Introduction
The diagnosis of congenital heart disease is a medical/clinical art that has been supported by vast improvement in imaging techniques over the past four decades. It was only in the early 1980s that two-dimensional cardiac ultrasound became a universally available technique to assess cardiac anatomy and function at the bedside. The first Doppler ultrasound machines with colour flow mapping became available in the mid-1980s. Before then, it was clinical diagnosis with a stethoscope, M-mode scans, chest X-ray and cardiac catheterization. We are now practicing in an era where we can generate real-time non-invasive and free of radiation exposure images of the congenitally malformed heart. These techniques have provided us with very detailed insights into cardiac morphology and function.
To undertake surgical treatment of congenital heart disease in the current era requires a sound understanding of the anatomy, physiology and surgical techniques and, above all, the ability to read and interpret the vast range of imaging information that is generated and provided by cardiologists.
Chest X-rays
It is rare that a plain chest X-ray is diagnostic of congenital heart disease. The traditional signs of a ‘ground glass’ lung field (obstructed total anomalous pulmonary venous drainage) and figure of three with rip notching (established coarctation) are rarely seen in an era of earlier diagnosis and treatment. Nonetheless, the plain chest X-ray remains a good technique in the assessment of cardiomegaly, associated lung pathology, spinal problems, etc. After all the advances in cardiac imaging, the plain chest x-ray remains one of the most powerful predictors of exercise tolerance.
Cardiac Ultrasound
Trans-thoracic Ultrasound.
Two-dimensional cardiac ultrasound has revolutionized the assessment of cardiac structure and function over the past three decades. This is a bedside technique which allows for the comprehensive assessment of virtually all neonates, infants and children with congenital heart disease. Limitations are experienced in adults, as ultrasound waves are hampered by intra-thoracic air and poor trans-thoracic windows. The addition of colour flow imaging (from the mid-1980s) has largely facilitated the rapid recognition of intra-cardiac shunt lesions and valvar dysfunction (stenosis and regurgitation).
Doppler ultrasound added a new dimension to cardiac ultrasound investigation of the heart. By measuring flow velocities across the cardiac valves or any existing shunt lesions, it became possible to quantitate intra-cardiac pressure differences during any particular phase of the cardiac cycle. The Bernouille equation, ΔP = 4(Vmax2), strictly applies for only pulsatile flow and instantaneous pressure gradients. Despite these restrictions, and with an awareness of the limitations, it is an extremely valuable bedside technique to assess intra-cardiac pressure differences and thereby postoperative results. In the assessment of aortic or pulmonary valve stenosis or a pulmonary artery band, it is more reliable to take the mean velocity rather than the peak velocity to calculate pressure gradients.
In the current era, it is possible to assimilate very detailed imaging information on the vast majority of neonatal and infant cases with congenital heart disease to plan surgical intervention. In fact, the quality and detail of the images provided render diagnostic cardiac catheterization superfluous in the majority of cases, other than for the detailed assessment of preoperative haemodynamics. The exception to this is assessment of the peripheral pulmonary arteries or collateral arteries, which are obscured by the lungs (air) during cardiac ultrasound imaging. In these cases, cardiac catheterization and angiography remain the gold standard.
Trans-oesophageal Ultrasound.
This became available for the study of young infants and later also neonates in the early 1990s. It has helped us to better understand and evaluate certain aspects of mostly atrial morphology, pulmonary and systemic venous return and the atrio-ventricular (A-V) valves. In the context of ever-increasing resolution and field of depth of trans-thoracic ultrasound (harmonic and 3D imaging), the advantages of trans-oesophageal ultrasound are frequently absent or just marginal. As trans-oesophageal ultrasound requires general anaesthesia in the young patient population, it is now confined mostly to the intra-operative monitoring of cardiac surgical and interventional cardiac catheterization procedures (Figure 3.1).
Figure 3.1 Trans-oesophageal imaging of a child with an atrial septal defect undergoing trans-catheter closure. (Above) Four-chamber view with the delivery sheath crossing the atrial septal defect. (Centre) Initial device position is perpendicular to the atrial septum behind the aortic root. (Below) Final device position after release, engaging all the rims of the defect and achieving a flat profile.
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