The concentration and second gas effects

The phenomenon by which the rise in the alveolar partial pressure of nitrous oxide is disproportionately rapid when it is administered in high concentrations. 

Nitrous oxide (N20), although relatively insoluble, is 20 times more soluble in the blood than nitrogen (N2). The outward diffusion of N20 from the alveolus into the blood is therefore much faster than the inward diffusion of N2 from the blood into the alveolus. Consequently, the alveolus shrinks in volume and the remaining N20 is concentrated within it. This smaller volume has a secondary effect of increasing alveolar ventilation by drawing more gas into the alveolus from the airways in order to replenish the reduced volume. 

The above concept can be described graphically by considering the fractional concentration of an agent in the alveolar gas (Fa) as a percentage of its fractional concentration in the inhaled gas (Fi) over time. 

After drawing and labelling the axis draw a series of build-up negative exponential curves with different gradients as shown. The order of the curves is according to the blood gas partition coefficients. The more insoluble the agent, the steeper the curve and the faster the rate of onset. The exceptions to this are the N20 and desflurane curves, which are the opposite way round. This is because of the concentration effect when N20 is administered at 

The phenomenon by which the speed of onset of inhalational anaesthetic agents is increased when they are administered with N20 as a carrier gas. 

---