Basic Sciences and Anatomy Flashcards

The mylohyoid nerve, a branch of the inferior alveolar nerve, provides sensory innervation to which structure that is clinically relevant when a standard inferior alveolar nerve block fails?

Answer: The anterior belly of the digastric muscle and the mylohyoid muscle

The mylohyoid nerve branches from the inferior alveolar nerve just before it enters the mandibular foramen and descends in the mylohyoid groove to supply motor innervation to the mylohyoid muscle and anterior belly of the digastric. It can provide accessory sensory innervation to the mandibular molars, which is why supplemental injections targeting this nerve are used when a standard IANB fails on lower molars.

During intravenous sedation, a patient's end-tidal CO₂ (EtCO₂) reading drops from 40 mmHg to 18 mmHg while SpO₂ remains normal. Which pathophysiologic mechanism BEST explains this pattern?

Answer: Pulmonary embolism causing increased dead space ventilation

A sudden drop in EtCO₂ with maintained SpO₂ is a classic sign of pulmonary embolism. When a PE occludes pulmonary vasculature, blood flow to ventilated alveoli is lost, creating alveolar dead space — CO₂ cannot be eliminated despite normal ventilation, so exhaled CO₂ concentration falls. SpO₂ may remain normal initially due to reflex compensatory hyperventilation and redistribution of blood flow. Hypoventilation would raise EtCO₂, not lower it.

Which structural feature of the mandibular ramus accounts for the risk of parotid gland injection and subsequent facial nerve block when performing an inferior alveolar nerve block with excessive posterior needle angulation?

Answer: The parotid gland wrapping around the posterior border of the ramus into the retromandibular space

The parotid gland extends around the posterior border of the mandibular ramus into the retromandibular (retromandibular fossa) space. When the needle is angled too far posteriorly during an IANB, it can penetrate the parotid capsule and deposit local anesthetic near the branches of the facial nerve (CN VII) that traverse the gland, causing a transient facial nerve palsy. This is a well-known complication of incorrectly directed inferior alveolar nerve blocks.

A 58-year-old patient with a history of chronic kidney disease (CrCl 22 mL/min) is scheduled for an oral surgical procedure under IV sedation. Which pharmacokinetic change poses the GREATEST risk if amide local anesthetics with vasoconstrictors are used in standard doses?

Answer: Decreased plasma protein binding of lidocaine due to uremia, increasing free drug fraction

In uremia (CKD), there is accumulation of endogenous compounds (uremic toxins) that compete with drugs for protein binding sites on albumin and α1-acid glycoprotein. Lidocaine is approximately 65–75% protein-bound, predominantly to α1-acid glycoprotein. Uremic displacement increases the free (pharmacologically active) fraction of lidocaine, raising the risk of systemic toxicity at doses that would normally be safe. Notably, lidocaine itself is hepatically metabolized, not renally excreted as the parent compound — so option C is incorrect.

The Bainbridge reflex is MOST accurately described as:

Answer: Tachycardia mediated by stretch receptors in the right atrium and great veins responding to increased venous return

The Bainbridge reflex (atrial reflex) involves stretch receptors located in the walls of the right atrium and the junctions of the great veins. When venous return increases — stretching these receptors — afferent signals travel via the vagus nerve to the medulla, resulting in a reflex increase in heart rate (tachycardia) via sympathetic stimulation. This is physiologically distinct from the carotid sinus baroreflex (which slows the heart) and is relevant in dental anesthesia when rapid IV fluid administration or positional changes alter venous return.

When assessing a patient's airway prior to sedation, a Mallampati Class IV classification indicates:

Answer: Only the hard palate is visible; neither soft palate nor uvula can be seen

The Mallampati classification grades oropharyngeal visibility with the patient seated, mouth fully open, and tongue protruded without phonating. Class I: soft palate, uvula, fauces, tonsillar pillars fully visible. Class II: soft palate, uvula, fauces visible. Class III: only soft palate and base of uvula visible. Class IV: only the hard palate is visible — the soft palate is completely obscured. Class IV predicts the highest difficulty for laryngoscopy and intubation, and is a critical flag during pre-sedation airway assessment.