How multiple agents share a single safety ceiling — and how to track cumulative dose burden
Clinical Decision Support Tool. This page is intended as a reference aid for licensed healthcare professionals. It does not replace clinical judgment. Always verify dosing against current guidelines, institutional protocols, and patient-specific factors before administration.
Fractional toxicity (FT) is a unitless number between 0 and 1 that quantifies how much of a patient's total local anesthetic capacity has been consumed. All local anesthetics — regardless of class (amide or ester) — share a common mechanism of systemic toxicity: sodium channel blockade. Their toxic effects are additive.
When a clinician uses two or more local anesthetics on the same patient, the doses do not exist in isolation. A patient who has received 50% of one agent's maximum and 50% of another agent's maximum has consumed 100% of their safe capacity. FT captures this relationship in a single number.
Core formula:
FT = Σ (administered_dosei / effective_maxi)
Where the sum runs over every local anesthetic agent administered to the patient, and effective_max is the conservative (lower bound) weight-based maximum dose for that agent.
Without fractional toxicity tracking, a clinician might administer a full dose of lidocaine for field infiltration, then add bupivacaine for a nerve block, reasoning that each agent is individually "under the max." But the systemic toxic load is cumulative. The CNS and cardiovascular system do not distinguish between agents — they see total sodium channel occupancy.
LAST (Local Anesthetic Systemic Toxicity) events are rare but can be catastrophic: seizures, cardiac arrhythmias, cardiovascular collapse. Most LAST events involve either a single large dose or, crucially, multiple agents given without tracking the cumulative burden. Fractional toxicity provides the bookkeeping that prevents this second failure mode.
Each agent contributes a fraction to the total toxicity score:
| Term | Definition |
|---|---|
| administered_dosei | The actual dose of agent i given to the patient, in milligrams |
| effective_maxi | maxMgKg.lower × weightBasis — the conservative maximum dose for agent i in milligrams |
| maxMgKg.lower | The lower bound of the published mg/kg dose range for the agent (e.g., 4.5 mg/kg for lidocaine without epinephrine) |
| weightBasis | The dosing weight for this patient (see Weight Basis section below) |
Why use the lower bound? The lower bound of the dose range is the conservative denominator. Using the lower bound means FT will reach 1.0 before the patient has reached the absolute pharmacological ceiling. This builds in a safety margin — the "conservative default" approach. Doses between the lower and upper bounds trigger a soft warning rather than a hard stop.
The effective_max is also subject to the absolute maximum cap for each agent. If the weight-based calculation exceeds the absolute cap (e.g., 300 mg for lidocaine without epinephrine), the cap applies instead.
The weight used in the effective_max calculation depends on patient age and body composition:
| Patient Category | Weight Basis | Rationale |
|---|---|---|
| Pediatric (< 18 years) | Total Body Weight (actual) | Pediatric dosing is established using actual weight; lean body weight equations are not validated in children |
| Adult, BMI < 30 | Total Body Weight | Body composition is within normal range; TBW is the standard dosing basis |
| Adult, BMI ≥ 30 | Lean Body Weight (Janmahasatian 2005) | Local anesthetics distribute primarily into lean tissue; dosing on TBW in obese patients overestimates safe doses |
Janmahasatian 2005 LBW equation: Lean body weight is calculated from height, weight, and sex using the Janmahasatian formula. MaxLocal computes this automatically when height and sex are provided and BMI ≥ 30.
Consider a 70 kg adult with BMI 24 (non-obese, so weight basis = Total Body Weight = 70 kg).
| Agent | maxMgKg.lower | Weight-Based Max | Absolute Cap | Effective Max |
|---|---|---|---|---|
| Lidocaine (no epi) | 4.5 mg/kg | 4.5 × 70 = 315 mg | 300 mg | 300 mg (cap applies) |
| Bupivacaine (no epi) | 2 mg/kg | 2 × 70 = 140 mg | 150 mg | 140 mg (weight-based applies) |
The clinician infiltrates 150 mg of lidocaine for a field block.
FT = 150 / 300 = 0.50
The patient has consumed 50% of their total local anesthetic capacity.
The clinician then administers 70 mg of bupivacaine for a nerve block.
FT = 150 / 300 + 70 / 140 = 0.50 + 0.50 = 1.00
At the limit. FT = 1.00 means the patient has reached their total conservative safe local anesthetic capacity. No additional local anesthetic should be administered without strong clinical justification and heightened monitoring for LAST signs.
Note that neither dose alone was at the agent's individual maximum — 150 mg is only half the lidocaine cap, and 70 mg is half the bupivacaine cap. But in combination, the cumulative burden is 100%. This is precisely the scenario fractional toxicity is designed to catch.
Not all injection sites carry equal risk for systemic absorption. Highly vascular sites (intercostal space, epidural space) produce higher peak plasma levels per mg injected than subcutaneous infiltration. MaxLocal adjusts CAUTION and DANGER thresholds accordingly:
| Risk Tier | Injection Sites | CAUTION | DANGER |
|---|---|---|---|
| High | IV regional (Bier block), intercostal, epidural, caudal | FT ≥ 0.80 | FT ≥ 0.90 |
| Medium | Brachial plexus, mucosal | FT ≥ 0.85 | FT ≥ 0.92 |
| Standard | Subcutaneous, lower extremity, fascial plane, penile, tumescent | FT ≥ 0.90 | FT ≥ 0.95 |
| Low | Intrathecal, topical skin | FT ≥ 0.95 | FT ≥ 0.98 |
MONITOR (FT > 1.0): When FT exceeds 1.0, MaxLocal displays an informational "ABOVE CONSERVATIVE" badge. This does not necessarily mean the patient is experiencing clinical toxicity — the conservative denominator intentionally builds in a margin. However, the patient has exceeded the recommended limit and requires heightened monitoring for prodromal LAST signs (tinnitus, perioral numbness, metallic taste, agitation).
The risk tier is determined by the highest-risk injection site used in the session. If one dose is subcutaneous (standard risk) and another is intercostal (high risk), the session uses high-risk thresholds.
Two formulations have modified FT coupling due to their distinct pharmacokinetics:
Exparel uses 100% FT coupling — it contributes its full dose fraction to the FT sum. This is per the FDA label, which states that liposomal bupivacaine has additive toxicity with other local anesthetics. Despite the extended-release kinetics, the manufacturer requires that Exparel be counted at full value against the patient's total local anesthetic capacity.
Tumescent lidocaine uses 25% FT coupling (attenuation factor 0.25). The pharmacokinetic basis: peak plasma concentration from tumescent infiltration occurs at 12–14 hours, compared to 30–60 minutes for standard subcutaneous lidocaine. This temporal separation means there is minimal overlap in peak systemic burden between tumescent lidocaine and any standard-formulation agents administered at the same session. The 25% coupling reflects this reduced temporal overlap while maintaining a conservative safety margin.
FT coupling example: If a patient has FT = 0.40 from standard agents and then receives tumescent lidocaine, the tumescent contribution to FT is attenuated: only 25% of the tumescent dose fraction is added to the running FT total. Conversely, when computing remaining tumescent capacity, the formula accounts for this attenuation.
Once FT is known, the remaining safe dose for any agent can be calculated:
Standard agents:
remainingMg = floor((1.0 − currentFT) × effectiveMaxMg.lower)
Tumescent lidocaine:
remainingMg = floor((1.0 − currentFT × 0.25) × tumescentMaxMg)
Floor rounding only. The remaining capacity is always rounded down (Math.floor), never up. This is a deliberate safety decision — rounding up a safe dose, even by 1 mg, moves in the wrong direction for a safety-critical calculation.
Using the worked example above (70 kg, BMI 24), after giving 150 mg lidocaine (FT = 0.50):
| Agent | Effective Max | Remaining Mg | Calculation |
|---|---|---|---|
| Lidocaine (no epi) | 300 mg | 150 mg | floor((1.0 − 0.50) × 300) = 150 |
| Bupivacaine (no epi) | 140 mg | 70 mg | floor((1.0 − 0.50) × 140) = 70 |
| Ropivacaine (no epi) | 210 mg | 105 mg | floor((1.0 − 0.50) × 210) = 105 |
These remaining capacities represent the maximum additional dose of each agent that could be given before FT reaches 1.0, assuming no other agents are administered.
MaxLocal computes fractional toxicity automatically and continuously throughout a dosing session:
Fractional toxicity (FT) is a unitless number between 0 and 1 that represents how much of a patient's total local anesthetic capacity has been consumed. It is calculated by summing the ratio of each administered dose to its effective maximum dose across all agents: FT = Σ(administered_dosei / effective_maxi). When FT approaches 1.0, the patient is at their total safe local anesthetic capacity regardless of which combination of agents was used.
For each local anesthetic administered, divide the dose given (in mg) by that agent's effective maximum dose (in mg). The effective maximum is the conservative lower bound of the mg/kg dose range multiplied by the patient's weight basis (total body weight for pediatric and non-obese adults, lean body weight for adults with BMI ≥ 30). Sum these fractions across all agents. For example, giving 50% of the lidocaine maximum and 50% of the bupivacaine maximum yields FT = 0.50 + 0.50 = 1.00, meaning the patient is at their limit.
Local anesthetics share a common mechanism of toxicity — sodium channel blockade — regardless of which specific agent is used. When multiple agents are administered to the same patient, their toxic effects are additive. A patient who receives half the maximum dose of two different local anesthetics is at the same systemic risk as a patient who received the full maximum dose of a single agent. Without fractional toxicity tracking, clinicians can inadvertently exceed safe cumulative limits by assuming each agent's dose is independently safe.
The danger threshold depends on the injection site risk tier. For high-risk sites (IV regional, intercostal, epidural, caudal), danger begins at FT 0.90. For medium-risk sites (brachial plexus, mucosal), danger begins at FT 0.92. For standard-risk sites (subcutaneous, lower extremity, fascial plane), danger begins at FT 0.95. For low-risk sites (intrathecal, topical skin), danger begins at FT 0.98. FT above 1.0 exceeds the conservative maximum but does not necessarily indicate clinical toxicity — it is flagged as "ABOVE CONSERVATIVE" for monitoring.
Weight-based dosing, real-time FT tracking across all agents, injection-site risk thresholds, remaining capacity calculations, and instant LAST protocol access — all offline.
Get MaxLocal on Google PlayClinical Decision Support Tool. This page is intended as a reference aid for licensed healthcare professionals only. It does not replace clinical judgment. Always verify information against current guidelines and institutional protocols before clinical use.