Objective: To explain why investigations of the measurement of central venous pressure (CVP) usually reveal a discrepancy between the clinician's estimate of CVP from physical diagnosis and supine measurement with a catheter. Data from MEDLINE search, personal files, and bibliographies of textbooks on physical diagnosis and cardiology were used.
Results: The most important reasons for this disagreement are the failure to standardize the external reference point used by the clinician to indicate "zero" venous pressure and the failure to recognize that venous pressure often depends on the position of the patient during examination. During physical examination clinicians tend to underestimate the CVP, as measured by a catheter in the same patient positioned supine, especially when the measured value is high. This occurs because the venous pressure of patients with heart failure, in contrast to that of healthy individuals, demonstrates an exaggerated postural fall when the patient is in the more upright positions that are necessary to visualize the elevated neck veins. The cause of this postural instability, increased venoconstriction from sympathetic tone, also helps explain two other physical findings of the jugular veins, the abdominojugular test, and Kussmaul's sign.
Conclusions: Clinicians should avoid making decisions about degrees of CVP elevation that are imprecise and difficult to reproduce. Instead, they should determine during physical diagnosis merely whether the CVP is elevated. Until further research is done, the best definition of elevated CVP is that of Sir Thomas Lewis-when the top of the external or internal jugular veins is >3 cm of vertical distance above the sternal angle, the CVP is abnormally high.