Just as its name implies, peak flow represents that part of the forced vital capacity where the air flows fastest out of the mouth. You can see this for yourself. Take in a deep breath and place a finger in front of your mouth. Now blow out as hard and as fast as you can. However, note that the speed of the air flowing past your finger decreased over time, and was fastest just after you began blowing out.
Peak flow is reduced during asthma attacks.
With the aid of easily portable apparatus designed to measure just peak flow, physicians now routinely use this test to determine both severity and degree of improvement from an asthma attack. Gas Exchange Tests of gas exchange tell how well your lungs and respiratory system are bringing in oxygen and getting rid of CO 2. Venous blood fills the blue veins of your arm and is easily obtained by syringe and needle. Except for blood gas analysis, all routine blood tests are done on venous blood. Venous blood is not useful for an ABG since it does not tell how much oxygen is being delivered to the tissues.