The reason behind why patients who are on aspirin still have heart attacks is an important topic in cardiovascular disease.
The VerifyNow(TM) is a simple test that was designed to measure whether aspirin therapy was working in patients with coronary artery disease, the number one killer in the Western world. Aspirin blocks an important enzyme, COX-1, in platelets that prevents them from sticking together to form blood clots. These clots can block blood flow in the heart's arteries causing heart attacks and death. COX-1 converts a molecule, arachidonic acid to thromboxane A2, and it is the latter molecule that triggers platelet clumping.
Patients on aspirin treatment who have sticky platelets after stimulation with arachidonic acid are termed "aspirin resistant" and this is the method used in the VerifyNow(TM) test. However, other molecules in the blood stream, in addition to arachidonic acid can trigger platelets to stick together and may also cause heart attacks.
To date, it is unknown whether patients who have sticky platelets after stimulation with arachidonic acid also have a similar response after stimulation with other molecules. Our study suggests that patients with aspirin resistance not only have highly sticky platelets after stimulation by arachidonic acid, but also have a similar response to other molecules that stimulate platelets to clump together. The platelets of these patients therefore have a tendency to clump in response to multiple stimuli. We studied one hundred ten patients with a history of coronary artery disease that were treated with aspirin.
Patients were randomly treated with 81mg, 162mg and 325mg of aspirin daily for four weeks each for a period of twelve weeks. We tested platelet stickiness using the VerifyNow(TM) and compared it to other methods. We made several important observations. First, patients who were resistant to aspirin also had exaggerated platelet stickiness in response to stimulation with other molecules. This suggested that not only were their platelets not fully blocked by aspirin, but that other pathways causing clot formation were highly active. Therefore, the VerifyNow(TM) test identified patients at potentially greater risk for heart attack because their platelets were highly reactive to many stimuli. Secondly, we discovered that a higher dosage of aspirin may decrease the platelet stickiness, prolonging life for the patient by preventing heart attacks.
Dr. Paul Gurbel, director of Cardiology and Cardiovascular Research at Sinai Hospital of Baltimore in Maryland and Associate Professor of Medicine, Johns Hopkins University School of Medicine said: "There now may be a more comprehensive explanation for why the patient who is aspirin resistant as measured by a simple bedside test may be at higher risk for heart attack. These patients have platelets that are highly reactive to many stimuli, not only to arachidonic acid. An easy and efficient way to identify those who are at increased risk may change current clinical practice. Larger scale studies linking the results of a bedside test to the occurrence of heart attack are imperative to enable cardiologists to identify high risk patients and potentially save their lives."