Pediatric Anesthesia: Free INBDE Prep Course
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LOCAL ANESTHETIC CALCULATIONS
pKa is the negative log of the acid dissociation constant or Ka value. Almost all pKa values of local anesthetics are above 7.4 (physiological pH). The closer the pKa of the anesthetic is to the physiologic pH, the more anesthetic exists in the uncharged base (non-ionized) form which can readily move in and out of cells. So the lower the pKa value, the more rapid the onset (higher proportion of LA is in the soluble form). Infections cause tissues to become acidic, lowering tissue ph from 7.4 to 5-6. This results in a higher concentration of the ionized form of anesthetic leading to poorer penetration through the nerve cell sheath and membrane, decreasing the effectiveness.
Lipid solubility affects the LA potency, with higher lipid solubility leading to increased potency. Protein binding affects the duration. The stronger the molecule binds to its receptor, the longer it takes to wear off. Sodium bicarbonate can be added to increase the pH of local anesthetic, from 3.5 closer to 7, in order to decrease pain when administering, and to increase effectiveness.
A dentist must be able to calculate safe dosages for use in kids, in light of their weight and any medical conditions or medications that could affect absorption and breakdown. Remember 1kg = 2.2 pounds. Start by calculating the patient’s weight. Then find the maximum dosage that is allowed to be given to a patient. This will be determined by the manufacturer (of the anesthetic) recommendation, which is usually 7mg/kg if there is a vasoconstrictor, and 4.4mg/kg if there is none. A maximum of 300mg is often indicated. The ADA recommends sticking to 4.4mg/kg regardless of the presence of a vasoconstrictor.
100% would be 1000mg/ml or 1g/ml, so 1% Lidocaine means there is 10mg/ml of solution. 2% means there is 20mg/ml of solution. 4% means 40mg/ml. First calculate the maximum dose specific to your patient:
4.4 (mg/kg max) x weight in kg (pounds divided by 2.2). For a 44lb child this will look like:
4.4 x 20 (44/2.2) = 88mg.
Then calculate how many cartridges you can give so as not to exceed the dose. Cartridges are usually 1.8mL. So for 2% Lidocaine:
20 (mg/ml) x 1.8 (volume of cartridge) = 36 mg/cartridge
88 (max dose) ÷ 36 (dose per cartridge) = 2.44 cartridges
2% Lidocaine is probably the safest anesthetic to use for kids, and a rough rule of thumb is max one cartridge for every 20 lb.
This number is based on the maximum local anesthetic dose, but you may need to be able to calculate the maximum vasoconstrictor dose in case this lowers the numbers of cartridges you can administer. In a healthy person the dose will largely be based on the anesthetic dose, in the case of cardiovascular disease it will depend more on the vasoconstrictor. The vasoconstrictor content is depicted as a ratio and the maximum dose is usually 0.2mg for normal patients and 0.04mg for patients with cardiovascular conditions where a vasoconstrictor could increase the risk of an unwanted cardiac event.
The concentration 1:100,000 means 1 g epinephrine in 100,000 ml solvent (i.e. 1000 mg/100,000 ml = 1 mg/100 ml = 0.01 mg/ml). 1:200,000 means 0.005mg/ml (the same vasoconstrictor in double the solution). Each 1.8mL cartridge of 1% lidocaine with 1:100,000 epine53phrine contains 0.018mg of epinephrine.
1.8 (mL of solution in the cartridge) x 0.01 (1:100,000 converted to mg/ml) = 0.018mg
This is the current recommendation from the American Academy of Pediatric Dentistry
LOCAL ANESTHESIA TECHNIQUES FOR CHILDREN
All dental and periodontal innervations are from branches of the trigeminal nerve (CN V), V2 for the maxillary teeth, and V3 for the lower teeth. All dental and periodontal arterial supply is from the maxillary artery, the branch names corresponding to the nerves. All dental and periodontal venous return drains to the pterygoid plexus of veins, which eventually turns into the maxillary vein.
The lower molars (all the lower teeth) are innervated by the inferior alveolar nerve. Anesthesia can be attained by an inferior alveolar nerve block, infiltration into the mandibular foramen area, or local infiltration with an anesthetic that exhibits good bony penetration (Articaine).
For an inferior alveolar nerve block the anesthetic syringe should bisect the deciduous lower molars on the opposing side. Compared to an adult, mesiodistally the target (the mandibular foramen) is roughly in the same position, but vertically the foramen sits closer to the occlusal plane. Deposition of LA at first entry or during the withdrawal should accomplish a lingual nerve block. A long buccal nerve block is required if buccal soft tissue anesthesia is required. Lower anterior teeth may receive nerve innervation from the opposite side (nerve anastomosis), so for teeth close to the midline additional infiltration is advisable.
Maxillary teeth are innervated by the branches of the superior alveolar nerve (posterior, middle, anterior). Local supraperiostial infiltration is effective for most procedures. A posterior superior alveolar (PSA) nerve block is available but mostly unnecessary. Palatal soft tissue is innervated by the anterior palatine and nasopalatine nerve. An infiltration is usually only required for extractions, not routine restorative work. An injection in the palate is quite painful.
Potential Complications
The maximum dose of anesthetic should not be exceeded to avoid toxicity. An overdose can cause central nervous system (CNS) and cardiac depression, and even death. Patients may experience dizziness, blurred vision, seizures, and a tingling sensation. Young patient’s often bite or chew on their anesthetized soft tissue causing trauma. Verbal instructions should be given to the patient and parents as a warning.
NITROUS OXIDE SEDATION
Children have a higher base metabolic rate, respiratory rate and heart rate, and lower blood pressure (compared to adults). They have narrower airways, and may present with enlarged tonsils and/or adenoids, and more oral secretions. Children are also at increased risk of desaturation (smaller oxygen reserve) and medications can have more variable effects.
Conscious sedation occurs when there is minimal suppression of the patient’s level of consciousness. The patient retains the ability to control their own airway and is able to respond to physical and verbal stimuli. The purpose of nitrous oxide sedation is to reduce fear/apprehension/anxiety and fatigue, while raising the patient’s pain threshold. Nitrous oxide has a rapid onset and recovery due to the very low plasma solubility. It is easily titratable and lacks serious adverse effects.
Minimum alveolar concentration (MAC) is a measure of the potency, defined as the concentration required to produce immobility in 50% of the patients. The MAC for NO is 105%. There is no biotransformation of the gas which is rapidly excreted by the lungs.
The total flow rate during the procedure is 4-6L/min, enough to keep the reservoir bag 1/3 to 2/3rds full. A mixture of oxygen and nitrous oxide is used. The proportion is increased in 10-20% increments until that patient reaches the drift plateau. The maintenance dose of nitrous oxide is usually around 30%. Oxygen should not strop below 30%, and the maximum dose of nitrous should not exceed 50%. Local anesthesia is still used since the gas only produces a little analgesia.
There are 4 stages of anesthesia. Patients undergoing conscious sedation are prevented from reaching the second stage. Within the first stage there are four plateaus.
- Stage I (stage of analgesia or disorientation): from induction of general anesthesia to loss of consciousness. Four plateaus of Stage I anesthesia (analgesia):
- Paresthesia: tingling of hands, feet. Lightheadedness.
- Vasomotor: warm sensations.
- Drift: euphoria, pupils centrally fixed, sensation of floating. The target plateau.
- Dream: eyes closed but will open in response to questions; difficulty in speaking; jaw sags open. This is where the dose would be adjusted to prevent drifting into stage II.
- Stage II (stage of excitement or delirium): from loss of consciousness to onset of automatic breathing. An anesthetist wants to pass through this stage quickly.
- Stage III (stage of surgical anesthesia): from onset of automatic respiration to respiratory paralysis. It is divided into four planes.
- Stage IV: respiratory paralysis till death. Anesthetic overdose causes medullary paralysis.
Nausea and vomiting are the most common side effects of nitrous oxide sedation. When the procedure is completed diffusion hypoxia can occur. The nitrous oxide is quick to push out of the tissues (high MAC) and can dilute the available oxygen in the lungs. For this reason patients are given 100% oxygen for 3-5 minutes following the procedure.
SEDATIVE HYPNOTICS
Chloral Hydrate is commonly used in pediatric sedation. It acts on the central nervous system to induce sedation and sleepiness but at normal doses does not affect breathing, reflexes or blood pressure. Onset is within 30 mins when given orally. There is often a period of excitement and irritability prior to sedation.
Short acting Barbiturates like Secobarbital (Seconal) and Pentobarbital (Numbutal) are occasionally used in conscious sedation, but of limited value. They can cause paradoxical excitement.
Benzodiazepines like Versed is the most commonly used pediatric premedication before general anesthesia.
