What does it mean when you "see stars" during some intense exercise?

Last night I was really wiped out. I did stiff-legged deadlifts, and that last rep was REALLY tough. I barely got it up. Good form though, just really intense. Well I think that maybe I wasn't breathing properly, or something. Afterwards I was seeing stars. What does this mean? Is it a bad sign? It's happened to me before, but very rarely (like only a couple of times before). This time it actually lasted longer than normal too, almost a whole minute.

Anyway just wondering what in the world this means, what it's caused by, and/or if it's something to be concerned about.

When you are exercising, more blood is in the extremities, muscles, limbs, etc... and less blood is being sent to the brain. Some blood is naturally "pooled" in your limbs from gravity. This pooling of blood plus the demands of the exercise means that less blood (and therefore, less oxygen) reaches the brain. When you don't breathe properly during exercise, even less oxygen can be taken up, resulting in a feeling of lightheadedness, seeing stars, darkening of vision, and if you go too far, fainting.

After you "saw stars", you probably felt a little shaky, and a higher heartbeat. That's because your body turned on a series of reflex responses designed to get you back on even keel. To make up for the lower amount of blood returning to the heart during exercise, the body will release norepinephrine and epinephrine (a.k.a., adrenaline). These substances cause the heart to beat a bit faster and more forcefully... you always feel this after you've been scared... but when you get faint, the adrenaline causes the blood vessels to tighten or constrict, increasing blood pressure. This forces more blood to return to the heart for oxygenation, then out to the brain to get rid of your lightheadedness.

-R

The biggest culprit with heavy weights is failing to breathe properly. If you hold your breath during a rep, usually you are trying to breathe out against your closed glottis (throat), increasing you intra-abdominal and intra thoracic pressure to help splint your torso. This reduces the venous return to your heart, therefore your cardiac output drops. As a result of the increased venous pressure in your torso, there is greater backpressure on your brain's veins. This, along with the decreased arterial inflow, gives your brain a bloodflow which is markedly reduced, and therefore its oxygenation suffers badly. The brain has little tolerance for interruptions in oxygen supply (it's one of the most metabolically active tissues we have), so you feel lightheaded.

A person's cardiac output (average 70kg male) is about 5 litres per minute. During intense exercise, thie can go up to 25 LPM, even higher with athletes, the extra being diverted to where it's needed most, your muscles (and skin to regulate your temperature). Your brain autoregulates its bloodflow, so that even though cardiac output is much higher, its bloodflow remains constant.

If someone is above their aerobic threshold, and producing lactic acid, this usually doesn't mean that they have a low oxygen saturation. The problem is that the muscles suck up so much oxygen that there's not enough to go around at that site. This totally blue blood returns to the heart and lungs, and is fully reoxygenated, before heading back out again, to brain, muscles etc. Your brain always has a fully oxygenated blood supply (unless there's major plumbing problems, or your lungs are toast). Following the cessation of exercise, your liver sorts it all out.

Andrew.

watch that breathing. i passed out once. :d_redface not pretty.

I've noticed my vision darkening when I do sprints during HIIT. Is this anything to worry about?