tiva & tci sedation dr james f peerless november 2012
TRANSCRIPT
TIVA & TCI Sedation
Dr James F PeerlessNovember 2012
Objectives
• TIVA & TCI– What, why, when, where and how
• Models, Monitoring & Limitations• Context-Sensitive Half-Time– Suitability of drugs for TCI
TCI
• “Target-Controlled Infusion”• An infusion system whereby the target
concentration of a particular agent is selected.
TIVA
• “Total IntraVenous Anaesthesia”– Anaesthesia provided solely by IV route– Generally as an infusion which is titrated at a
specific rate to achieve a specific concentration• Cp – plasma concentration
• Ce – effect site concentration
Indications/Benefits
• When:– Inhalational agents unavailable– Administering inhalational agents is difficult– Inhalational agents are contraindicated– Patient has severe PONV
• Also:– Reduces staff exposure to inhalational agents– Reduces pollution
• NOT:– Unmonitored sedation at home
Pharmacokinetics
Pharmacokinetics
Why do we use models?
• At present in clinical use there is no method of measuring drug concentrations real time analogous to the end tidal volatile agent concentration
• Models only – no actual plasma measurements• Most models describe healthy volunteers and
there is poor correlation in unwell patients• The trend of increasing obesity does alter
pharmacokinetics and accuracy of the pumps
Models
• Propofol– Marsh model
• 3-compartment model optimised by weight, gender & age
– Schnider model– Diprivan 2% prefilled syringes
• Remifentanil– Minto model– Made up to 50 mcg mL-1 (2 mg in
40 mL 0.9% N. Saline)
Induction of Anaesthesia
• Good IV access – visible at all times!!– Dedicated line where possible– Ensure a dripping drip– Anti-reflux valves– Minimise dead-space
Induction of Anaesthesia
• Select a target concentration• Press ‘go’• Ensure adequate O2• Change settings/increase the target
concentration in slow, small stages
Numbers!
• Propofol TCI– Sedation: 1 – 2 mcg mL-1
– Anaesthesia EC50: 6 – 7 mcg mL-1
• Remifentanil– 0.05 – 0.5 mcg kg-1 min-1
– TCI: 4 – 8 ng mL-1
• Bear in mind individual variations in pharmacokinetics and drug interactions
Limitations
• Inability to monitor actual drug concentration• Slow recovery/wake-up after long operations
due to distribution across compartments• Increased cost compared with volatile agents• Interruption to TCI delivery may go unnoticed
longer than with volatiles• [fighting with other theatres for use of PK
pumps]
Context-Sensitive Half-Time
• The CSHT is:“The time taken for the drug concentration
to reduce by half once an infusiondesigned to maintain a constant plasma
concentration is stopped.”• CSHT for a specific drug will vary depending on
the length of the infusion
CSHT
• During an infusion, drugs will accumulate and equilibrate within all the tissues/compartments.
• The longer the duration of the infusion, the higher the degree of accumulation, which will maintain plasma levels once the infusion is stopped.
• As a result, some drugs are better suited to infusions than others
Upon Stopping the Infusion
Ideal drugs
• Small VD
• Rapid metabolism (no active metabolites)• High Cl• Short CSHT
• Propofol, alfentanil, remifentanil
Remifentanil – wow!
• Rapidly metabolised– Non-specific plasma and tissue esterases
• Short t1/2elim
– 1.3 minutes• High Cl– 2.5 L kg-1 hr-1
• Small VD
– 0.35 L kg-1
= context-“insensitive” half-time
Context-Sensitive Half-TimeCS
HT
(min
)
Duration of Infusion (hr)
Fentanyl
Thiopental
Alfentanil
Propofol
Remifentanil
Summary
• Target concentrations are calculated, not measured
• TCI pumps maintain three superimposed infusions, one at a constant rate to replace drug elimination and two exponentially decreasing infusions to match drug removed from central compartment to other peripheral compartments of distribution
Questions?