It's said that up to 20% of patients experiencing an acute MI will have a normal EKG. With over 25 years of emergency medical experience under my belt, I would dispute this statistic. I think it would be more accurate to say that a high percentage of these patients will have a "non-diagnostic" EKG. However, the totally normal EKG does occasionally happen, as in the case I am about to relate.
I recently treated a very pleasant and healthy older lady. I would say...a young eightly-year old. She came into the ER with complaints of chest pain that had been bothering her for the past 3 days. The pain was a mid-sternal pressure than radiated to her back and both axilla. She said it "cut off her wind," but didn't notice any specific provacative factors. It had been intermittant, but on the day she decided to come visit with me in the ER, she had been experiencing pain for a little over 6 hours. EMS placed her on a cardiac minitor and had started an IV.
There were no interventions because the patient had a normal EKG and told the paramedics that she had a normal stress test only 3 months ago.
When I saw this nice lady, she was still having pain and still had a normal EKG. Not a "non-disgnostic" EKG, but normal like it had been copied out of a textbook of normal EKGs. Despite this, her description of symptoms was pretty alarming, and her blood pressure was significantly elevated, she was started on typical cardiac chest pain treatment. She had already taken an aspirin at home and remained on oxygen. She was started on SL nitroglycerin and IV lopressor, which resulted in a modest reduction of her pain. Subsequently, she was started on IV nitroglycerin and titrated doses of morphine with more improvement. The EKG remained "normal," but the lab returned CK-MB and Troponin-I values well into the positive for MI ranges. Her BP and pain continued to improve with the nitrates, now in concert with Lovenox and Integrilin. Finally, now on maximal therapy, her pain was reduced to zero.
I consulted with her primary physician and cardiology and wrote orders to admit her to the ICU. The expectation was that she would undergo cardiac catheterization within 12 hours if she remained pain-free.
A few take-to-work messages:
1) The history is the most important diagnostic tool in your box
2) AMI can certainly happen, even with a stone-normal EKG
3) A negative stress test (treadmill, nuclear, or stress-echo) means nothing. These studies are only significant if they are abnormal (positive).
4) Don't delay treatment waiting on enzymatic testing
One more thing regarding cardiac catheterization reports. A normal cath may rule out CAD, but I have still seen patients die from arrhythmias and suffer MI caused by coronary vasospasm (printzmetals). In younger patients (<40) especially, screen for cocaine abuse. A cath with small (20-40%) lesions without critical stenosis does NOT reduce the risk of AMI. It is these younger and less stable lesions that will often rupture and cause an acute thrombus. Larger lesions (>50%) tend to be more stable (less likely to rupture), but may cause anginal pain and occlude with smaller thrombi.