Calculation skills for nurses: Local anaesthetic agents

Abstract

M clinical scenarios require the administration of local anaesthetic. Local anaesthetics are generally viewed as safe drugs, with a low risk of systemic adverse drug reactions when prescribed and administered correctly (Faccenda and Finucane, 2001). However, if toxicity arises—as a result of exceeding the maximum safe dose (MSD), inappropriate use, or inadvertent intravascular administration—the consequences for the neurological and/or cardiovascular systems can be serious (Cox et al, 2003). The aim of this article is to assist nurse prescribers to administer safe levels of injectable local anaesthetic agents. Allergic reactions are not included, as they are unrelated to the calculation of safe doses. Agents predominantly for dental use and topical agents are also excluded. All local anaesthetics reversibly block nerve conduction at a local level without impeding consciousness. Most local anaesthetics in routine use in the UK are amide compounds (e.g lidocaine and bupivacaine), which undergo hepatic metabolism and predominantly renal excretion (Edgcombe and Hocking, 2006). The speed of onset, potency, and duration of action depend upon the active agent, the percentage concentration and the addition of vasoconstrictors. Vasoconstrictors, such as adrenaline, alter drug kinetics and hence may impact on the permitted dose. They shrink local vasculature and slow the rate of absorption of the anaesthetic into the circulation, prolonging the anaesthetic effect. A temporal decrease in systemic absorption means that toxic concentrations are less likely, and this is reflected in higher MSDs for some agents (Table 1). Formally, there is no change for bupivacaine with adrenaline, to minimise the cardiotoxic risk. Despite this, some sources advise that the MSD can increase to 2.5 mg/kg with adrenaline (Anaesthesia UK, 2009).