Pharmacology for Anaesthesia and Intensive Care

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8 General anaesthetic agents

(b)

(c)

FA/F I= 12/20 = 0.6

FA/F I= 59.5/70 = 0.85

Augment

Augment

28%

70%

37.8%

59.5%

43%

54%

2% 3% 2.7%

35%

Inspired Alveolar

78%

20%

86.7%

2% 2.2%

10%

11.1%

85.8%

12%

2.2%

Inspired Alveolar

(a) N

2 O 70%

N 2 O 20%

FA/FI

1

(^0) time
a
b
t
Figure 8.8.The concentration effect. See text for details.
that was in the conducting airways is drawn down into the alveoli so that the various
alveolar concentrations change.
Inthe final analysis, when comparing two separate scenarios, the first using high
concentrations of N 2 O(Figure8.8b) and the second using low concentrations of N 2 O
(Figure8.8c), the FA/FIratio is disproportionately high (pointaat timet,Figure8.8a)
where large concentrations of N 2 Oareused, compared to the FA/FIratio when lower
concentrations of N 2 Oareused (pointbat timet,Figure8.8a).