Medtronic Visia ICD with AF Diagnosis
As seems to be the way in the world of pacemakers and ICD’s, CE approval comes before FDA approval for new hardware, so Europe has access before the USA. The first implant has already taken place in the united kingdom.
In the media the Visia ICD is being heralded for its ability to detect and report on the occurrence of atrial fibrillation (Medtronic themselves are using this as a key marketing strategy). Firstly we know that atrial fibrillation increases the risk of a person having a stroke so early and accurate detection can enable medical intervention to help reduce this risk (for example the introduction of anticoagulation medication). This is not what makes it groundbreaking or unique as many devices have this functionality. The beauty of the Visia ICD is that it can do this with just one standard ICD lead. The Biotronik Lumax DX for example, has atrial diagnostics in a single lead ICD but requires the Linox Smart lead – a specialist ventricular lead with an atrial sensing electrode.
The Visia ICD is able to achieve this thanks to the ‘sophisticated’ algorithm a software previously used in the Medtronic Linq reveal that uses R-R intervals (time between heartbeats) to detect atrial fibrillation and not direct sensing of atrial activity.
How Does the AF Algorithm Work?
During atrial fibrillation the intervals from one beat to the next is always varying – the heartbeats are irregular. Whilst rhythm irregularity is not unique to atrial fibrillation, the manner in which it is irregular (the correlation structure) is unique. The sophisticated algorithm is able to recognise the correlation structure of atrial fibrillation with 95% accuracy.
In plain english the pattern of heartbeats during atrial fibrillation is unique and the Visia ICD is able to identify this pattern.
We can display the pattern (correlation structures) of different rhythms using scatter plot graphs (Lorenz plots) which makes it much easier to understand what I am talking about but more importantly how the algorithm works.
As we can see on these scatter plots the correlation structures of different rhythms are very different, the Visia ICD uses correlation structures to identify AF. Cool stuff!
We know that the ability for a device to identify and report on AF is so important but why is it important to do this with one lead? Less leads means less complications one study found that dual chamber ICD’s have a 40% increase risk of lead complications (Ref 1).
So Why Do So Many ICD’s Have Two Leads?
Please remember not to confuse ICD’s with pacemakers. In standard bradycardia pacing we prefer two leads because it helps us achieve a physiological heart rate and synchronicity between atrial and ventricular contractions. People with ICD’s often have fully functioning conduction systems and do not require any of these pacemaker functions. Instead the device is implanted ‘just in case’ and the ICD is there in the background ready to defibrillate a dangerous rhythm if it should occur – an outcome that is achievable with a single lead and many doctors do prefer to implant single lead ICD’s into this patient population.
However many doctors prefer to implant dual chamber (two lead) ICD’s in these patients.
“Given the increased risk of complication, why would doctors often choose to implant dual chamber ICD’s?”
Well there are benefits of course…
- Traditionally two leads have been required to detect and report on atrial arrhythmias such as AF.
- It is also handy if the patient develops the need for a dual chamber pacemaker and one is already in situ.
- Two lead ICD’s have a significantly lower incidence of inappropriate shocks compared to single chamber devices.
Inappropriate shocks are where the device shocks the patient because it believes a dangerous ventricular arrhythmia is occurring, when in fact it is not. Dual chamber ICD’s tend to have a lower incidence of inappropriate shocks as they have information from the atria AND the ventricles when identifying what heart rhythm is taking place.
Discriminators in ICD’s
What is a discriminator?
The main role of the ICD is to deliver therapy (an electrical ‘shock’) to cardiovert dangerous arrhythmias – ventricular tachycardia and ventricular fibrillation (VT and VF). Delivering this therapy is the easy part, the challenge is recognising when these dangerous rhythms are occurring (detection). The main criteria an ICD uses to detect VT or VF is the ventricular heart rate – a fast heart rate in the ventricles is a characteristic of these dangerous arrhythmias.
However when you consider that a VT could have a rate of 150bpm… and so could running for a bus – ICD’s cannot depend on heart rate alone to discriminate between a dangerous rhythm and a perfectly normal one. Otherwise we would have lots of people being electrocuted near bus stops.
To overcome this issue ICD’s use software to recognise characteristics associated with different rhythms to tell them apart – these are called discriminators. Dual chamber devices have ‘better’ discriminators because they have access to data provided from the atria AND the ventricles.
The concern with the Visia ICD would be that having only one lead would hamper its ability to discriminate between a dangerous arrhythmia and a safe one… leading to inappropriate shocks. For me, this is what really stands the Visia ICD apart, 12 months post implant the Visia ICD has only a 2.5% incidence of inappropriate shocks (Ref 3.) thanks to Smartshock Technology. This is comparable to devices using dual lead discriminators, for example Sorin ICD’s that use the PARAD+ discriminator algorithm had a 2.6% incidence of inappropriate therapy after 12 months (Ref 2.).
In summary the Visia ICD appears to offer all the diagnostic and discrimination benefits of a dual chamber ICD with only one lead. Great news when reducing the risk of lead complications for all of our patients.
Oh yes, one last thing. Paired with the Medtronic Sprint Quattro lead the device is MRI compatible.
Thank you for reading…
- Dewland TA. et al., Dual-chamber implantable cardioverter-defibrillator selection is associated with increased complication rates and mortality in the NCDR, JACC, 2011.
- Kolb C. et al., Reduced risk for inappropriate implantable cardioverter-defibrillator shocks with dual-chamber therapy compared with single-chamber therapy: results of the randomized OPTION study in the JACC, 2014.
- Aurrichio A. et al., Low inappropriate shock rates in patients with single and dual/triple-chamber implantable cardioverter-defibrillators using a novel suite of detection algorithms: PainFree SST trial primary results in Heart Rhythm, 2015.