazards regression
model analyzes the influence of independent variables on the survival
time. In this case, survival is defined as time spent receiving
ventilation; therefore, when a regression equation is formed, a
negatively signed regression coefficient indicates that increasing
values of this variable are associated with prolonged time receiving
ventilation. Significance testing for mortality data was done using the
2 test. A post hoc comparison of
patient ages was performed using the unpaired t test. A statistical
software package (NCSS 2000; NCSS; Kaysville, UT) was used for all
analyses. A p value < 0.001 was chosen as significant to decrease the
type I errors associated with testing multiple variables.
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Results |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionConclusionAppendix 1References |
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When the patients were separated according to operating surgeon, there was no significant difference in postoperative duration of mechanical ventilation.
Table 1
shows the variables categorized into demographic factors, chronic
medical problems (those present or occurring > 30 days prior to
surgery), previous cardiac interventions, acute medical problems
(present during the same hospitalization), type of operation, operative
urgency, and STS score. For each risk factor in Table 1
, the total
number of patients having the risk factor is given, as well as the
percentage requiring mechanical ventilation 4 to 14 days after CABG and
> 14 days after CABG. The operative mortality for patients with each
risk factor is also provided.
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Chronic Medical Problems
When the medical history was considered, patients with renal
failure, preexisting cardiovascular disease (cardiomyopathy, valvular
dysfunction, congestive heart failure), prior stroke, or COPD required
prolonged postoperative mechanical ventilation more often than patients
without these risk factors. Other chronic medical conditions such as
prior MI, prior transient ischemic attack, and diabetes had no
significant influence on duration of mechanical ventilation. Tobacco
use was not independently associated with prolonged postoperative
mechanical ventilation.
Patients with preoperative renal failure were the most likely to require prolonged mechanical ventilation after CABG; 10.8% of these patients received ventilation from 4 to 14 days, compared with the normal 4.2%, while 9.7% of these patients remained receiving ventilation > 14 days compared with the normal 1.7% (Fig 3 ).
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Interestingly, the presence of hyperlipidemia and a positive family history of early cardiac disease were both significantly associated with a lower incidence of prolonged postoperative ventilation. The mean age at the time of CABG for these patients (Table 2 ) is also significantly lower than that of the overall study population.
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Acute Medical Problems
Not surprisingly, the patients who were in medically unstable
condition with respiratory or cardiac failure prior to surgery had the
highest incidence of prolonged mechanical ventilation following
surgery. There were 33 patients (0.7%) who received ventilation prior
to their surgery. Of these, only eight patients (24.2%) were extubated
in the typical 1 to 3 days after CABG, while two patients died in this
period. The remaining patients required a prolonged duration of
mechanical ventilation (39.4% from 4 to 14 days, 30.3% > 14 days)
and had a higher operative mortality, 21.2% overall.
Those patients with cardiogenic shock requiring IV pressor or inotropic agents or the use of an intra-aortic balloon pump had significantly increased postoperative ventilation requirements, with approximately 30% receiving ventilation from 4 to 14 days, and 11.6 to 13.8% receiving ventilation > 14 days. Other active cardiovascular problems, including arrhythmia or recent MI, also carried increased risk, but to a lesser degree.
Type of Operation and Operative Urgency
The incidence of prolonged postoperative ventilation
increased when the CABG was combined with concomitant valve surgery, as
well as when the operation was considered urgent or emergent. Of the 10
patients in our series who had CABG by the minimally
invasive approach, there were 2 patients who required ventilation
beyond the typical 3 days; 1 patient was extubated on day 4, and 1
patient was extubated on day 7.
STS Score
An increased preoperative predicted mortality from the STS
logistic regression equation was associated with an increased incidence
of prolonged postoperative mechanical ventilation. The risk of
prolonged ventilation dramatically increases when the predicted
mortality is > 10%.
Regression Analysis
Many of the variables studied are interrelated. To analyze the
relative influence of the significant variables on the duration of
mechanical ventilation, Cox proportional hazards regression analysis
was performed (Table 3
). Several factors from each of the categories were important. Among the
demographic factors, increasing age and female gender were identified
as important predictors of increased postoperative ventilatory
requirements. Among chronic medical problems, the presence of mitral
valve disease, renal failure, a history of a previous CABG, and a
history of a cerebrovascular accident were all significant
independent factors, but morbid obesity and COPD were not. When factors
present immediately prior to surgery were considered, the use of
pressors or inotropic agents, the operative urgency, preoperative
mechanical ventilation, and the presence of an acute MI were all
important independent predictors of prolonged mechanical ventilation.
With regards to operative factors, concomitant valve surgery was also
an independent risk factor.
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Discussion |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionConclusionAppendix 1References |
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Those risk factors that reflect preoperative medical instability, especially cardiac or respiratory insufficiency, were associated with the highest risk of prolonged postoperative mechanical ventilation. In general, the predicted mortality score from the STS model was the best single predictor for prolonged postoperative ventilation. When regression analysis was performed with multiple variables, the STS score had the largest absolute Z value, and only the operative urgency and the age of the patient added significantly to its predictive ability. When the STS score was withheld from the analysis, the most important independent risk variables were the presence of mitral valve disease, advanced age, the use of preoperative vasopressors and/or inotropes, renal failure, operative urgency, and whether or not valve surgery was performed in addition to the CABG.
Time Receiving Ventilation
We have characterized the distribution of time receiving
ventilation after CABG for our hospital. Because > 94% of the
patients were extubated within the first 3 postoperative days, we have
defined the term prolonged postoperative mechanical
ventilation as mechanical ventilation beyond the third day after
CABG. Many of these patients were successfully extubated within 2 weeks
after surgery; however, 1.7% of our patients had greatly prolonged
(> 14 days) mechanical ventilation. With the number of CABG surgical
procedures approaching 600,000/yr in the United States,1
a
complication such as prolonged postoperative mechanical ventilation can
significantly increase hospital costs and diminish overall quality of
life. We do not document the specific reasons for each patient’s
prolonged ventilator dependence, but note that, in addition to causes
of isolated respiratory failure such as pneumonia or pulmonary edema,
many patients had multiple organ system involvement.
Risk Factors
This study attempted to relate certain variables with the
patient’s risk of requiring ventilation from 4 to 14 days and the risk
for ventilation > 14 days. Several prior studies have created
predictive models for postoperative morbidity based on
preoperative4
5
6
7
8
and/or early postoperative6
clinical factors, with varying degrees of success. Prediction models
usually take the form of complex equations derived from logistic
regression analysis,4
5
7
8
or of simplified additive
scores where each variable is assigned a numeric
ranking.5
7
8
Such models are not usually relied on
clinically, however, and recent research would demonstrate that even
very sick, high-risk patients may derive significant survival benefit
from CABG.9
Although we comment on increased risk
associated with these factors, we make no suggestion that CABG should
be withheld from any particular patient, but rather that these
conditions should be addressed prior to surgery, when possible.
Patients at particularly high risk should understand this risk as part
of the informed consent process.
In this study, we have classified the preoperative variables into general categories: demographic factors, chronic medical problems, previous cardiac interventions, acute medical problems, type of operation, operative urgency, and STS-predicted mortality score. Factors from each of these categories were found to be significant predictors of prolonged postoperative mechanical ventilation.
Demographic Factors
Prolonged postoperative ventilation was significantly associated
with female gender, increasing age, and low BMI. Increased operative
risk in women has been noted before.2
10
11
The
explanation for this finding is not clear, but has been thought to be
related to a female patient’s average smaller size and smaller
diameter of coronary arteries compared with male
patients,11
or due to possible selection or referral bias
in the coronary surgery population.8
One other possibility
is that women, in general, tend to have CAD later in life, and would
therefore be older at the time of their surgery. Table 2
confirms that
the women in our study population were generally older than the men. If
the increased risk is real, it is probable that these factors are
additive (ie, that the risk is due to the tendency to be
older and to have smaller body size with smaller coronary arteries),
since it is unlikely that any one factor is solely responsible.
It is not surprising that the risk of prolonged postoperative ventilation, as well as risk of general postoperative morbidity and mortality,4 5 7 increases with increasing age, since these patients generally have more preoperative comorbid conditions and less physiologic reserve.
A low (< 20 kg/m2) BMI was associated with an increased duration of prolonged postoperative ventilation, while obesity (> 30 kg/m2) or even morbid obesity (> 40 kg/m2) was not. Previous studies of CABG operative risk have been divided on the impact that patient BMI has on risk; often obesity is included as a risk factor,7 10 as well as low BMI7 or low weight.5 One would generally expect obesity to increase the complexity of the operation, the complexity of postoperative nursing care, and be associated with more preoperative comorbidities. A low BMI may reflect the poor nutritional state found with more advanced heart disease ("cardiac cachexia") and other comorbidities. A recent prospective study12 in patients undergoing lung volume reduction surgery found preoperative measurement of BMI to be a sensitive measure of nutritional state, and a low BMI to correlate with prolonged postoperative ventilation and other complications.
Chronic Medical Problems
Among preoperative chronic medical problems, the presence of renal
failure has previously been shown to be an important predictor of
postoperative morbidity and mortality,5
7
and in the
current study conferred a high risk of prolonged duration of
postoperative mechanical ventilation. Valvular dysfunction also carried
an increased risk for morbidity after CABG,5
which is
likely a result of greater cardiac dysfunction. A history of cardiac
dysfunction (clinical diagnosis of congestive heart failure,
cardiomyopathy, or documented decreased EF) is known to carry a
significant increase in operative risk,2
5
7
and has been
previously related to increased postoperative duration of mechanical
ventilation.13
It is interesting that among our patients the duration of postoperative ventilation was only modestly increased by the presence of COPD, and was not significantly affected by a history of tobacco use, regardless of whether the patient had quit smoking. In the only other study of ventilatory requirements after CABG that we are aware of, Spivack et al13 reported the lack of predictive ability that a clinical diagnosis of COPD, history of tobacco use, or preoperative use of bronchodilators have for time receiving ventilation following elective CABG. They also reported that no parameter of preoperative spirometry or arterial blood gas sampling is useful in predicting postoperative ventilator time.13
This study failed to show any significant relationship between diabetes and postoperative time receiving ventilation, although it is commonly considered to increase operative morbidity and mortality.5 7 10 It is interesting that the patients with hyperlipidemia or a positive family history of cardiac disease, both considered major risk factors for CAD, had significantly less time receiving ventilation. It is possible that these factors are associated with an earlier onset of cardiac disease, and thus lead to CABG at an earlier, less risky, age (Table 2) .
Previous Cardiac Interventions
A well-recognized risk factor for operative morbidity and
mortality is the history of previous cardiac
surgery.2
4
5
8
10
CABG or valve "redo’s" not only
had an increased duration of postoperative ventilation in our study,
but also carried higher in-hospital mortality. CABG is a more
technically difficult procedure in the setting of prior
cardiac surgery, given the areas of scarring which must be crossed, the
limited options for graft vessels, and the generally more advanced
stage of coronary disease found in such patients.
Approximately 1% of our patients had a previously placed permanent pacemaker or implanted cardioverter/defibrillator. These patients had a significant separation of the early part of their Kaplan-Meier curves, representing a delay in postoperative extubation of several days. These patients also had an increased in-hospital mortality at 11%, although it is not clear from our data whether these deaths were attributable to a lethal arrhythmia. Previous angioplasty did not prove to be significantly related to postoperative ventilation.
Acute Medical Problems
The highest risk for prolonged postoperative mechanical
ventilation and the highest in-hospital mortality were seen in those
patients who were in medically unstable condition prior to their
surgery. Of the 33 patients with preoperative respiratory failure
requiring mechanical ventilation, one third were extubated between 4
days and 14 days, and one third remained receiving ventilation > 14
days.
Patients with preoperative cardiogenic shock, determined in this study by the use of IV vasopressor or inotropic agents or by the use of an intra-aortic balloon pump, have significantly increased morbidity (including prolonged ventilation) and mortality. Greater than 30% of these patients received ventilation into the 4- to 14-day period, and > 10% received ventilation for > 14 days. There was an increased overall operative mortality of approximately 15%, but this is much lower than the mortality of 47% reported by others.9 A recent study by Hochman and coworkers9 suggested that early revascularization (CABG or angioplasty) can play a significant role in long-term survival in the patient with shock complicating acute MI.
Of the other preoperative medical problems, there was a small increase in risk for patients with arrhythmia (atrial or ventricular), acute MI, and those receiving IV nitroglycerin, but only acute MI was independently associated with increased risk of prolonged postoperative ventilation by the multivariate analysis.
Type of Operation
The 10 patients who underwent MIDCAB were generally extubated in
the first few days; however, 2 of our patients required ventilation
slightly beyond this period. Among those patients who had concomitant
valve surgery with CABG, there was an increase of 3- to 5-times the
percentage of patients requiring prolonged or greatly prolonged
postoperative mechanical ventilation, compared with those patients who
had CABG alone; they also had an operative mortality that was 3.6 times
higher. This increased risk with combined CABG and valve surgery is
consistent with other studies of morbidity and mortality after
CABG.8
Operative Urgency
Urgent or emergent operations usually are performed when a patient
is in unstable condition; therefore, it is expected that the operative
morbidity and mortality are increased. CABG done under these conditions
has been noted to carry a particularly high
risk.2
4
5
7
8
10
Patients undergoing urgent or emergent
procedures necessarily have higher degrees of preoperative instability,
and may not have had an opportunity for preoperative evaluation and
risk-factor modification. In our population, patients undergoing CABG
urgently or emergently had a significant and independent elevation in
risk for postoperative ventilation > 3 days. The risk of ventilation
> 14 days was increased, but to a lesser degree. Operative mortality
in this group was also significantly higher (4.4 to 4.5%) than in
those patients undergoing elective procedures (1.7%).
STS Score
The preoperative predicted mortality from the STS logistic
regression equation is significantly and independently associated with
increasing probability of prolonged postoperative mechanical
ventilation. Although this model was designed chiefly for the purpose
of mortality prediction, it has been shown previously to have a high
degree of correlation with postoperative morbidity and length of
stay.2
14
Because many centers performing CABG already
routinely calculate this score for their patients, it would be useful
to have further validation for its ability to predict morbidity as well
as mortality.
Comparing Populations Requiring Ventilation for 4 to 14 Days
and Those Requiring Ventilation > 14 Days
In general, the same factors were associated with postoperative
ventilation of 4 days and 14 days and for > 14 days with a few
notable exceptions. Nearly 20% of the 58 patients with EFs < 25%
required ventilation for 4 to 14 days, but none required ventilation
> 14 days. In addition, 15% of 54 patients with a permanent
pacemaker required ventilation for 4 to 14 days, but none for > 14
days.
Limitations of the Study
This study was conducted in a large referral center
performing many operations per year, and therefore the results may not
be completely generalizable to the more typical center with a lower
volume of CABG cases and different practice patterns. Selection bias in
our population is possible, with a greater number of high acuity cases
seen. We have chosen a p value of 0.001 for significance testing to
reduce the type I errors that occur with testing multiple variables.
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Conclusion |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionConclusionAppendix 1References |
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These data lend support to the practice of preoperative optimization of acute and chronic medical problems prior to CABG surgery to minimize postoperative morbidity. Patients at high risk should be appropriately counseled as to their risk of prolonged postoperative mechanical ventilation prior to CABG surgery.
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Appendix 1 |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionConclusionAppendix 1References |
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Footnotes |
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This work was supported in part by the Saint Thomas Foundation, Nashville, TN, and by National Institutes of Health grant HL 07123.
Received for publication March 9, 2000. Accepted for publication July 18, 2000.
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionConclusionAppendix 1References |
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