Is it possible to have atrial fibrillation and not detect it by a bp monitor or taking the pulse? my husband feels weak and lethargic.

Could miss it.: Generally atrial fib is detected by checking the pulse which is "irregularly irregular." Sometimes if the rate is fast it could be hard to tell if it is indeed irregular. BUT, if your husband feels weak and lethargic one would expect that his pulse would not be "normal." It should be too fast too, too slow or too irregular...

Yes: Blood pressure machines can detect an irregular pulse but cannot diagnose atrial fibrillation and they are far from perfect. Same goes for taking your pulse. At times, atrial fibrillation and especially atrial flutter can be fairly regular and not even show up as an irregular rhythm by these methods. He needs an EKG or a heart monitor that shows the rhythm