Intricacies of blood pressure measurement: reexamining the rituals

Critical care clinicians are often frustrated when blood pressure values derived via direct monitoring methods (i.e., arterial lines) do not "correlate" with values obtained via indirect methods (e.g., auscultatory). Precious time has been spent attempting to troubleshoot monitoring systems and ascertain why these discrepancies exist. Greater insight into the intricacies of blood pressure monitoring reveals that both direct and indirect methods are subject to many external and physiologic influences that have the ability to significantly affect the value ultimately accepted as the "true" blood pressure. Direct blood pressure monitoring is influenced by normal physiologic changes in the pressure pulse configuration as it travels to the periphery, as well as by properties of the external monitoring system. Indirect monitoring is also influenced by a variety of factors, and may be unreliable in the very clinical situations where it is used the most. Finally, the relationship between blood pressure and blood flow, particularly in critically ill patients, suggests that it is unreasonable to expect that pressures obtained by direct monitoring methods will be the same as those derived by indirect methods that are flow dependent.