Interventional cardiology – definition, usage and role

Interventional cardiology is a branch of cardiology that deals specifically with catheter-based treatments of structural heart diseases via a minimally invasive approach¹.

The first cardiac catheterisation was performed by Werner Forssmann, a German physician, in 1929 when he was still a young surgical trainee.

He put himself under local anaesthesia and passed a urinary catheter through a large needle inserted in the antecubital vein of his arm. Dr Forssmann then walked with his nurse to the radiology department on the floor below, where, under fluoroscopic guidance, he advanced the catheter the full 60cm into his right ventricular cavity and injected contrast.

The image of the catheter lying in his heart has become an historical testimonial of such a pioneering experiment.

Dr Forssmann described the passage and removal of the catheter as entirely painless, accompanied only by occasional sensations of warmth and a cough secondary to the stimulation of the vagal trunk². Following this test, Dr Forssmann faced a disciplinary action for self-experimentation and he was forced to leave the institution for not meeting scientific expectations.

Nevertheless, he continued his medical career and, in 1956, was awarded the Nobel Prize in Medicine, together with André Frédéric Cournand and Dickinson W Richards, for “their discoveries concerning heart catheterisation and pathological changes in the circulatory system”³.

Another important advancement in interventional cardiology was reported in the 1950s, when Sven Ivar Seldinger described a percutaneous vascular access using a catheter advanced over a guidewire. The Seldinger technique is now the method of choice for vascular access in most interventional cardiac procedures, which are also referred to as “pinhole surgery” (as opposed to “keyhole surgery”)¹.

Many cardiac procedures can be performed by catheterisation, while the main advantages of using interventional cardiology, rather than traditional cardiac surgery, is the avoidance of pain, scar formation and long postoperative recovery times.

In humans, coronary angioplasty is the most common interventional cardiology procedure, which represents the gold standard of care for acute myocardial infarction. This involves the use of a tiny balloon catheter to dilate a blocked coronary artery, often combined with the placement of a stent to keep the artery open and decreasing its chance of narrowing again.

In small animal cardiology, coronary artery disease is not a common clinical presentation; therefore, interventional cardiology is reserved for the correction of congenital cardiac defects, cardiac pacing and arrhythmia ablation.

Facilities and equipment

Cardiac interventions require a well-equipped catheterisation laboratory, commonly referred to as a “cath lab”.

Although some academic institutions have a dedicated facility, the majority of veterinary cath labs are multipurpose surgical theatres with a fluoroscopy unit as well as lead-lined walls and doors, to protect staff and patients from the effects of radiation.

Fluoroscopy provides real-time x-ray imaging for guiding a variety of diagnostic and interventional procedures with a continuous series of images produced at a rate of 25 to 30 complete images per second, similar to the frame rate of conventional televisions.

Digital fluoroscopes are preferred over traditional fluoroscopes for their faster turnaround times and immediate image preview; the ability to digitally transfer images to other systems for analysis; and increased image enhancement and detail.

However, the radiation dose rates for fluoroscopy can be relatively high, and exposure times depend on the complexity of the procedure and the skill and experience of the cardiologist performing the procedure. Consequently, a thorough understanding of the technology and radiation doses from fluoroscopy is essential.

Digital subtraction angiography and road mapping are valuable tools in many endovascular interventions, although the availability of these techniques is less relevant for standard cardiac procedures.

It is important to acknowledge that, in human hospitals, interventional cardiology is ranked as the occupation with the highest level of radiation exposure in the world, with providers facing levels that are typically three times higher than those faced by workers in nuclear power plants.

As a result, cardiologists and their assistants are required to use appropriate personal protective equipment during procedures – such as aprons, thyroid shields and glasses – made using lead-equivalent materials. A personal dosimeter should be worn throughout the entire procedure⁴.

The room should also be equipped with a radio-transparent operating table to allow the radiographic visualisation of the heart and vessels.

Pressure injectors (or power injectors) allow the opacification of cardiac chambers and vessels using high-flow, high-volume and fixed-rate injections of contrast medium delivered with relatively high pressures. However, manual injections can also be considered for interventional procedures where a low and variable flow rate is sufficient.

Finally, the dedicated room for cardiology interventions should also be equipped with a vital signs monitor with cardiac defibrillation and pacing modalities connected to self-adhesive monitor/defibrillation pads.

Transoesophageal echocardiography (TOE) is becoming widely available in veterinary interventional cardiology, and TOE is particularly helpful for a more accurate measurement of anatomical structures, assessing positioning of catheters and occlusion devices, and evaluating blood flows in real time during procedures.

Consumables

A basic interventional cardiology starter pack should include a variety of vascular sheaths, vessel dilators, catheters, guidewires and stents.

Hydrophilic-coated vascular sheaths, also called “introducers”, are used to obtain arterial or venous access and facilitate the insertion of catheters and guidewires. They have a collateral lock for easy flushing and a haemostatic valve to prevent bleeding through the device. Vessel dilators allow progressive dilation of the access site when needed.

Catheters are available in different sizes, lengths and conformation for a variety of applications. They can be divided into angiographic catheters to deliver contrast medium for angiographic studies, and non-angiographic catheters for passing guidewires or other devices. Their tip can be straight, curved or curled, with an end hole for passing guide wires.

Angiographic catheters also have smaller side holes to optimise contrast medium distribution during the injection, minimise catheter movements and reduce the risk of arrhythmias or myocardial staining by directing dye away from the myocardial or vascular wall.

The Berman angiographic catheter represents an exception because this does not have an end hole, but instead is equipped with an inflatable balloon to facilitate the catheterisation of the right ventricle and pulmonary artery.

Conversely, standard balloon catheters are coaxial catheters that incorporate a balloon at the leading end, which can be inflated for therapeutic dilations, such as balloon valvuloplasty (BVP) to palliate pulmonic stenosis.

Guidewires also come in a variety of sizes, lengths, strengths and conformations, and are used to select a desired route and offer support for advancing catheters⁵.

All these instruments are single use and need to be thoroughly flushed with standard or heparinised saline solution before their use, to reduce the risk of injecting air bubbles or small blood clots into arteries or veins.

Dedicated bowls are also used to keep catheters and guidewires moistened and clean at all times.

Common interventional procedures in small animal cardiology

Several interventional procedures have become commonly observed in small animal cardiology.

Patent ductus arteriosus

Patent ductus arteriosus (PDA) is one of the most common congenital cardiac defects observed in dogs, although it is also sometimes observed in cats. If not corrected at a young age, it can cause cardiac dilation, congestive heart failure and death⁶.

The first minimally invasive PDA closure in a dog was performed in 1994 using a detachable spring coil. A variety of occlusion devices have been used over the years, including thrombogenic coils, vascular plugs and the Amplatz canine duct occluder (ACDO), using both transvenous and transarterial routes for deployment of occlusion devices⁷.

The ACDO is generally superior in terms of simplicity of the technique, degree of occlusion and complication rate, but it requires an arterial approach, which in very small dogs represents a limiting factor. Therefore, for small dogs and cats, coils and vascular plugs inserted via a venous access may represent a better option.

The ACDO is made of a double nitinol mesh, which is a nickel-titanium alloy with superelasticity and shape memory properties. It has a short waist that separates a flat distal disc, which – once deployed – will sit firmly in the wall of the pulmonary artery, and a cupped proximal disc that will remain inside the ductal ampulla.

This unique design allows close-fitting to various duct morphologies. The ACDO procedure involves arterial access via the femoral artery, followed by insertion of an angiographic catheter (such as a pigtail angiographic and sizing catheter) to perform an aortogram and determine PDA morphology and dimension.

Transoesophageal echocardiography can also be used for this purpose, alone or in combination with traditional selective angiography. Following identification of the duct, a long vascular sheath is introduced through the duct and positioned in the main pulmonary artery.

At this point, an ACDO with adequate waist diameter is advanced into the vascular sheath with its delivery cable until the flat distal disc opens completely within the main pulmonary artery. The vascular sheath is then retracted into the duct until the cupped proximal disc opens inside the ductal ampulla.

Once all the components of the ACDO are firmly positioned, the successful closure of the patent ductus can be verified via disappearance of the continuous murmur, using an oesophageal stethoscope, or disappearance of the shunt assessed via Doppler interrogation with TOE or with a small contrast injection through the vascular sheath.

Finally, the delivery cable is detached and removed with the vascular sheath, leaving the ACDO firmly in place (Figure 1)⁸.

In dogs smaller than 3.5kg and in cats, the ACDO technique may be infeasible due to the tiny lumen of their femoral arteries, which may not accommodate the size of the required vascular sheath. Therefore, duct occlusion via thrombogenic coils can be considered, using either a transarterial or transvenous approach.

However, valid alternatives to coil deployment are currently available using a transvenous approach via the jugular vein, although some cardiologists may still prefer a femoral vein access. Nit-Occlud PDA , a spiral nitinol coil mounted in a straightened fashion on a flexible delivery system, is the author’s favourite option for small dogs or cats with a minimal duct diameter less than 2.5mm.

The procedure involves transcutaneous vascular access via the jugular vein, followed by catheterisation of the right ventricle and, subsequently, advanced into the main pulmonary artery until the tip of the catheter is near the duct. In this position, a soft guidewire is advanced inside the catheter and then into the duct until it is clearly visible in the descending thoracic aorta.

The Nit-Occlud implantation catheter is then passed over the guidewire into the aorta before the coil is passed through the catheter. All the loops of the spiral coil, except the last one, are deployed into the aorta.

The entire system is then retracted into the ductal ampulla before releasing the final loop in the pulmonary side of the duct, anchoring the coil in position. The device can then be released by rotating the delivery cable (Figure 2).

For small dogs with a minimal duct diameter bigger than 2.5mm, vascular plugs may represent a better option for a transvenous approach.

Although several types of self-expanding vascular plugs exist, the Amplatz vascular plug II, made out of three lobes of a densely braided multilayer nitinol mesh, probably represents the safest option. The procedure is very similar to the one previously described for the Nit-Occlud, with the vascular plug positioned with two lobes into the PDA ampulla and the third one into the main pulmonary artery to anchor it in position while still connected to its delivery cable.

A pre-release angiography can confirm the successful closure and the plug can eventually be left in position by rotating its delivery cable⁹.

The major potential risk associated with all PDA closure techniques is represented by the embolisation of the occlusion device, which tends to lodge in a small branch of the pulmonary artery. However, this is a relatively rare complication that is seldom accompanied by significant complications.

BVP

Severe valvular stenosis affecting either the pulmonic or aortic valve may benefit from BVP.

This is currently the treatment of choice for pulmonic stenosis (PS) in both humans and dogs, while some limitations and controversies still exist for the treatment of aortic stenosis. The first successful BVP was performed on an English bulldog in 1980, two years before the first BVP in a child. Acknowledgement exists that a fatal outcome in that dog would have caused indefinite postponement of BVP in human patients – a procedure that currently benefits more than 25,000 patients a year worldwide¹⁰.

Indeed, bulldogs with PS often display another congenital defect characterised by an aberrant left coronary artery originating from a single right branch, and balloon inflation can cause disruption or avulsion of the aberrant artery with a potentially fatal outcome.

The BVP procedure for PS involves catheterisation of the right ventricle, through either a femoral or jugular venous approach, where contrast medium is injected to obtain a right ventriculogram and opacification of the main pulmonary artery (MPA) to visualise the stenotic valve.

A catheter is then advanced into the MPA and a soft guidewire is inserted inside the catheter until reaching a distal arterial branch. The catheter is then advanced over the wire to reach the same peripheral position and the soft guidewire is exchanged with a stiffer guidewire.

The catheter is subsequently retracted, leaving only the stiff guidewire in position. This will provide a steady support for the balloon catheter, which is advanced over the wire until the middle part of the balloon, marked by two radiopaque dots, is positioned at the level of the stenotic lesion.

At this point, the balloon is inflated either manually with a syringe prefilled with diluted contrast medium or via a dedicated inflation device with a manometer to avoid overinflation and the risk of balloon rupture.

During inflation, a central waist is observed as the balloon engages the stenotic valve and, continuing inflation, the waist disappears, and the balloon takes its proper inflated shape as the valve fusion is resolved. Right ventricular pressure can be measured before and after inflation to quantify the amount of pressure gradient reduction obtained following BVP (Figure 3).

The BVP procedure for aortic stenosis is very similar to the one described previously, with the major difference that the vascular access is obtained via surgical cutdown of the right carotid artery. This allows insertion of catheters and guidewires into the left ventricle, passing through the stenotic aortic valve. This procedure can be successful for valvular aortic stenosis, which is not a very common condition in dogs.

Indeed, the majority of aortic stenosis cases in dogs have a subvalvular component characterised by a ventricular fibrous ridge, just beneath the valve, that does not dilate with a simple balloon inflation.

For this reason, a cutting balloon dilation catheter can be used to reduce the fibrous ring prior to balloon dilatation of the stenosis. Cutting balloons contain embedded blades (microtomes) designed to create radially oriented cuts when inflated. The combined cutting technique followed by high-pressure valvuloplasty represents a relatively safe and feasible therapeutic option for dogs with severe subaortic stenosis; although, particular attention should be paid to avoid damage of the nearby chordae tendineae with the potential risk of severe mitral valve damage (Figure 4)¹¹.

Pulmonic stent angioplasty

Although BVP represents the treatment of choice for dogs with severe valvular PS, the presence of hypoplasia of the MPA, frequently observed in the bulldog and other brachycephalic breeds, represents a serious limitation for traditional BVP.

However, transvalvular pulmonic stent angioplasty has been recently described as a novel technique to reduce the pressure gradient across the valve and improve clinical signs of affected dogs, including cases with supravalvular PS, characterised by a fibrous lesion in the MPA above the pulmonic valve, and even those with coronary malformation¹². Stents for angioplasty are hollow mesh tubes that, once deployed in the desired position, will permanently expand and support the arterial wall, preventing it from re-stenosing.

The procedure is similar to that described for traditional BVP, but instead of using a balloon catheter, a pre-mounted balloon-expandable metallic stent is advanced inside a vascular sheath over the guidewire and positioned across the stenotic lesion. The vascular sheath is then withdrawn over the stent, before inflating the balloon, which, in turn, dilates the stent and anchors it to the MPA wall.

The balloon is then rapidly deflated and withdrawn, leaving the stent in place (Figure 5).

Cor triatriatum dexter

Cor triatriatum dexter (CTD) is a relatively rare congenital heart defect in which the embryologic right sinus venosus valve persists as a septum within the right atrium, literally dividing the chamber into two compartments divided by a small orifice.

The defect causes increased pressure in the caudal vena cava with subsequent hepatic vein congestion and ascites, although pleural effusion may also develop due to increased pressure in the upper right atrial chamber.

CTD is considered the most common causes of ascites in puppies and it can be palliated with a balloon dilation of its orifice. The procedure involves catheterisation of the femoral vein and insertion of a high-pressure balloon catheter through the CTD orifice. Following balloon dilation, the orifice is opened widely, reducing the pressure gradient across the two atrial chambers.

To obtain a sufficient dilation of the orifice, several dilations are often necessary, using incremental balloon diameters. However, sometimes, the separating membrane may be too thick and fibrotic, limiting its dilation with standard balloon catheters.

Therefore, the use of a cutting balloon prior to balloon dilation may provide better results of the subsequent dilatation¹³.

Another alternative is represented by the insertion of a vascular stent across the abnormal partition, improving caudal venous return to the right ventricle and reducing the right to left shunt¹⁴.

Ventricular and atrial septal defects

Although ventricular septal defects (VSDs) and atrial septal defects are rarely haemodynamically significant abnormalities in small animals, particularly large defects may eventually cause volume overload and secondary cardiac remodelling.

In such cases, a transcatheter septal defect closure can be attempted with an Amplatzer muscular VSD occluder using a retrograde aortic approach¹⁵ or an Amplatzer atrial septal occluder using a right jugular venous approach¹⁶.

Conclusions

In summary, interventional cardiology is rapidly replacing traditional cardiac surgery for the treatment or palliation of a variety of congenital cardiac defects both in humans and small animals.

Challenges remain, including (with the exception of the ACDO) the lack of dedicated device design for dogs or cats.

Furthermore, since the majority of these procedures are carried out in young patients, a lack of clinical studies exist to evaluate the potential consequences of body growth.

Nevertheless, minimally invasive technologies are now offering remarkable opportunities for the successful management of congenital cardiac defects in dogs and cats, although involvement of experienced human cardiologists and collaboration among veterinary surgeons operating in this field remains an indispensable factor to maximise optimal outcomes.