I am a 75 years old man, who underwent deep brain stimulation surgery 2 years ago for essential tremors. For the past 18 months or so, I am experiencing breathlessness, specially while talking. Cannot talk more than 8-10 words without pausing for a breath. Do an hours brisk walk up inclines 3-4 times a week, without a problem. Resting ecg and 2d echo are wnl. Pft and x ray chest performed recently were normal. What could be the problem?