06/14/2026
☕ 𝗠𝗼𝗱𝗶𝗳𝗶𝗲𝗱 𝗦𝗴𝗮𝗿𝗯𝗼𝘀𝘀𝗮 𝗖𝗿𝗶𝘁𝗲𝗿𝗶𝗮: 𝗔 𝗗𝗲𝗲𝗽 𝗗𝗶𝘃𝗲
Grab your coffee. Let’s spend a few minutes talking about one of the most misunderstood ECG topics in emergency and critical care medicine.
The Modified Sgarbossa Criteria.
Most providers can recite the criteria. Fewer can explain why they work.
That matters.
When you understand the electrophysiology behind the criteria, you stop memorizing rules and start understanding what the heart is actually telling you.
If you transport critically ill patients, work in emergency medicine, fly, ride, or stand at the bedside in an ICU, this is worth understanding.
🫀 𝗜𝘁 𝗔𝗹𝗹 𝗦𝘁𝗮𝗿𝘁𝘀 𝘄𝗶𝘁𝗵 𝗟𝗕𝗕𝗕
The challenge isn’t finding STEMI.
The challenge is finding STEMI hiding inside a Left Bundle Branch Block.
In a normal heart, electrical activation travels down both bundle branches simultaneously. The ventricles activate together. The QRS stays narrow.
In LBBB, the left bundle is blocked.
The impulse travels down the right bundle first. The right ventricle activates. Then the impulse slowly spreads across the septum into the left ventricle through cell-to-cell conduction.
That delay changes everything.
You get:
• A QRS duration ≥120 ms
• Broad or notched R waves in I, aVL, V5, and V6
• Deep S waves in V1 through V3
• Delayed left ventricular activation
• Secondary ST-segment and T-wave abnormalities
Those ST and T wave changes are the reason diagnosing acute coronary occlusion becomes difficult.
⚡ 𝗪𝗵𝘆 𝗧𝗵𝗲 𝗦𝗧 𝗦𝗲𝗴𝗺𝗲𝗻𝘁𝘀 𝗟𝗼𝗼𝗸 𝗪𝗿𝗼𝗻𝗴
The QRS represents ventricular depolarization.
The ST segment and T wave reflect ventricular recovery and repolarization.
When depolarization becomes abnormal, repolarization follows.
That creates what we call appropriate discordance.
In LBBB:
• Positive QRS complexes usually have ST depression and T-wave inversion.
• Negative QRS complexes usually have ST elevation and upright T waves.
Those findings are normal in LBBB.
Read that again.
ST elevation can be completely normal in a patient with LBBB.
That is exactly why so many providers struggle with these ECGs.
The question is not whether ST elevation exists.
The question is whether the ST elevation is appropriate for the underlying conduction abnormality.
🚨 𝗪𝗵𝗮𝘁 𝗔𝗰𝘂𝘁𝗲 𝗢𝗰𝗰𝗹𝘂𝘀𝗶𝗼𝗻 𝗗𝗼𝗲𝘀
Acute coronary occlusion changes myocardial membrane behavior.
ATP production falls.
Potassium leaks from injured cells.
Normal ion gradients begin to fail.
The injured myocardium develops a different electrical potential than surrounding healthy tissue.
That creates an injury current.
The injury current shifts the ST vector toward the infarcted region.
When that happens, the normal repolarization pattern of LBBB begins to break down.
That breakdown is exactly what the Modified Sgarbossa Criteria are designed to identify.
🚨 𝗖𝗿𝗶𝘁𝗲𝗿𝗶𝗼𝗻 𝟭
𝗖𝗼𝗻𝗰𝗼𝗿𝗱𝗮𝗻𝘁 𝗦𝗧 𝗘𝗹𝗲𝘃𝗮𝘁𝗶𝗼𝗻 ≥ 𝟭 𝗺𝗺
This is the easiest criterion to recognize.
If the QRS is upright and the ST segment is also elevated, something is wrong.
In LBBB, upright QRS complexes should usually have ST depression.
When ST elevation appears instead, the injury current is overpowering the expected repolarization pattern.
This finding carries very high specificity for acute coronary occlusion.
When you see it, your suspicion should immediately rise.
🚨 𝗖𝗿𝗶𝘁𝗲𝗿𝗶𝗼𝗻 𝟮
𝗖𝗼𝗻𝗰𝗼𝗿𝗱𝗮𝗻𝘁 𝗦𝗧 𝗗𝗲𝗽𝗿𝗲𝘀𝘀𝗶𝗼𝗻 ≥ 𝟭 𝗺𝗺 𝗶𝗻 𝗩𝟭-𝗩𝟯
In LBBB, V1 through V3 typically show deep S waves with some degree of ST elevation.
When ST depression appears instead, the expected pattern has been reversed.
This often points toward posterior wall occlusion.
Posterior injury vectors move away from V1 through V3 and create ST depression in those leads.
A posterior STEMI may never announce itself loudly.
Sometimes this criterion is the clue that saves the patient.
🚨 𝗖𝗿𝗶𝘁𝗲𝗿𝗶𝗼𝗻 𝟯
𝗘𝘅𝗰𝗲𝘀𝘀𝗶𝘃𝗲 𝗗𝗶𝘀𝗰𝗼𝗿𝗱𝗮𝗻𝘁 𝗦𝗧 𝗘𝗹𝗲𝘃𝗮𝘁𝗶𝗼𝗻
This is where Smith’s modification changed everything.
The original Sgarbossa Criteria used a fixed cutoff of 5 mm of discordant ST elevation.
The problem?
Large QRS complexes naturally produce larger ST shifts.
Five millimeters means very different things depending on the size of the S wave.
Smith recognized this.
Instead of using a fixed number, he compared ST elevation to the depth of the S wave.
The ratio matters.
If the ST elevation is at least 25% of the depth of the S wave, concern for acute coronary occlusion increases significantly.
Example:
• S wave depth = 20 mm
• ST elevation = 6 mm
• Ratio = 0.30
That exceeds the threshold.
That ECG deserves your attention.
📊 𝗛𝗼𝘄 𝗚𝗼𝗼𝗱 𝗜𝘀 𝘁𝗵𝗲 𝗘𝘃𝗶𝗱𝗲𝗻𝗰𝗲?
Pretty strong.
The original Sgarbossa Criteria were highly specific but lacked sensitivity.
Many occlusions were missed.
Smith and colleagues improved diagnostic performance by introducing proportional discordance.
The 2015 validation study demonstrated sensitivity approaching 80% while maintaining specificity near 99%.
That is impressive performance for a notoriously difficult ECG diagnosis.
The evidence supporting Modified Sgarbossa is significantly stronger than the evidence that once supported treating new LBBB as an automatic STEMI equivalent.
That older approach has largely disappeared from modern guidelines.
Current practice focuses on identifying acute coronary occlusion rather than simply identifying LBBB.
🚑 𝗙𝗶𝗲𝗹𝗱 𝗔𝗽𝗽𝗹𝗶𝗰𝗮𝘁𝗶𝗼𝗻
For EMS, flight, and critical care transport clinicians:
• Confirm the presence of true LBBB.
• Apply the Modified Sgarbossa Criteria.
• Obtain serial ECGs.
• Compare with prior ECGs whenever possible.
• Consider posterior leads.
• Consider bedside ultrasound if available.
• Look for dynamic changes.
• Always place the ECG into clinical context.
The ECG matters.
The patient matters more.
A positive Modified Sgarbossa ECG in a diaphoretic patient with crushing chest pain should make you nervous.
A positive Modified Sgarbossa ECG in an asymptomatic patient deserves investigation, but the urgency may be different.
The tracing is part of the story.
It is never the whole story.
☕ 𝗔 𝗙𝗶𝗻𝗮𝗹 𝗧𝗵𝗼𝘂𝗴𝗵𝘁
I’ve stared at plenty of ugly LBBB tracings over the years.
Sometimes the diagnosis is obvious.
Sometimes it isn’t.
The Modified Sgarbossa Criteria give us a reliable way to identify acute coronary occlusion when normal STEMI criteria fail.
More importantly, they help us understand when the heart’s usual LBBB pattern stops behaving like LBBB.
When you understand the physiology behind the criteria, the ECG starts making a lot more sense.
The next time a wide-complex ECG lands in your lap, slow down for a second.
Look at the QRS.
Look at the ST segment.
Ask yourself whether the ST changes fit the expected pattern.
If they don’t, your patient may be telling you something important.
Listen carefully.
📚 References
Sgarbossa EB, Pinski SL, Barbagelata A, et al. Electrocardiographic Diagnosis of Evolving Acute Myocardial Infarction in the Presence of Left Bundle Branch Block. N Engl J Med. 1996;334(8):481-487. DOI: 10.1056/NEJM199602223340801
Smith SW, Dodd KW, Henry TD, Dvorak DM, Pearce LA. Diagnosis of ST-Elevation Myocardial Infarction in the Presence of Left Bundle Branch Block Using the ST-Elevation to S-Wave Ratio in a Modified Sgarbossa Rule. Ann Emerg Med. 2012;60(6):766-776. DOI: 10.1016/j.annemergmed.2012.07.119
Meyers HP, Limkakeng AT Jr, Jaffa EJ, et al. Validation of the Modified Sgarbossa Criteria for Acute Coronary Occlusion in the Setting of Left Bundle Branch Block. Am Heart J. 2015;170(6):1255-1264. DOI: 10.1016/j.ahj.2015.08.016
Di Marco A, Anguera I, Schmitt M, et al. New Electrocardiographic Algorithm for the Diagnosis of Acute Myocardial Infarction in Patients With Left Bundle Branch Block. J Am Heart Assoc. 2020;9:e015573. DOI: 10.1161/JAHA.119.015573
