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Cardiovascular measures are related to your heart, and are mainly about looking at rhythms and how they change. These measures are heart rate, inter beat interval, heart rate variability, and blood pressure.
The first three measurements are again all about measuring an electrical signal, and are measured by attaching electrodes to the chest. Although nowadays there are many heart rate monitoring belts and high-techT-shirts which are designed for athletes (available the commercial fitness market) which can easily be used as well.
Blood pressure on the other hand requires a cuff to measure, much like at your doctor's office, and does not involve a measurement of electric current.
To go into a bit more detail, heart rate is the number of heat beats you have per unit of time (say per minute), and this will typically initially increase with increased workload and emotional arousal.
Inter beat interval is basically the same measure as heart rate, but refers to the time between beats and therefore tends to initially decrease with increased effort and emotional arousal. This is because there are more beats (heart rate is increasing) therefore the time that passes between those beats is shorter (so inter beat interval decreases).
With relatively cheap and easy heart rate measurement in every sporting goods store and gym, we have come a long way from this (image from Wikipedia).
Heart rate variability is a little more completed in that it is not measured directly, but is rather the variability of the inter beat interval. In other words, it is derived from measurements of inter beat interval over time. This again initially decreases with increased effort and emotional arousal and it is generally seen as a more sensitive measure to changes in workload than inter beat interval and heart rate -- especially if the 10hz frequency band is examined.
However, it is also quite vulnerable to any artifacts in the underlying data source, and can be relatively complex to calculate. That said, if you are recording inter beat interval then you should also attempt to calculate heart rate variability as it increases the richness of your data. It is therefore important that whatever device you use does provide you with information on inter beat interval, something that some of the mass market exercise belts do not do.
One of the great things about cardiovascular measures is that you can often see changes in variables with the naked eye. For example here you can clearly see an increase in heart rate and a decrease in inter beat interval when moving from a resting to a task completion state.
The change in heart rate and inter beat interval can be clearly seen when the task begins.
Finally, blood pressure simply measures the pressure that your blood is under, and therefore how hard your heart is working. This also initially increases with arousal but it is a less common measurement due it is intrusiveness in that you need to have a cuff to restrict blood flow, and then release it again.
There are of course some caveats to using cardiovascular measures. The first is that I have said that the changes above are initial reactions. This is because after about 20 minutes or so on a task, the trends I have described above tend to reverse due to the body's natural defense mechanisms attempting to return the body state to normal. This means that after long periods of emotional arousal or workload, heart rate actually starts to decrease and inter beat interval increases. This can cause problems with your data if you are not aware of it.
The other two problems are ones I have mentioned previously with other measures, and they are of specificity and individual differences. Some people may have particularly high or low natural heart rates, or irregular rhythms -- again this can be overcome through the use of individual resting measurements which are then used as comparison points for each individual.
As an aside, cardiovascular measures can also sometimes get you in a bit of a stressful situation, where you may detect irregular rhythms or particularly high blood pressure in your participants which may indicate underlying heart problems that they themselves may not be aware of. You have to use your own judgment when dealing with this. However, I would personally lean towards calmly advising any such participant that it is probably nothing (some people just have irregular patterns) but suggesting that they may want to have a checkup at their doctor's office.
Finally there are many things that can increase or decrease cardiovascular measures -- yes, perhaps your game is really exciting and demanding at his exact moment, but it is also possible that your player just took a deep breath or yawned (which causes your heart rate to increase as to distribute the oxygen in that breath around the body). Artifacts are also a problem, so again, no talking, and be careful that there isn't interference from excessive moment or other electrical sources.
The final measure I want to discuss is respiration. Respiration refers to the measurement of breathing, and is actually part of the cardiovascular system. However, I want to mention it separately because it is an often-overlooked metric. This is probably due the fact that it is not as sensitive as some of the others mentioned here. However, it is relatively easy and cheap to measure, requiring only a respiration belt (or one of the high-tech T-shirts used to measure information from the heart will also work).
Although it should be noted that respiration belts (and the belts used to measure heart rate and inter beat interval) work best when next to your skin on your chest. Therefore, even though respiration belts and heart rate measuring belts are easy to use, they are somewhat intrusive in terms of people often having to show experimenters at least part of their bare upper bodies – a situation can particularly be an issue if the player being tested is of the opposite sex to the tester.
However, respiration is also important to measure because breathing has a strong effect on other physiological measure such as EDA and the cardiovascular measures. This means that if these other measures are being used then respiration should also be recorded, if only to control for its effect.
In terms of gaming, the general effect of workload on respiration is to cause an increase in respiration. However engagement in a game or activity can also cause periods where players breath is held as this is a natural preparatory action as part of the "fight or flight" response setup by the sympathetic nervous system -- certainly an ex-girlfriend of mine used to shout at me to "remember to breathe!" whenever I would play particularly tense games.
Respiration is of course also vulnerable to artifacts (again no talking or excessive movement) although because it is not as sensitive as other measures, and is not measuring an electrical current, this somewhat less of a problem.