Dissertation Defence: Assessment of the Stability and Quantification of Agitated Saline Contrast to Assess Blood Flow Through Intrapulmonary Arteriovenous Anastomoses

Lindsey Boulet, supervised by Dr. Glen Foster, will defend their dissertation titled “Assessment of the Stability and Quantification of Agitated Saline Contrast to Assess Blood Flow Through Intrapulmonary Arteriovenous Anastomoses” in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Interdisciplinary Studies.

An abstract for Lindsey Boulet’s dissertation is included below.

Examinations are open to all members of the campus community as well as the general public. Please email glen.foster@ubc.ca to receive the Zoom link for this defence.

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ABSTRACT

Substantial research has been conducted on right-to-left shunts in healthy humans using transthoracic agitated saline contrast echocardiography (TTSCE), however, TTSCE is limited by an inherent instability and an imprecise method of quantification. The goal of this dissertation is twofold: (1) to assess the stability of TTSCE using an in vitro model of the pulmonary circulation under varying environmental conditions, and (2) to determine the feasibility of measuring shunt fraction by modelling ultrasound contrast data. Study 1 aims to determine if the stability of saline contrast microbubbles is altered under gas conditions that match mixed venous (5.9 % CO2, 4.9% O2) blood and hyperoxic conditions (100% O2) at varying flow rates (1.8, 2.8, 4.3 & 6.8 l/min). At higher flow rates (4.3 & 6.8 l/min), less ultrasound contrast was lost in the hyperoxic vs mixed venous gas condition whereas no significant difference was observed at low flow rates (1.8 –2.8 l/min). The purpose of study 2 was to determine if hypobaria (simulated altitude: 5050m) affects microbubble stability. Using the experimental design from study 1, it was determined that reduced barometric pressure did not significantly alter microbubble stability. Finally, study 3 was designed to determine if mathematical modelling can be applied to saline contrast data from TTSCE to quantify a simulated shunt fraction. Saline contrast was injected into an in vitro model of the pulmonary circulation that was adapted to include an adjustable shunt vessel. Results show a good agreement between shunt flow measured using modeled data and actual shunt flow at higher flow rates but was less reliable at low flow rates. In summary, we determined that the stability of saline contrast microbubbles generated using TTSCE is not affected by hypobaria and is only slightly increased in hyperoxic conditions compared to mixed venous conditions suggesting the technique can be employed in a wide variety of environmental conditions. Furthermore, this work demonstrates that mathematical modelling applied to saline contrast ultrasound data can be used to assess shunt fraction, however it loses some fidelity at low flow rates and with small shunt fractions.