I read with interest the study by Martin et al1–
(December 2002) describing correlations between the findings of portable chest radiographs and fluid balance in critically ill patients. In the study, the authors suggested that the vascular pedicle width (VPW) is an underutilized tool for the assessment of intravascular volume status. However, although the relationships reported in the study are statistically significant, those between the VPW and the wedge pressure or the fluid balance are weak (r2 range, 0.25 to 0.5), such that a given VPW value cannot be recommended to accurately predict intravascular volume status. Moreover, from a methodologic point of view, demonstrating that a parameter is sensitive to changes in volume status does not allow one to conclude that this parameter is useful in assessing intravascular volume. For example, central venous pressure goes up during fluid loading, goes down during fluid depletion, but cannot be used to assess intravascular blood volume,2
simply because intravascular volume is not the only determinant of central venous pressure. It must also be pointed out that chest radiographs were performed under the best conditions (prospective clinical study) and were interpreted by one experienced thoracic radiologist, which is usually not the case in real life.