Evaluation of external reference levels for central venous pressure measurements of severely obese patients in the supine position.

PURPOSE: A proper reference level is important for measuring intracardiac pressures, especially for parameters with small normal values such as central venous pressure (CVP). Although several external zero reference levels (eZRLs) have been proposed for non-obese patients, none has been reported for severely obese patients. The aim of this study was to investigate an appropriate eZRL for CVP measurements of severely obese patients.

METHODS: Chest computed tomography images of 65 patients with body mass index (BMI) ≥ 35 kg/m2 were retrospectively reviewed. The anteroposterior thoracic diameter and height of the mid-right atrium (RA) were measured. Four reported eZRLs for CVP measurements (midthoracic level, two-thirds and four-fifths of the thoracic diameter above table level, and 5 cm below the anterior thorax) were examined for error when predicting the midpoint of the RA.

RESULTS: The median BMI was 36.9 kg/m2 [interquartile range (IQR), 36.0-39.2]. There was a significant difference in the calculated errors for the midpoint of the RA among the four eZRLs (Kruskal-Wallis test, P < 0.001). Two-thirds of the thoracic diameter above table level was the most accurate reference level for CVP measurement (Steel-Dwass post hoc analysis, P < 0.001). The Bland-Altman plot showed acceptable agreement for clinical use (mean difference, - 7 mm; 95% limit of agreement, - 23 to 9 mm).

CONCLUSION: The most accurate eZRL for CVP measurements of severely obese patients in the supine position was two-thirds of the thoracic diameter above table level. This result is consistent with that of a previous report of non-obese patients.