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This is Dr. D.J., experienced Internal Medicine Specialist.
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Variation in blood pressure can be due to the underlying heart disease.
Myocardial infarction can cause this.
Also unequal BP between 2 arms can be due to the atherosclerosis of the blood vessels.
Subclavian artery blockage can cause this inequality in Bp as well.
Also, measurement error is still possible, since with aging the blood vessels are not compressible resulting in falsely high BP.
However, given these changes in her BP, I will suggest a sooner clinical evaluation to rule out any MI.