The cardiovascular system (CVS) appears to be more resistant to the toxic effects of local anaesthetic agents than the central nervous system (CNS). 'Toxicity' symptoms occurring subsequent to injection into the epidural space are due to:
1. accidental intravenous injection producing immediate, high plasma levels of local anaesthetic. This produces mainly CVS symptoms; or
2. slower systemic absorption of an excessive dose of correctly-placed local anaesthetic. Here, the CNS toxicity symptoms predominate (Table E.2).
Animal experiments have been undertaken to investigate the relative toxicity of several local anaesthetics. Their CVS and CNS toxicity thresholds are usually expressed as a ratio (Table 37.2) (1, 2). The CVS:CNS toxicity ratio is obtained by administering the local anaesthetic until convulsions ensue and then continuing until circulatory collapse follows. The lower the ratio, the greater the likelihood of selective cardiotoxicity.
In non-pregnant sheep, the CVS:CNS toxicity ratio is low following bupivacaine (3.7) but not lignocaine (7.1).
Ropivacaine may be less cardiotoxic than bupivacaine because it has a higher ratio.
References:
1. LAISHLEY RS. Local Anaesthetic Toxicity.In:
Reynolds F Editor. Epidural and Spinal Blockade in Obstetrics.
London: Bailliere Tindall 1990: 81-94
2. Reynolds F. In defence of bupivacaine. Int. Journal of Obstetric Anesthesia 1995; 4:93-108