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Ann Thorac Surg 1998;66:220-224
© 1998 The Society of Thoracic Surgeons
a Department of Anesthesiology and Critical Care Medicine, Memorial Sloan-Kettering Cancer Center, New York, New York, USA
Accepted for publication February 19, 1998.
Address reprint requests to Dr Melendez, Department of Anesthesiology and Critical Care Medicine, Memorial Sloan-Kettering Cancer Center, 1275 York Ave, New York, NY 10021
e-mail: (melendej{at}mskcc.org)
| Abstract |
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Methods. Sixty-one consecutive patients undergoing pulmonary resections were evaluated. All patients underwent spirometry, carbon monoxide diffusion capacity, split lung function testing, and room air blood gas analysis at rest and after a 2-minute step climb. The thoracic prospective data base and patient charts were reviewed for length of hospitalization, postoperative length of stay, and complications requiring therapy. Logistic regression analysis of the preoperative data, operation and postoperative outcome was used to develop a new postoperative predictive index: the predictive respiratory complication quotient (PRQ). We describe the design of the equation for the probability of serious pulmonary complications, hospital stay, and hospital charges based on PRQ.
Results. Ten of 12 patients with a PRQ less than 2,200 suffered serious pulmonary complications of pneumonia, respiratory insufficiency, hypoxemia, and death. Forty-nine patients with a PRQ more than 2,200 did not experience any pulmonary complications. Postoperative length of stay and hospital charges correlated with the PRQ.
Conclusions. A construct such as the PRQ may provide a better prediction of outcome than its individual parts. We identified an important underlying relationship between intensive care unit stay, hospital stay and charges, and our index. A PRQ of less than 2,200 was associated with an increased risk of pulmonary complications and mortality.
| Introduction |
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Investigators have attempted to predict outcome using spirometry, blood gas analysis, carbon monoxide diffusion capacity, and split lung function testing. Individual parameters have shown some promise at predicting pulmonary complications including death. In 1955, Gaensler and colleagues [1] identified a relationship between spirometric testing and outcome. Boysen and associates [2] expanded the resectability criteria to include patients previously considered too sick to survive thoracotomy by using split lung function tests to estimate predicted postoperative forced expiratory volume in 1 second (ppoFEV1). However, their measurements of ppoFEV1 failed to predict pulmonary morbidity. Ferguson and colleagues [3] used logistic regression to demonstrate that predicted postoperative carbon monoxide diffusion capacity percent (ppoDLco%) was inversely related to the incidence of complications and an important predictor of pulmonary morbidity. Markos and associates [4] provided evidence that a ppoFEV1 percentage could be useful in estimating complications and outcome. Pierce and colleagues [5] were the first investigators to demonstrate the usefulness of a composite index, the predicted postoperative product (PPP). The PPP, or the algebraic product of the ppoFEV1 percentage and the ppoDLco%, incorporated values for ventilation, gas exchange, lung perfusion, and the proportion of the remaining lung into one index. They attributed the predictive power of the PPP to the accurate reflection of ventilation and perfusion abnormalities in the postoperative period.
We have developed an index, based on the methodology of Pierce and colleagues [5], that provides not only a measure of mortality but also one of severe respiratory complications. We constructed the predictive respiratory complication quotient (PRQ) to predict the probability of pulmonary morbidity and mortality in thoracic surgical patients.
| Material and methods |
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Pulmonary function tests were performed on a Collins GS unit using Collins Plus software (Warren E. Collins, Inc, Braintree, MA). Normal values for spirometry were based on studies reported by Knudson and colleagues [6]. The resting single breath diffusion capacity was determined using the method of Ogilvie and associates and adjusted for hemoglobin [7, 8]. The normal range of DLco in the pulmonary laboratory was 80 to 100 mL · min-1 · mm Hg-1. Testing and exercise arterial blood gases were measured (ABL 520; Radiometer, Copenhagen, Denmark) after a radial artery puncture; the alveolararterial oxygen gradient (room air) (A-a PO2) was calculated using the alveolar gas equation corrected for barometric pressure and age.
All patients had a quantitative lung perfusion scan after intravenous injection of 4 mEq of technetium-99m-labeled microaggregated albumin. Intravenous radiotracer was injected in the supine position and immediately imaged using a large-field-of-view dual-headed gamma camera (Adac Genesys; Adac Lab, Milipitas, CA) in the anterior and posterior projections. This was followed by a variable-dose krypton-81m ventilation scan imaged in the same projections. The quantitative ventilation perfusion scan was read by dividing the hemithorax in thirds in the lateral projection.
The predicted postoperative function was calculated by the equation:
. Patients underwent operation if their ppoFEV1 was more than 800 mL. We excluded from study those patients who underwent chest wall resection. Five surgeons performed all operations. Data were acquired from the pulmonary laboratory data base in conjunction with the prospective thoracic surgical complication data base and verified by chart review. Charts were reviewed for length of hospitalization (LOS) and postoperative LOS. Readmission LOS were added to the initial hospitalization LOS in patients who required a second admission for treatment complication. Surgical and medical complications were considered if they occurred within 60 days of operation. Pulmonary complications were defined as: pneumonia (temperature, >38.5°C, purulent sputum, and chest roentgenographic findings requiring antibiotic therapy), hypoxemia, atelectasis (requiring bronchoscopy), respiratory failure (requiring reintubation and prolonged ventilatory support), and death. Surgical complications were persistent air or chyle leak and reoperation. Medical and cardiac complications included all nonpulmonary and nonsurgical morbidity and mortality. We recalculated the PPP index by substituting (ppoDLco%)2 for ppoDLco%. Hospital charges were derived from the financial management system of Memorial Sloan-Kettering Cancer Center.
Statistics
Wilcoxon nonparametric tests were performed to compare respiratory functions between patients with and without complications. When applicable, results are expressed as mean ± standard deviation. Relative odds ratio for pulmonary complications was calculated. Stepwise forward logistic regression (p < 0.05) (SPSS 7.5 for Windows; SPSS Inc, Chicago, IL) were used to determine the respiratory parameters for predicting pulmonary, cardiac, surgical, and all complications. The 16 variables tested alone and in combination were age, smoking history, FEV1, FEV1 percent predicted, ppoFEV1 percent predicted, DLco, DLco percent predicted, ppoDLco% predicted, arterial partial pressure of oxygen (PO2), arterial partial pressure of carbon dioxide (PCO2), O2 saturationrest, O2 saturationexercise, O2 saturation change, A-a PO2 (room air), A-a PO2exercise (after exercise), and A-a PO2 difference. The output was used to construct the equation describing the probability of pulmonary complications. The PRQ was designed to conform to the relative weighted contribution (SPSS Inc) of selected variables where the weight function was 1/X-Y (only Y = integers were accepted). Receiver operating characteristic curves (MedCalc 4.2 for Windows, Mariakerke, Belgium) were used to select the PRQ cut-off for pulmonary complications. Curve estimation (SPSS Inc) was used to construct the equation depicting the relation between PRQ and postoperative LOS, intensive care unit admissions, and hospital charges.
| Results |
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) is described by the equation:
; p = 0.02, where the weight constant (Y) for ppoFEV1% was 1, for ppoDlco percent was 2 and for A-a PO2 was -1, thus,
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The mean postoperative LOS was 6.4 ± 1.4 days for patients without complications with a mean hospital stay of 8.8 ± 1.8 days. The mean postoperative LOS was 23.3 ± 18.3 days for patients with pulmonary complications with a mean hospital stay of 27.8 ± 20.2 days, including two deaths. The relationship between PRQ and postoperative LOS is shown in Figure 2. The postoperative LOS is described by the equation:
; p = 0.009. Five of 10 patients with pulmonary complications required admission to the intensive care unit. Other complications were successfully treated on the surgical floors. Patients without pulmonary complications did not require intensive care unit care. The average stay in the intensive care unit was 19 days with a range of 4 to 46 days. The probability of intensive care unit admission (ß) is described by the equation:
; p = 0.05. The mean hospital charges were $77,387.00 ± 11,114.00 or 291% higher in patients with than in patients without pulmonary complications (p = 0.011).
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| Comment |
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The PRQ appears to provide a very accurate prediction of serious pulmonary complications after thoracic operation for cancer. This index not only incorporates the ppoFEV1 percent and the ppoDLco%, but adds a variable, the A-a PO2. The PRQ also weighs the relative contributions of each of the three variables. Our data showed that ppoDLco% was the most important contributor in the model, therefore the square adjustment. Previous studies seem to agree that ppoDLco% is an important factor in postthoracic surgical patients developing pulmonary complications [35]. Excluding possibly exercise testing, investigators found DLco variables to provide the best single prediction of outcome when compared with other pulmonary laboratory parameters. Recalculating the data by Pierce and colleagues [5] by substituting (ppoDLco%)2 for ppoDLco% results in a construct that improves the PPP predictive power for respiratory failure and mortality. The ppoFEV1 is considered by many researchers to be an important variable in predicting outcome; a value of 800 mL is commonly quoted as the margin of resectability [9]. Markos and colleagues [4] demonstrated that a ppoFEV1 percentage more than 40% was associated with no postoperative deaths, whereas a value less than 40% was associated with 50% mortality. The ppoFEV1 percentage was superior to the ppoFEV1 at predicting complications, hence its inclusion in the index. The A-a PO2 was the measure of oxygenation that showed statistically significant predictive value; it was inversely related to outcome. There is no precedent for using A-a PO2 in the prediction of outcome. However, there is some evidence to suggest that a patient with hypoxemia, not attributable to right-to-left intrapulmonary shunting caused by areas of resectable postobstructive atelectasis and pneumonia is more likely to do worse after thoracic operation [4, 10].
The PRQ uses the rationale that the margin of resectability be based on an algebraic combination of multiple variables. As Pierce and colleagues [5] explained, there is an advantage to this kind of algorithm: a patient may undergo operation despite one poor individual measurement as long as another measurement is well above the minimum. For example, if a patient has a ppoFEV1 percentage and a ppoDLco% of 50%, and a PO2 more than 50 mm Hg with a corresponding A-a PO2 of less than 57, the PRQ is close but above the resectability margin of 2,200 identified by receiver operating characteristic curves. Such a patient should be capable of tolerating the planned procedure.
We failed to corroborate the significance of a number of factors described by other investigators as predictors of outcome (see Table 1) [4, 9, 1117]. The ability to successfully exercise has been used to assess the cardiopulmonary risk of thoracotomy. Studies have also used exercise testing with cycle ergometry to assess surgical risk. Most, but not all, exercise testing studies have shown that they can be used to assess the operative risk for cardiopulmonary complications [4, 18]. Markos and colleagues [4] used staged bicycle ergometry to show the significance of exercise desaturation in predicting outcome. Rao and associates [11] used stair climbing to show similar results. Although we used stair climbing, we were unable to show any relationship between exercise desaturation and outcome. Some investigators believe a PCO2 more than 45 mm Hg is a predictor of poor outcome after lung resection, whereas others believe that PCO2 plays no role [1417]. The PCO2 did not have any importance in the prediction of outcome in our patients. There was no difference in the PCO2 between the pulmonary complication group and the noncomplication group. In addition, there were 7 patients with a PCO2 more than 45 mm Hg and a PRQ more than 2,200 who experienced no complications. Kearney and coworkers [9] showed smoking history to be a predictor of outcome. We were unable to describe any relation between smoking history and complications. The significance of advanced age as a predictor of complications is controversial [15]. We had 29 patients who were older than 65 years; 5 patients experienced pulmonary complications, and another 12 experienced other types of complications. In total, 17 of the 27 patients experiencing complications were older than 65 years. Advanced age was a marginally important parameter in the prediction of all complications (p = 0.06). Pierce and coworkers [5] reported a difference in the relation of the PPP and operation to the accuracy of the prediction; the predictive power of the PPP was better for lobectomies than for pneumonectomies. Our results do not corroborate this finding. In addition, our population did not experience a higher incidence of complications in pneumonectomy patients. Although sample size may play a part in our findings, we believe that postoperative pulmonary complications are better associated with the remaining lung function than with the nature of the operation. The PRQ with its various weighted variables better corrects for the operation bias.
There was a statistically significant increase in the postoperative LOS between patients with and without pulmonary complications. Pulmonary complications studied resulted in prolongation of postoperative LOS in hospital as well as the requirement for intensive care unit care. This outcome clearly has a financial impact, underscoring the importance of the PRQ. Patients with pulmonary complications spent an average of 9.5 days in the intensive care unit. This was associated with a threefold increase in the mean charge for hospitalization. Clearly, it is of great importance that we identify patients who are likely to incur such high expenses. The task was accomplished thanks to the application of well-defined criteria for complications. Similar analysis may be possibly performed for other complications aiding hospitals in identifying potential sources of increased expenditures. A PRQ of less than 2,200 is associated with an increased risk of pulmonary complications and mortality.
Although this study lacks prospective validation, we believe our results have significant legitimacy. Predicting postoperative outcome is a very complex process partly the result of vaguely defined complications. It is of utmost importance that precise definitions for complications are used when examining outcomes, so that useful comparisons can be made between studies. Many reviews have even attempted to correlate nonpulmonary complications with preoperative pulmonary parameters making the process of understanding outcome almost impossible to sort.
In conclusion, the PRQ can predict outcome after lung resection better than other well-studied parameters. We describe the design of the equations for the probability of serious pulmonary complications, postoperative LOS, and probability of admission to the intensive care unit based on the PRQ. Hospital expenditures were correlated to the PRQ. We believe that our hypothesis deserves serious consideration and prospective corroboration.
| Acknowledgments |
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| References |
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