Day Three After the Arterial Switch: Open Sternum, Rising Lactate, and a 3.2 m/s Jet

Setup and safety first: sterile probe cover and sterile gel, hands cleaned, and do not disturb or lift the wound VAC dressing. Warm the gel, minimize handling, and coordinate with the bedside nurse for analgesia and monitoring - this infant is febrile and unstable, and the study must be shorter, not more thorough at any cost.

Sequence, working from what is available outward:

1. Subcostal coronal and sagittal views from just below the dressing margin - often the best remaining window in a ventilated infant, and good for ventricular function, septal position, and any pericardial effusion.
2. High parasternal short axis with a small-footprint 8-12 MHz probe for the coronary origins; the same window gives the neo-aortic valve and root.
3. Apical four- and two-chamber views for function and regional wall motion, mitral regurgitation, and right ventricular pressure estimation.
4. Suprasternal notch last, working around the tube and lines for the anastomosis and arch.

Priority order is dictated by the question:

• Left ventricular systolic function and any regional wall motion abnormality (the fastest route to coronary insufficiency).
• Coronary flow, if a window exists.
• Neo-aortic and anastomotic velocities, arch, and branch pulmonary artery velocities - after the LeCompte maneuver the pulmonary bifurcation lies anteriorly and branch pulmonary stenosis is common, so a high velocity near the aorta may not be aortic at all.

Documentation: state explicitly which views were unobtainable and why, and recommend transesophageal imaging if the transthoracic study cannot answer the question in an unstable patient. Do not let the request for speed silently convert an incomplete study into a negative one.

Technique:

• High-frequency small-footprint probe, narrow color box, low Nyquist (roughly 15-25 cm/s), single focus at the coronary depth.
• Identify the left coronary origin from the left-facing sinus of the neo-aorta and the right from the anterior sinus; the reimplanted anatomy is not the native pattern, so trace from the sinus outward rather than hunting a memorized location.
• Use color to find the vessel, then PW with a small sample volume and angle correction only when the angle is acceptable.

Interpretation:

• Normal infant coronary peak velocity is roughly 30-60 cm/s with diastolic predominance. Focal aliasing with peak velocity above about 1 m/s, turbulence, or a visible narrowing suggests stenosis at or near the anastomosis.
• A proximal vessel that is dilated with reduced or reversed flow suggests insufficiency or distal obstruction.
• Absent diastolic flow, or flow reversal, is a strong abnormal signal.

Pitfalls to avoid:

• Coronaries are 1-2 mm in a 5 kg infant; color blooming and angle dependence generate false stenosis and false patency with equal ease.
• A prominent septal branch or a coronary venous signal can mimic an artery.

Triangulate rather than rely on the vessel alone: a regional wall motion abnormality or reduced regional strain in the left coronary territory, new mitral regurgitation, and worsening ventricular function all support the diagnosis, and in practice coronary obstruction after the switch often presents this way with no measurable gradient at all.

First, the arithmetic: 4 x (3.2) squared = 41 mmHg peak gradient. The number is easy; the attribution is not.

Distinguish fixed obstruction from a high-flow state. This infant is febrile at 38.6 degrees C and tachycardic at 168 bpm, so cardiac output and therefore gradients are elevated. Features favoring true stenosis:

• A late-peaking, prolonged jet that persists and lengthens into the cardiac cycle, rather than a brief early peak.
• A step-up in velocity across a visible anatomical narrowing in two dimensions, with a proximal-to-distal velocity-time integral ratio consistent with loss of area.
• Diastolic runoff or a 'diastolic tail' in the descending aorta, and a systolic blood pressure gradient between the right arm and the leg.
• Reduced distal arch or descending aortic pulsatility.

What else to measure before reporting:

• Two-dimensional measurement of the anastomotic site and neo-aortic root, and comparison with the valve level, so you can say where the acceleration begins.
• Branch pulmonary artery and pulmonary bifurcation velocities - the LeCompte maneuver places the pulmonary arteries adjacent to the ascending aorta, and supravalvular or branch pulmonary stenosis is a more common early postoperative finding than anastomotic aortic stenosis. Misattributing a pulmonary jet to the aorta is a classic error.
• Right ventricular systolic pressure from the tricuspid regurgitation jet, and septal geometry.
• Ventricular function, mitral regurgitation, and any pericardial effusion.

Reporting judgment: a rising lactate with fever and a new 41 mmHg gradient in an infant with an open chest should be communicated directly and urgently to the surgical team as a possible hemodynamic problem, with the caveat that fever and high output exaggerate gradients and that a repeat study when the fever resolves will help separate the two. Do not simply file a velocity; state the differential and the recommended next step.