Welcome back to the tasty morsels of critical care podcast.
Cardiac pacing comes in a variety of flavours in critical care but a reasonable list of class I indications for permanent pacing might include:
- 2nd/3rd block with symptoms and bradycardia, pauses or in people with NM disease (who are a bunch of patients oddly prone to collapse of the conduction system)
- sinus node dysfunction and bradycardia and symptoms. Sinus node dysfunction here meaning that the SA node either temporarily or permanently goes AWOL leaving the bundle of His or ventricles to keep things ticking along in some form of escape rhythm.
- recurrent syncope due to carotid stimulation (eg neck tie syncope)
- pause dependant VT (which seems to be a rare beast that is a form of torsades)
From the ED context we see the syncopal patient with heart block that might come with an acute MI or perhaps more commonly in an older patient with a degenerative conducting system that either presents in complete heart block or develops intermittent pauses as part of sinus node dysfunction. Commonly in the intensive care unit we are presented with a patient with intermittent pauses, perhaps from autonomic involvement of GBS or as a complication of infective endocarditis with an aortic root abscess. We are often forced to use a short period of external pacing, which while occasionally life saving, is aesthetically very unsatisfying as we inevitably end up watching someone’s pectorals twitching away for hours on end.
The majority of these patients will end up with some form of emergent invasive pacing from cardiology and it is rare for the emergency physician or intensive care physician to place these in Irish practice. The pacing wires here are typically placed via an introducer sheath in one of the central veins. They have little balloons on the end that allow them to be floated along on the stream of returning blood to the heart. Once in the heart the idea is to get the tip to make sustained contact with the deep recesses of the RV allowing systole to be triggered from the RV. The ECG waveform is therefore in an LBBB pattern as if the left bundle isn’t working.
Perhaps the most common form of pacing will be in the cardiac ICU where we often receive patients from the operating theatre with epicardial wires and a pacing box hanging from the drip stand. Passing familiarity with your units box of tricks is well worth it, especially simple things like how to unlock it to change the settings which is by no means obvious especially when the patient is asystolic and unlocking said box is a critical part of their management.
These epicardial wires are slender little things that are generally poorly visible on CXR and tend to only last 5-10 days before either dislodging or malfunctioning. Of note they can be a troublesome cause of tamponade upon removal in the coagulopathic patient.
There is a strange little secret code used to describe the type of pacing being delivered. Its existence seems to be primarily to confuse novices like myself.
There are 5 described “positions” used in pacing but in reality we just need to know 3 of them
- chamber paced (A,V or D)
- chamber sensed (A, V, D or O for no sensing)
- pacemaker response to sensed beat (I=inhibit, T=triggered, D= dual where both inhibition and triggered responses can happen depending where it was sensed, O= nothing)
For example in VVI pacing the chamber paced is the ventricle, the chamber sensed is the ventricle and response to a sensed native beat is to inhibit the pacer. This is a good example to use as it covers 99% of what cardiac surgery patients end up in the ICU. It is primarily used as a back up mode for a conduction system too stoned on fentanyl, propofol and hypothermia to grace the recently traumatised heart with its presence. The main downside of VVI is loss of AV synchrony which means no atrial kick and perhaps a worse output. As a result VVI is often set at a bit of a higher rate than you might do otherwise to ensure cardiac output is OK.
DDD is also commonly used, especially in AVR/MVR surgery where loss of the native rhythm is common. The dual leads (on both the atrium and the ventricle) and dual pacing/sensing gives a better cardiac output as AV synchrony can be restored.