A Biomedical Engineering Study Guide: From Cellular Electrophysiology to Artificial Pacemakers
After completing this study guide, you should be able to:
The heart's intrinsic conduction system consists of specialized myocardial cells that generate and conduct electrical impulses. These cells display automaticity (the ability to spontaneously depolarize).
CARDIAC CONDUCTION SYSTEM
βββββββββββββββββββββββββββββββββββββββ
β Sinoatrial (SA) Node β β Primary Pacemaker (60-100 bpm)
β (Right atrium near SVC) β
ββββββββββββββββ¬βββββββββββββββββββββββ
β
βββββββββββββββββββββββββββββββββββββββ
β Atrioventricular (AV) Node β β Secondary Pacemaker (40-60 bpm)
β (Interatrial septum) β
ββββββββββββββββ¬βββββββββββββββββββββββ
β
βββββββββββββββββββββββββββββββββββββββ
β Bundle of His β
β (AV bundle) β
ββββββββββββββββ¬βββββββββββββββββββββββ
β
βββββββββββββββββββββββββββββββββββββββ
β Right & Left Bundle Branches β
ββββββββββββββββ¬βββββββββββββββββββββββ
β
βββββββββββββββββββββββββββββββββββββββ
β Purkinje Fibers β β Tertiary Pacemaker (20-40 bpm)
β (Ventricular myocardium) β
βββββββββββββββββββββββββββββββββββββββ
Hierarchy of intrinsic cardiac pacemakers with their typical intrinsic rates
The SA node normally serves as the primary pacemaker because it has the fastest intrinsic depolarization rate. Lower pacemakers are "overdrive suppressed" by faster upstream activity. If the SA node fails, lower pacemakers take over (escape rhythms).
The heart's pacemaker hierarchy represents a biological example of fault tolerance through redundancyβa critical concept in medical device design. Multiple backup systems ensure function even if the primary system fails.
Unlike contractile cardiomyocytes that maintain a stable resting potential, pacemaker cells slowly depolarize during diastole until they reach threshold and fire an action potential.
| Feature | Pacemaker Cells (SA node) | Contractile Cardiomyocytes (Ventricular) |
|---|---|---|
| Resting Membrane Potential | Unstable (-60 mV to -40 mV) | Stable (-85 mV to -90 mV) |
| Phase 4 (Diastole) | Slow depolarization (pacemaker potential) | Stable resting potential |
| Depolarization (Phase 0) | CaΒ²βΊ influx (L-type channels) | Fast NaβΊ influx |
| Repolarization | KβΊ efflux (delayed rectifier) | KβΊ efflux (multiple channels) |
| Automaticity | Yes (intrinsic) | No (requires stimulation) |
The pacemaker potential results from the interplay of three key currents:
Pacemaker cells function as biological oscillators. The "funny current" acts like a current source charging a capacitor (cell membrane), while potassium channels act as a discharge pathway. This creates a relaxation oscillator similar to electronic timer circuits.
Heart rate is dynamically regulated by the autonomic nervous system to meet metabolic demands:
AUTONOMIC REGULATION OF HEART RATE
βββββββββββββββββββββββββββββββββββββββββββββββ
β Sympathetic Stimulation (β Heart Rate) β
β β’ Norepinephrine β Ξ²β-adrenergic receptors β
β β’ β cAMP β β If and ICa currents β
β β’ β Slope of phase 4 depolarization β
β β’ β Time to reach threshold β
βββββββββββββββββββββββββββββββββββββββββββββββ
β
β
βββββββββββββββββββββββββββββββββββββββββββββββ
β Parasympathetic Stimulation (β Heart Rate) β
β β’ Acetylcholine β Mβ muscarinic receptors β
β β’ β cAMP β β If current β
β β’ β KβΊ conductance (IK,ACh) β
β β’ Hyperpolarization & β phase 4 slope β
βββββββββββββββββββββββββββββββββββββββββββββββ
Neurotransmitter effects on pacemaker cells
The ECG provides a non-invasive window into pacemaker function:
Artificial pacemakers replace or supplement the heart's intrinsic pacing system through electrical stimulation:
| Component | Function | Engineering Considerations |
|---|---|---|
| Pulse Generator | Contains battery, circuitry, and microcontroller | Low power consumption, long battery life (5-15 years), hermetic sealing |
| Leads | Conduct impulses to heart and sense intrinsic activity | Biocompatible materials, fatigue resistance, stable electrical interface |
| Electrodes | Interface with cardiac tissue | High surface area, low polarization, minimal fibrosis |
| Sensing Circuit | Detects intrinsic cardiac depolarizations | High input impedance, appropriate filtering, sensitivity/threshold programming |
| Output Circuit | Generates pacing pulses | Constant voltage/current, programmable amplitude and pulse width |
Standard 5-letter code describes pacemaker function:
Example: A VVI pacemaker paces the ventricle, senses the ventricle, and is inhibited by sensed ventricular events.
Question 1: Explain why the SA node serves as the primary pacemaker despite all pacemaker cells having automaticity.
Question 2: Compare the ionic mechanisms of phase 0 depolarization in SA node cells versus ventricular myocytes. Why does this difference matter clinically?
Question 3: A patient with complete heart block receives a VVI pacemaker. What does this code mean, and how would the pacemaker behave if intrinsic ventricular activity is detected at 50 bpm with a pacing rate set to 60 bpm?
Question 4: Describe how Ξ²-blockers (which block Ξ²-adrenergic receptors) affect SA node function at the cellular level. What ECG changes might you expect?
Question 5: As a biomedical engineer, what design considerations would you prioritize for a pacemaker lead intended for a pediatric patient who will grow significantly?