Key Formulas & Clinical Formulas
Alveolar Gas Equation: PAO2 = FIO2 × (Patm − PH2O) − (PaCO2 / RQ)
where RQ (respiratory quotient) ≈ 0.8, PH2O = 47 mmHg, Patm = 760 mmHg
where RQ (respiratory quotient) ≈ 0.8, PH2O = 47 mmHg, Patm = 760 mmHg
Cardiac Output (Fick): CO = VO2 / (CaO2 − CvO2)
Systemic Vascular Resistance: SVR = (MAP − CVP) × 80 / CO
Systemic Vascular Resistance: SVR = (MAP − CVP) × 80 / CO
Ideal Body Weight (IBW): Male: 50 + 2.3 × (height in inches − 60); Female: 45.5 + 2.3 × (height in inches − 60)
Minimum Alveolar Concentration (MAC): The end-tidal concentration of inhaled anesthetic at 1 ATM that prevents movement in 50% of patients to surgical stimulus.
Fluid Maintenance (4-2-1 Rule): 4 mL/kg/hr for first 10 kg + 2 mL/kg/hr for next 10 kg + 1 mL/kg/hr for each kg above 20 kg
Core Concepts
1. General Anesthesia
A drug-induced reversible state of unconsciousness, amnesia, analgesia, and muscle relaxation. Components: hypnotics (propofol), opioids (fentanyl), muscle relaxants (rocuronium), and inhaled agents (sevoflurane).
A drug-induced reversible state of unconsciousness, amnesia, analgesia, and muscle relaxation. Components: hypnotics (propofol), opioids (fentanyl), muscle relaxants (rocuronium), and inhaled agents (sevoflurane).
2. Regional Anesthesia
Blocking sensation in a specific body region. Includes spinal anesthesia (subarachnoid injection), epidural anesthesia (catheter in epidural space), and peripheral nerve blocks (brachial plexus, femoral).
Blocking sensation in a specific body region. Includes spinal anesthesia (subarachnoid injection), epidural anesthesia (catheter in epidural space), and peripheral nerve blocks (brachial plexus, femoral).
3. ASA Physical Status Classification
ASA I: Healthy patient. ASA II: Mild systemic disease. ASA III: Severe systemic disease. ASA IV: Severe disease that is a constant threat to life. ASA V: Moribund patient not expected to survive without surgery.
ASA I: Healthy patient. ASA II: Mild systemic disease. ASA III: Severe systemic disease. ASA IV: Severe disease that is a constant threat to life. ASA V: Moribund patient not expected to survive without surgery.
4. Pharmacokinetics of Anesthetics
Uptake and distribution of inhaled anesthetics depend on blood-gas solubility coefficient, alveolar ventilation, and cardiac output. Less soluble agents (desflurane) have faster induction and emergence.
Uptake and distribution of inhaled anesthetics depend on blood-gas solubility coefficient, alveolar ventilation, and cardiac output. Less soluble agents (desflurane) have faster induction and emergence.
5. Airway Management
Skills include bag-mask ventilation, direct laryngoscopy, video laryngoscopy, supraglottic airways, and surgical airway (cricothyroidotomy). The difficult airway algorithm guides management when intubation fails.
Skills include bag-mask ventilation, direct laryngoscopy, video laryngoscopy, supraglottic airways, and surgical airway (cricothyroidotomy). The difficult airway algorithm guides management when intubation fails.
Core Definitions
Induction: The process of administering drugs to achieve a state of general anesthesia, typically via IV (propofol) or inhalation (sevoflurane in pediatrics).
Emergence: The transition from anesthetic unconsciousness to consciousness at the end of surgery, requiring careful reversal of neuromuscular blockade and hemodynamic stability.
Malignant Hyperthermia: A rare but life-threatening pharmacogenetic disorder triggered by volatile anesthetics and succinylcholine, causing hypermetabolism, muscle rigidity, and rapid temperature rise. Treatment: dantrolene.
Neuromuscular Blockade: Pharmacological paralysis using non-depolarizing agents (rocuronium, vecuronium) or depolarizing agents (succinylcholine). Reversed with neostigmine or sugammadex.
Local Anesthetic Systemic Toxicity (LAST): Excessive plasma concentration of local anesthetics causing perioral numbness, seizures, arrhythmias, and cardiovascular collapse. Treatment: intravenous lipid emulsion (Intralipid).
Worked Examples
Example 1: Alveolar Gas Equation
Calculate PAO2 with FIO2 = 0.5, PaCO2 = 40 mmHg:
PAO2 = 0.5 × (760 − 47) − (40/0.8) = 0.5 × 713 − 50 = 356.5 − 50 = 306.5 mmHg
Calculate PAO2 with FIO2 = 0.5, PaCO2 = 40 mmHg:
PAO2 = 0.5 × (760 − 47) − (40/0.8) = 0.5 × 713 − 50 = 356.5 − 50 = 306.5 mmHg
Example 2: Maintenance Fluid Calculation
For a 70 kg patient: (4 × 10) + (2 × 10) + (1 × 50) = 40 + 20 + 50 = 110 mL/hr
For a 70 kg patient: (4 × 10) + (2 × 10) + (1 × 50) = 40 + 20 + 50 = 110 mL/hr
Example 3: MAC Calculation
For a 45-year-old, Sevoflurane MAC ≈ 2.0%. For a 70-year-old: MAC decreases ~6% per decade → 2.0% × (1 − 0.18) ≈ 1.64%
For a 45-year-old, Sevoflurane MAC ≈ 2.0%. For a 70-year-old: MAC decreases ~6% per decade → 2.0% × (1 − 0.18) ≈ 1.64%
Practice Problems
Problem 1: A patient receives FIO2 = 0.8 and has PaCO2 = 50 mmHg. Calculate PAO2.
Answer: PAO2 = 0.8 × 713 − 62.5 = 570.4 − 62.5 = 507.9 mmHg
Answer: PAO2 = 0.8 × 713 − 62.5 = 570.4 − 62.5 = 507.9 mmHg
Problem 2: Calculate the maintenance fluid rate for a 25 kg child.
Answer: (4 × 10) + (2 × 10) + (1 × 5) = 40 + 20 + 5 = 65 mL/hr
Answer: (4 × 10) + (2 × 10) + (1 × 5) = 40 + 20 + 5 = 65 mL/hr
Problem 3: What is the first-line treatment for malignant hyperthermia?
Answer: Dantrolene 2.5 mg/kg IV, repeated until symptoms resolve. Also: stop triggering agents, hyperventilate with 100% O2, active cooling.
Answer: Dantrolene 2.5 mg/kg IV, repeated until symptoms resolve. Also: stop triggering agents, hyperventilate with 100% O2, active cooling.
Problem 4: Calculate MAP for BP 90/60 mmHg.
Answer: MAP = DBP + 1/3(SBP − DBP) = 60 + 1/3(30) = 70 mmHg
Answer: MAP = DBP + 1/3(SBP − DBP) = 60 + 1/3(30) = 70 mmHg
Problem 5: Which inhaled anesthetic has the lowest blood-gas solubility and therefore fastest emergence?
Answer: Desflurane (blood-gas partition coefficient 0.42), followed by nitrous oxide (0.47), sevoflurane (0.65).
Answer: Desflurane (blood-gas partition coefficient 0.42), followed by nitrous oxide (0.47), sevoflurane (0.65).