Risk Factors for LAST

Local anesthetic systemic toxicity (LAST) can be triggered by a variety of risk factors, categorized broadly into patient-related, pharmacological, procedural, and practice-setting factors. Understanding these risk factors helps clinicians identify high-risk patients and situations, allowing for appropriate precautions and monitoring to reduce the likelihood of systemic toxicity.

Patient Factors

Certain patient characteristics make individuals more susceptible to LAST. Age plays a significant role, as patients at both extremes of the age spectrum—those younger than 16 and those older than 60—are at higher risk. Neonates and infants have low muscle mass, underdeveloped metabolic pathways, and immature hepatic and renal systems, all of which reduce their capacity to metabolize and excrete local anesthetics. Elderly patients often have reduced organ function, lower muscle mass, and comorbidities, all of which increase susceptibility to toxicity. Frailty in elderly patients also decreases their ability to buffer and metabolize these agents, raising the risk for adverse effects.

Gender has also been identified as a risk factor, with females having a higher likelihood of experiencing LAST than males. Pregnancy increases susceptibility to LAST due to several physiological changes, including increased sensitivity to local anesthetics, higher cardiac output, and venous engorgement, which enhances the spread of local anesthetics to the systemic circulation. Pregnant women also exhibit decreased protein binding, resulting in more unbound, active drugs available to exert toxic effects.

Patients with pre-existing cardiac disease, such as conduction abnormalities, ischemic heart disease, or congestive heart failure, are more vulnerable to LAST. These conditions make the heart less able to withstand the sodium channel blockade effects of local anesthetics, increasing the likelihood of severe cardiac events, such as arrhythmias or hypotension.

Liver disease is another significant risk factor, as the liver is the primary organ responsible for the metabolism of local anesthetics. In conditions like cirrhosis or hepatic failure, the capacity to metabolize anesthetics is impaired, leading to prolonged drug action and accumulation in tissues. Metabolic diseases, such as diabetes, isovaleric acidemia, mitochondrial disease, and carnitine deficiency, further compromise the body's ability to handle local anesthetics, increasing the risk of toxicity.

Patients with central nervous system (CNS) diseases are also at higher risk, as pre-existing neurological dysfunction can exacerbate the neurotoxic effects of local anesthetics. Lastly, conditions resulting in low plasma protein binding states—such as malnourishment, liver disease, pregnancy, or infancy—are associated with an increased risk of LAST. When plasma protein levels are reduced, there is a higher concentration of free, unbound local anesthetic in circulation, increasing the potential for systemic toxicity.

Pharmacologic Factors

The pharmacologic properties of the local anesthetic itself are critical in determining the risk of LAST. Some local anesthetics, such as bupivacaine, have a narrower therapeutic index between CNS and cardiac toxicity. Bupivacaine binds to sodium, potassium, and calcium channels in the heart with high affinity, leading to severe cardiac toxicity, often before the onset of CNS symptoms. This "fast-in/slow-out" binding of bupivacaine means it accumulates in myocardial sodium channels, prolonging its toxic effects on the heart.

Total local anesthetic dose and dose per kilogram of body weight are important factors. Larger doses increase the concentration of the anesthetic in plasma and tissues, heightening the likelihood of systemic toxicity. However, certain clinical scenarios can lead to higher plasma levels of anesthetics regardless of body weight. For example, blocks performed at sites with high vascularity, such as the intercostal space or the brachial plexus, can result in faster systemic absorption and higher plasma levels. Additionally, continuous local anesthetic infusions—such as those used for post-surgical analgesia—pose an increased risk of LAST, particularly after 1 to 4 days of infusion. Plasma levels can continue to rise over time, reaching toxic levels even with seemingly modest dosing.

Procedural Factors

The technique used to administer local anesthetics significantly influences the risk of LAST. Procedures involving ultrasound guidance reduce the risk of toxicity compared to traditional localization methods, such as nerve stimulation or anatomical landmark techniques. Ultrasound guidance allows for more precise placement of the anesthetic, reducing the likelihood of inadvertent intravascular injection, which is a common cause of LAST.

The type of block performed also plays a role in determining risk. Peripheral nerve blocks are associated with a higher risk of LAST compared to neuraxial blocks (such as spinal or epidural anesthesia). This is partly due to the increased vascularity of peripheral nerve block sites. Among peripheral blocks, the risk of systemic toxicity correlates with the vascularity of surrounding tissues. For instance, blocks in the lower limbs have a lower risk of LAST compared to upper limb blocks, which in turn have a lower risk than paravertebral blocks, where the highly vascularized surrounding tissues contribute to increased absorption and systemic exposure to local anesthetics.

Practice Setting

The practice setting can also influence the risk of LAST. Notably, up to 20% of LAST cases occur outside of hospital settings, where access to emergency equipment and specialized personnel may be limited. Local anesthetics are frequently used in outpatient surgical centers, dental practices, and emergency rooms, where monitoring may not be as rigorous as in a hospital setting. This lack of immediate access to advanced resuscitative measures can lead to delayed recognition and treatment of LAST, increasing the risk of severe outcomes.