2019A09 Draw on a single graph wash-in curves for the first half hour administration of 70% N2O, 6% desflurane and 2% sevoflurane.
Explain why the curves are different for each agent (80%).
Describe and explain the effects on these curves of A) doubling of alveolar ventilation B) doubling of cardiac output (20%).

 

List:

·         Graph

·         Reasons for difference

·         Effect of physiological changes as described

 

Graph:

 

Reasons for difference:

Blood-gas partition coefficient (BGPC)

·   ↓BGPC -> ↓solubility in blood -> rate of saturation -> ↑rate of rise FA/FI

·   i.e. des 0.42 > N2O 0.47 > sevo 0.69

MAC

·   ↓Potency -> ↑safe inspired concentration -> ↑rate of delivery to alveoli -> ↑rate of rise FA/FI

·   Analogous to the Bowman principle

·   N2O 105% > des 6.6% > sevo 2%

Concentration effect

·   Occurs with high volume carrier gas (e.g. N2O)

·   N2O is ~30x more soluble than N2 (BGPC 0.47 cf. 0.014)

·   Hence rate of N2O uptake >> rate of N2 output

·   Rapid N2O uptake -> ↓alveolar pressure -> ↑rate of delivery to alveoli -> ↑rate of rise FA/FI

 

Changes:

2 x VA

·   Hence 2x ratio VA:FRC

·   ↑Rate of delivery to alveoli -> ↑rate of rise FA/FI

·   More effect on soluble agents (sevo > N2O > des)

 

·   Note: desflurane irritant to airway -> ↓VA -> ↓rate or rise FA/FI
(avoided for gas induction)

·   Note: N2O causes less respiratory depression -> ↑VA -> ↑rate of rise FA/FI

2 x CO

·   ↑Rate of drug uptake; but also

·   ↑Rate of distribution -> ↓rate of rise venous partial pressure -> ↓rate of rise FA/FI

·   More effect on soluble agents (sevo > N2O > des)

 

 

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