The 3-to-5 Mhz sectoral ultrasound transducer is generally the most suitable and it is recommended to use the transcranial factory preset. Then, the transducer is positioned in the temporal window, with the transducer index pointed to the patient’s right, anterior to the tragus of the ear and moving in small steps, until the flow signal towards the transducer is recorded by adjusting the frequency (2.0 MHz and 2.5 MHz), gain and depth (35-60mm) settings to obtain clear images of the middle cerebral artery (MCA) (Figures 1A, 1B, 1C).

Figure 1 MCA: middle cerebral artery.

In detail, the contrast agent ideally consists of 8 mL of normal saline combined with 1 mL of air and 1 mL of the patient’s blood. The presence of red blood cells in the agitated saline solution enhances ultrasonic wave reflection, thereby improving visibility of microbubbles in TCD. The mixture is then vigorously shaken approximately 10 times using a system comprising two 10 mL syringes connected by a 3-way stopcock. The stirred solution is then administered into the antecubital vein via an 18 gauge, preferably.

During the procedure, the patient is instructed to perform the Valsalva maneuver, which involves forced expiration against a closed glottis for at least 10 seconds (strain phase), followed by relaxation (release phase). Hemodynamically, this maneuver causes a sudden increase in blood volume in the right atrium, exceeding the flow from the pulmonary veins to the left atrium, thereby promoting flow through the shunt.^6,7

Simultaneously, in an insonation of the curve is conducted using Pulsatile Doppler on the middle cerebral artery (MCA), with reduced scanning speed to allow continuous recording for 60 seconds from the start of the Valsalva maneuver. During this time period, spikes of microbubbles will be visualized, being evidenced as HITS due to reflection of the ultrasound signal (Figure 1C).

The test is considered positive when at least one early hyperintense signal is detected (up to 10 seconds after microbubble infusion). In tachycardic patients, counting cardiac cycles may be a more accurate approach, as it considers the patient’s individual heart rate.⁸ The detection of microbubbles within the first 3-5 cardiac cycles indicates an intracardiac shunt, whereas their appearance after the fifth cardiac cycle suggests a pulmonary shunt.⁹

The number of embolic spikes (HITS) recorded provides critical information; a higher number of microbubbles correlates with a larger foramen ovale passage size.^10,11 Such data is essential for determining the hemodynamic significance of PFO according to the Spencer logarithm scale (adapted from Table 1 and Figure 2).^12,13

Table 1

SPENCER’S LOGARITHMIC SCALE
GRADE 0	Absence of HITS	Absence of shunt
GRADE 1	1 to 10 HITS	Intermittent shunt (during Valsalva only)
GRADE 2	11 to 30 HITS	Small shunt
GRADE 3	31 to 100 HITS	Moderate shunt
GRADE 4	101 to 300 HITS	Significant shunt
GRADE 5	> 300 HITS	Very large shunt, continuous appearance of hits ("curtain effect")

HITS: High-Intensity Transient Signals.

Figure 2

Case report demonstrating the value of TCD in the initial screening of a patient with suspected PFO. J.R.C., a 56-year-old male patient, without comorbidities, but with sequelae of ischemic stroke. The 24-hour Holter did not show atrial fibrillation. The patient sequentially underwent TTE, TCD and TEE associated with the Valsalva maneuver to evaluate the cardioembolic source.

The TTE showed an aneurysm of the interatrial septum with the early detection of a large amount of saline contrast in the left atrium (Figure 3A). TEE was performed to adequately visualize the characteristics of the interatrial septum and PFO, aiming for percutaneous closure (Figures 3B and 3C). Using the MCA spectral Doppler, a number of severe HITS were detected during the first three cardiac cycles, defining the presence of an important venous-arterial shunt (Figure 4).

Figure 3

Figure 4