12.3 Cardiac Conduction System & Cardiac Cycle

12.3 Cardiac Conduction System & Cardiac Cycle

The coordinated mechanical pumping of the human heart is governed by an exquisite electrical timing system. While skeletal muscle requires somatic motor nerve impulses to initiate contraction, the heart exhibits intrinsic autorhythmicity (automaticity): specialized non-contractile cardiac muscle cells generate spontaneous, rhythmic action potentials that propagate across the myocardium. This section examines the neuroelectrical pathways of the conduction system, correlates these impulses with electrocardiographic waveforms, details the mechanical phases of the cardiac cycle, and explores the hemodynamics governing cardiac output.

---

The Intrinsic Cardiac Conduction System

Approximately 1% of cardiac muscle fibers are specialized non-contractile autorhythmic (pacemaker) cells. Unlike contractile myocytes that maintain a stable resting membrane potential (around -90 mV) until externally stimulated, pacemaker cells possess an unstable resting membrane potential that continuously drifts toward threshold. This spontaneous drifting depolarization is termed the pacemaker potential.

Ionic Basis of the Pacemaker Potential

1. Slow Inward Sodium Flux ($I_f$ "Funny" Channels): Following repolarization, hyperpolarization-activated cyclic nucleotide-gated (HCN) cation channels open, permitting a slow, continuous influx of sodium ($Na^+$) ions while potassium ($K^+$) permeability declines. This gradual inward positive charge brings the membrane potential from -60 mV up toward threshold (-40 mV).
2. Transient Calcium Influx: As threshold approaches, voltage-gated transient T-type calcium ($Ca^{2+}$) channels open briefly, providing the final depolarizing boost.
3. Rapid Depolarizing Spike (L-Type Calcium Channels): At threshold (~ -40 mV), voltage-gated long-lasting L-type $Ca^{2+}$ channels open widely. A massive influx of extracellular $Ca^{2+}$ drives the rapid upstroke of the cardiac action potential (peaking at approximately +10 mV). Note the critical difference: skeletal muscle depolarization is driven by rapid $Na^+$ influx, whereas pacemaker depolarization is driven primarily by $Ca^{2+}$ influx.
4. Repolarization (Potassium Efflux): Voltage-gated $K^+$ channels open while $Ca^{2+}$ channels inactivate. Potassium rapidly diffuses out of the cell, returning the membrane potential to its negative baseline (-60 mV), which immediately re-triggers the opening of funny channels to initiate the next cycle.

The Five Sequential Components of Cardiac Conduction

Sequence of Electrical Excitation
1. Sinoatrial (SA) Node [Superior wall of right atrium] (~75 bpm pacemaker)
       │
       ▼ (Interatrial / Internodal Pathways & Bachmann's Bundle)
2. Atrioventricular (AV) Node [Inferior interatrial septum] (~0.10 s delay)
       │
       ▼
3. Atrioventricular (AV) Bundle (Bundle of His) [Superior interventricular septum]
       │
       ▼
4. Right and Left Bundle Branches [Interventricular septum to apex]
       │
       ▼
5. Purkinje Fibers (Subendocardial Network) [Apex sweeping upward into ventricular walls]

1. Sinoatrial (SA) Node ("The Primary Pacemaker"):
   • Location: A crescent-shaped cluster of autorhythmic cells situated in the superior anterolateral wall of the right atrium, immediately inferior to the entrance of the superior vena cava.
   • Electrophysiological Role: The SA node exhibits the fastest intrinsic rate of spontaneous depolarization in the entire heart (~75 to 100 action potentials per minute). Because it depolarizes first, it sets the sinus pacing rhythm for the rest of the heart. In a healthy resting individual, continuous parasympathetic stimulation delivered via the vagus nerve (CN X) releases acetylcholine to slow this intrinsic firing rate to approximately 70 to 75 beats per minute (vagal tone).
   • Propagation: The action potential spreads rapidly from the SA node across both atria through gap junctions within intercalated discs. Specialized internodal pathways traverse the right atrium, while Bachmann's bundle (the interatrial tract) conducts the impulse directly across to the left atrium, ensuring simultaneous, coordinated contraction of both atria from superior to inferior.
2. Atrioventricular (AV) Node ("The Delay Station"):
   • Location: Located in the posteroinferior interatrial septum immediately superior to the tricuspid valve.
   • The Critical 0.10-Second (100 ms) Delay: As the electrical wavefront reaches the AV node, impulse transmission slows down dramatically, imposing an obligatory delay of approximately 0.10 second. This delay occurs because AV nodal fibers have a substantially smaller cell diameter and contain significantly fewer gap junctions, creating higher internal electrical resistance.
   • Crucial Physiological Purpose: This 0.10-second delay is biologically essential: it provides sufficient time for the atria to complete their mechanical contraction (atrial systole) and fully empty their blood volume into the ventricles before the ventricles begin contracting. Without this delay, atria and ventricles would contract simultaneously, slamming the AV valves shut and preventing ventricular filling.
   • Backup Pacemaker: If the SA node is damaged or suppressed, the AV node can take over as secondary pacemaker, firing at an intrinsic rate of 40 to 60 impulses per minute (junctional / nodal rhythm).
3. Atrioventricular (AV) Bundle (Bundle of His):
   • Location: Located in the superior portion of the muscular interventricular septum.
   • The Sole Electrical Bridge: The atria and ventricles are structurally separated by the dense connective tissue of the cardiac fibrous skeleton, which acts as an electrical insulator preventing stray impulses from crossing between chambers. The AV bundle is the ONLY anatomical electrical connection between the atria and ventricles.
4. Right and Left Bundle Branches:
   • Location: Within the interventricular septum, the AV bundle bifurcates into two distinct cords: the Right Bundle Branch and the Left Bundle Branch.
   • Pathway: These branches plunge down the length of the interventricular septum toward the cardiac apex. The left bundle branch is thicker, rapidly distributing impulses to the massive left ventricle, while the right bundle branch courses to the right ventricle (with fibers traversing the moderator band to anterior papillary muscles).
5. Purkinje Fibers (Subendocardial Conducting Network):
   • Location and Structure: Large-diameter, barrel-shaped cardiac myocytes containing abundant glycogen and few myofibrils, equipped with exceptionally dense arrays of gap junctions. They constitute the fastest conducting tissue in the heart, transmitting impulses at 2 to 4 meters per second.
   • Pathway and Vector: Originating at the apex, Purkinje fibers turn sharply superiorly, fanning out into the subendocardial myocardium of both ventricular lateral walls. This anatomical distribution ensures that ventricular depolarization begins at the apex and sweeps upward toward the base of the heart. Consequently, ventricular contraction wrings the blood upward toward the great arterial outlets at the base of the heart (the pulmonary trunk and aorta).
   • Tertiary Pacemaker: In complete heart block where no supraventricular impulses reach the ventricles, Purkinje fibers fire at an intrinsic rate of 20 to 40 impulses per minute (idioventricular rhythm), which is typically insufficient to sustain normal cerebral perfusion.

Cardiac Conduction Pathway Reference Table

---

Electrocardiography (ECG / EKG): Waveforms & Intervals

An Electrocardiogram (ECG or EKG) is a composite clinical recording of all electrical action potentials generated by cardiac muscle cells during each heartbeat, recorded via surface electrodes attached to the patient's skin. It is critical to recognize that an ECG records electrical depolarization and repolarization events, NOT mechanical contractions directly; however, mechanical systole and diastole follow predictably in the wake of electrical excitation.

Standard Normal ECG Waveform (Lead II)
Voltage
   ^             R
   │            / \
   │           /   \
   │    P     /     \        T
   │  ┌───┐  /       \     ┌───┐
───┴──┘   └──Q       S─────┘   └───> Time
     ├───┤   ├───┬───┤     ├───┤
     P Wave     QRS        T Wave
     ├───────┤             ├───────┤
    PR Interval           QT Interval

The Characteristic ECG Waves and Deflections

1. P Wave: A small, smooth upward deflection measuring approximately 0.08 to 0.10 second in duration. It represents Atrial Depolarization initiated by the SA node and spreading across both atria. Atrial mechanical contraction (atrial systole) begins approximately 0.1 second after the P wave commences.
2. PR Interval: Measured from the beginning of the P wave to the beginning of the QRS complex (normally 0.12 to 0.20 second, corresponding to 3 to 5 small grid squares on standard ECG paper). It represents the total time required for the electrical impulse to travel from the SA node, depolarize the atria, traverse the AV node (accounting for the 0.10 s delay), and pass through the AV bundle into the ventricular bundle branches. Clinical Pearl: Prolongation of the PR interval beyond 0.20 second indicates delayed AV nodal conduction, diagnostic of first-degree atrioventricular block.
3. QRS Complex: A tall, sharp tripartite spike lasting less than 0.08 to 0.10 second (under 0.12 second). It consists of an initial downward deflection (Q wave), a tall upward spike (R wave), and a secondary downward deflection (S wave). The QRS complex represents Ventricular Depolarization immediately preceding ventricular systole. Its amplitude is vastly larger than the P wave because the ventricular muscle mass is immensely greater than the atrial muscle mass.
   • Where is Atrial Repolarization? The electrical repolarization of the atria occurs simultaneously during the identical time window as ventricular depolarization; however, because the voltage of ventricular depolarization is so overwhelmingly powerful, the subtle electrical signal of atrial repolarization is completely masked and hidden within the QRS complex.
4. ST Segment: The horizontal, isoelectric plateau line between the end of the S wave and the beginning of the T wave. It represents the period during which the entire ventricular myocardium is completely depolarized (corresponding to the prolonged plateau phase of the ventricular action potential). Clinical Pearl: Elevation of the ST segment above the isoelectric baseline is the hallmark electrocardiographic sign of acute transmural myocardial infarction (ST-Elevation Myocardial Infarction / STEMI); depression below baseline indicates subendocardial myocardial ischemia.
5. T Wave: A rounded, dome-shaped upward deflection measuring roughly 0.16 second. It represents Ventricular Repolarization. The T wave is broader and more blunted than the QRS complex because myocardial repolarization occurs more slowly and less synchronously than depolarization.
6. QT Interval: Measured from the beginning of the Q wave to the end of the T wave (normally 0.36 to 0.44 second at a heart rate of 70 bpm). It represents the total electrical duration of ventricular systole—encompassing both ventricular depolarization and ventricular repolarization. Clinical Pearl: Pharmacological or congenital prolongation of the QT interval predisposes patients to life-threatening polymorphic ventricular tachycardia known as Torsades de Pointes.

ECG Waveforms & Physiological Events Table

---

The Mechanical Cardiac Cycle: Phases & Heart Sounds

The cardiac cycle encompasses all electrical and mechanical events that take place during a single complete heartbeat. At a normal resting heart rate of 75 beats per minute, each complete cardiac cycle lasts approximately 0.8 second (calculated as $60\text{ seconds} / 75\text{ beats} = 0.8\text{ s/beat}$). The cycle is divided into two primary phases:

• Systole: The period of myocardial contraction and active blood chamber emptying.
• Diastole: The period of myocardial relaxation and passive chamber filling.

Timeline of the 0.8-Second Cardiac Cycle
0.0 s ───────────── 0.4 s ───────── 0.5 s ───────────────────── 0.75 s ─── 0.8 s
[  Ventricular Filling   ][ Isovol.  ][   Ventricular Ejection   ][ Isovol. ]
[ (Mid-to-Late Diastole) ][ Contract.][        (Systole)         ][ Relax.  ]
  AV Open; SL Closed        ALL Closed    AV Closed; SL Open        ALL Closed
  80% Passive + 20% Kick    ★ S1 Sound                              ★ S2 Sound

The Four Consecutive Phases of the Cardiac Cycle

Phase 1: Ventricular Filling (Mid-to-Late Diastole, ~0.4 Second)

• Mechanical State: Atria and ventricles are both relaxed. Ventricular pressure is extremely low (0 to 5 mmHg), significantly below the venous pressure in the atria. Consequently, the atrioventricular (tricuspid and mitral) valves hang loosely open, while the semilunar valves are tightly closed.
• Rapid Passive Filling (80%): Blood returning through the venae cavae and pulmonary veins flows continuously through the relaxed atria directly into the ventricles. Approximately 80% of total ventricular filling occurs passively during this quiet diastolic period.
• Atrial Systole / "Atrial Kick" (20%): Following SA node depolarization (marked by the P wave on the ECG), the atria contract (atrial systole, lasting ~0.1 s). Atrial contraction squeezes the remaining 20% of blood into the ventricles. The maximum volume of blood contained within each ventricle at the conclusion of ventricular diastole is designated the End-Diastolic Volume (EDV), which averages 120 to 130 mL in an adult at rest.

Phase 2: Isovolumetric Contraction (Ventricular Systole Onset, ~0.05 Second)

• Mechanical State: Ventricular depolarization (marked by the QRS complex) triggers powerful myocyte contraction. As intraventricular pressure spikes sharply above atrial pressure, blood surges backward toward the atria, slamming the atrioventricular (tricuspid and mitral) valves firmly shut.
• Genesis of the First Heart Sound ($S_1$): The abrupt closure of the AV valves and the reverberating vibrations of the ventricular walls and tense chordae tendineae produce the First Heart Sound ($S_1$, the "lub"). The $S_1$ sound is louder, longer, and deeper-pitched than $S_2$, signaling the beginning of ventricular systole.
• The Isovolumetric Period: For approximately 0.05 second, ventricular pressure exceeds atrial pressure (keeping AV valves closed) but has not yet climbed high enough to exceed arterial pressure in the aorta (~80 mmHg) or pulmonary trunk (~10 mmHg) to open the semilunar valves. All four cardiac valves are closed. Because blood is an incompressible liquid and cannot escape, the volume of blood inside the ventricles remains strictly unchanged (isovolumetric), while intraventricular pressure climbs steeply.

Phase 3: Ventricular Ejection (Ventricular Systole Peak, ~0.25 Second)

• Mechanical State: Left ventricular pressure exceeds 80 mmHg (and right ventricular pressure exceeds 10 mmHg), overcoming arterial back-pressure and forcing the aortic and pulmonary semilunar valves to burst open.
• Blood Ejection: Blood is rapidly and forcefully propelled out of the ventricles into the aorta and pulmonary trunk. Peak systolic pressure reaches approximately 120 mmHg in the left ventricle (and ~25 mmHg in the right ventricle). As blood leaves, ventricular volume declines sharply.
• End-Systolic Volume (ESV): Ventricular ejection does not empty the chambers completely. The volume of blood remaining in each ventricle at the end of systole is termed the End-Systolic Volume (ESV), which averages 50 to 60 mL in a resting adult.

Phase 4: Isovolumetric Relaxation (Early Ventricular Diastole, ~0.05 Second)

• Mechanical State: Following ventricular repolarization (marked by the T wave on the ECG), the ventricles relax. Intraventricular pressure drops precipitously below the arterial pressure in the aorta and pulmonary trunk.
• Genesis of the Second Heart Sound ($S_2$): Blood in the aorta and pulmonary trunk briefly surges backward toward the heart, filling the crescent cusps of the semilunar (aortic and pulmonary) valves and snapping them shut. The abrupt closure of the semilunar valves generates the Second Heart Sound ($S_2$, the "dup"). The $S_2$ sound is sharper, shorter, and higher-pitched than $S_1$, signaling the start of ventricular diastole. On an aortic pressure tracing, the brief arterial pressure rebound caused by semilunar valve closure creates a characteristic notch termed the dicrotic notch.
• The Isovolumetric Period: For roughly 0.05 second, intraventricular pressure remains higher than atrial pressure (keeping AV valves closed) but lower than arterial pressure (keeping SL valves closed). All four valves are closed once again. Ventricular volume remains constant at the ESV level while internal pressure plummets. When ventricular pressure drops below atrial pressure, the AV valves fall open, and Phase 1 (ventricular filling) begins the cycle again.

Summary Table of Cardiac Cycle Phases

---

Hemodynamics & Cardiac Output Dynamics

Cardiac Output (CO) is defined as the volume of blood ejected by each ventricle into the systemic or pulmonary circuit per minute. It represents the ultimate measure of cardiac pumping efficacy.

The Cardiac Output Equation

Cardiac output is calculated as the product of Heart Rate (HR) and Stroke Volume (SV): $\text{Cardiac Output (CO)} = \text{Heart Rate (HR)} \times \text{Stroke Volume (SV)}$

• Heart Rate (HR): The number of cardiac contraction cycles per minute (normal resting average: 75 beats/min).
• Stroke Volume (SV): The volume of blood pumped out by one ventricle with each single beat (normal resting average: 70 mL/beat).

Calculating Normal Resting Values

$\text{CO} = 75\text{ beats/min} \times 70\text{ mL/beat} = 5,250\text{ mL/min} = 5.25\text{ L/min}$ Because normal adult blood volume is approximately 5.0 liters, the heart pumps an entire human blood volume through the systemic and pulmonary circuits every single minute at rest! During strenuous aerobic exercise in trained athletes, cardiac output can increase four- to seven-fold (reaching 20 to 35 L/min). The difference between a person's maximum cardiac output and their resting cardiac output is termed the cardiac reserve.

Stroke Volume Calculation & Ejection Fraction

Stroke volume is determined mathematically by subtracting the blood remaining after contraction from the blood accumulated before contraction: $\text{Stroke Volume (SV)} = \text{End-Diastolic Volume (EDV)} - \text{End-Systolic Volume (ESV)}$ $\text{SV} = 120\text{ mL} - 50\text{ mL} = 70\text{ mL/beat}$

• Ejection Fraction (EF): The percentage of end-diastolic blood volume that is actively ejected during systole: $\text{Ejection Fraction (EF)} = \left(\frac{\text{SV}}{\text{EDV}}\right) \times 100 = \left(\frac{70\text{ mL}}{120\text{ mL}}\right) \times 100 \approx 58\%$ In clinical cardiology, a normal resting ejection fraction ranges from 55% to 65%. An ejection fraction below 40% serves as a primary diagnostic hallmark of systolic heart failure.

The Three Primary Factors Regulating Stroke Volume

Stroke volume is dynamically regulated on a beat-to-beat basis by three physiological variables: preload, contractility, and afterload.

Determinants of Stroke Volume
┌────────────────────────────────────────────────────────┐
│ 1. Preload (↑ Venous Return → ↑ EDV → ↑ Stretch → ↑ SV) │
│    ★ Governed by Frank-Starling Law of the Heart       │
├────────────────────────────────────────────────────────┤
│ 2. Contractility (Inotropic State, independent of EDV) │
│    ★ Positive Inotropes (Ca2+, Sympathetic) → ↑ SV     │
│    ★ Negative Inotropes (Acidosis, Beta Blockers) → ↓ SV│
├────────────────────────────────────────────────────────┤
│ 3. Afterload (Arterial back-pressure opposing ejection)│
│    ★ Hypertension / Aortic Stenosis → ↑ Afterload → ↓ SV│
└────────────────────────────────────────────────────────┘

1. Preload (Degree of Stretch Before Contraction):
   • Definition: Preload represents the degree of mechanical stretch placed upon cardiac muscle cells immediately prior to contraction. In the intact heart, preload is directly proportional to the End-Diastolic Volume (EDV).
   • The Frank-Starling Law of the Heart: States that within physiological limits, the greater the volume of blood entering the ventricle during diastole, the greater the stretch of the ventricular muscle fibers, and the greater the force of contraction during systole. Under resting conditions, cardiac myocytes sit at a sarcomere length (approx. 1.8 to 2.0 micrometers) that is shorter than optimal. An increase in venous return (e.g., during exercise or leg elevation) fills the ventricle with more blood, stretching sarcomeres toward their optimal length (~2.2 micrometers). This optimizes actin-myosin cross-bridge overlap and increases troponin's affinity for $Ca^{2+}$, producing a significantly more forceful systolic contraction and increasing stroke volume.
   • Takeaway: Preload ensures that when venous return increases, cardiac output automatically surges to match it, balancing outputs between the right and left ventricles.
2. Contractility (Inotropic State):
   • Definition: Contractility is the intrinsic contractile strength developed by the myocardium at any given muscle length or preload. It is completely independent of muscle stretch or EDV and is determined by the availability of intracellular free ionized calcium ($Ca^{2+}$) within the cytoplasm.
   • Positive Inotropic Agents (Increase Contractility): Sympathetic nervous system activation releases norepinephrine (and adrenal medulla epinephrine), which bind to $\beta_1$-adrenergic receptors on myocytes. This activates cyclic AMP (cAMP) and protein kinase A, opening L-type calcium channels and stimulating calcium release from the sarcoplasmic reticulum. Elevated cytoplasmic $Ca^{2+}$ enables more cross-bridge cycles, dramatically increasing contractile force and emptying the ventricle more completely (reducing ESV and increasing SV). Other positive inotropes include glucagon, digitalis/digoxin, and hypercalcemia.
   • Negative Inotropic Agents (Decrease Contractility): Acidosis (excess $H^+$ ions compete with $Ca^{2+}$ for troponin binding), hyperkalemia, hypoxia, and pharmacological calcium channel blockers (e.g., diltiazem, verapamil) or beta-blockers impair $Ca^{2+}$ mobilization, dampening contractility and lowering stroke volume.
3. Afterload (Resistance to Ejection):
   • Definition: Afterload is the back-pressure exerted by arterial blood that the contracting ventricles must overcome to force open the semilunar valves and eject blood.
   • Hemodynamics: For the left ventricle, afterload is represented by the diastolic pressure within the aorta (normally ~80 mmHg); for the right ventricle, it is the diastolic pressure in the pulmonary trunk (normally ~8 mmHg).
   • Clinical Impact: In healthy individuals, afterload is relatively constant. However, in patients with chronic systemic hypertension (elevated arterial blood pressure) or aortic valve stenosis (narrowed aortic orifice), afterload rises substantially. The left ventricle must expend much more time and energy generating pressure just to crack open the aortic valve. Consequently, the duration of ejection shortens, more blood remains in the ventricle at the end of contraction (elevating ESV), and stroke volume declines. To compensate for chronically elevated afterload, the left ventricular myocardium undergoes pathological concentric hypertrophy, eventually progressing to heart failure.