Factors which affect the plasma leel of epidurally administered local anaesthetics
| Total dose |
* Risk of toxicity is increased with increasing dosage.
* Tachyphylaxis may require increased dosage and result in cumulative toxicity (short acting drugs).
|
| Rate of administration |
* Rapid rate of administration increases plasma leels of lignocaine and etidocaine.
* Bolus administration results in higher plasma leels than continuous infusion, particularly with lignocaine.
|
| Epidural fat deposition (drug reservoir) |
* Bupivacaine - high fat uptake; minimal plasma increase with repeat bolus or infusion compared to lignocaine or etidocaine (accumulation) .
|
| Vertebral venous pleus dynamics |
* Rate of blood flow influences drug uptake.
|
| Physiological variable |
* dependent on cardiac output & renal & hepatic function
|
| Adrenaline |
* Delays absorption of lignocaine and etidocaine proportionately more than longer acting agents.
|
| Sodium Bicarbonate |
* Increases rate of absorption of lignocaine and bupivacaine |
Plasma leels of local anaesthetic usually peak 15-20 mins after an epidural bolus.