Monday, 15 August 2016

How to perform an optimal saline bubble contrast echo study

Saline bubble contrast echo studies, sometimes called agitated saline studies, are used in echocardiography to test for the presence of a right-to-left shunt at the intracardiac or intrapulmonary level. But what's the best way to perform such a study? Here are some tips.

Saline bubble contrast echo study showing PFO (right atrium opacified,
and red arrow shows small number of bubbles crossing into left atrium)

In order to perform a saline bubble contrast echo study, we need to create macroscopic bubbles in a syringe of saline. To prepare this, we draw up 8 mL of sterile normal saline in a 10 mL syringe, plus 0.5 mL of room air. We also add 1 mL of the patient's blood to the mixture, which helps create smaller bubbles and thereby provides better opacification of the right atrium.

To create the macroscopic bubbles, the saline/air/blood mixture needs to be agitated, and this is effectively done using a second 10 mL syringe attached to the first syringe via a 3-way tap. Always use Luer-lock syringes to avoid one of the syringes becoming detached and spraying everyone with the mixture!

By agitating the saline/air/blood mixture back and forth between the two syringes, a suspension of macroscopic bubbles is created. This is the saline bubble contrast.

This contrast is then immediately injected as an intravenous bolus via a cannula in an antecubital vein (ideally at least 20-gauge), while the echo images are obtained. A saline bubble contrast procedure can be performed during transthoracic or transesophageal echo. An echo window should be chosen that gives a clear view of both atria and the interatrial septum.

If the patient is suspected to have a persistent left-sided superior vena cava, then both left and right arm injections should be used to help clarify the diagnosis - in PLSVC, a right arm injection will opacify the right atrium normally, but a left arm injection will opacify the coronary sinus before the right atrium.

As soon as the bolus of saline bubble contrast is seen to arrive in the right atrium, the patient should perform a manoeuvre to transiently raise right atrial pressure - if there is a right-to-left shunt, this will increase the likelihood of bubbles crossing into the left atrium, and therefore this increases the sensitivity of the study. Suitable manoeuvres include:
  • coughing
  • release of a Valsalva manoeuvre

The timing with which bubbles appear in the left atrium (if at all) should be noted. With an intracardiac shunt (e.g. a patent foramen ovale), bubbles will normally appear in the left atrium within three cardiac cycles of the right atrium opacifying. However with an intrapulmonary shunt, there will normally be at least five cardiac cycles before bubbles appear in the left atrium.

Useful guidelines on the performance of contrast echo (including both saline bubble contrast and also transpulmonary ultrasound contrast agents) have been published by the ASE and can be found by clicking here.

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