The reason i was under the impression that the bradycardia caused the unconsciousness not the deoxygenation of the brain, is i thought the brain could survive for some time with the oxygenated blood already in the brain before needing to shut down consciousness to conserve oxygen. Is it possibe for the person to go unconscious within say 3 seconds through the blood constriction alone?
Oxygen delivery is characterized by the oxygen hemoglobin dissociation curve. In the blood, oxygen is carried by hemoglobin.
There's some molecular biology and hematology behind this, but basically, when the curve shifts to the right, you get oxygen unloading of hemoglobin and delivery to the tissues. Factors that shift the curve to the right are listed on that graph: increase in 2,3 diphosphoglycerate, decrease in pH, and increase in temperature.
The effect of exercise on 2,3 diphosphoglycerate has not been elucidated, some studies think exercise increase it, some say decrease, and some say no change. Let's just ignore it for now. A decrease in pH means acidosis, and this is evident in exercise, as hypercarbia results from muscle work. Increase in temperature is due to ****bolism, and again work by tissues. So in exercise, not only is there increased stroke volume (blood pumped from heart, increases due to increases in contractility) and heart rate (sympathetic nervous system response) to increase cardiac output (CO = SV x HR) as well as increased breathing to ventilate, but oxygen unloading is also increased to meet oxygen demands.
So in the brain, which is an oxygen and glucose hungry organ to begin with, exercise would increase unloading even more. Thus, deoxygenation of blood that is in the brain during RNC compression would be faster. While I believe the amount of time needed to exhaust oxygen supply of this remaining blood is quick, the exact time (whether it can be three seconds or ten or twenty) is hard to say, probably differs from person to person, and depends on many, many factors - ie the aforementioned O2-Hgb curve, PO2 of the person's blood, and even the existence of hemoglobin defects or impairments in blood flow make the time of deoxygenation vary. I still also think that bradycardia, even though mechanical activation to severe levels could lead to loss of consciousness, is highly unlikely to contribute to the effects of an RNC in the face of increased sympathetic activity.
id say dont try it.. but yes it can kill someone... would u think someone could live underwater?
I should also point out that this is a little different than the scenario where you are drowning - which has been referred to on this thread twice already. If you are not breathing and are suffering from some sort of respiratory failure, if your heart is still pumping, your brain is STILL getting oxygen for some time, because it is still being perfused.
Mixed venous blood, or blood returning to the heart, is not completely devoid of oxygen. The PO2 of this blood is generally around 40 mmHg. On the oxygen hemoglobin dissociation curve above, that's an oxygen saturation of 70%. So as long as the heart continues to pump this blood, even if you aren't breathing, there is still enough oxygen in your blood to perfuse the brain (and the rest of your body) for a period of time. TyroneSlothRop on page 2 mentioned fishing kids out of freezing water. Remember that a decrease in temperature shifts that dissociation curve to the left - a PO2 of 40 mmHg would be even higher oxygen saturation and decreased unloading. This is why in CPR, the chest compressions are more important than the rescue breathing. Eventually, all the oxygen in your blood would be used up and you would die, but for some time, there is still a window of rescue. There are other autoregulatory mechanisms in the body such as vasoconstriction that actually shunts blood to critical areas like the brain to help widen this window.
In the RNC compression scenario, you are blocking 80% of the flow to the brain. It would be the equivalent of having an instant clot in both your internal carotid arteries. There is no continued perfusion other than the 20% through the vertebral arteries - and in a period of exercise where oxygen unloading is increased significantly, ischemia of the brain can generally occur much sooner.
So using the drowning example to define the timing of brain damage and death in comparison to RNCs (even though both result in ischemia) may not be appropriate because the oxygen unloading and perfusion effects are different.