You can pass out while lifting weights through a temporary decrease in blood flow to the brain. When performing a heavy lift like deadlift or squat, holding your breath increases the tension in your mid-section. This will allow you to lift heavier. However, there is the risk of passing out. To reduce the risk of passing out when lifting, focus on proper breathing. If you are going to hold your breath on a heavy rep, make sure you have sucked in a couple of big ones first. This is where some conditioning work also comes into play. Although weight lifting is not considered a cardio vascular activity, you do need to have some form of a gas tank. Other factors that could cause you to pass out in the gym are diet and hydration. Training with low blood sugar not only increases the risk of passing out, it will also reduce your strength and muscular endurance. When you are dehydrated your blood volume is reduced. This could potentially restrict blood flow to the brain.