Key Clinical Formulas
Cardiac Output: CO = HR × SV
HR = heart rate (bpm), SV = stroke volume (mL/beat). Normal CO ≈ 4–8 L/min
HR = heart rate (bpm), SV = stroke volume (mL/beat). Normal CO ≈ 4–8 L/min
Mean Arterial Pressure: MAP = DBP + ⅓(SBP − DBP)
Normal MAP ≈ 70–100 mmHg. MAP = CO × SVR
Normal MAP ≈ 70–100 mmHg. MAP = CO × SVR
Ejection Fraction: EF = (EDV − ESV) / EDV × 100%
EDV = end-diastolic volume, ESV = end-systolic volume. Normal EF ≥ 55%
EDV = end-diastolic volume, ESV = end-systolic volume. Normal EF ≥ 55%
Fick Equation: CO = VO2 / (CaO2 − CvO2)
VO2 = oxygen consumption (mL/min), Ca-Cv = arteriovenous O2 difference
VO2 = oxygen consumption (mL/min), Ca-Cv = arteriovenous O2 difference
Systemic Vascular Resistance: SVR = (MAP − CVP) × 80 / CO
Normal SVR ≈ 800–1,200 dyn·s·cm⁻⁵
Normal SVR ≈ 800–1,200 dyn·s·cm⁻⁵
Core Concepts
1. Coronary Circulation
The heart's own blood supply via left and right coronary arteries. Atherosclerotic plaque buildup narrows vessels, causing angina or myocardial infarction (heart attack) when completely occluded.
The heart's own blood supply via left and right coronary arteries. Atherosclerotic plaque buildup narrows vessels, causing angina or myocardial infarction (heart attack) when completely occluded.
2. Cardiac Cycle
Systole (ventricular contraction/diastole (relaxation-filling). The Wiggers diagram illustrates pressure-volume relationships. Heart sounds: S1 (AV valve closure), S2 (semilunar valve closure).
Systole (ventricular contraction/diastole (relaxation-filling). The Wiggers diagram illustrates pressure-volume relationships. Heart sounds: S1 (AV valve closure), S2 (semilunar valve closure).
3. Electrical Conduction System
SA node (pacemaker) → AV node → Bundle of His → bundle branches → Purkinje fibers. Coordinates atrial then ventricular contraction. ECG records this electrical activity.
SA node (pacemaker) → AV node → Bundle of His → bundle branches → Purkinje fibers. Coordinates atrial then ventricular contraction. ECG records this electrical activity.
4. Heart Failure
The heart's inability to pump sufficient blood. Left-sided failure: pulmonary edema, dyspnea. Right-sided failure: peripheral edema, jugular venous distention. Classified by EF: HFrEF (≤40%), HFmrEF (41–49%), HFpEF (≥50%).
The heart's inability to pump sufficient blood. Left-sided failure: pulmonary edema, dyspnea. Right-sided failure: peripheral edema, jugular venous distention. Classified by EF: HFrEF (≤40%), HFmrEF (41–49%), HFpEF (≥50%).
5. Arrhythmias
Abnormal heart rhythms: atrial fibrillation (irregular, rapid atrial activity), ventricular fibrillation (chaotic, lethal without defibrillation), bradycardias, and heart blocks.
Abnormal heart rhythms: atrial fibrillation (irregular, rapid atrial activity), ventricular fibrillation (chaotic, lethal without defibrillation), bradycardias, and heart blocks.
Core Definitions
Myocardial Infarction (MI): Death of cardiac muscle due to prolonged ischemia from coronary artery occlusion. STEMI: ST elevation on ECG, complete occlusion. NSTEMI: partial occlusion, elevated troponin.
Fractional Shortening (FS): FS% = (LVEDD − LVESD) / LVEDD × 100%. Normal range 25–45%. Used to assess systolic function by echocardiography.
Afterload: The resistance the heart must overcome to eject blood. Determined mainly by systemic vascular resistance and arterial pressure.
Preload: The volume of blood in the ventricle at end-diastole (LVEDV). Governed by venous return and blood volume. Starling's law: increased preload increases SV (up to a point).
STEMI Equivalents: ECG patterns indicating acute coronary occlusion without ST elevation: de Winter T-waves, Wellens syndrome, posterior MI, and new LBBB with clinical symptoms.
Worked Examples
Example 1: Cardiac Output Calculation
HR = 75 bpm, SV = 70 mL: CO = 75 × 70 = 5,250 mL/min = 5.25 L/min
HR = 75 bpm, SV = 70 mL: CO = 75 × 70 = 5,250 mL/min = 5.25 L/min
Example 2: MAP from Blood Pressure
BP = 120/80 mmHg: MAP = 80 + ⅓(120−80) = 80 + 13.3 = 93.3 mmHg
BP = 120/80 mmHg: MAP = 80 + ⅓(120−80) = 80 + 13.3 = 93.3 mmHg
Example 3: Ejection Fraction
EDV = 120 mL, ESV = 50 mL: EF = (120−50)/120 × 100 = 70/120 × 100 = 58.3% (normal)
EDV = 120 mL, ESV = 50 mL: EF = (120−50)/120 × 100 = 70/120 × 100 = 58.3% (normal)
Practice Problems
Problem 1: Calculate CO for HR = 60 bpm and SV = 80 mL.
Answer: CO = 60 × 80 = 4,800 mL/min = 4.8 L/min
Answer: CO = 60 × 80 = 4,800 mL/min = 4.8 L/min
Problem 2: A patient has BP 140/90. Calculate MAP.
Answer: MAP = 90 + ⅓(140−90) = 90 + 16.7 = 106.7 mmHg
Answer: MAP = 90 + ⅓(140−90) = 90 + 16.7 = 106.7 mmHg
Problem 3: What does EF of 30% indicate?
Answer: Severely reduced ejection fraction (HFrEF). Indicates systolic heart failure. Normal ≥ 55%.
Answer: Severely reduced ejection fraction (HFrEF). Indicates systolic heart failure. Normal ≥ 55%.
Problem 4: Differentiate STEMI from NSTEMI.
Answer: STEMI: complete coronary occlusion, ST elevation on ECG, urgent reperfusion (PCI). NSTEMI: partial occlusion, troponin elevation, no ST elevation, managed medically ± delayed intervention.
Answer: STEMI: complete coronary occlusion, ST elevation on ECG, urgent reperfusion (PCI). NSTEMI: partial occlusion, troponin elevation, no ST elevation, managed medically ± delayed intervention.
Problem 5: What is the pacemaker of the heart and its intrinsic rate?
Answer: SA node, 60–100 bpm. AV node backup: 40–60 bpm. Ventricular backup: 20–40 bpm.
Answer: SA node, 60–100 bpm. AV node backup: 40–60 bpm. Ventricular backup: 20–40 bpm.