Category Archives: Pulmonary Function : Part 8

Evaluation of Different Criteria for the Separation of Pleural Transudates From Exudates: Discussion

Evaluation of Different Criteria for the Separation of Pleural Transudates From Exudates: DiscussionTable 3 lists the percentages of misclassified pleural effusions in every diagnostic group for every parameter studied. It shows that the criteria of Light et al, with their high sensitivity for exudates of any diagnostic origin, afforded the smallest percentage (5 percent) of misclassified pleural efiusions.
Looking for new cutoff points that could improve the discriminating capacity of the criteria of Light et al, we found that using the following as new cut-off points — 0.6 for the pleural fluid to serum protein ratio, 280 IU/L for LDH pleural fluid concentration, and 0.9 for pleural fluid to serum LDH ratio—the accuracy of the test remained practically unmodified (95 percent to 94 percent) while the specificity rose from 77 percent to 93 percent (Table 4).
In Table 5, patients with transudative efiusions due to CHF are grouped according to previous therapeutic use of diuretics. One patient was excluded because we could not ascertain previous use of diuretics. It shows that the use of diuretics does not significantly change the mean values of the different parameters used. Among the 21 patients with diuretics, the criteria of Light et al correctly classified 14 patients and misclassified 7 patients. The proportion of patients receiving long-term diuretics (more than a week) was not significantly different between groups.
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Evaluation of Different Criteria for the Separation of Pleural Transudates From Exudates: Ratio of Pleural Fluid LDH to Serum LDH

Using a dividing line of 0.6, 267 of 293 patients were correctly classified (accuracy, 91.1). Among the 44 patients with transudates, 8 patients with pleural effusions secondary to CHF were incorrectly classified (specificity, 81.8 percent). Eighteen of 249 exudative pleural effusions were misclassified using this parameter (sensitivity, 92.8 percent). The causes of the misclassified exudates were as follows: nine neoplastic, seven parapneumonic, and one each postsurgery and posttraumatic.
The Criteria of Light et aP (Pleural Fluid to Serum Protein Ratio >0.5, or Pleural Fluid LDH >307 U/Ly or Pleural Fluid to Serum LDH Ratio >0.6yfor Exudates)
Using these criteria for segregating transudates from exudates, 14 of 293 were misclassified (accuracy, 95 percent). All 10 incorrectly classified of the 44 transudates (specificity, 77.3 percent) were secondary to CHF. Among the 249 exudates, 4 were misclassified (sensitivity, 98.4 percent); among these 2 were parapneumonic, and 2 were secondary to breast cancer.
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Evaluation of Different Criteria for the Separation of Pleural Transudates From Exudates: Results

Evaluation of Different Criteria for the Separation of Pleural Transudates From Exudates: ResultsThree hundred fifty-one patients with pleural effusion were evaluated. In 54 patients, despite extensive evaluation, the cause of the pleural effusion was either indeterminate (31 patients) or was due to multiple superimposed diseases (23 patients). These patients were excluded from the study.
The average age of the 297 remaining patients with an effusion of single and known cause was 57 ± 18.6 years (range, 8 to 95 years); there were 160 men and 137 women. Forty-four were defined as having transudates and 253 were identified as having exudates. Among the 44 patients with transudates, 28 (64 percent) were men and 16 (36 percent) were women, with an average age of 68 ± 11 years (range, 36 to 86 years). Among the 253 patients with exudates, 132 (52 percent) were men and 121 (48 percent) were women, with a average age of 55 ± 19 years (range, 8 to 95 years).
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Evaluation of Different Criteria for the Separation of Pleural Transudates From Exudates: Statistical Analysis

Effusions were considered malignant if one of the following criteria was met: (1) demonstration of malignant cells at cytologic examination or in a biopsy specimen; or (2) histologically proven primary malignancy with exclusion of any other cause known to be associated with pleural effusions. A pleural effusion was considered to be parapneumonic when there was an acute febrile illness with purulent sputum and pulmonary infiltrate in the absence of malignancy or diseases causing transudates. Tuberculous pleurisy was diagnosed with positive culture for Mycobacterium tuberculosis or pleural biopsy specimen showing typical epithelioid cell granuloma. A diagnosis of pulmonary embolus or infarction was made w hen there w as a strong clinical suspicion and a high-probability perfusion scan or abnormal angiogram.
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Evaluation of Different Criteria for the Separation of Pleural Transudates From Exudates

Evaluation of Different Criteria for the Separation of Pleural Transudates From ExudatesDetermining the cause of a pleural effusion is not always easy. It frequently requires the use of different diagnostic techniques, some of which are invasive and not completely free of morbidity. Invasive local procedures, such as pleural biopsy, are indicated only in patients with pleural effusions secondary to pleural abnormalities, in which case the effusion is usually exudative. Therefore, the first step in the diagnosis of pleural effusions should be to classify them as transudates or exudates.
The criteria established by Light et al for segregating transudates from exudates have been widely accepted. In their original article, the use of these criteria led to the correct classification of the pleural effusions tested in 99 percent of the cases. However, several recent reports have shown that the low specificity of the criteria of Light et al may lead to unwarranted invasive interventions in up to 20 to 30 percent of patients with transudates. This cast some doubt on the universal applicability of these criteria. Several alternative criteria have been proposed, and some of these, such as cholesterol level, seem to have higher diagnostic accuracy for identifying transudates.
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The Control of Breathing during Weaning from Mechanical Ventilation (17)

The Control of Breathing during Weaning from Mechanical Ventilation (17)Dynamic hyperinflation and frequency-related changes in energy demand are bound to set limits on respiratory rate responses to C02. Alveolar hypoventilation may, thus, be one mechanism that enables the respiratory pump to cope with a large intrinsic load. Whether and how long such a breathing strategy can be sustained without resulting in overt pump failure remains unclear. Nevertheless, C02 retention (relative to C02RT) may allow the pressure output of respiratory muscles (ie, intrinsic load and drive) to fall to a lower level, not unlike that seen in patients without respiratory distress.

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The Control of Breathing during Weaning from Mechanical Ventilation (16)

Crs (a measure of the inspiratory elastic load) does not account for the pressure which the respiratory muscles must generate to oppose intrinsic PEEP, while VP6 reflects only the expiratory resistance of the respiratory system but not the inspiratory resistive load. Finally, none of these weaning indices would detect a cardiovascular limitation to weaning.
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The Control of Breathing during Weaning from Mechanical Ventilation (15)

The Control of Breathing during Weaning from Mechanical Ventilation (15)If respiratory pump failure is truly the consequence of an imbalance between load and strength, why could groups 1 and 2 not be distinguished on the basis of respiratory system impedance and maximal inspiratory muscle pressures? Although many patients with weaning-induced respiratory distress had profound inspiratory muscle weakness and suffered from severe airways obstruction, there was sufficient overlap among the measurements of strength and load (Table 2) such that a clear separation between groups was not possible. Pimax is effort dependent and may, there-
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The Control of Breathing during Weaning from Mechanical Ventilation (14)

The Determinants of C02SB
In a mechanistic sense, weaning from mechanical ventilation is a test of the load response of the respiratory pump. The finding that most patients with weaning-induced respiratory distress retained C02SB compared with C02RT suggests that in them the net effect of intrinsic loading on alveolar ventilation was greater than the effect of mechanical feedback inhibition on C02RT. At the end of the weaning trial, seven of nine subjects in group 2 had a minute ventilation less than would have been necessary to maintain C02SB within 2 mm Hg of C02RT. Only one patient (subject 8) had already developed respiratory acidosis by the time he met predefined weaning failure criteria.

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The Control of Breathing during Weaning from Mechanical Ventilation (13)

The Control of Breathing during Weaning from Mechanical Ventilation (13)By first adjusting the ventilator settings until phasic respiratory muscle output disappears, one imposes a high level of background inhibition on the respiratory pump. The measurement loses its uniqueness, however, if PaC02 is raised by lowering ventilator rate or tidal volume instead of supplementing C02 to the inspired gas. Prechter et al considered C02RT to be an estimate of the transition zone between a C02 responsive and a C02 unresponsive region of the ventilatory C02 response curve. As such, any increase in PaC02 above C02RT represents an error signal that induces a compensatory increase in respiratory drive.

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