The Relationship Between Volume Status, Hydration, and Radiographic Findings in the Diagnosis of Community-Acquired Pneumonia

All continuous variables were analyzed with a 2-sample Student t test for equal means between the P and NP groups. We used stepwise logistic regression to determine associations between potential predictor variables and between the groups. Continuous variables were graphed against the logit to access linearity, and we examined the Hosmer-Lemeshaw statistic¹¹ to help determine adequacy of the model. We analyzed data using the PROC t test and the PROC logistic in the SAS 6.12 software.¹²

Results

A total of 125 records satisfied study admission criteria. Our record review revealed that 42 patients had radiographic worsening (P), and the remaining 83 had no change or improvement radiographically (NP). The cumulative data for each group is shown in [Table 1].

We found that admission BUN levels were significantly greater for the P group (mean=12.24 mmol/L [34.3 mg/dL]) than the NP group (mean=8.57 mmol/L [24.0 mg/dL]). Fluid intake during the first 48 hours after admission was also significantly greater for the P group (mean=5824.0 mL) than the NP group (mean=4764.3 mL). There was marginal significance (P=.053) between the 2 groups for age. There was no significant difference between the 2 groups for sodium levels on admission. Thus, those patients in the P group were more likely to have a higher admission BUN level, a higher fluid intake in the first 48 hours following admission, and to be younger than the NP patients.

Results of the logistic regression model showed that few factors were related to the change in radiographic findings, and no combination of factors seemed to be a good predictor for this change. The model poorly predicted which patients would show progressive radiographic infiltrates on the basis of age and admission BUN level. Although a significant difference was found between the 2 groups with respect to the first 48-hour fluid intake, this factor was dropped from the logistic regression model because it paralleled the admission BUN level [Table 2]. On the basis of a patient’s age and admission BUN level, the model could correctly predict only 28% of patients who would show progressive radiographic changes. In contrast, the same model correctly predicted 92% of patients who would show no change or improvement on subsequent radiographic studies. The c statistic (which is equivalent to the area under the curve for the receiver-operating characteristic) equals 0.68.

Discussion

The diagnosis of CAP is usually made by the presence of signs and symptoms of lower respiratory tract infection coupled with the finding of at least one acute opacity on the chest radiograph.^1,13 Some clinicians believe that such an opacity is necessary for the diagnosis of CAP, and they consider it the gold standard for the diagnosis in conjunction with supporting data from the history and physical examination.^2,4,5 However, radiographic and clinical findings are not always consistent. Heckerling¹⁴ found that no clinical findings other than auscultory abnormalities could be used to predict the presence of pneumonia on chest radiography. A review of the literature by Metlay and colleagues¹⁵ found that there is no reliable combination of findings that can rule in the diagnosis of pneumonia. Up to 25% of young adults may not have auscultory findings of pneumonia.⁴ It is well documented that elderly patients may initially manifest no fever, minimal respiratory symptoms, and minimal radiographic infiltrates despite laboratory documentation of pneumonia.^16,17 Patients with neutropenia or infections with atypical organisms or viruses may not manifest an infiltrate on chest radiograph until several days after clinical symptoms appear.^5,9,18 Dehydration or volume depletion has also been shown to produce this phenomenon.^4,5,10

Pneumonia has been defined as inflammation and consolidation of lung tissue due to an infectious agent. When sufficient fluid has collected in the normally air-filled spaces, X-ray beam attenuation occurs and an opacity appears on a chest radiograph. It has been postulated that decreased pulmonary hydrostatic pressure and increased pulmonary oncotic pressure secondary to dehydration or volume depletion may diminish the flux of fluid into the alveoli and interstitium and thus delay or alter the radiographic findings of pneumonia.¹⁹ Cooligan and coworkers²⁰ found that small increases in pulmonary capillary wedge pressure increased wet lung weight in euvolemic dogs with pneumococcal pneumonia, but no radiographic evaluation was performed. Caldwell and colleagues¹⁹ found that acute intravascular volume depletion did not affect the radiographic evolution of pneumococcal pneumonia in dogs, but they did not evaluate the effects of fluid volume repletion. Caldwell and coworkers also noted that on retrospective chart review of 20 consecutive human patients with pneumonia and the diagnosis of dehydration, all had an infiltrate apparent radiographically at the time of admission. The effects of rehydration were not addressed in these patients.