A methodological approach for quantifying and characterizing the stability of agitated saline contrast: implications for quantifying intrapulmonary shunt

Agitated saline contrast echocardiography is often used to determine blood flow through intrapulmonary arteriovenous anastomoses (Q_IPAVA). We applied indicator dilution theory to time-acoustic intensity curves obtained from a bolus injection of hand-agitated saline contrast to acquire a quantitative index of contrast mass. Using this methodology and an in vitro model of the pulmonary circulation, the purpose of this study was to determine the effect of transit time and gas composition [air vs. sulphur hexafluoride (SF₆)] on contrast conservation between two detection sites separated by a convoluted network of vessels. We hypothesized that the contrast lost between the detection sites would increase with transit times and be reduced by using contrast bubbles composed of SF₆. Changing the flow and/or reducing the volume of the circulatory network manipulated transit time. Contrast conservation was measured as the ratio of outflow and inflow contrast masses. For air, 53.2 ± 3.4% (SE) of contrast was conserved at a transit time of 9.25 ± 0.02 s but dropped to 16.0 ± 1.0% at a transit time of 10.17 ± 0.06 s. Compared with air, SF₆ contrast conservation was significantly greater (P < 0.05) with 114.3 ± 2.9% and 73.7 ± 3.3% of contrast conserved at a transit time of 10.39 ± 0.02 s and 13.46 ± 0.04 s, respectively. In summary, time-acoustic intensity curves can quantify agitated saline contrast, but loss of contrast due to bubble dissolution makes measuring Q_IPAVA across varying transit time difficult. Agitated saline composed of SF₆ is stabilized and may be a suitable alternative for Q_IPAVA measurement.

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