Original Research

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

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BACKGROUND: Many clinicians believe the radiographic expression of community-acquired pneumonia (CAP) is affected by the fluid volume status of the patient. However, there are very few data to support or refute this concept. With this study we began to examine the relationship between admission fluid volume status and the radiographic expression of CAP.

METHODS: Using a retrospective chart review, we examined 376 consecutive inpatient encounters with the diagnosis of pneumonia at discharge from a community teaching hospital. Patients were evaluated by age, sex, admission serum sodium, blood urea nitrogen (BUN) level, creatinine, and fluid administered in the first 48 hours of treatment. We classified these patients as either showing radiographic progression (P) or no radiographic progression (NP) by comparison of admission and follow-up radiographs.

RESULTS: A total of 125 patient encounters satisfied inclusion criteria for the study. Using the Student t test we noted a statistically significant difference between the P and NP groups for BUN level (P=.02), volume of fluid administered during the first 48 hours (P=.04), and marginally for age (P=.05). The P group had higher BUN levels (mean=34 vs 24), more 48-hour fluid intake (mean=5824 mL vs 4764 mL), and younger age (mean=59 years vs 66 years) than the NP group. Logistic regression poorly predicted which patients would have worsening infiltrate on the second radiograph.

CONCLUSIONS: Elevated admission BUN level and higher fluid volume administered in the first 48 hours of admission were associated with worsening radiographic findings of pneumonia after hydration. Prospective studies are needed for confirmation of our results.

More than 2.5 million cases of pneumonia are diagnosed each year in the United States,1 and the hospitalization rate ranges from fewer than 1 of every 1000 persons aged 35 to 44 years to more than 11 of every 1000 persons aged 75 years or older.2,3 The annual direct cost of treating pneumonia is estimated to be $14 billion, of which $8 billion is for inpatient care. Pneumonia is also responsible for $9 billion in lost wages4-6 and is the sixth leading cause of death in the United States6—and these numbers may be increasing.7,8 It is the most frequently encountered life-threatening infectious disease in the United States.9 Accurate diagnosis, treatment, and follow-up are essential for treating this serious disease.

Physicians often obtain chest radiographs in the course of evaluating acutely ill febrile patients with suspected pneumonia. Many of these patients have some degree of fluid volume depletion secondary to the effects of the acute illness, as well as chronic illnesses and concomitant medication use. It is widely believed that the radiographic findings of community-acquired pneumonia (CAP) may be masked by volume depletion and that repletion may facilitate the expression of infiltrates on posthydration chest radiographs.4 This phenomenon has been noted in a recent publication about CAP,5 and we have encountered this clinical concept in discussions with colleagues.

Unfortunately, few data are available to support or refute the concept that fluid volume status affects the radiographic findings in CAP. A MEDLINE database search of the literature from 1966 to the present involving numerous combinations of key words yielded no human studies on this subject. One case study was reported of a dehydrated older person with a normal chest radiograph on admission for suspected pneumonia who developed a lobar infiltrate after rehydration.10

Our goal for this initial retrospective study was to determine if a relation exists between initial volume depletion, repletion, and subsequent radiographic findings of CAP. This information could be useful in determining the appropriate use of repeat radiographs in the evaluation of patients with clinical pneumonia.

Methods

The hospital records of 376 consecutive inpatient encounters with the discharge diagnosis of CAP between June 1996 and June 1998 were drawn from a 500-bed community teaching hospital. We reviewed them for the following inclusion criteria: 18 years of age or older; nonpregnant; chest radiographs obtained on admission and within 96 hours of admission; and at least 2 clinical indicators to support the radiographic diagnosis of pneumonia (fever >37.8 °C, leukocytosis, tachypnea, vomiting, pleuritic pain, productive cough, positive blood cultures.). A total of 125 records met the inclusion criteria. Each of these records was evaluated for reported chest radiograph results, fluids administered during the first 24 hours (intravenous and oral), blood urea nitrogen (BUN) level, repeat BUN level after 24 to 48 hours, admission creatinine level, repeat creatinine level after 24 to 48 hours, age, sex, race, weight, and admission serum sodium level.

We grouped the patients according to reported change in chest radiographs. The progression group (P) had chest radiograph reports indicating worsening appearance of infiltrates after fluid administration. The no-progression (NP) group had reports indicating either no change or improvement in the appearance of the infiltrates after fluid administration.

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