Smiths Modification and Pacemakers – is that a STEMI?

Ghu-roos. Prehospital diagnosticians. Call yourselves what you will.

We are here today to look through the pacemaker.
Like Alice through the looking glass; through to find any ischemia that potentially lurks within that paced rhythm. Don’t let that grey hair tell you about the futility of interpreting paced rhythms….just like they said with LBBB…..mmmm
So what can we do…

Example. 85 yr old caucasian male pt who initiates 911 after being unable to mitigate his chest pain with rest &/or nitrates as per usual. Today pt. pre administers nitrates prior to his daily walk as he knows he “will get chest pain to walk the small rise out of his home to gain the flats“. Pt. has an extensive cardiac history with quintuple bypass surgery a decade previous and just last year further stenting although he is not sure which artery. He presents to you with 6 on 10 central chest pain that will not resolve and feels like both pressure and burning. No radiation. No mitigation and very familiar in its presentation to his regular and frequent cardiac chest pain. No other associated symptomology. HR60 RR14 BP129/71 SPO297RA ETCO238mmHg BGL5.8 The pt. is taking all their medications as scheduled and no recent changes to Rx other than the addition of Plavix post angiography/stenting.

Your partner hands you the initial 12 lead that reveals the following…Applying Smith Modified Sgarbossa criteria provides you with the following;

  • V2 = T4.36/ST10 = 0.43
  • V3 = T6.54/ST12 =  0.54
  • V4 = T3.58/ST9 = 0.39

 

 

 

 

 

 

 

 

 

According to Smith Modified Criteria any T/ST variance >0.25 = both sensitive and specific for acute MI/STEMI.

 

 

Conundrum…you are closest to a non PCI capable Level 1 receiving facility and have access to a PCI facility although via diversion criteria is for STEMI only. Your organization does not support Paramedic’s interpreting their own 12 lead ECG and ECG must say STEMI  as interpreted by the LP15 algorithm for inclusion, transmission and thus diversion….what to do….

As you have a relationship with the ERP at your closest facility you reach out to them by phone and you are lucky enough to be able to have a clinical conversation with the receiving ERP. They agree with your treatment plan and ask you to divert to the PCI capable facility. You also consult with your online medical physician (EPOS) available to you who also agrees. You now transport your pt. to a cath/PCI capable facility and is accepted by cardiology upon arrival.

Learning points:

1. Use the modified Sgarbossa criteria in paced rhythms.  The specificity is good. The sensitivity is unknown but probably similar to sensitivity in LBBB, which I believe to be as good as ST elevation in normal conduction, probably about 70-75% sensitive for coronary occlusion (though the sensitivity was much higher in our case control studies, which probably do not accurately reflect clinical practice).

2. One difference between LBBB and Paced rhythm is that, in paced rhythm, the QRS in V5 and V6 is almost always negative (but positive in LBBB).  Therefore, any STEMI that manifests in V5 and V6 in LBBB will usually manifest by concordant ST elevation in these leads, whereas in paced rhythm, it must be excessively discordant ST elevation.