Invasive arterial BP monitoring in trauma and critical care - Effect of variable transducer level, catheter access, and patient position

Objectives: (1) To determine the validity of current recommendations for di rect arterial BP measurement that suggest that the transducer (zeroed to at mosphere) be placed level with the catheter access regardless of subject po sitioning: and (2) to investigate the effect of transducer level, catheter access site, and subject positioning on direct arterial BP measurement. Design: Prospective, controlled laboratory study. Setting: Large animal laboratory. Subjects: Five Yorkshire pigs. Interventions: Anesthetized animals had 16F catheters placed at three acces s sites: aortic root, femoral artery, and distal hind limb. Animals were pl aced in supine, reverse Trendelenburg 35 degrees, and Trendelenburg 25 degr ees positions with a transducer placed level to each access site while in e very position. Measurements and main results: For each transducer level, five systolic and diastolic pressures were measured and used to calculate five corresponding mean arterial pressures (MAPs) at each access site. When transducers were at the aortic root, MAP corresponding to aortic root pressure was obtained in all positions regardless of catheter access site. When transducers were moved to the level of catheter access, as current recommendations suggest, significant errors in aortic MAP occurred in the reverse Trendelenburg posi tion. The same trend for error was noted in the Trendelenburg position but did not reach statistical significance. Conclusions: (1) Current recommendations that suggest placing the transduce r at the level of catheter access regardless of patient position are invali d. Significant errors occur when subjects are in nonsupine positions. (2) V alid determination of direct arterial BP is dependent only on transducer pl acement at the level of the aortic root, and independent of catheter access site and patient position.