Key Formulas & Equations
Cardiac Output: CO = HR × SV
Ejection Fraction: EF = (EDV - ESV)/EDV × 100%
Systemic Vascular Resistance: SVR = (MAP - CVP)/CO × 80
Osmolality: Osm = 2[Na⁺] + Glucose/18 + BUN/2.8
Corrected Calcium: Cacorr = Ca + 0.8(4 - albumin)
GFR (Cockcroft-Gault): GFR = (140 - age) × weight × (0.85 if female) / (72 × Cr)
Shunt Equation: Qs/Qt = (CcO₂ - CaO₂)/(CcO₂ - CvO₂)
Oxygen Content: CaO₂ = 1.34 × Hb × SaO₂ + 0.003 × PaO₂
PA-a Gradient: P(A-a)O₂ = PAO₂ - PaO₂
Body Surface Area (Du Bois): BSA = 0.007184 × W0.425 × H0.725
Ideal Body Weight: IBW = 50 + 2.3(inches > 60) (male); 45.5 + 2.3(inches > 60) (female)
Parkland Formula: Fluid = 4ml × weight(kg) × %TBSA
IV Flow Rate: Rate = Volume/Time
Clearance: Cl = Dose/AUC
Half-Life: t½ = 0.693 × Vd/Cl
Core Concepts
Homeostasis
The body's ability to maintain a stable internal environment despite external changes. Regulated by negative feedback loops involving the nervous and endocrine systems.
Pathophysiology
The study of functional changes in the body that result from disease processes. Explains how cellular injury leads to organ dysfunction and clinical symptoms.
Pharmacokinetics
The study of how the body processes drugs — absorption, distribution, metabolism, and excretion (ADME). Determines dosing regimens and drug interactions.
Evidence-Based Medicine
The integration of clinical expertise with the best available research evidence and patient values to make medical decisions. Uses systematic reviews and RCTs.
Immunology
Study of the immune system, including innate and adaptive immunity, antibodies, cytokines, and the body's defense against pathogens.
Genetics in Medicine
The study of heredity and gene expression in health and disease. Includes Mendelian inheritance, genetic mutations, and personalized medicine.
Clinical Reasoning
The cognitive process of analyzing patient data to arrive at a diagnosis. Includes differential diagnosis, pattern recognition, and hypothetico-deductive reasoning.
Medical Ethics
The moral principles guiding medical practice: autonomy, beneficence, non-maleficence, and justice. Guides end-of-life care, consent, and resource allocation.
Worked Examples
Example 1: Anion Gap Calculation
Problem: Patient labs: Na⁺ = 140, K⁺ = 4, Cl⁻ = 100, HCO₃⁻ = 14. Calculate anion gap and interpret.
Solution:
Anion Gap = 140 - (100 + 14) = 26 (elevated; normal = 8-12)
Elevated AG suggests metabolic acidosis with unmeasured anions (e.g., lactate, ketones, toxins).
Example 2: Cardiac Output
Problem: A patient has heart rate 80 bpm and stroke volume 70 mL/beat. Calculate cardiac output and index (BSA = 1.8 m²).
Solution:
CO = HR × SV = 80 × 70 = 5600 mL/min = 5.6 L/min
CI = CO/BSA = 5.6/1.8 = 3.1 L/min/m² (normal: 2.5-4.0)
Example 3: Drug Dosage Calculation
Problem: Order: Morphine 4 mg IV. Available: 10 mg/mL. How many mL to administer?
Solution:
Volume = (Desired/Have) × Volume = (4/10) × 1 mL = 0.4 mL
Practice Problems
Problem 1
Calculate MAP for BP 120/80 mmHg.
Answer: MAP = 80 + (120-80)/3 = 80 + 13.3 = 93.3 mmHg
Problem 2
A 70-year-old male (weight 80 kg, serum creatinine 1.2 mg/dL) needs GFR estimation. Use Cockcroft-Gault.
Answer: GFR = (140-70) × 80/(72×1.2) = 70×80/86.4 = 64.8 mL/min
Problem 3
Calculate BMI for a person weighing 90 kg and 1.75 m tall. Classify the result.
Answer: BMI = 90/(1.75)² = 90/3.06 = 29.4 kg/m² (overweight: 25-29.9)
Problem 4
A patient has Hb 10 g/dL, SaO₂ 95%, PaO₂ 100 mmHg. Calculate arterial oxygen content.
Answer: CaO₂ = 1.34 × 10 × 0.95 + 0.003 × 100 = 12.73 + 0.3 = 13.0 mL O₂/dL
Problem 5
Calculate the dose for a 15 kg child if the adult dose is 500 mg (using Clark's rule: child dose = weight/70 × adult dose).
Answer: Dose = (15/70) × 500 = 107 mg
Problem 6
A burn patient (70 kg) has 30% TBSA burns. Calculate total Parkland formula fluid for first 24 hours.
Answer: 4 × 70 × 30 = 8400 mL (half in first 8 hrs, half in next 16 hrs)
Problem 7
Calculate corrected calcium for a patient with Ca 7.5 mg/dL and albumin 2.5 g/dL.
Answer: Cacorr = 7.5 + 0.8(4 - 2.5) = 7.5 + 1.2 = 8.7 mg/dL