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📋 Ephedrine — Drug Note

Basic Classification

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Mechanism of Action

1. Direct: Stimulates α₁, β₁, and β₂ adrenergic receptors directly.
2. Indirect: Enters presynaptic sympathetic nerve terminals → displaces norepinephrine from synaptic vesicles → released NE activates postsynaptic receptors.

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Pharmacokinetics

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Pharmacological Effects

Cardiovascular

• ↑ Heart rate, contractility, cardiac output
• ↑ Systolic and diastolic blood pressure (via vasoconstriction + cardiac stimulation)
• Peripheral resistance variably increased

Respiratory

• Bronchodilation via β₂ receptor activation (less potent and slower than epinephrine/isoproterenol)

CNS

• Mild CNS stimulant: ↑ alertness, ↓ fatigue, prevents sleep
• Raises minimum alveolar concentration (MAC) of anesthetic agents

Urological

• Stimulates α₁ receptors in bladder neck → ↑ urinary outflow resistance

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Clinical Uses

Dosing (Anesthesia Context)

• Adults: Bolus of 2.5–10 mg IV
• Children: Bolus of 0.1 mg/kg IV
• Subsequent doses may need to be increased to overcome tachyphylaxis

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Adverse Effects / Toxicity

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Special Considerations

• Tachyphylaxis: Rapid tolerance develops with repeated doses; dose escalation may be required.
• Herbal preparations (Ma huang/Ephedra): Highly variable ephedrine content → risk of inadvertent overdose. The FDA has banned ephedra-containing dietary supplements.
• Obstetric use: Previously preferred vasopressor in obstetric anesthesia (did not reduce uterine blood flow in animal models), but phenylephrine is now widely preferred.
• Illicit use: Ephedrine and pseudoephedrine are precursors for illicit methamphetamine synthesis → sales regulated by the Combat Methamphetamine Epidemic Act (2005) in the US.
• Sports: Completely banned from all sports competitions since 2003.
• Bitter orange (p-synephrine): Marketed as a "safer" herbal alternative; contains demethylated ephedrine, but appears to have minimal stimulant/cardiotoxic effects at usual doses.

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Comparison: Ephedrine vs. Epinephrine

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💊 Ephedrine — MOA, Adverse Effects with Sources & Reasons

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Mechanism of Action (MOA) — Detailed

Indirect Action (Predominant)

"Endocytosis of ephedrine into alpha- and beta-adrenoceptor presynaptic postganglionic nerve terminals displaces norepinephrine from the synaptic vesicles. The displaced norepinephrine is then released to activate the corresponding postsynaptic receptors to cause arterial and venous vasoconstriction and increased myocardial contractility." — Barash Clinical Anesthesia, 9e

Direct Action

"Ephedrine is an agonist at both α and β receptors; in addition, it enhances release of NE from sympathetic neurons and thus is a mixed-acting sympathomimetic." — Goodman & Gilman's Pharmacological Basis of Therapeutics

Summary of Receptor Actions

"Ephedrine and pseudoephedrine are not catecholamines and are poor substrates for COMT and MAO. Therefore, these drugs have a long duration of action." — Lippincott Illustrated Reviews: Pharmacology

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Adverse Effects — with Source + Mechanistic Reason

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1. 🔴 Hypertension

"Untoward effects of ephedrine include hypertension and insomnia."

• α₁ agonism → arterial and venous vasoconstriction → ↑ peripheral vascular resistance
• β₁ agonism → ↑ heart rate + ↑ cardiac output
• Both mechanisms together drive blood pressure up. Because ephedrine is a noncatechol, it is not rapidly degraded by COMT/MAO → sustained pressor effect.

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2. 🟡 Tachyphylaxis (Rapid Tolerance)

"Tachyphylaxis to ephedrine's hemodynamic effects occurs with repetitive administration of the drug because presynaptic norepinephrine stores are rapidly depleted and ephedrine is then released from synaptic vesicles as a false neurotransmitter."

• Ephedrine's indirect mechanism depends on NE stores. With repeated dosing, the vesicular NE is exhausted.
• Once stores are depleted, ephedrine itself is taken up into vesicles and released as a false neurotransmitter — it occupies receptors but produces little to no response.
• This is why epinephrine does NOT show tachyphylaxis — it acts directly on receptors without needing NE displacement.

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3. 🟡 Insomnia / CNS Stimulation

"Ephedrine is a potent CNS stimulant."

"Ephedrine produces a mild stimulation of the CNS. This increases alertness, decreases fatigue, and prevents sleep."

• Ephedrine crosses the blood-brain barrier (it is a noncatecholamine with high lipophilicity relative to catecholamines).
• Promotes NE and dopamine release in the CNS → activates arousal pathways → insomnia, hyperactivity, headache.

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4. 🟡 Urinary Retention (especially in BPH)

"Stimulation of the α adrenergic receptors of smooth muscle cells in the bladder base may increase the resistance to the outflow of urine... the drug may cause urinary retention, particularly in men with benign prostatic hyperplasia (BPH)."

• α₁ receptor activation in the bladder neck/internal urethral sphincter → smooth muscle contraction → increased urethral resistance.
• In BPH, the prostate is already compressing the urethra. Adding α₁ stimulation compounds this obstruction → acute urinary retention.

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5. 🔴 Cardiovascular Toxicity / Death (with high doses / herbal preparations)

"Usual or higher-than-recommended doses may cause important adverse effects in susceptible individuals, especially in patients with underlying cardiovascular disease that might be unrecognized. There can be considerable variability in the content of ephedrine in these preparations... leading to significant toxicity and death."

"Ephedra has been associated with strokes and deaths in adolescent users."

• Massive sympathomimetic surge → severe hypertension → hemorrhagic stroke
• Intense cardiac stimulation → arrhythmias, myocardial infarction
• Susceptible individuals (undiagnosed hypertension, coronary artery disease) are at highest risk.

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6. 🟠 Arrhythmias

• β₁ receptor overstimulation in the heart → ↑ automaticity and ↑ conduction velocity in ectopic foci → ventricular tachycardia, atrial arrhythmias.
• Combined with hypertension-induced increase in cardiac wall stress, arrhythmia risk rises.

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7. 🟠 Drug Interactions (Cocaine, Reserpine)

"Predictably, drugs that block the ephedrine uptake into adrenergic nerves (e.g., cocaine) and those that deplete norepinephrine reserves (e.g., reserpine) attenuate its cardiovascular effects."

• Cocaine blocks the transporter (NET) that ephedrine uses to enter the nerve terminal → ephedrine cannot displace NE → indirect action is blocked.
• Reserpine depletes NE stores → nothing left for ephedrine to release → indirect action is abolished.

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MOA Clarification Summary Table

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• Goodman & Gilman's Pharmacological Basis of Therapeutics (MOA, adverse effects)
• Barash, Cullen & Stoelting's Clinical Anesthesia, 9e (tachyphylaxis mechanism, drug interactions)
• Lippincott Illustrated Reviews: Pharmacology (mixed-action classification, CNS effects)
• Rosen's Emergency Medicine (herbal ephedra toxicity)
• Morgan & Mikhail's Clinical Anesthesiology, 7e (clinical use, obstetric context)

1. Basic Classification — drug class, chemical type, origin
2. Mechanism of Action — indirect NE displacement, direct receptor agonism, COMT/MAO resistance, receptor action table (with textbook quotes)
3. Adverse Effects — each effect with a direct textbook quote, source citation, and mechanistic reason (hypertension, tachyphylaxis, insomnia, urinary retention, cardiovascular toxicity, arrhythmias, drug interactions)
4. MOA Clarification Summary Table
5. Pharmacokinetics
6. Clinical Uses + Dosing
7. Ephedrine vs. Epinephrine comparison

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🫁 High-Flow Nasal Cannula (HFNC) — Complete Note

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1. What is HFNC?

"High flow nasal cannula (HFNC) consists of oxygenated gas heated and humidified to body conditions, delivered to the nares at flows up to 40 to 80 L/min." — Murray & Nadel's Textbook of Respiratory Medicine

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2. Components of HFNC

"Gas is delivered through an air-oxygen blender system to achieve a FiO₂ of up to 100%. Humidification of gases is a standard addition and is required for tolerability of the flow rates... heated to increase humidity to nearly 100% at 37°C." — Murray & Nadel's Textbook of Respiratory Medicine

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3. Physiological Mechanisms / Principles

3.1 High FiO₂ Delivery

• Standard nasal cannula draws in room air during inspiration (dilutes FiO₂). HFNC flows at 40–80 L/min, which exceeds the patient's peak inspiratory flow → virtually all inspired gas comes from the device → accurate, high, predictable FiO₂.

"High flow rates more closely match patients' inspiratory flow and volume demands, so more inspired gas comes from the device than ambient air." — Rosen's Emergency Medicine

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3.2 Anatomical Dead Space Washout (CO₂ Clearance)

• The nasopharynx/oropharynx (≈150 mL) normally holds expired CO₂-rich gas which is re-inspired first in the next breath.
• HFNC's continuous high flow physically flushes this dead space with fresh O₂-rich gas, reducing CO₂ rebreathing and improving ventilatory efficiency.

"The high flow rates of HFNC also decrease physiologic dead space by reducing the fraction of rebreathed air in the upper airway, thereby providing a small fraction of ventilation support through carbon dioxide washout." — Murray & Nadel's Textbook of Respiratory Medicine

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3.3 Low-Level PEEP Generation

• The high flow through partially occluded nares creates a small but real positive end-expiratory pressure.
• Each 10 L/min increase in flow ≈ 0.3–1.0 cm H₂O of CPAP. At 60 L/min → up to 4–6 cm H₂O.
• This recruits collapsed alveoli and increases functional residual capacity (FRC).

"HFNC results in the application of a low level of CPAP. It has been variably estimated that each 10 L/min increase in flow generates from 0.3 to 1.0 cm H₂O CPAP." — Murray & Nadel's Textbook of Respiratory Medicine

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3.4 Reduced Work of Breathing

• Matching or exceeding inspiratory demand means the patient's respiratory muscles work less to pull gas.
• Studies show ↓ respiratory rate, ↓ minute ventilation, ↓ pressure-time product (measure of respiratory effort), with ↑ end-expiratory lung volume and more equitable ventilation distribution.

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3.5 Mucociliary Clearance & Airway Conditioning

• Conventional O₂ at >6 L/min dries out mucous membranes → epistaxis, crusting, patient discomfort.
• HFNC delivers gas at 37°C with ~100% relative humidity → keeps airways moist → enhances mucus mobilization and ciliary function.

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4. Apnoeic Oxygenation — Principle

What is Apnoeic Oxygenation?

"Apneic oxygenation is a physiologic phenomenon by which oxygen from the oropharynx or nasopharynx diffuses down into the alveoli as a result of the net negative alveolar gas exchange rate resulting from oxygen removal and carbon dioxide excretion during apnea." — Miller's Anesthesia, 10e

Physiology — Why Does It Work?

• O₂ leaves alveoli into blood faster than CO₂ enters alveoli from blood.
• This creates a net negative pressure gradient in the alveolus → alveolar volume shrinks slightly → this "vacuum" pulls gas from above (nasopharynx → trachea → alveoli).
• If O₂ is insufflated into the nasopharynx, it travels passively down this gradient into the alveoli without any breathing effort.

"Even in the absence of ventilation, oxygen is able to travel down the tracheobronchial tree to the alveoli and diffuse into the bloodstream... more oxygen leaves the alveoli than carbon dioxide enters. This creates a pressure gradient that causes oxygen to travel from the nasopharynx to the alveoli." — Roberts and Hedges' Clinical Procedures in Emergency Medicine

HFNC as Apnoeic Oxygenation (THRIVE)

• HFNC allows higher flow rates (up to 70 L/min) during apnoea vs. standard nasal cannula (max 15 L/min).
• This extends safe apnoea time AND improves CO₂ clearance (average CO₂ rise of only 1.1 mmHg/min vs. 3–4 mmHg/min with standard apnoeic oxygenation).

"THRIVE involves the administration of warmed, humidified oxygen, allowing higher oxygen flow rates — up to 70 L/min. These higher flows extend the safe apnea time even further and improve the clearance of carbon dioxide... In 25 patients with a difficult airway, THRIVE achieved a median apnea time without desaturation to <90% of 14 minutes, range 5 to 65 minutes." — Miller's Anesthesia, 10e

Practical Apnoeic Oxygenation with HFNC

• Place HFNC (or nasal cannula) beneath the preoxygenation mask during RSI
• At onset of apnoea: keep HFNC in place → oxygen continues passively flowing to alveoli
• Standard nasal cannula: 15 L/min → adequate but not humidified
• HFNC: 30–70 L/min → superior apnoeic oxygenation + CO₂ clearance

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5. Advantages of HFNC

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6. Disadvantages / Limitations of HFNC

"HFNC should not be used in place of NIV in patients needing primary ventilation support." — Murray & Nadel's Textbook of Respiratory Medicine

"Relative contraindications include depressed mental status, facial injury, inability to manage secretions, or respiratory arrest." — Rosen's Emergency Medicine

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7. Indications Summary

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• Miller's Anesthesia, 10e — Apnoeic oxygenation, THRIVE
• Roberts and Hedges' Clinical Procedures in Emergency Medicine — Apnoeic oxygenation mechanism, HFNC components
• Murray & Nadel's Textbook of Respiratory Medicine — HFNC physiology, advantages, indications, PEEP generation
• Rosen's Emergency Medicine — Clinical use, contraindications
• Barash Clinical Anesthesia, 9e — HFNC in acute respiratory failure
• Fishman's Pulmonary Diseases — Components, dead space clearance

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When Do We Use HFNC? (Simple Language)

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1. 🫁 Patient whose oxygen level is dangerously low (Acute Hypoxaemic Respiratory Failure)

• Pneumonia — lungs filled with infection/pus → can't pick up O₂
• COVID-19 — severe lung inflammation
• ARDS — both lungs badly injured, stiff, and flooded

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2. 🛏️ Patient just taken off the breathing machine (Post-Extubation)

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3. 💉 During intubation — keeping oxygen levels up while the doctor places the tube (Apnoeic Oxygenation / THRIVE)

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4. 🧬 Cancer patients / immune-compromised patients (Immunosuppressed)

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5. 😮‍💨 COPD patients who can't tolerate a face mask (NIV-Intolerant)

⚠️ NIV is still first choice for COPD with high CO₂. HFNC is an alternative, not a replacement.

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6. 👶 Children with bronchiolitis (RSV lung infection in babies)

⚠️ In very premature newborns, CPAP is still preferred over HFNC.

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Quick One-Line Summary Table

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🧲 Magnesium Sulphate (MgSO₄) in Anaesthesiology — Complete Note

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1. Basic Facts

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2. Mechanism of Action — How Magnesium Works

2a. Calcium Channel Antagonism (L and P channels)

• Mg²⁺ competes with Ca²⁺ at voltage-gated calcium channels.
• At the neuromuscular junction (NMJ): blocks P-type calcium channels → reduces Ca²⁺ entry into the nerve terminal → less acetylcholine (ACh) release → muscle relaxation.
• At the vascular smooth muscle: blocks L-type channels → vasodilation → ↓ blood pressure.

"Higher-than-normal concentrations of bivalent inorganic cations (e.g., magnesium) can block the entry of calcium through P channels and profoundly impair neuromuscular transmission. This mechanism is behind the typical muscle weakness and potentiation of muscle relaxants when magnesium sulfate is administered to treat preeclampsia." — Miller's Anesthesia, 10e

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2b. NMDA Receptor Antagonism (Analgesic Mechanism)

• Mg²⁺ is a physiological voltage-dependent blocker of the NMDA (N-methyl-D-aspartate) receptor channel.
• NMDA receptors are central to pain transmission and central sensitisation (wind-up pain).
• Blocking NMDA → ↓ central sensitisation → ↓ pain → ↓ opioid requirements.

"The mechanism of analgesia is thought to be mediated by NMDA receptor antagonism as well as regulation of calcium influx into the cell, resulting in suppression of neuropathic pain and inhibition of central sensitization, potentially making this drug useful in the opioid-tolerant patient." — Barash Clinical Anesthesia, 9e

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2c. NMJ — Presynaptic + Postsynaptic Effects

• Presynaptic: ↓ ACh release (via Ca²⁺ channel blockade)
• Postsynaptic: reduces sensitivity of the motor end-plate to ACh
• Combined → potentiates ALL non-depolarising muscle relaxants (NDMRs)

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2d. CNS Depression / Anticonvulsant

• Reduces excitatory neuronal transmission by NMDA blockade and Ca²⁺ antagonism
• Raises seizure threshold → prevents/treats eclamptic seizures

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2e. Smooth Muscle Relaxation

• Via calcium channel blockade in smooth muscle
• → Bronchodilation (airway smooth muscle)
• → Uterine relaxation (tocolysis)
• → Vasodilation (↓ BP, useful in hypertensive crises)

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3. Uses in Anaesthesiology

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3.1 Pre-eclampsia and Eclampsia (Most Classic Use)

"Magnesium sulfate (4 g IV loading dose, followed by 1–2 g/hour) is considered first-line therapy for all cases of preeclampsia and eclampsia. It relaxes the smooth muscle (partly through calcium antagonism), leading to some decrease in blood pressure." — Rosen's Emergency Medicine

• Potentiates NDMRs → reduce dose of rocuronium/atracurium by 30–50%
• Use objective neuromuscular monitoring (TOF) — do not rely on clinical assessment alone
• Sugammadex still works for reversal of rocuronium
• Extubate carefully — residual neuromuscular block risk is real

"In the case of administration of magnesium sulfate, a distinct potentiation of the effect of any nondepolarizing agents occurs, with subsequently prolonged recovery time... neuromuscular monitoring based on an objective monitoring technique should be used." — Miller's Anesthesia, 10e

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3.2 Perioperative Analgesia (Opioid-Sparing)

"IV magnesium infusion decreases pain scores, is opioid sparing in the first 24 hours following surgery and is devoid of any serious adverse effects." — Barash Clinical Anesthesia, 9e

• Opioid-tolerant/opioid-dependent patients
• Chronic pain patients
• Patients at high risk for opioid-related adverse effects

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3.3 Attenuation of Haemodynamic Responses

• Laryngoscopy and intubation → Mg given before induction ↓ hypertensive and tachycardic response
• Surgical stimulation
• Pneumoperitoneum (laparoscopic surgery)

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3.4 Bronchospasm / Intraoperative Bronchospasm

"Mechanisms [of magnesium in bronchospasm] include calcium channel-blocking properties, inhibition of cholinergic neuromuscular transmission, stabilization of mast cells and T-lymphocytes, and stimulation of nitric oxide and prostacyclin." — Rosen's Emergency Medicine

• Used as adjunct in acute severe asthma and intraoperative bronchospasm refractory to bronchodilators.
• Dose: 1.2–2 g IV over 20 minutes
• Also useful in status asthmaticus in ICU/emergency setting.

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3.5 Cardiac Arrhythmias — Torsades de Pointes

"Manifest cardiac arrest caused by torsades de pointes is managed by the intravenous administration of magnesium... magnesium may effectively control the arrhythmia without normalising the QT interval." — Braunwald's Heart Disease, 15e

• Torsades de Pointes (TdP): life-threatening polymorphic VT in setting of prolonged QT
• Dose: 1–2 g IV over 5–10 minutes (bolus) → then infusion if needed
• Also used for digoxin toxicity arrhythmias and hypomagnesaemia-induced arrhythmias

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3.6 Fetal Neuroprotection (Obstetric Anaesthesia)

"Benefits have been shown for antenatal use for its fetal neuroprotective effects... fetoprotective effects probably result from noncompetitive antagonism of the NMDA receptor or through antiapoptosis and prevention of neuronal cell loss." — Roberts and Hedges' Clinical Procedures in Emergency Medicine

• Given to mothers with preterm labour (<32 weeks) to protect the fetal brain from hypoxic-ischaemic injury.
• Reduces incidence of cerebral palsy.

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3.7 Muscle Relaxant Potentiation (NMJ Effect)

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4. Pharmacokinetics

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5. Toxicity — Signs by Serum Level

"The first sign of magnesium toxicity, a decrease in the patellar reflex, typically occurs as serum magnesium levels exceed 4 mEq/L. The dosing and ongoing maintenance of magnesium therapy should be guided by the clinical status of the patient rather than by laboratory values." — Roberts and Hedges' Clinical Procedures in Emergency Medicine

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6. Monitoring During Mg Therapy

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7. Antidote for Magnesium Toxicity

"If respiratory depression develops, inject 10 mL of a 10% solution of calcium gluconate or calcium chloride over a 3-minute period as an antidote. For severe respiratory depression or arrest, prompt endotracheal intubation may be lifesaving." — Roberts and Hedges' Clinical Procedures in Emergency Medicine

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8. Contraindications

"Magnesium sulfate is contraindicated for treatment of eclampsia in myasthenia gravis because of its neuromuscular blocking effects; barbiturates or phenytoin should be used instead." — Rosen's Emergency Medicine

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9. Drug Interactions in Anaesthesia

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10. Quick Summary Table — Uses in Anaesthesiology

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• Miller's Anesthesia, 10e — NMJ mechanism, NDMR potentiation, obstetric use
• Barash Clinical Anesthesia, 9e — Perioperative analgesia, NMDA antagonism, opioid-sparing
• Roberts and Hedges' Clinical Procedures in Emergency Medicine — Toxicity signs, antidote, monitoring, dosing
• Rosen's Emergency Medicine — Bronchospasm mechanism, preeclampsia dosing, contraindications
• Braunwald's Heart Disease, 15e — Torsades de Pointes management
• Tintinalli's Emergency Medicine — Arrhythmia use, calcium antidote