Understanding Systemic Blood Flow Changes with Near Infrared Spectroscopy
A delivery limitation means that cardiac output is not sufficient to meet the demand for all of the organs and muscles. The body places a very high priority on preserving blood flow for the brain and heart and If the athlete doesn’t pass every time they hit a hard metcon this mechanism is working. When the body is in a scenario where the demand for blood flow is higher than the cardiac system is capable of supplying it selectively reduces blood flow to low priority areas. This ensures that skeletal muscle vasodilation does not 'outstrip' cardiac output. Arterial blood pressure needs to be tightly regulated, and if a large % of skeletal muscle was vasodialted, cardiac output would need to increase drastically to maintain the pressure gradient needed to keep arterial blood pressure within it's stable range. This is dangerous to the delivery limited athlete. Arterial pressure is calculated as... P(pressure)= F(flow) x R(resistance), where resistance = k(constant)/r(radius)^4. This means that small changes in the radius of a blood vessel have a huge impact on resistance. I.e., if an athlete vasodilation even a little bit resistance will decrease and flow needs to increase to keep pressure fixed. So, when I'm assessing athletes one of things I look at is to see if there is a progressive vasoconstriction trend over the course of a workout. If so, this tells me then cardiac system may be a limiter. I'll also look at heart rates trends and blood flow over the course of the session as well as the rate of oxygen delivery versus utilization. In the picture above we have a vasodilation trends on the top and a vasoconstriction trend on the bottom. On the top you can see a progressive increase in THb every rest interval as CO2 accumulates in the muscle acting as a vasodilator (respiratory limitation from diagram fatigue). On the bottom we see baseline resting THb values decreases from pre--> post workout as the athlete progressively vasoontricts over the course of the session. This basic understanding of physiology goes a long way when trying to understand and interpret NIRS data.