It's unfortunate, but weight lifting puts tremendous stress on a heart that has to pump its entire output through a tiny hole. Your brain and muscles scream for extra oxygen and nutrients during lifting, but the pipelines are constricted at the valve. It puts you at risk for your heart to go arrhythmic from the stress, a potentially fatal problem while under that kind of load. And if you have any coronary blockage at all, you are at an even higher risk.
As you know, you can't tell whether you have any coronary blockage without someone actually looking, such as through a catheter angiogram or an MRA. Stress echoes (AKA stress tests, exercise stress tests, etc.), particularly for someone with AS, are highly unreliable in predicting coronary blockages. Not that thet're so accurate for people without AS, either. Former President Clinton had it done by some of the best in the nation at Columbia in NY, and was declared fit. Three weeks later he was in the hospital for chest pains due to clogged coronary arteries that required intervention. And he's far from alone in that result.
I'm not a medical professional or an exercise maven, but I might think a good scenario would be to start right now by greatly lowering the weight (to below 50lbs., e.g. going to hand weights) and working in shorter sessions to avoid stressing your heart. This is assuming you have no aortic aneurism tendencies, which would simply mean no lifting whatsoever, starting yesterday.
Basically, if the weight or the machine resistance is enough to make you grunt, you need to go still lighter. Absolutely, you must practice breathing through any lifting, including in regular life situations, and always avoiding holding your breath when pushing with your muscles. This is something you probably already practice, but rates mentioning.
The goal would be to rapidly and continuously work to scale down all lifting as your heart needs you to, while you set your body up to be able to deal with the worsening valve constriction. Because you are weight training, your body weight may well be somewhat higher than someone else your size. Even though you're fit, that does mean extra work for the heart. So, you probably want to reduce your overall bulk. If you can manage to lower your weight, you can avoid having that muscle go to fat on you in the meantime by reducing some bulk as you go along.
You will want to come to a complete stop by the time your valve opening reaches, say, 1.5 cm² or less, or if your left ventricle begins to approach the top of the normal size limit.
You might also consider moving to yoga or other stretching/movement-related body training to increase your limberness, although you will need to avoid the more rigorous or aerobic aspects of the training. Limberness may help with sternum recovery, and possibly reduce body pains, such as costochondritis attacks during recovery, and may allow you to longer maintain your base muscle tone.
You will likely be able to pick up moderate lifting and unrestricted aerobic body disciplines again later, after your heart and sternum are healed. Disappointing, I'm sure. But as you point out, the value of lifting would be lost on a corpse.
Just my thoughts. There are others who will probably come along here here who are very weight-training literate.
Best wishes,
