Background: Many reference levels have been proposed for the measurement of intracardiac pressures, but none have met with universal acceptance. In the first part of our study, we evaluated 10 cardiologists' understanding of how hydrostatic pressure influences intracardiac pressures as measured with fluid-filled catheters. In the second part, we proposed and validated a new zero level (H): the uppermost blood level in the left ventricular (LV) chamber relative to the anterior chest wall for a patient in the supine position. A comparison was made of LV minimum diastolic pressure measured by reference to H versus measurements made with the zero level at midchest. Methods and Results: Using two-dimensional echocardiography, we determined H in the LVs of seven normal patients (five male, two female; age. 49±9 years) undergoing routine cardiac catheterization. H was determined from a left parasternal short-axis view and calculated as the average distance between end diastole and end systole of the endocardium of the uppermost segment of the LV anterior wall below the fourth or fifth intercostal space of the left sternal border on the anterior surface of the chest wall, with the patient in the supine position. A micromanometer/fluid-filled lumen catheter was then positioned in the LV, and we compared the micromanometer LV minimum pressure (LVP(min)) obtained when the reference fluid-filled transducer was aligned at midchest with the LVP(min) obtained when the reference fluid-filled transducer was aligned at H. LVP(min) referenced to a midchest fluid-filled external transducer was measured as 5.1 ± 1.6 mm Hg (range, 2.4 to 7.2 mm Hg) versus -0.6±0.6 mm Hg (range, -1.6 to 0.4 mm Hg) when referenced to H (P<.001). A significant linear relation was found to exist between patient anterior-posterior chest diameter and the magnitude of hydrostatic pressure influences related to pressure referenced at midchest (r=.88; P<.01). Conclusions: External fluid-filled transducers should be used with the goal of removing hydrostatic pressure and other influences so that the presence of subatmospheric pressure during diastole in any of the cardiac chambers is accurately measured. To achieve this goal, intracardiac pressure should be referenced to an external fluid-filled transducer aligned with the uppermost blood level in the chamber in which pressure is to he measured. The current practice of referencing the zero level of LV diastolic pressure to an external fluid-filled transducer positioned at the midchest level results in systematic overestimation due to hydrostatic effects and produces physiologically significant error in the measurement of diastolic intracardiac pressure.