In pacemaker science , any pacemaker that maintains AV synchrony is often referred to as physiological pacemaker. This is of course , a wrong reasoning .None of the pacemakers available today can be claimed to be completely physiological .All pacemakers which paces the right ventricle induces IVS dysynchrony (Including the modern DDD)
Single chamber physiological pacing
AAI
Paradoxically , the most primitive of pacemakers AAI can be the near perfect physiological pacemaker . The simple explanation is , In AAI mode , expect for the origin of pacemaker impulse the entire depolarisation and repolarisation is through the normally existing physiological conducting system .(AV node, HIS, Purkinje etc)
(It not only has atrio ventricular synchrony but also has ventriculo ventricular and intra ventricular synchrony )
So, technically AAIR is most physiological pacemaker possible .But the practical utility of such a pacemaker is limited.It can be used only in isolated sinus node dysfunction with intact AV conduction . (The problem is the AV nodal conduction can develop later ) To over come this DDDR pacemaker can be programmed to AAIR as a default mode.
VVIR
This rate adaptive pacemaker , to a certain extent can be termed physiological as the heart rate can improve with exercise . (Still it is unphysiological as it paces the RV )
VVD
This is based on the concept , for pacing to be physiological , it requires atria to be at least sensed not necessarily paced.This mode which has a floating sensor attached to the lead as it crosses the atria.This facilitates atrial sensed ventricular pacing .But many believe the atrial sensing is not consistent in VDD mode.Currently this mode is not popular.There is scope for improving the atrial sensor technology .
Dual chamber physiological pacing
DDD, DDRR
Both these are the prototype dual chamber physiological pacing modes.
Bi-Ventricular or triple chamber pacing ( one atria two ventricle) are our elusive answers for attaining perfect physiological pacing . it need to be realized, we simply , can not mimic the natural cardiac conduction system.It is estimated to be more than 10 miles long specialized fibers .
Final message
In our quest for physiological pacemaker we often forget the fact , AAI is the most physiological pacemaker mode available .(It even has VV synchrony ! )
We should use it liberally whenever possible .Of course ,we cannot use it in complete heart block .Still 50 % the permanent pacemaker we implant is for sinus node dysfunction. Many of them could be candidates for AAI mode .If current generation cardiac physicians feel out dated to insert a AAI pacemaker, at the least they should program the DDDR into AAI mode with a mode switching to ventricular pacing modes whenever required.
In spite of all advantages , why atrial based pacemakers are not gaining popularity ?
- Ignorance
- Lack of expertise
- Technical difficulty of fixing atrial lead
- Perceived fear of lead dis-lodgement.
- The fact remains the ventricular based pacing is always safe in case of sudden AV block due to any reason .
Dr.S.Venkatesan MD 