NURS 203 Lecture Notes - Amyotrophic Lateral Sclerosis, Perfusion, Anion Gap
Document Summary
Alveolar o2 and arterial po2 are never the same. The difference between the two is called alveolar arterial gradient. Reasons for it: (1) ventilation and perfusion are not evenly matched in the lungs. When standing up the ventilation is better than perfusion in the apex, whereas perfusion is better than ventilation at lower lobes. This explains why almost all pulmonary infarctions are in the lower lobes perfusion is greater there. Also, this explains why reactivation tb is in the apex tb is a strict aerobe and needs as more o2, and there is more ventilation in the upper lobes (higher o2 content). Normally, alveolar o2 is 100 and the arterial po2 is 95. As you get older, the gradient expands, but not that much. Most people use their upper limit of normal in other words, have a very very high specificity of 30 mmhg. If you have an a-a gradient of 30 mmhg or higher there is a problem.
