2.1 Cardiac Anatomy, Electrophysiology & Hemodynamics
Key Takeaways
- The cardiac wall comprises the epicardium (carrying coronaries), myocardium (syncytium linked by intercalated discs and connexin 43 gap junctions), and endocardium; the subendocardium experiences the highest wall stress and is most vulnerable to ischemia.
- Coronary perfusion occurs primarily during diastole (~80-85%); the LAD supplies the anterior two-thirds of the septum and anterior LV wall, while the PDA (arising from the RCA in 85-90% of individuals) supplies the inferior wall and the solitary posteromedial papillary muscle.
- Fast-response action potentials comprise five phases (0 to 4) governed by fast sodium influx (Phase 0), transient potassium efflux (Phase 1), L-type calcium plateau influx triggering calcium-induced calcium release (Phase 2), delayed rectifier potassium repolarization (Phase 3), and resting potential (Phase 4).
- Nodal pacemaker automaticity is driven by Phase 4 spontaneous diastolic depolarization via inward funny currents (If) and low-threshold calcium channels without functional fast sodium channels.
- Stroke volume determinants include preload (Frank-Starling law), afterload (Laplace's law), contractility, and compliance; invasive hemodynamic profiles (CI, MAP, SVR, PVR, CVP, PAOP) guide clinical differentiation of shock states.
2.1 Cardiac Anatomy, Electrophysiology & Hemodynamics
[!NOTE] ANCC Blueprint Context: Domain I (Assessment and Diagnosis) requires cardiovascular nurses to command an in-depth understanding of normal and abnormal cardiac anatomy, coronary circulation territories, electrical conduction pathways, cellular ion dynamics, and invasive hemodynamic monitoring. These foundational concepts form the baseline for clinical reasoning during acute ischemia, heart failure, and cardiogenic shock.
Cardiovascular nursing requires an integrated understanding of gross cardiac anatomy, microvascular coronary perfusion, cellular electrophysiology, and invasive bedside hemodynamics.
Myocardial Wall Architecture & Pericardial Layers
The heart is enclosed within the pericardial sac and structured into three tissue layers:
- Pericardium: A fibroserous sac consisting of an outer fibrous pericardium (anchoring the heart to the sternum and diaphragm) and an inner serous pericardium. The serous pericardium has a parietal layer lining the fibrous sac and a visceral layer (epicardium) adhering to the myocardium. The pericardial space contains 15 to 50 mL of lubricating serous fluid. Rapid accumulation of fluid (as little as 150–250 mL acutely) exceeds pericardial compliance, causing cardiac tamponade.
- Epicardium (Visceral Pericardium): Outer connective tissue and adipose layer housing the main epicardial coronary arteries, coronary veins, autonomic nerve fibers, and lymphatics.
- Myocardium: The muscular middle layer composed of striated cardiomyocytes arranged in a helical syncytium. Cardiomyocytes are joined end-to-end by intercalated discs, which contain desmosomes (mechanical anchoring) and gap junctions (primarily Connexin 43) for low-resistance electrical coupling. Cardiomyocytes have a high mitochondrial volume (up to 40% of cell volume), reflecting an obligate dependence on aerobic oxidative metabolism.
- Endocardium & Subendocardial Layer: Smooth endothelial lining continuous with cardiac valves and vascular endothelium. The subendocardial space houses the Purkinje conduction network. Because intramyocardial compressive wall tension is highest at the endocardial border, and coronary perfusion flows from epicardium to endocardium, the subendocardium is the first tissue layer to suffer ischemia during hypoperfusion, presenting on the 12-lead ECG as ST-segment depression.
Coronary Circulation, Territories & Anatomical Dominance
Coronary arteries originate from the aortic sinuses of Valsalva. Because intramyocardial systolic pressure compresses intramural microvessels, approximately 80% to 85% of left ventricular coronary blood flow occurs during diastole. Tachycardia critically shortens diastolic filling time, impairing coronary perfusion.
Left Coronary Artery System
The Left Main Coronary Artery (LMCA) arises from the left aortic sinus and bifurcates into:
- Left Anterior Descending (LAD) Artery: Travels down the anterior interventricular sulcus. Its septal perforators supply the anterior two-thirds of the interventricular septum, bundle of His, and bundle branches. Its diagonal branches supply the anterior and anterolateral left ventricular wall and cardiac apex. Proximal LAD occlusion ("widowmaker") can infarct up to 40–50% of the left ventricle.
- Left Circumflex (LCx) Artery: Courses along the left AV groove. Its obtuse marginal (OM) branches supply the lateral and posterolateral LV free wall and left atrium. The LCx supplies the SA node in ~40% of individuals and the AV node in ~10%.
Right Coronary Artery System
The Right Coronary Artery (RCA) originates from the right aortic sinus and courses down the right AV groove. Key branches include:
- Conus Artery: Supplies the right ventricular outflow tract (RVOT).
- Sinus Node Artery: Originates from the proximal RCA in 55% to 60% of individuals.
- Acute Marginal Branches: Supply the right ventricular free wall.
- AV Nodal Artery: Arises at the crux of the heart in approximately 90% of individuals.
- Posterior Descending Artery (PDA): Courses along the posterior interventricular sulcus, supplying the posterior one-third of the interventricular septum, inferior LV wall, and the posteromedial papillary muscle.
Coronary Dominance & Papillary Muscle Vulnerability
Coronary dominance is defined by which vessel gives off the Posterior Descending Artery (PDA):
- Right-Dominant (85% to 90%): PDA arises from the RCA.
- Left-Dominant (8% to 10%): PDA arises from the terminal LCx.
- Co-Dominant (1% to 2%): Branches from both RCA and LCx share posterior perfusion.
[!IMPORTANT] Papillary Muscle Vulnerability: The anterolateral papillary muscle has a dual blood supply from the LAD and LCx. Conversely, the posteromedial papillary muscle relies on a single blood supply from the PDA. In acute inferior or posterior STEMI (usually RCA occlusion), the posteromedial papillary muscle is vulnerable to ischemic rupture, leading to catastrophic acute mitral regurgitation, flash pulmonary edema, and cardiogenic shock.
| Coronary Artery | Major Branches | Anatomical Territory Supplied | 12-Lead ECG Leads |
|---|---|---|---|
| LAD | Septals, Diagonals | Anterior LV wall, anterior 2/3 septum, apex, bundle branches | V1–V2 (Septal), V3–V4 (Anterior) |
| LCx | Obtuse Marginals | Posterolateral LV wall, lateral LV wall, left atrium | I, aVL, V5–V6 (Lateral) |
| RCA | SA nodal (60%), Acute Marginal, AV nodal (90%), PDA | Right atrium, right ventricle, inferior LV wall, posterior 1/3 septum, SA/AV nodes | II, III, aVF (Inferior); V3R–V4R (RV) |
| PDA | Posterolateral | Inferior/posterior LV wall, posterior septum, posteromedial papillary muscle | II, III, aVF; V7–V9 (Posterior) |
Cardiac Conduction System & Action Potential Dynamics
The conduction system coordinates cardiac contraction:
- Sinoatrial (SA) Node: Primary pacemaker in the high right atrium (intrinsic rate: 60 to 100 bpm).
- Internodal Pathways & Bachmann's Bundle: Rapid conduits traversing the right and left atria.
- Atrioventricular (AV) Node: Located in the triangle of Koch in the inferior interatrial septum (intrinsic rate: 40 to 60 bpm). Introduces a delay of 0.04 to 0.10 seconds, allowing atrial emptying to contribute an "atrial kick" (15% to 30% of total stroke volume) and protecting ventricles during rapid atrial arrhythmias.
- Bundle of His & Bundle Branches: Traverses the central fibrous skeleton, dividing into the Right Bundle Branch and Left Bundle Branch (which splits into the thin Left Anterior Fascicle [single LAD supply] and broad Left Posterior Fascicle [dual LAD/PDA supply]).
- Purkinje Fibers: Subendocardial network with fast conduction velocity (2 to 4 m/s) and an intrinsic escape rate of 20 to 40 bpm.
Fast-Response vs. Slow-Response Action Potentials
| Phase | Fast-Response Myocyte (Atria, Ventricles, Purkinje) | Slow-Response Pacemaker (SA & AV Nodes) |
|---|---|---|
| Phase 0 | Rapid Depolarization: Fast voltage-gated $Na^+$ channels open ($I_{Na}$); $V_m$ shifts from -90 mV to +20 mV. | Slow Depolarization: Inward $Ca^{2+}$ influx through L-type calcium channels ($I_{Ca-L}$); fast $Na^+$ channels are absent/inactive. |
| Phase 1 | Early Rapid Repolarization: Inactivation of fast $Na^+$ channels; transient outward $K^+$ efflux ($I_{to}$). | Absent. |
| Phase 2 | Plateau Phase: Inward $Ca^{2+}$ influx via L-type channels ($I_{Ca-L}$) balances outward $K^+$ efflux ($I_K$). Inward calcium triggers Calcium-Induced Calcium Release (CICR) from the sarcoplasmic reticulum via Ryanodine Receptors (RyR2). | Absent. |
| Phase 3 | Rapid Repolarization: Inactivation of $Ca^{2+}$ channels; outward $K^+$ efflux via delayed rectifier channels ($I_{Kr}, I_{Ks}$). | Repolarization: Outward delayed rectifier $K^+$ efflux ($I_K$). |
| Phase 4 | Resting Potential: Stable at -90 mV via $Na^+/K^+$-ATPase and inward rectifier $K^+$ channels ($I_{K1}$). | Spontaneous Diastolic Depolarization: Unstable resting potential (-60 mV). Inward funny currents ($I_f$) via HCN channels ($Na^+$ influx) and low-threshold T-type $Ca^{2+}$ influx generate automaticity. |
- Refractory Periods: The Absolute/Effective Refractory Period (ARP/ERP) (Phase 0 to mid-Phase 3) prevents tetanic contraction. The Relative Refractory Period (RRP) (late Phase 3) allows strong stimuli to trigger disorganized action potentials. The vulnerable period (R-on-T phenomenon) on the T-wave peak/downslope can provoke polymorphic VT or ventricular fibrillation.
Cardiac Cycle Mechanics & Stroke Volume Determinants
Cardiac output is calculated as $\text{CO} = \text{HR} \times \text{SV}$. Stroke volume (SV = End-Diastolic Volume - End-Systolic Volume, normally 60 to 100 mL/beat) is governed by four determinants:
- Preload: End-diastolic wall tension/stretch, estimated by End-Diastolic Volume. According to the Frank-Starling law, optimal sarcomere stretch (2.2 $\mu$m) increases troponin C calcium sensitivity and cross-bridge formation, augmenting stroke volume. Excessive volume overstretches myofilaments, causing plateauing of SV and congestion.
- Afterload: Impedance or resistance against which ventricles must contract. Governed by Laplace's Law: $\text{Wall Stress} = (P \times r) / (2h)$ (where $P$ is pressure, $r$ is radius, $h$ is wall thickness). Elevated afterload (SVR or PVR) increases myocardial work and reduces stroke volume.
- Contractility (Inotropy): Force and velocity of myocardial shortening independent of loading conditions, regulated by intracellular free $Ca^{2+}$ during Phase 2. Sympathetic $\beta_1$-adrenergic stimulation increases cAMP, enhancing inotropy and lusitropy (relaxation velocity).
- Compliance: Distensibility during diastole ($\Delta V / \Delta P$). A stiff LV (hypertrophy, ischemia, amyloidosis) exhibits reduced compliance, shifting the diastolic pressure-volume curve upward and causing elevated filling pressures with small volume increments.
Bedside Hemodynamic Parameters & Clinical Equations
| Parameter | Clinical Formula | Normal Range | Clinical Significance |
|---|---|---|---|
| Cardiac Output (CO) | $\text{HR} \times \text{SV}$ | 4.0 – 8.0 L/min | Total blood volume pumped into circulation per minute. |
| Cardiac Index (CI) | $\text{CO} / \text{BSA}$ | 2.5 – 4.2 L/min/m² | Standardized perfusion parameter; $<2.2\text{ L/min/m}^2$ indicates hypoperfusion. |
| Stroke Volume Index (SVI) | $\text{CI} / \text{HR} \times 1000$ | 33 – 47 mL/beat/m² | Volume ejected per beat indexed to BSA. |
| Mean Arterial Pressure (MAP) | $[\text{SBP} + (2 \times \text{DBP})] / 3$ | 70 – 105 mmHg | Driving capillary perfusion pressure; target $\ge 65$ mmHg in shock. |
| Central Venous Pressure (CVP) | Direct RA transducer reading | 2 – 8 mmHg | Right ventricular preload; elevated in RV failure and fluid overload. |
| Pulmonary Artery Pressure (PAP) | PA catheter measurement | 15–30 / 4–12 mmHg (Mean: 9–18) | Pulmonary vascular pressures; MPAP $>20$ mmHg defines pulmonary hypertension. |
| PAOP / PCWP | Balloon occlusion in distal PA | 6 – 12 mmHg | Left ventricular preload and LA pressure; $>18$ mmHg reflects pulmonary congestion. |
| Systemic Vascular Resistance (SVR) | $[(\text{MAP} - \text{CVP}) / \text{CO}] \times 80$ | 800 – 1200 dynes·s/cm⁵ | Left ventricular afterload; elevated in cardiogenic shock, low in distributive shock. |
| Pulmonary Vascular Resistance (PVR) | $[(\text{MPAP} - \text{PAOP}) / \text{CO}] \times 80$ | 50 – 250 dynes·s/cm⁵ | Right ventricular afterload; elevated in PE, hypoxia, and ARDS. |
| Mixed Venous Saturation ($SvO_2$) | Distal PA co-oximetry | 60% – 75% | Global balance between oxygen delivery ($DO_2$) and consumption ($VO_2$). |
[!TIP] Transducer Rules: Level the transducer air-fluid interface to the phlebostatic axis (fourth intercostal space, mid-axillary line). Record all vascular pressures (CVP, PAP, PAOP) at the end of expiration to eliminate respiratory pleural pressure artifacts.
A patient admitted with an acute ST-elevation myocardial infarction exhibits ST-segment elevations in leads II, III, and aVF. Twelve hours later, telemetry displays a heart rate of 38 beats/minute with normal P waves, a regular PR interval of 0.28 seconds, and occasional dropped QRS complexes following a progressive prolongation of the PR interval (Mobitz I / Wenckebach). Blood pressure is 88/54 mmHg. Which coronary artery is the primary culprit, and which anatomical structures are acutely ischemic?
A critical care patient with acute respiratory distress and anterior myocardial infarction is being monitored via a pulmonary artery catheter. The nurse records the following resting hemodynamic profile: Blood Pressure 86/52 mmHg, Heart Rate 112 bpm, CVP 16 mmHg, PAP 46/26 mmHg (Mean 33 mmHg), PAOP 24 mmHg, Cardiac Output 3.2 L/min, Cardiac Index 1.7 L/min/m², and SVR 1750 dynes·s/cm⁵. Which pathophysiological state is demonstrated, and what is the primary hemodynamic objective?
During the ventricular cardiomyocyte action potential, which cellular electrophysiological mechanism characterizes Phase 2 and directly enables mechanical myocyte contraction?