Although in terms of anatomy, a child’s heart does not differ significantly from an adult’s heart, each organ has its specific characteristics from the electrophysiological point of view. What are the secrets of the youngest patients’ hearts and what should be remembered regarding electrophysiological diagnostics and therapy, reveals Artur Baszko, MD, PhD (2nd Department of Cardiology and Pediatric Electrophysiology Centre, Poznań University of Medical Sciences).
Let’s start with a fundamental issue: is a child’s heart, apart from its size, the same as an adult’s?
From the moment of birth through all stages of further development until adulthood, the child’s heart changes very little in terms of quality. It is, of course, much smaller, has thinner walls and large vessels that supply and drain blood from the heart. However, anatomically it is practically the same as an adult heart. The difference lies in the heart’s functionality, where significant differences exist. A child’s heart works much faster than an adult’s, and children have also lower blood pressure.
Your main specialty is cardiac electrophysiology. What differences do you observe between children and adults in this area?
It is worth knowing what electrophysiology is. It is the science of the electrical activity of the heart. We can compare the action of the heart to the action of an automobile engine – without electric sparks, the heart would not be able to contract, i.e. pump blood. These sparks are generated in the sinus node at the base of the heart’s right atrium and then propagate through a specialized conduction system to the atria and ventricles, leading to a synchronized contraction, first of the atria and then of the ventricles. However, there are few differences in the electrical properties between a child’s heart and an adult’s heart. One of the most significant is a considerably higher heart rate. Whereas the heart rate for adults fluctuates around 60-80 beats per minute, it is well over 100 beats for children. The heart rate can be up to 140 beats per minute in the newborn period!
What about other differences?
Another difference concerns the speed of impulse conduction and the so-called refractory time. Impulses are conducted much faster in children, and the individual heart structures have significantly shorter refractory times, leading to substantially higher heart rates in the case of tachycardia.
It is worth to mention that the neonatal period favors the development of rhythm disturbances that rarely occur in adults. Moreover, some of these rhythm disturbances may disappear as the child’s heart develops and the autonomic system matures.
Do children have different heart rhythm disorders than adults?
Rhythm disorders related to an accessory pathway (overt or concealed WPW syndrome) are more common in children than in adults. Ectopic tachycardia (especially atrial) and nodal reentrant tachycardia are also more frequent. The type of predominant rhythm disturbances changes with age. In adolescents and young adults, nodal reentrant tachycardia and tachycardia related on the accessory pathway are most frequent. Still, ventricular disturbances in the absence of structural heart disease (arrhythmias from the right and left ventricular outflow tract and the so-called fascicular tachycardia) are more common.
Which arrhythmias are more likely to develop in adults, including seniors?
In adults, atrial fibrillation and arrhythmia caused by other heart diseases (such as myocardial infarction or cardiomyopathies) are important electrophysiological problems. There is also an increasing incidence of bradyarrhythmias in the elderly population, i.e. diseases of the stimulus conduction system that slow down heart rate (sinus bradycardia) or block conduction through atrioventricular node or the left bundle branch. Many patients with sinus node or atrioventricular node disease require the implantation of a pacemaker. In children’s, these problems are rare and are usually congenital or result from a corrective operation on a heart defect. Out of more than 1100 ablation procedures in our centre, we have implanted 168 epicardial pacemakers in infants and small children and 48 endocardial systems in older children.
In terms of implantable devices, how common is it to safeguard children with cardioverter-defibrillators?
The most common risk of sudden cardiac death among adults is coronary artery disease with acute myocardial infarction, its consequences and various types of cardiomyopathies such as dilated, hypertrophic, and arrhythmogenic. Cardiomyopathies also occur in children but are rarely associated with a risk of sudden death. This group is dominated by genetic diseases, the so-called channelopathies: long QT syndrome, catecholaminergic ventricular tachycardias and Brugada syndrome). For life protection, a cardioverter-defibrillator is implanted in adults in strictly defined cases, whereas the need for such a system in children is relatively rare.
What are the critical differences in treating arrhythmia patients according to age group?
On the one hand, most of the youngest patients respond well to pharmacological treatment, so it is not always necessary to undertake interventional therapy or implant a device. Secondly, it can happen that children “grow out” of certain rhythm disorders over time. In the case of children under 5, we sometimes prefer to prescribe antiarrhythmic drugs rather than perform an ablation procedure. These are generally accepted recommendations. At the same time, however, there is a group of children for whom drug therapy is ineffective, poorly tolerated or has serious side effects. In these cases, ablation is necessary. The most common issues are WPW syndrome, atrial and nodal tachycardias. In Poznań Pediatric Department, we have performed almost 90 such procedures on small children. The youngest patients were 3-4 weeks old. I can immodestly admit that we have a very high ablation efficiency in this group of patients, and so far, we have had no complications.
Sounds impressive. Let us stop at the stage of diagnosing heart rhythm disorders in children. What does a diagnosis of arrhythmia mean for the youngest patients?
A good example are children actively involved in sports who have been diagnosed with WPW syndrome at periodic check-ups. The presence of an accessory pathway connecting the atria to the ventricles, bypassing the conduction system, may in some situations result in the risk of tachycardia and sudden death. Fortunately, the latter happens rarely. However, the problem of WPW syndrome is essential in qualifying for sports. In the case of symptomatic patients, i.e. those with paroxysmal tachycardia or atrial fibrillation, ablation is necessary. In the absence of parental consent for this procedure, sports participation is not recommended, as it is potentially risky for the child.
The management is slightly different in children who show features of pre-excitation on ECG but are entirely asymptomatic. Exercise stress test or Holter test, in which we find only intermittent pre-excitation (we call it intermittent WPW syndrome), suggests a relatively safe form of the accessory pathway. If there is constant pre-excitation, only an electrophysiological study can show whether the pathway is safe or potentially dangerous, i.e. whether it can lead to tachycardia or has a very short refractory time, associated with the risk of cardiac arrest in case of atrial fibrillation.
What does a children electrophysiological study look like?
Electrophysiological examination of children is usually performed under general anaesthesia. We should bear in mind that anaesthesia affects the haemodynamic parameters and can also influence electrical heart properties. In order to exclude the sedative effect of anaesthesia on the heart, the patient is administered a short-acting beta-mimetic intravenously – isoprenaline – a drug acting similarly to adrenaline. After its application, it is possible to see how the accessory pathway behaves under “stress” conditions. It allows a more reliable assessment of the electrophysiological properties of the accessory pathway and the stimulus-conduction system.
In this group of patients with WPW syndrome, we also have a subgroup with the so-called parahisian accessory pathway. This term means that the accessory pathway runs very close to the conductive system, especially the His bundle. The ablation in these patients always carries an increased risk of atrioventricular block. The use of a 3D cardiac reconstruction system with electroanatomical mapping improves the precision of the procedure, and the use of cryoablation improves its safety. We have performed cryoablation in over 120 children with a parahisian arrhythmia without observing any atrioventricular conduction system impairment. However, the efficacy of this technique is slightly lower than the efficacy of classical radiofrequency ablation. Nevertheless, we assume that the procedure can be repeated if necessary – the most important thing is to avoid complications that may remain for life.
Which groups of youngest patients are the most challenging to treat?
A category of children problematic in this context is the subgroup with rhythm disturbances and congenital heart defects, especially after previous cardiac surgical interventions, often of multiple stages. Among patients with particular difficulties performing ablation procedures, I would select children with a common atrioventricular canal with a hypoplastic heart due to underdevelopment of the left or right side. After many hospital stays, these children often have an occluded part of the venous system due to previously inserted catheters for administering drugs or performing cardiac catheterization. It significantly limits access to the heart.
How does the strategy differ in the electrophysiological surgical treatment of adults and children?
The child’s heart anatomically differs slightly from the adult heart, as I mentioned before. The differences mainly concern the size and thickness of walls of the individual heart cavities, the diameter of venous and arterial vessels. The distances between unique structures are also smaller. It is important when performing an electrophysiological examination and ablation procedures. Firstly, the puncture of the vessels is more difficult due to the size and degree of filling the vessels with blood. Such procedures are almost always performed under general anaesthesia, and anaesthetic drugs reduce blood pressure. We often use ultrasound to puncture the vessels, especially in young children. We use practically the same diagnostic electrodes for electrophysiological examination as in adults. Sometimes the catheters are slightly thinner. Due to narrow veins in children, we have to limit their number to the necessary minimum. When introducing catheters into the heart, we must be cautious not to cause any perforation of the vessel wall or heart.
What are the other things the operator should pay attention to?
It is essential to be aware of the minimal distances between the endocardium and the arteries running on its surface. The spaces between the structures forming the Koch’s triangle, at the top of which runs the bundle of His, are also very small. For left-sided arrhythmias, it is necessary to pass the ablation electrode through the aortic valve, which has very thin and delicate leaflets in children, unlike in adults. The literature has described the aortic valve dysfunction after ablation procedures performed with the transaortic technique. There has also been damage to the femoral artery wall requiring reparative surgery.
For this reason, for more than 10 years we have been performing the majority of procedures – if not almost all of them – using the transseptal technique. Here, the atrial septum has to be pierced with a transseptal set – the same set used in adults whose hearts are several times bigger. The structure that needs to be punctured has a few millimetres in diameter. It is located close to the aorta and the atrial free wall. We have already performed several hundred such operations, fortunately without any complications. Conditions resulting from the anatomy of a child’s heart and equipment dedicated to performing procedures in adults significantly impact the safety of procedures performed in this area.
Is it necessary to perform additional tests before the procedure regarding children?
Indeed, sometimes the changed anatomy of the heart also requires additional imaging studies before the planned ablation. We use cardiac tomography or rotational angiography performed during the procedure. The advancement of the electrophysiological system allows us to perform a 3D reconstruction of these examinations, which dramatically improves the orientation of the highly altered anatomy of the heart and vessels. The most challenging group of patients are children after the Fontan operation. All venous flow is redirected directly from the upper and lower body to the pulmonary artery (so-called TSPS). Suppose the tunnel turning blood from the lower body to the pulmonary artery is connected to the atrial part by a small hole (so-called fenestration). In that case, some procedures can be performed by slightly extending this opening or using the access through the aorta. Needless to say, the manipulation of ablation electrode in such altered anatomy is considerably tricky, even in the ablation of focal tachycardias or those related to WPW syndrome. The degree of difficulty is exceptionally high when dealing with macro-reentry tachycardias dependent on previous surgical scars. It is then necessary to perform an exact bipolar and propagation map with the identification of scar and slow conduction zones. After analysing such an electroanatomical map, we plan the application system to eliminate all possible tachycardias.
You’ve drawn attention to the aspect of using electroanatomical systems during paediatric procedures. What is their role?
I’ve mentioned before that one of the most important principles when performing ablation procedures in children is safety. Electroanatomical systems improve the precision of ablation procedures and are therefore often used in more difficult ones. We were the first in Poland to start performing “freezing” or cryoablation procedures in children for safety reasons. So far, we have performed over 120 such procedures in cases where the classical ablation procedure was utterly impossible from the standpoint of complication risk. We were the first to use the Ensite system to perform ablation in children to entirely or almost entirely eliminate X-rays. We have now completed practically 800 such procedures. The system allows us to map and perform ablation procedures with all available electrodes accurately. Our initial idea was to eliminate potentially harmful X-rays, assuming that the procedures could be prolonged by this, as the initial publications indicated. However, after years of using this technique, we discovered that not only did we reduce the dose of X-rays tenfold, but in 70% of cases, we performed the procedure wholly fluoroscopically free. It turned out that treatments with the Ensite system are significantly shorter and that we achieve an effective ablation procedure by performing fewer applications.
Thank you for this talk!