Cardiac Physiology and ECG Interpretation

Cardiac Excitability: Heart Rate and ECG

• Cardiac cells contract due to specific electrical signals.
• Understanding these signals helps study the heart's function and identify potential issues.
• Two main types of cardiac action potentials exist: non-pacemaker (myocyte) cells and pacemaker cells (autorhythmic).

Two Types of Cardiac Action Potentials

Type 1: Non-Pacemaker Cells (Myocytes)

• Characterized as "fast response" action potentials; rapid depolarization upon receiving a signal.
• Contractile cells need signals to contract.
• Make up most of the atrial and ventricular muscle walls.

Type 2: Pacemaker (Autorhythmic) Cells

• Unstable resting potential leads to spontaneous triggering.
• Non-contractile cells; "generals" providing signals that other cells follow.
• Found in the sinoatrial (SA) and atrioventricular (AV) nodes.

Action Potentials in Cardiac Autorhythmic Cells

• Funny current channels (If) allow both potassium (K+) and sodium (Na+) to influence the unstable resting potential.
• The pacemaker potential gradually becomes less negative until it reaches threshold, triggering an action potential.
• Ion movements are crucial during pacemaker and action potentials.

Role of Na+ and Ca2+

Role of Na+

• Rapid depolarization phase in cardiac muscle (non-pacemaker cells) due to sodium (Na+) channel opening.
• Gradual depolarization in pacemaker cells results in net sodium (Na+) influx.

Role of Ca2+

• Calcium (Ca2+) influx prolongs the duration of action potentials in cardiac muscle (non-pacemaker cells), creating a plateau phase.
• In pacemaker cells, calcium (Ca2+) is important in initial depolarization.

Electrical Conduction to Myocardial Cells

• Autorhythmic signals spread quickly to adjacent contractile cells via gap junctions.
• Electrical activity originates in the SA node and spreads through the heart.

All Cells of the Intrinsic Conduction System

• All intrinsic conduction cells can produce spontaneous action potentials, i.e., are autorhythmic.
• Structures include the SA node, AV node, bundle of His, bundle branches, and Purkinje fibers.

Nodes (Control Points)

• SA node: sets heart rate at roughly 70 bpm.
• AV node: routes electrical signals and delays their transmission.

Conductive Fibres

• Conductive fibres are often sheathed and separated from myocyte connections, except for specialized contact regions of atria and ventricles.
• Atrial and ventricular myocyte syncytia are separated by inert fibrous tissue; no gap junctions exist.

Heart Rate is Controlled by Both Sympathetic and Parasympathetic Nerves

• Parasympathetic activity slows heart rate; vagus nerve stimulates SA node.
• Sympathetic activity speeds heart rate; norepinephrine (NE) binds to beta-adrenergic receptors (BARs) on SA node cells.

Heart Rate Regulation - SA Node Action Potential Firing Rate

• Regulated by both sympathetic and parasympathetic fibers.
• Acetylcholine (Ach) binds to muscarinic receptors, lowering heart rate.
• Norepinephrine (NE) binds to beta-adrenergic receptors, raising heart rate.
• Various receptors and channels are involved at the cellular level.

Control of Heart Rate

• To increase heart rate (beyond intrinsic rate), sympathetic nerves need activation.
• Catecholamines released by the adrenal gland can also stimulate heart rate.

Modulation of Heart Rate by the Autonomic Nervous System

• Sympathetic stimulation causes pacemaker potentials to reach threshold quickly.
• Parasympathetic stimulation causes pacemaker potentials to reach threshold more slowly.

Review Question – Autorhythmic Cells

• Statement "c": The membrane potential is unstable, drifting between -90mV and -55 mV, is true.

Control of Heart Rate vs. Contraction Strength

• Vagus nerve activity (parasympathetic) influences heart rate, but not contraction force.
• Sympathetic fibers increase heart rate and contraction force.

ECG (Electrocardiogram): Normal Waves

• Three major waves: P wave, QRS complex, and T wave.

Electrical Activity – Overview

• The ECG tracing correlates with electrical events in the heart.
• Each wave represents specific electrical events in the heart.

Electrical Events of the Cardiac Cycle

• P wave: atrial depolarization.
• PQ or PR segment: AV nodal and A-V bundle conduction.
• QRS complex: ventricular depolarization.
• ST segment: ventricular depolarization.
• T wave: ventricular repolarization.

Comparison of ECG and Myocardial Action Potential

• ECG reflects summed electrical activity across the whole heart.
• Myocardial action potentials are recorded intracellularly.

Tips for ECG Analysis

• Analyze rate, rhythm, and wave forms.
• Verify that QRS appears with each P wave.
• Check both P-R and P-R timings.

ECG: Normal and Abnormal Electrocardiograms

• Various abnormal patterns represent different heart conditions.