Q4 LONG ANSWER 10M ★ High-yield

Describe Pharmacokinetics & Vasoconstrictors in detail.

General Surgery · Final Professional

Absorption & Fate

After injection, a local anaesthetic acts locally but is progressively absorbed into the circulation; it is the peak plasma concentration that determines the risk of systemic toxicity. Absorption depends on the site (vascularity), the dose and concentration, and whether a vasoconstrictor is added. The drug is then metabolised (amides in the liver, esters by plasma cholinesterase) and excreted.

Effect of Injection Site

The more vascular the site, the faster the absorption and the higher the peak plasma level (and toxicity risk). A rough order of decreasing absorption is intercostal > caudal/epidural > brachial plexus > subcutaneous. This is why the maximum safe dose varies with the block, and why intercostal blocks carry a higher toxicity risk.

Vasoconstrictors (adrenaline)

Adding a vasoconstrictor (adrenaline) to a local anaesthetic causes local vasoconstriction that slows systemic absorption — which prolongs and intensifies the block, reduces the peak plasma level (raising the safe dose), and reduces surgical bleeding. It is most useful with lignocaine (safe dose rises from 3 to 7 mg/kg) and adds little to bupivacaine.

DANGER / REMEMBER

Never use adrenaline-containing local anaesthetic in areas supplied by end-arteries — fingers, toes, penis, nose and ears (the ‘extremities’) — as the vasoconstriction can cause ischaemic necrosis. Use plain solutions there.

CLINICAL PEARL

Adrenaline’s benefits — longer, denser block, less bleeding, higher safe dose — come from slowing absorption; but it is contraindicated in end-artery territories (digits, penis, nose, ears) and used cautiously in ischaemic heart disease.

Other Factors

The dose (mass) of drug, its concentration and volume, addition of vasoconstrictor, the site, and patient factors (weight, cardiac output, hepatic function, pregnancy, extremes of age) all influence plasma levels and effect. Onset can be quickened by warming or alkalinising the solution (more unionised drug).

Systemic & Local Balance

Every regional injection sets up a balance between the desired local effect on the target nerve and the unwanted systemic effect of the fraction that is absorbed, and understanding what shifts that balance is the key to safe practice. A larger mass of drug, a higher concentration, a more vascular site and the omission of a vasoconstrictor all push towards higher plasma levels and greater toxicity risk, whereas the smallest effective dose, a less vascular site and the judicious use of adrenaline push the other way. The clinician therefore chooses the lowest dose and concentration that will do the job, at the appropriate site, and adds a vasoconstrictor where it is safe and helpful.

Patient Factors

The same dose of local anaesthetic does not carry the same risk in every patient, because absorption, distribution and elimination all vary with the individual. Reduced hepatic function or blood flow slows the clearance of the amide agents; low plasma protein raises the free, active fraction; the extremes of age and pregnancy alter both pharmacokinetics and sensitivity; and a low cardiac output changes distribution. These factors mean that maximum doses are guides rather than guarantees, and that the dose is reduced in the frail, the very young or old, and those with significant hepatic or cardiac disease.

CLINICAL PEARL

Remember that toxicity tracks the peak plasma level, so the levers you control are dose, concentration, site vascularity and the use of adrenaline — and that adrenaline’s help stops at the end-artery territories, where only plain solution is safe.

DANGER / REMEMBER

Never treat the printed maximum dose as a fixed safe number in every patient: it must be reduced in the elderly, the very young, the pregnant, and those with hepatic or cardiac impairment or low plasma protein, in whom the same dose produces a higher free plasma concentration and greater toxicity.

Avoid adrenaline in end-artery areas — digits, penis, nose, ear.

KEY POINT

Key points TO remember

Systemic toxicity depends on peak plasma level → governed by site vascularity, dose/concentration, vasoconstrictor.
Absorption: intercostal > epidural/caudal > brachial plexus > subcutaneous.
Adrenaline slows absorption → longer/denser block, less bleeding, higher safe dose (esp. Lignocaine).
Adrenaline contraindicated in end-artery areas (fingers, toes, penis, nose, ears) — necrosis risk.
Onset quickened by warming/alkalinising; amides metabolised hepatically, esters by cholinesterase.

EXAM TIP

Sources: Morgan & Mikhail’s Clinical Anesthesiology; Miller’s Anesthesia; Ajay Yadav’s Short Textbook of Anaesthesia.

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