anesthesia study guide

Larynx and Pharynx Anatomy and Innervation

  • Larynx
    • Motor
    • External Superior Laryngeal Nerve: Innervates the Cricothyroid muscle. Stimulation of the Superior Laryngeal Nerve leads to laryngospasm.
    • Recurrent Laryngeal Nerve: Innervates all other intrinsic laryngeal muscles.
    • Sensory
    • Internal Superior Laryngeal Nerve: Provides sensation above the vocal cords.
    • Recurrent Laryngeal Nerve: Provides sensation below the vocal cords.
    • Muscle Functions
    • Posterior Cricoarytenoid: Abducts vocal cords. "You take it out back."
    • Lateral Cricoarytenoid: Adducts vocal cords. "You bring it in from the side."
    • Cricothyroid: Tenses the vocal cords.
    • Thyroarytenoid: Relaxes the vocal cords.

Laryngeal Cartilages

  • Epiglottis
  • Superior Thyroid
  • Cuneiform (Posterior)
  • Corniculate (Posterior)
  • Arytenoids (Posterior)
  • Cricoid (Inferior)

Hemoglobin Physiology

  • Oxyhemoglobin Dissociation Curve
    • P50: The oxygen partial pressure at which hemoglobin is 50% saturated. Normal P50 = 26-27 mmHg.
    • Left Shift: Indicates increased affinity for O2 due to factors such as:
    • Decreased PCO2
    • Decreased H+
    • Increased pH
    • Decreased temperature
    • CO poisoning
    • Increased 2-3 DPG (Carboxyhemoglobin)
    • Sickle cell anemia
    • Fetal hemoglobin
    • Methemoglobin
    • Right Shift: Indicates decreased affinity for O2 with factors like:
    • Increased PCO2
    • Increased H+
    • Decreased pH
    • Increased temperature
    • Loss in affinity ("Bohr effect")

Gas Transport Summary

  • Dissolved Oxygen: Dissolved O2 = 0.003 X PaO2
  • Oxygen Bound to Hemoglobin: O2 bound to HgB = (1.34 X HgB) (SaO2)
  • Total Oxygen Content: Total O2 Content = (0.003 X PaO2) + (1.34 X HgB) (SaO2)
  • Oxygen Consumption: O2 Consumption = 250 ml/min (or 3-4 ml/kg/min)

CO2 Transport in Blood

  • Dissolved CO2: Dissolved CO2 = 0.067 X PaCO2
  • CO2 Produced and Eliminated: 200 ml/min
  • Normal CO2 Content: Arterial blood = 48 ml CO2/100 ml blood, Venous blood = 52 ml CO2/100 ml blood, Difference = 4 ml CO2/100 ml blood.

Control of Ventilation

  • Brainstem
    • Primary Respiratory Centers:
    • Dorsal Respiratory Group (DRG): Pacemaker; controls phrenic and external intercostals (Medulla)
    • Ventral Respiratory Group (VRG): Controls internal intercostals.
    • Secondary Respiratory Centers:
    • Apneustic Center: Regulates deep and prolonged respiration (Pons).
    • Pneumotaxic Center: Shuts off respiration (Hering-Breuer reflex, typically around 1.5 L).

Neurochemical Control of Respiration

  • Central Chemoreceptors: Located in medulla, respond primarily to changes in H+ and PCO2 in CSF.
  • Peripheral Chemoreceptors: Respond to O2 (when PaO2 < 60 mmHg).
    • Carotid: Innervated by Glossopharyngeal Nerve (CN IX)
    • Aortic: Innervated by Vagus Nerve (CN X)
  • Stretch Receptors: Respond to lung volume, inhibit further lung stretch via the vagus nerve.

Physiologic Responses to Ventilation Changes

  • Hering-Breuer Reflex: Inhibits over-stretching (normal tidal volume around 1.5L).
  • Intrapleural Pressure: Generally negative; becomes positive during Valsalva maneuver and forced expiration.
  • Intrapulmonary Pressure: Negative during inspiration; positive during expiration.

Ventilation and Perfusion (V/Q) Ratio

  • A normal V/Q ratio is approximately 0.8.
  • Physiologic Dead Space: Combines Anatomic and Alveolar Dead Space.
    • Anatomic Dead Space: Typically 2 ml/kg.
    • Alveolar Dead Space: Ventilation without perfusion.
  • Ventilation without perfusion indicates misallocated air and blood flow.

Respiratory Physiology Complications

  • Factors Affecting Pulmonary Compliance:
    • Conditions like obesity, pregnancy, and certain lung diseases (like pulmonary fibrosis) can alter compliance.
  • Minute Ventilation (MV): MV = Tidal Volume (TV) x Respiratory Rate (RR).
  • Compliance Formula: Compliance = ΔV / ΔP.

Pulmonary Function Tests

  • Obstructive Diseases (e.g., COPD, Asthma):
    • FEV1: For a normal person 4 L; more than 70% of FVC in normal.
    • FEV1/FVC Ratio: < 0.7 indicates obstructive disease.
  • Restrictive Diseases (e.g., pulmonary fibrosis):
    • Normal FEV1/FVC Ratio; FEV < 80% indicates decrease in FVC.

Diffusion and Gas Exchange

  • Control of Gas Exit: Factors like body temperature, pressure adjustments, and solubilities affect the degree of gas exchange and diffusion.

Anesthesia Considerations

Difficult Intubation Characteristics
  1. Short, muscular neck.
  2. Receding mandible.
  3. Protruding maxillary incisors.
  4. Uvula visualization difficulty.
  5. Limited temporomandibular joint mobility.
  6. Limited cervical mobility (flexibility).

Anesthesia Medications and Techniques

  • Aminophylline: A phosphodiesterase inhibitor, expands the airways, causing bronchodilation, improves diaphragmatic contractility.
  • Xanthines: Release norepinephrine; contraindicated with halothane.
  • Beta-2 Agonists: Activate adenylate cyclase, increasing cAMP and causing bronchodilation.
  • Cromolyn Sodium: A mast cell stabilizer preventing histamine release.

Pediatric and Geriatric Considerations

  • Differences in anatomy and physiology necessitate altered anesthetic doses and approaches.
    • Geriatrics: Generally slower metabolism, altered volume distribution, and sensitivity to agents.
    • Pediatrics: Higher metabolic rates, fluid management differences, and developmental disparities.

Conclusion

This guide encompasses various physiology, pharmacology, and anesthetic considerations ranging from oxygen transport to postoperative complications. The details provide comprehensive knowledge essential for practical applications in anesthesia and critical care settings.