Abstract P226: The Analysis Of Regional Placental Hemodynamics Using Combined Photoacoustic Imaging And Contrast-enhanced Ultrasound

Placental insufficiency is often associated with pregnancies at risk for fetal growth restriction or preeclampsia. We have previously demonstrated that three-dimensional ultrasound-guided photoacoustic imaging (PAI) is a sensitive tool for assessing regional variations in placental oxygenation in mice. However, whether changes in placental oxygenation determined by the PAI correlate with changes in placental perfusion is unknown and was tested in this study. PAI combines optical contrast of photoacoustic laser technology with a high spatial resolution of ultrasound. Placental vascular oxygen saturation (sO2) was quantified in C57Bl/6 mice by the PAI using Vevo 2100 LAZR instrument (Fujifilm, VisualSonics). 50 microliters of Vevo MicroMarker contrast agent was delivered via a bolus tail-vein injection. Placental perfusion parameters were determined at day 15 of gestation in C57Bl/6 mice. Our data demonstrated significantly greater oxygenation in the placental labyrinth versus maternal triangle area (53.01±1.7 vs. 42.7±4.2 % sO2; n=6, p<0.05). No regional differences were detected in relative blood flow in these mice (dB or a.u.). However “time-to-peak” (perfusion kinetics parameter) was greater in the labyrinth compared with maternal triangle area (2.3 times, p<0.05, n=5). Our data show that the overall blood flow is similar throughout the placenta at mid-late gestation, while the perfusion is greater in the labyrinth (fetal) side of the placenta consistent with greater vascularization of this area. Our data suggest that placental sO2 measured by the PAI is associated with coordinated changes in placental perfusion during late stages of mouse pregnancy. This approach provides a novel tool for non-invasively investigating placental environment and pregnancy outcomes particularly in pregnancies complicated by IUGR or preeclampsia.

Constructing quasi-linear V̇o2 responses from nonlinear parameters

Oxygen uptake (V̇o2) kinetics have been shown to be governed by a nonlinear control system across a range of work rates. However, the linearity of the V̇o2 response to ramp incremental exercise would appear to be the result of a linear control system. This apparent contradiction could represent a balancing of changing V̇o2 kinetics parameter values across a range of work rates. To test this, six healthy men completed bouts of ramp incremental exercise at 15, 30, and 60 W/min (15R, 30R, 60R, respectively) and four bouts of an extended-step incremental exercise. V̇o2 parameter values were derived from the step exercise using two monoexponential models: one starting at time zero and encompassing the entire stage (MONO), and the other truncated to the first 5 min and allowing a time delay (5TD). The resulting parameter values were applied to an integrative model to estimate the ramp responses. As work rate increased, gain values increased ( P < 0.001 for MONO and 5TD), as did mean response time (or time constant) values (MONO: P < 0.001; 5TD: P = 0.003). Up to maximal V̇o2 (V̇o2 max), the gains of the estimated ramp responses from both models were not different from the gains of the actual observed V̇o2 responses for 15R and 30R (15R: 11.3 ± 1.2, 11.7 ± 0.7, 10.9 ± 0.3; 30R: 10.5 ± 0.8, 11.0 ± 0.5, 10.7 ± 0.3 ml O2·min−1·W−1, for actual, MONO, 5TD, respectively) but were significantly greater for 60R (8.7 ± 1.0, 9.9 ± 0.4, 10.3 ± 0.3 ml O2·min−1·W−1 for actual, MONO, 5TD, respectively). Up to 80%V̇o2 max gain values were not significantly different for any ramp rate ( P > 0.05 for all). We conclude that the apparent linearity of the V̇o2 response to ramp incremental exercise is consequent to a balancing of increasing time constant and gain parameter values.