CALCULATION OF VOLUME FLOW-RATE BY THE PROXIMAL ISOVELOCITY SURFACE-AREA METHOD - SIMPLIFIED APPROACH USING COLOR DOPPLER ZERO BASE-LINE SHIFT

Objectives. The goal of this study was to develop an accurate, simplif ied proximal isovelocity surface area (PISA) method for calculating vo lume flow rate using lower blue-red interface velocity produced by a c olor Doppler zero baseline shift technique. Background. The Doppler co lor proximal isovelocity surface area method has been shown to be accu rate for calculating the volume flow rate (Q) across a narrowed orific e by the formula Q = PISA x Blue-red interface velocity. A hemispheric model is generally used to calculate proximal isovelocity surface are a (PISA = 2pia2, where a = the radius corresponding to the blue-red in terface velocity). Although a hemispheric model is simple, requiring m easurement of one radius, it may underestimate the actual volume flow rate because, in the general case, the shape of a proximal isovelocity surface area is hemielliptic. Although a hemielliptic model is genera lly more accurate for calculating proximal isovelocity surface area, i t is more complex, requiring measurement of two orthogonal radii. Meth ods. Sixteen in vitro constant flow model studies were performed using planar circular orifices (diameter range 6 to 16 mm). The blue-red in terface velocity was changed from 3 to 54 cm/s using color Doppler zer o baseline shift. Results. 1) With decreasing blue-red interface veloc ity, the size of the proximal isovelocity surface area was increased, and its shape changed from hemielliptic to hemispheric. 2) With the bl ue-red interface velocity in the range 11 to 15 cm/s, the proximal iso velocity surface area became nearly hemispheric; however, it was diffi cult to determine the blue-red interface radius at a blue-red interfac e velocity < 10 cm/s because of interface fluctuations. 3) Calculated volume flow rate using the hemispheric proximal isovelocity surface ar ea model with a single radius was relatively accurate at a blue-red in terface velocity of 11 to 15 cm/s (mean percent difference from actual volume flow rate was -3.6%). Conclusions. Because the shape of the pr oximal isovelocity surface area is nearly hemispheric at a blue-red in terface velocity of 11 to 15 cm/s, volume flow rate can be accurately calculated in this proximal isovelocity surface area interface velocit y range (produced by zero baseline shift) by measuring a single-interf ace radius. This approach should be clinically useful for calculating the volume flow rate across stenotic and regurgitant valves and across shunt defects.