Non-selective contrast echocardiography
is achieved by the injection of a suspension of microbubbles into a peripheral
vein and observing the 2-D image for the presence of shunts.
The microbubbles can be created by pushing
intravenous fluid such as saline or a colloid solution (e.g. Haemacel) rapidly
to and fro between two syringes connected by a three-way tap. When a suspension
of bubbles has been produced, the three-way tap is opened to a previously placed
intravenous cannula, and the fluid injected. Only a small amount of fluid is
required, e.g. 5-10 ml for a medium-sized dog.
This technique requires an additional
assistant to perform the injection, while the echocardiographer maintains adequate
visualisation of the area of interest. The difficulty in obtaining good visualisation
of the right heart complicates the procedure. It is recommended that the procedure
be recorded on video, so that repeat viewings of the injection may be studied.
The microbubbles are quickly seen to
opacify the right atrium, ventricle and pulmonary artery.
The presence of right to left shunts
can be seen as the microbubbles shunt into the left side of the heart.
If there is a left to right shunt, negative contrast may sometimes be seen.
Occasionally bubbles may pass through the pulmonary circulation and return to
the left side of the heart; this can be misinterpreted as indicating a shunt.
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