Anatomy Relevant to Pacemakers

by Adam Lawson BA, MSc and Terra Doucette Hiller BA, BSN, RN

Conduction System of the Heart

The conductive tissues of the heart transmit electrical signals in a hierarchical fashion. For each heart beat, the signals originate in the sinu-atrial node and terminate in the ventricular myocardium.

Sinu-atrial node (a.k.a., sinoatrial node, SA node) - generates the initial electrical signals of sinus rhythm.
- It is located in the posterior wall of the right atrium near the entrance of the superior vena cava.
- It can receive blood from:
  - the right coronary artery exclusively (~80% cases)
  - the left coronary artery exclusively (~15% cases)
  - both the left and right coronary arteries (~5% cases)
- It rhythmically initiates impulses around 60-100 beats per minute (BPM).
- Electrical conduction from the SA node travels through the myocardium of the atrium causing the right and left atrium and the interatrial septum to contract simultaneously and depolarize.
Atrioventricular node (a.k.a., AV node) - conducts electrical signals from the depolarized atria to the ventricular nerve fibers.
- It is located at the bottom portion of the right atrium in the interatrial septum, between the tricuspid valve and the mitral valve and next to the coronary sinus.
- It can receive blood from:
  - the right coronary artery (~90% cases) - a specific branch called the AV nodal artery supplies the node in these cases.
  - the left circumflex artery (~10% cases)
- It delays conduction to allow time for ventricular filling and to receive all action potentials arriving from the atria.
  
  The refractory period of a functioning AV node prevents rapid irregular ventricular heart rhythms (e.g., ventricular defibrillation) by delaying the conduction of rapid atrial impulses.
  Heart Conduction
  (Click to view animation)
  
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Bundle of His - conducts electrical signals from the AV node to the bundle branches.
- It runs through the intraventricular septum and divides into a right and left bundle branch.
Bundle Branches
- Right bundle branch - runs along the right side of the septum to the heart apex and carries the impulse to the right ventricle.
- Left bundle branch - runs along the left side of the septum to the heart apex and carries the impulse to the left ventricle.
The left bundle branch, which is thicker than the right, branches from the bundle of His at almost a right angle and further divides into the anterior and posterior fascicles. There is also a central (median) fascicle division in about 60% of the population.
- The anterior fascicle travels to the anterolateral wall of the left ventricle and terminates in purkinje system.
- The posterior fascicle is an extension of the main bundle which further divides into smaller sections that travels through the inferoposterior portion of the left ventricle and terminates in the papillary muscle.
Purkinje fibers - conduct electrical signals from the bundle branches to the ventricular muscle.
- Purkinje fibers arise from contributions of the right bundle branch and the anterior and median fascicle of the anterior bundle branch.
- The purkinje fibers divide multiple times before terminating in the myocardial cells themselves in the ventricles.
- The fastest conduction velocity of all the heart tissue happens in the purkinje fibers, which contributes to the depolarization of the ventricular muscle.

Nervous Control of the Heart Rate

Extrinsic hormones and neurons can influence the heart rate.

Parasympathetic control - decreases heart rate.
- Rate changes can be modified by control centers found in the medulla oblongata.
- Vagus nerve - has branches which innervate cardiac tissue near the atria, SA node, and AV node.
- The ends of the parasympathetic fibers release acetylcholine.
Atropine administration will influence vagal tone (i.e., responsiveness) during symptomatic bradycardia and increase the heart rate at least 20-40 beats.
Sympathetic control - increases heart rate.
- Nerve fibers branching from the spinal cord at the level of T1-L2 vertebrae primarily innervate and influence the ventricular contraction rate.
- The sympathetic fibers release norepinephrine which increases the rate of nodal depolarization and strengthens myocardial contractility (i.e., a positive inotropic effect).
- Other sympathetic control comes from the release of norepinephrine and epinephrine into the bloodstream from the adrenal medulla.

Cardiopulmonary Receptors

Specialized cells at various locations in the body provide feedback that influences cardiopulmonary function.

Baroreceptors (pressure receptors) - monitor arterial pressure and arterial wall conformation and signal the nervous system to responsively raise or lower blood pressure, breathing rate, and heart rate.
- They are located in the:
  - Aortic arch - between the branching of the brachiocephalic trunk and the right common carotid artery.
  - Carotid sinuses - in the internal carotid arteries near their branching from the common carotid arteries.
The heart rate will decrease when blood pressure rises, and vice versa when the blood pressure decreases.
Chemoreceptors - monitor the concentrations of carbon dioxide and oxygen ions in:
- Blood flowing through the carotid arteries and aortic arch
- Cerebrospinal fluid surrounding the medulla oblongata
Chemoreception of Carbon Dioxide (CO₂)
- Chemoreceptors are especially responsive to CO₂ because the fluid concentrations of carbon dioxide are correlated to the concentration of hydrogen ion levels (i.e., acidity).
- CO₂ concentrations in the cerebrospinal fluid and blood are similar, as CO₂ easily diffuses across the blood-brain barrier.
- Increased CO₂ levels cause medullary regions to signal an increased breathing rate; these regions also signal the sympathetic ganglia to release norepinephrine at the SA node and thereby increase the heart rate. These combined effects increase the rate of CO₂ excretion during expiration.
- Chemoreception of low CO₂ levels causes the heart and breathing rate to slow and thereby decrease CO₂ excretion.
In the Bainbridge reflex (a.k.a., the atrial reflex):
1. Right atrial receptors monitor stretch of the right atrial wall resulting from increase in venous return.
2. Overstretching of the atrial wall signals the medulla.
3. Medullary control centers send signals that equalize pressures in the heart by inducing tachycardia.

Electrocardiogram

An electrocardiogram (a.k.a., ECG, EKG) monitors the electrical activity of the heart to diagnose and/or locate the site of ischemia, infarctions, or other life threatening cardiac events. The likelihood of detecting cardiac ischemia will increase if more EKG leads are used.

3 Lead EKG placement
(Click to view animation)

3 Lead EKG placement
- Uses three electrodes (white, black, and red)
- Gives three views of the heart
- Quick placement reference: White to the right, smoke over fire
- Lead I: refers to the lateral view of the heart, which includes activity of the left ventricle and left atrium.
- Lead II: refers to the inferior view of the heart, which includes activity of the left and right ventricle.
- Lead III: refers to the inferior view of the heart, which includes the activity of the right and left ventricles.
- Anterior wall myocardial ischemia cannot be adequately monitored with a three lead; a 5 lead EKG is better suited when ischemia is suspected.
5 Lead EKG placement
(Click to view animation)

5 Lead EKG placement
- Uses five electrodes (white, green, black, red, and brown)
- Gives seven views of the heart
- Quick placement reference: Clouds over grass, smoke over fire, brown in the middle
- Includes monitor of leads I, II and III (the same angles of the heart as described for 3-lead EKG).
- V1 is an additional view of the heart in a 5-lead EKG. It refers to both ventricles and the right atrium. It is an important lead in analyzing ventricular ectopy. V1 can be used to differentiate between the ventricular tachycardia (VT) and supraventricular tachycardia (SVT).
- The combination of V1 and Lead II can be used to differentiate between a right or left bundle branch block (RBBB vs. LBBB).
- All atrial dysrhythmias, ventricular dysrhythmias, and anterior wall ischemias can be observed and differentiated with a 5-lead EKG; monitoring posterior wall ischemia requires 12 leads.
12 Lead EKG placement
(Click to view animation)

12 Lead EKG placement
- Uses ten electrodes
- Gives twelve views of the heart
- Includes monitor of leads I, II and III.
- An additional 6 angles V1-V6 (refer to image for anatomical reference of each lead).
- Augmented leads - three additional angles: aVF, aVL and aVR. Combination of these leads with other lead angles allows for diagnosis of ischemic events in a variety of walls.
  - aVF refers to the inferior wall of the heart.
  - aVL refers to the lateral wall of the heart
  - aVR refers to the right ventricle of the of the heart.

References