A Study of Variability in Locating the External Reference Point for Central Venous Pressure Determination

This exploratory study was conducted to determine the variability occurring among intensive care nurses in locating, with two methods, the external reference point for central venous pressure (CVP) determinations. In the literature on CVP and on CVP value discrepancies, no reports were found of studies related to variability among medical personnel in locating the point. The nurse participants in this study were thirty-one intensive care nurses who routinely were involved in CVP monitoring and marking reference points on patients in two surgical intensive care units. Thirty-two volunteer, non-hospitalized subjects, ages 18-85, sixteen males and sixteen females, with varying body builds and without clearly abnormal chest structure or vertebral columns were used for the location of the reference points. Eight subgroups were formed for data collection with four nurses and four subjects per subgroup. However, subgroup one had only three nurses. Each nurse located, measured, and recorded in writing an external reference point for each of the two methods used on the four subjects in her subgroup. Method I designated the working criteria each nurse used for the location of the external reference point for CVP monitoring. Method II consisted of following the criteria specified by the researcher for locating an anterior axillary line reference point. For both methods nurses described the reference point in terms of centimeters above the posterior surface of the flat, supine subject (Vertical) and centimeters down from the top of the shoulder (Horizontal) . They used special measuring devices developed for this purpose. Data were analyzed for (1) estimates of variability in centimeter units, (2) variation within the nursing units between the methods, and (3) variation within the methods between the nursing units. Both methods used showed variability existing in the located reference points. The greatest variability occurred when nurses used Method II for locating the horizontal component of the reference point. In the analysis between the methods the age and height of the subject were found to be significant factors affecting the nurses’ variability in locating both the vertical and horizontal components of the reference point. Sex was found to be a significant factor when nurses located the vertical component, while weight was a significant factor affecting the nurses’ variability when they located the horizontal component. In the analysis between the nurses of the two units the only variable having significance was the method used in determining the horizontal component. The 95% confidence intervals for all nurses using Method I to locate the CVP reference point was 6.78 centimeters for the vertical component and 7.06 centimeters for the horizontal component. The 95% confidence intervals for all nurses using Method II was 7.09 cm. for the vertical component and 8.52 cm. for the horizontal component. Possible reasons for the amount of variability observed are discussed in the study