🫀 Hemodynamics: Cardiac Cycle & Wiggers Profile
Cardiac physiology governs rhythmic mechanical contraction and hemodynamic perfusion driven by spontaneous sinoatrial pacemaker electrophysiology.
1. Cardiac Pacemaker vs Ventricular Action Potentials
| Phase | Sinoatrial (SA) Node Pacemaker | Ventricular Myocyte Action Potential |
|---|---|---|
| Phase 0 | Depolarization via L-type $Ca^{2+}$ channels (Cav1.2, slow upstroke) | Rapid depolarization via voltage-gated $Na^+$ channels (Nav1.5, fast upstroke) |
| Phase 1 | Absent | Transient early repolarization via transient outward $K^+$ current ($I_{to}$) |
| Phase 2 (Plateau) | Absent | Prolonged plateau (~200–300 ms) balancing inward $Ca^{2+}$ (Cav1.2) with outward $K^+$ ($I_{Kr}, I_{Ks}$) preventing tetanic contraction |
| Phase 3 | Repolarization via delayed-rectifier $K^+$ efflux ($I_K$) | Rapid repolarization via inward rectifier $K^+$ channels ($I_{K1}$) |
| Phase 4 | Unstable pacemaker potential: Funny current ($I_f$, hyperpolarization-activated HCN channels conducting $Na^+$) + T-type $Ca^{2+}$ | Stable resting potential (-90 mV) maintained by high resting $K^+$ conductance ($I_{K1}$) |
2. Wiggers Diagram & Mechanical Heart Phases
- Isovolumetric Contraction: Ventricular pressure exceeds atrial pressure → Mitral and Tricuspid (AV) valves snap shut (generating the First Heart Sound, S1 “lub”). All 4 valves closed; ventricular pressure skyrockets while volume remains constant (End-Diastolic Volume $approx$ 120 mL).
- Rapid Ejection: Ventricular pressure exceeds aortic/pulmonic pressure (~80 mmHg in left ventricle), forcing semilunar valves open. Blood shoots into aorta.
- Isovolumetric Relaxation: Ventricular pressure falls below aortic pressure → Aortic and Pulmonic semilunar valves snap shut (generating the Second Heart Sound, S2 “dub” with dicrotic incisura notch). End-Systolic Volume (ESV) $approx$ 50 mL.
- Stroke Volume ($SV$): $SV = EDV – ESV approx 120text{ mL} – 50text{ mL} = 70text{ mL}$. Ejection Fraction: $EF = frac{SV}{EDV} times 100% approx 58%$.