Action Potential (AP) in Heart Muscle
- Two Types:
- Myocardium & Purkinje fibers
- Nodal tissue (SA node & AV node)
Myocardium & Purkinje Fiber AP
Nodal Tissue (SA & AV Node) AP
- Starting Membrane Voltage: -65 mV.
Pacemaker Potential /
Diastolic Depolarization
Pre-potential / Pacemaker Potential / Diastolic Depolarization:
- Voltage rise (-65 mV → -40 mV)
- before AP upstroke
- SA node
- Pacemaker of the heart
- It generates impulses at the highest rate
- Mnemonic: Echis (HCN/H channel) in a Camp (Hyperpolarisation CAMP Sodium channels) → Nodiyidayil (Nodal) Funny (Funny channel) ayirunnu
Steps | Channels |
Slow Na+ entry | • HCN channels ↳ (funny channels/small H channels) • Ligand-gated (CAMP) & hyperpolarization-induced. • Responsible for spontaneous rhythm |
Ca2+ entry | • TT type calcium channels ↳ (transient rapid Ca2+ current) • Helps reach threshold |
K channels closes | • Gradual closure |
Threshold Voltage:
- -40 mV.
Action Potential
ㅤ | ㅤ | ㅤ |
Phase 0 | Depolarization | • Calcium (Ca2+) entry • L-type calcium channels ↳ (long-time, slow Ca2+ current) • Rapid rise (-40 mV → +10 mV). • Actual AP |
Phase 1 & 2 | Absent | ㅤ |
Phase 3 | Repolarization | • Return to -65 mV Cause • Ca2+ channels close • Potassium channels open → K+ exit |
Na⁺/K⁺ ATPase in Cardiac Muscle
- Maintains
- Ionic balance
- Resting membrane potential
Na⁺/K⁺ ATPase Inhibition
- Mechanism:
- Less sodium exits
- Less potassium enters
- Disrupts resting membrane potential.
- Result:
- Depolarization of cardiac cells
- Membrane potential becomes
- Less negative
- More positive
Depolarization & Repolarization in Whole Heart
- Atrium:
- Depolarization & Repolarization:
- Both start near SA node.
ㅤ | Ventricle Depolarization: | Ventricle Repolarization: |
Path | • Septum → Apex → Base | • Starts from epicardial surface ↳ (opposite to depolarization). • Apex → Base |
First part | • Middle part of (L) endocardial surface of IVS | • Apical epicardial surface |
Last part | • Uppermost IVS • Postero-basal epicardial surface of LV | • Base |
Cardiac Cycle
Cardiac Output (CO)
- Fick's Law Formula:
- Oxygen Consumption
(Arterial Oxygen Content - Venous Oxygen Content) - Gold standard for CO
- Invasive
- Requires pulmonary artery catheterization
- to get mixed venous blood
- Example
- O2 Consumption=300 mL/min
- Arterial O2=20 mL/dL
- Venous O2=16 mL/dL
- CO = 300 / (20-16) = 75 dL/min = 7.5 L/min.
- Echo, Thermodilution, Dye dilution method
- Also be used to find CO
Standard Formula | Formula |
Cardiac Output | Stroke Volume x Heart Rate |
Stroke Volume | EDV - ESV |
EDV | EDV = ESV / (1 - EF) |
ESV | ESV = EDV (1 - EF) |
Ejection Fraction | • (Stroke Volume / End Diastolic Volume) x 100 • Normal EF: 55 - 60% |
Bazett's Formula | • QTc (Corrected QT) = QT interval / √(RR interval) • Account for QT interval length variation with heart rate |
JVP (Jugular Venous Pulse)
- TR → Lancisi sign (Large V Wave) → Viva (V wa) Try () but Regret ()
- CP → Y da → Fried Chappathi
- CT & CP → Yes Da → Chapati + Chicken curry
Cardiac Tamponade
- Ewart sign
- Over the left infrascapular area.
- Dullness on percussion
- Bronchial breath sounds
- Large pericardial effusion
- Cause
- Left lower lobe compression
- Consolidation-like findings
Constrictive Pericarditis
- P a4 qrs1 cx 2Tdn v3y
HIS Bundle Electrocardiogram
Wave | Meaning |
A | AV node activation |
H | His bundle activation |
V | Ventricular depolarization |
Interval | Corresponding Event | Recording |
PA interval | SA node → AV node conduction time | Atrial depolarization → A wave |
AH interval | AV nodal conduction time | A wave → start of H spike |
HV interval | Conduction in His bundle and bundle branches | Start of H spike → Start of QRS in ECG |
Heart Sounds
Pressure-Volume (PV) Loop (Left Ventricle)
Shifts in PV Loop:
- Dicrotic notch → D/t closure of aortic valve
- P a4 qrs1 cx 2Tdn v3y
Wiggers Diagram
Cardiac Output (CO) Regulation
- Aortic Pressure = CO x TPR
- CO = Stroke Volume x Heart Rate
SV Regulation (Main Parameters):
Aspect | Preload | Afterload |
Definition | Load on ventricle → Initial stretch | Resistance to ejection |
SV Relationship | More preload → More SV | More afterload → Less SV |
Best Index | End-Diastolic Volume | Mean Aortic Pressure |
Best Determinant | ↑ Venous Return → ↑ EDV | Total Peripheral Resistance |
Frank-Starling Law
- ↑ Preload → ↑ Muscle fiber length → Stronger contraction
- More contractility → Higher SV
- Regulation:
- sympathetic ↑
- Parasympathetic ↓
Other Effects:
- Bowditch effect / Treppe phenomena
- ↑ HR → ↑ Contractility
- Anrep effect
- ↑ Afterload → Transient ↑ Contractility
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