My Husband Had A Quadruple Bypass In Nov 9,2015. In

My husband had a quadruple bypass in Nov 9,2015. In January 25 he was back in the hospital because 3 of the 4 failed. One of them had a Stent put in it, one had 2 Stent put in it and one would not take a Stent so they angioplastied it. Tuesday he starting coughing and it is getting worse. His oxygen levels are good when at rest but even getting dressed can his levels drop, they usually recover guickly, but today it about an hour to recover. His stats are 77 to 88 when they drop. At rest he is 92 to 97. Is this normal? He does not feel sick with a cold and the produces phlegm.