2.1 Cardiac Anatomy, Coronary Circulation & Hemodynamics

2.1 Cardiac Anatomy, Coronary Circulation & Hemodynamics

Cardiovascular rehabilitation professionals require a rigorous foundation in cardiac anatomy, coronary topography, and cardiovascular hemodynamics. Clinical exercise prescription, continuous telemetry monitoring, and acute symptom evaluation all depend upon a precise understanding of myocardial blood supply and mechanical pump function.

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Cardiac Chamber Architecture & Valve Dynamics

The human heart operates as two coordinated in-series muscular pumps separated by the central fibrous skeleton. The right heart operates within a low-pressure, high-capacitance pulmonary circuit, whereas the left heart powers a high-pressure, high-resistance systemic circuit.

• Right Atrium (RA): Receives systemic venous return from the superior vena cava, inferior vena cava, and coronary sinus. Normal mean resting pressure is 2–6 mmHg. The RA contains the sinoatrial (SA) node at the superior vena caval junction and the fossa ovalis on the interatrial septum.
• Right Ventricle (RV): A crescent-shaped chamber that wraps around the anterior left ventricle. Its free wall is relatively thin (3–5 mm). Peak RV systolic pressure normally ranges between 15–25 mmHg, with an end-diastolic pressure of 0–8 mmHg. The RV ejects blood through the pulmonary valve into the low-resistance pulmonary vascular bed (mean pulmonary artery pressure: 10–20 mmHg).
• Left Atrium (LA): Receives oxygenated blood from four pulmonary veins. Normal mean LA pressure is 6–12 mmHg (mirrored clinically by pulmonary capillary wedge pressure, or PCWP). LA enlargement is a major predisposing substrate for atrial fibrillation.
• Left Ventricle (LV): A conical, thick-walled muscular chamber (8–12 mm in diastole). Normal LV peak systolic pressure is 100–130 mmHg, with an end-diastolic pressure (LVEDP) of 4–12 mmHg. The LV generates the mechanical force necessary to overcome systemic afterload.

Heart Valve Architecture & Auscultatory Correlates

Cardiac valves maintain unidirectional flow and are classified into two structural groups:

1. Atrioventricular (AV) Valves: The tricuspid valve (three leaflets: anterior, posterior, septal) separates the RA and RV. The mitral (bicuspid) valve (two leaflets: anterior and posterior) separates the LA and LV. Leaflets are tethered to subvalvular chordae tendineae, which originate from ventricular papillary muscles. Papillary muscle contraction during ventricular systole tensions the chordae, preventing leaflet eversion (prolapse) into the atria. Mitral valve closure produces the first heart sound (S1).
2. Semilunar Valves: The pulmonic valve (three cusps: anterior, left, right) and the aortic valve (three pocket-like cusps: right coronary, left coronary, non-coronary). Closure of the semilunar valves when ventricular pressure falls below arterial pressure at the onset of diastole produces the second heart sound (S2). Physiological splitting of S2 occurs during inspiration because increased venous return delays RV ejection and pulmonic valve closure ($P_2$), widening the interval from aortic closure ($A_2$).

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The Pericardium: Biomechanics & Pathology

The heart is enclosed within the pericardium, a fibroserous sac consisting of an outer tough fibrous pericardium and an inner two-layered serous pericardium:

• Parietal layer: Lines the internal surface of the fibrous pericardium.
• Visceral layer (Epicardium): Firmly adheres to the epicardial myocardial surface.
• Pericardial space: Contains 15–50 mL of clear transudative pericardial fluid that eliminates friction during cardiac excursion.

Clinical Pericardial Syndromes in Rehabilitation

• Acute Pericarditis: Inflammation characterized by sharp, pleuritic retrosternal chest pain that worsens when supine and improves when leaning forward. A high-pitched, scratchy pericardial friction rub is audible at the left sternal border. Electrocardiography (ECG) reveals hallmark widespread concave ST-segment elevation with reciprocal PR depression in lead aVR.
• Cardiac Tamponade: Accumulation of excess fluid under high pressure compromises ventricular diastolic filling. Signs follow Beck's triad: hypotension, jugular venous distention (JVD), and muffled heart sounds. Pulsus paradoxus—an abnormal inspiratory drop in systolic blood pressure exceeding 10 mmHg—is pathognomonic. Tamponade is an absolute emergency contraindicating exercise.

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Cardiac Conduction Hierarchy & Electrophysiology

Synchronized myocardial contraction depends upon a specialized, non-contractile conduction system:

1. Sinoatrial (SA) Node: Located in the epicardial groove at the junction of the superior vena cava and the high lateral right atrium. As the primary pacemaker, its intrinsic depolarization rate is 60–100 bpm due to spontaneous phase 4 diastolic depolarization mediated by pacemaker funny channels ($I_f$).
2. Internodal Pathways: Depolarization propagates across the RA via anterior (Bachmann), middle (Wenckebach), and posterior (Thorel) tracts. Bachmann's bundle crosses the interatrial septum to activate the left atrium.
3. Atrioventricular (AV) Node: Located subendocardially in the inferior right atrium at the apex of the triangle of Koch (bounded by the coronary sinus ostium, the septal tricuspid leaflet, and the tendon of Todaro). The AV node conducts slowly, introducing a physiological delay of 0.09–0.12 seconds. This pause allows complete atrial mechanical emptying ("atrial kick") to maximize end-diastolic ventricular filling. The intrinsic AV junctional pacemaker rate is 40–60 bpm.
4. Bundle of His & Bundle Branches: Depolarization rapidly crosses the non-conductive atrioventricular fibrous trigone via the Bundle of His. It bifurcates into the Right Bundle Branch (RBB) and the broader Left Bundle Branch (LBB). The LBB divides into a thin left anterior fascicle (traversing anterior LV) and a stout, dual-blood-supplied left posterior fascicle (traversing posterior-inferior LV).
5. Purkinje Fiber Network: Extensive subendocardial arborization exhibiting conduction velocities of 2–4 m/s (the fastest in the heart). This ensures near-instantaneous depolarization of ventricular myocardium from the endocardium outward and from the apex toward the base, optimizing mechanical ejection. The Purkinje network possesses an intrinsic escape rate of 20–40 bpm.

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Coronary Artery Topography & Vascular Territories

The coronary arterial tree originates from the aortic root immediately superior to the aortic valve cusps at the sinuses of Valsalva.

                    [Aortic Root]
                     /        \
        [Left Main / LMCA]     [Right Coronary / RCA]
           /          \           |-- Conus & Sinoatrial Nodal (60%)
          /            \          |-- Acute Marginal Branches (RV)
  [LAD]                 [LCx]     |-- AV Nodal Artery (90%)
   |-- Diagonals (LV)    |-- Obtuse Marginals (OM)  \-- Posterior Descending (PDA, 85%)
   \-- Septals (2/3)     \-- Posterolateral (PL)

Left Main Coronary Artery (LMCA) & Branches

The LMCA arises from the left aortic sinus, coursing between the pulmonary trunk and left atrial appendage before bifurcating into the LAD and LCx:

• Left Anterior Descending (LAD) Artery: Known colloquially as the "widowmaker." Travels down the anterior interventricular groove toward the apex. It provides:
  • Diagonal branches: Supply the anterolateral left ventricular free wall.
  • Septal perforators: Penetrate deeply to supply the anterior two-thirds of the interventricular septum, including the bundle of His and bundle branches.
  • Territory: Anterior wall, anteroseptal myocardium, cardiac apex, and bundle branch conduction system. Occlusion yields extensive anterior/septal STEMI (leads V1–V4), carrying high risk of acute pump failure, cardiogenic shock, and bundle branch block.
• Left Circumflex (LCx) Artery: Courses along the coronary sulcus around the left cardiac border to the posterior surface. Gives off:
  • Obtuse Marginal (OM) branches: Supply the lateral and posterolateral LV myocardium and the anterolateral papillary muscle.
  • Territory: Lateral and high lateral LV wall (leads I, aVL, V5, V6).

Right Coronary Artery (RCA) & Dominance

The RCA arises from the right aortic sinus, coursing down the anterior right atrioventricular groove:

• Conus artery & Sinoatrial (SA) Nodal Branch: Supplies the SA node in 60% of human hearts (the LCx supplies it in the remaining 40%).
• Acute Marginal Branches: Supply the right ventricular free wall.
• Atrioventricular (AV) Nodal Branch: Arises at the crux of the heart to supply the AV node in 90% of human hearts.
• Posterior Descending Artery (PDA): Runs in the posterior interventricular groove supplying the inferior wall of the LV and the posterior third of the interventricular septum.

Coronary Dominance Patterns

Coronary dominance is defined strictly by the artery giving origin to the posterior descending artery (PDA) and posterolateral branches:

• Right-Dominant (85% of population): PDA arises from the distal RCA.
• Left-Dominant (10% of population): PDA arises from the distal LCx.
• Co-Dominant (5% of population): RCA supplies the PDA, while the LCx supplies a parallel posterior descending or large posterolateral branch.

Clinical Exam Tip: Because the RCA supplies the inferior myocardium (leads II, III, aVF) and the AV node in 90% of individuals, inferior STEMI is frequently accompanied by vagally mediated or ischemic sinus bradycardia, Mobitz I (Wenckebach) AV block, or complete third-degree AV block.

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Coronary Perfusion Physiology & Diastolic Mechanics

Unlike all other systemic vascular beds, myocardial blood flow in the left ventricle occurs predominantly during ventricular diastole:

$\text{Coronary Perfusion Pressure (CPP)} = \text{Aortic Diastolic Pressure (AoDP)} - \text{Left Ventricular End-Diastolic Pressure (LVEDP)}$

1. Systolic Extravascular Compression: During ventricular systole, myocardial contraction generates high intramyocardial tissue compressive forces. In the subendocardium of the LV, tissue pressure approaches or exceeds intracavitary pressure (~120 mmHg), compressing and occluding intramural capillaries. Consequently, >80% of left ventricular coronary blood flow occurs during diastole.
2. Tachycardia & Diastolic Curtailment: At resting heart rates (60–70 bpm), diastole comprises approximately two-thirds (~65%) of the cardiac cycle, and systole comprises one-third. With exertional tachycardia (e.g., 140–160 bpm), diastolic duration shortens disproportionately to less than 35% of each cycle. This precipitous drop in diastolic filling time drastically curtails coronary perfusion time precisely when metabolic oxygen demand ($MVO_2$) surges.
3. Subendocardial Ischemia Vulnerability: The subendocardium is the most metabolically demanding and mechanically compressed layer, rendering it the first zone to become ischemic when coronary driving pressure falls or downstream stenoses prevent compensatory autoregulation.

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Cardiovascular Hemodynamics: Formulas & Clinical Calculations

Rehabilitation clinicians regularly calculate and interpret hemodynamic parameters during diagnostic testing and exercise sessions:

1. Cardiac Output (CO) & Cardiac Index (CI)

$\text{CO} = \text{Heart Rate (HR)} \times \text{Stroke Volume (SV)}$

• Normal Resting CO: 4.0 to 8.0 L/min.
• During maximal aerobic exercise in healthy trained individuals, CO can rise to 20–35 L/min, mediated by increases in both HR and SV.
• Cardiac Index (CI): Standardizes CO to Body Surface Area (BSA in $m^2$):

$\text{CI} = \frac{\text{CO}}{\text{BSA}} \quad (\text{Normal: } 2.5\text{--}4.0\text{ L/min/m}^2)$

A CI $<2.2\text{ L/min/m}^2$ indicates cardiogenic hypoperfusion.

2. Stroke Volume (SV) & Ejection Fraction (EF)

$\text{SV} = \text{End-Diastolic Volume (EDV)} - \text{End-Systolic Volume (ESV)}$

• Normal SV: 60 to 100 mL/beat.

$\text{EF (\%)} = \left( \frac{\text{SV}}{\text{EDV}} \right) \times 100 = \left( \frac{\text{EDV} - \text{ESV}}{\text{EDV}} \right) \times 100$

• Normal EF: 55% to 70% (indicates normal resting systolic function).

3. Determinants of Stroke Volume: Preload, Afterload, Inotropy

• Preload: The resting wall tension or stretch exerted on cardiomyocytes at the end of diastole, clinically reflected by EDV or LVEDP. Under the Frank-Starling law, increased venous return stretches sarcomeres toward their optimal length (~2.2 $\mu m$), expanding actin-myosin overlap and augmenting contractility and stroke volume up to a physiological ceiling.
• Afterload: The resistance or impedance against which the ventricle must eject blood during systole, clinically reflected by Systemic Vascular Resistance (SVR) and mean arterial pressure. According to the Law of Laplace:

$\text{Myocardial Wall Stress} (\sigma) = \frac{P \times r}{2h}$

(where $P$ is intraventricular pressure, $r$ is ventricular internal radius, and $h$ is wall thickness). Elevated afterload (e.g., severe hypertension, aortic stenosis) increases wall stress and oxygen demand, retarding stroke volume.

• Contractility (Inotropy): The intrinsic velocity and force of myocardial fiber shortening independent of preload and afterload, driven by intracellular calcium transient availability.

4. Rate Pressure Product (RPP) / Double Product

$\text{RPP} = \text{Heart Rate (bpm)} \times \text{Systolic Blood Pressure (mmHg)}$

(Often divided by 100 for clinical charting: $[\text{HR} \times \text{SBP}] / 100$).

• Normal Range: Resting RPP is typically 7,000–12,000 (70–120); peak exercise RPP exceeds 25,000 (>250).
• Clinical Significance: RPP correlates linearly with Myocardial Oxygen Consumption ($MVO_2$). In patients with stable coronary disease, myocardial ischemia develops at a consistent, highly reproducible RPP—the ischemic threshold. Exercise training heart rates must be prescribed at least 10 beats per minute below the heart rate at which ischemia occurs.

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Hemodynamic & Coronary Vascular Comparison Table

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Clinical Application: Exercise Prescription at the Ischemic Threshold

A 63-year-old post-CABG patient demonstrates exercise-induced horizontal ST depression (1.5 mm) at a heart rate of 134 bpm and blood pressure of 164/88 mmHg during an intake graded exercise test:

1. Ischemic RPP Calculation: $134 \times 164 = 21,976$ (or 219.8).
2. Prescription Directive: The maximum training heart rate for endurance conditioning must be capped at $134 - 10 = \mathbf{124\text{ bpm}}$ to maintain an adequate safety margin below the ischemic threshold.
3. Patient Education: Counsel the patient to avoid sudden isometric straining or upper-extremity overhead loads that cause rapid surges in systolic blood pressure, as sudden pressure spikes accelerate RPP toward the ischemic threshold even at modest heart rates.