THE use of minimally invasive catheter-based interventions has revolutionised the treatment of cardiovascular disease in people and is now becoming increasingly common in veterinary practice with the first balloon valvuloplasty being performed in 1985.
Although the majority of vets aren’t likely to be performing these interventions in general practice (for lack of the correct equipment if nothing else), learning about these techniques is still fascinating as I discovered after watching last month’s webinar organised by The Webinar Vet.
Cardiologist Alistair Gibson led the webinar, discussing advances in veterinary minimally-invasive cardiothoracic surgery, having performed his first catheter-based intervention 25 years ago.
Pacemaker implantation for the treatment of bradydysrhythmias was one of the first of these techniques used but Alistair was keen to stress this could be an entire CPD topic by itself and so made a decision to concentrate on the more common congenital cardiac conditions encountered: pulmonic stenosis, subaortic stenosis and patent ductus arteriosis (PDA).
Pulmonic stenosis
Pulmonic stenosis is a common congenital heart defect in dogs and is associated with the malformation of the pulmonic valve and accounts for 11-18% of congenital cardiac defects.
Clinical signs vary markedly from case to case with some being asymptomatic to others suffering from syncope, fatigue and right sided heart failure. Most cases are older than one year old prior to developing clinical signs unless the stenosis is severe.
Assessing the severity of the disease can be achieved by a number of diagnostic techniques but performing an echo is the gold standard option.
It can both diagnose and assess the severity of the stenosis by determining the pressure gradient across the pulmonic valve.
Knowing when to intervene with these cases is a contentious topic and continues to be debated by cardiologists. It is accepted with a pressure gradient <50mmhg that the pulmonic stenosis is considered mild and often patients will be asymptomatic. However, a pressure gradient of >80mmhg is considered severe with clinical signs such as congestive heart failure being expected or even sudden death.
So when should we intervene: 60, 70 or 80mmhg? This is where it very much depends on the cardiologist overseeing the case and Alistair advised that, in his opinion, the patient should be considered as a whole, with decisions being based on the severity of clinical signs and the pressure gradient.
Until the last 25 years, treatment for this condition involved a thoracotomy and surgical correction; today, however, the treatment of choice is a balloon angioplasty which is a safe and non-invasive procedure involving the introduction of a low-profile balloon through an introducer placed into the jugular or femoral vein. It is then passed through the right atrium, the right ventricle, the RVOT to the pulmonary artery where it is wedged into position. The balloon is then dilated and as demonstrated by a video shown on the webinar, a waist develops within the balloon at the point of the stenosis which after 2-3 inflations impressively disappears.
The risks associated with this method include the temporary occlusion of the RVOT which can lead to dysrhythmias and hypotension. For this reason Alistair places cases on beta blockers one week prior to the placement of the balloon. Balloon rupture can also cause a problem and become stuck in the pulmonic artery but the risk of this happening in experienced hands is small.
Alistair explained that by using this method there is usually a reduction of the pressure gradient across the valve of up to 50%. However, even if there is just a small reduction in pressure, often there will be a marked improvement in clinical signs. This technique may, however, need to be repeated.
Aortic stenosis
Aortic stenosis is the most common congenital cardiac disease in dogs with subaortic stenosis being the most common presentation. Boxers appear to be particularly prone to this disease.
Unfortunately, treatment for this disease is a lot more limited. Beta blockers can be used and balloon dilatation as per the treatment of pulmonic stenosis is a possibility, but all this technique tends to do is push the lesion up which then drops down again, offering no long-term benefit.
There are also significant risks attached which include the development of ventricular arrythmias and the risks attached with occluding the left ventricular out ow tract, even for a brief period of time. However, with a 70% mortality rate in the first three years of life, in severe cases of aortic stenosis an alternative to the very risky open heart surgery has been, and is being, actively sought.
Recently a new technique using a combination of balloons has been developed. Initially cutting balloons are used (i.e. balloon with a razor blade) to create a controlled tear in the fibrous muscular lesion followed by dilation of the lesion with high-pressure balloons. Alistair suggests that this is an option for young dogs with severe SAS (> 80mmhg pressure gradient).
Patent ductus arteriosus (PdA)
PDAs are another of the more commonly encountered congenital heart defects resulting in left to right shunting and pulmonary over circulation secondary to the ductus between the aorta and pulmonary artery failing to close.
Patients present with a classic machinery murmur and a palpable thrill on the left side of the chest with the majority of cases developing congestive heart failure by the age of one year old.
