Gravimetric measurement of lunq water

Gravimetric measurement of extravascular lung water is considered to be the gold standard against which other methods are compared. However, since it is a postmortem technique, it has only experimental rather than clinical value. The technique requires the determination of the wet and dry weights of the lung sample. The normal range of lung water measured in this way is between 4 and 7 ml/kg.

H3>Clinical examination and chest radiography

Bedside assessment of pulmonary edema may be difficult in the intensive care unit (ICU). Clinical examination can be misleading. Furthermore, although lung function tests such as pulmonary compliance and measures of oxygenation may be abnormal, these abnormalities are not specific for pulmonary edema. Although chest radiography is the most common technique used in the further assessment of pulmonary edema, there are a variety of factors which reduce its accuracy. Radiographic diagnostic accuracy depends upon technical consistency. This is often difficult to achieve in the ICU. Furthermore, it is important to appreciate that the characteristic radiographic appearance of pulmonary edema is dependent on the structural integrity of the lungs. In the intact lung, excess water will accumulate in a predictable fashion. If lung architecture is severely distorted by the presence of various pulmonary diseases, the recognition of excess water becomes more difficult. Also, gravity may contribute to an atypical distribution of edema in the supine patient. It has also been shown that, paradoxically, the chest radiograph may show no evidence of edema despite hemodynamic evidence of congestive heart failure. It appears that the radiographic appearance may occasionally lag behind the clinical state (Herman.. . et_ a/ 1990).

Despite these limitations chest radiography has several advantages. It is portable, inexpensive, and non-invasive. In most circumstances it is relatively sensitive if somewhat non-specific. This is particularly true of the detection of early pulmonary edema when other techniques such as dye dilution give values for lung water that are still within the normal range. In view of these advantages it is not surprising that considerable effort has been made to standardize this investigation. Numerous scoring systems have been devised to quantify pulmonary edema, and many studies have been carried out to assess their reliability. However, the correlation between radiographic quantification and other techniques such as CT and double-indicator dye dilution have been variable ( r = 0.62-0.89). Chest radiography is still the most popular technique for the evaluation of extravascular lung water and this is likely to remain the case for the near future.

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