Electrocardiography (ECG) Notes

I. Introduction

• The heart functions as a dual pump, circulating blood through two circuits:

  • Pulmonary circuit: oxygenates blood in the lungs and removes carbon dioxide.

  • Systemic circuit: delivers oxygen and nutrients to tissues and removes carbon dioxide.

• Three types of cells are required for the heart to beat:

  • Rhythm generators: produce an electrical signal (SA node, the normal pacemaker).

  • Conductors: spread the pacemaker signal.

  • Contractile cells (myocardium): mechanically pump blood.

II. Electrical and Mechanical Sequence of a Heartbeat

• Cardiac tissue possesses inherent rhythmicity or automaticity due to specialized pacemaker cells.

• The sinoatrial node (SA node) generates an electrical signal that spreads through specific conducting pathways:

  • Internodal pathways and atrial fibers

  • Atrioventricular node (AV node)

  • Bundle of His

  • Right and left bundle branches

  • Purkinje fibers (Fig. 5.1)

• Systole: the mechanical event of contraction of contractile cells upon the arrival of a depolarization signal.

• Diastole: the mechanical event of relaxation of myocardial cells upon the arrival of a repolarization signal.

• Electrical signals precede and cause the mechanical pumping action of the heart (Fig. 5.2).

• The SA node initiates each electrical and mechanical cycle as the normal pacemaker.

• SA node depolarization leads to atrial muscle contraction.

• The AV node delays the electrical signal for approximately 0.20 seconds, allowing atrial contraction before relaying the signal to the ventricles.

• The signal is then relayed to ventricles via the bundle of His, right and left bundle branches, and Purkinje fibers.

• Purkinje fibers stimulate ventricular muscle contraction (ventricular systole).

• Ventricles repolarize and enter diastole during ventricular systole (Fig. 5.2).

• The autonomic nervous system modifies heart rate (beats per minute or BPM) and contraction strength through its sympathetic and parasympathetic divisions.

III. Sympathetic and Parasympathetic Influence

• Sympathetic division:

  • Increases automaticity and excitability of the SA node, increasing heart rate.

  • Increases conductivity of electrical impulses through the atrioventricular conduction system.

  • Increases the force of atrioventricular contraction.

  • Sympathetic influence increases during inhalation.

• Parasympathetic division:

  • Decreases automaticity and excitability of the SA node, decreasing heart rate.

  • Decreases conductivity of electrical impulses through the atrioventricular conduction system.

  • Decreases the force of atrioventricular contraction.

  • Parasympathetic influence increases during exhalation.

IV. Electrocardiogram (ECG)

• Echoes of depolarization and repolarization are detectable via electrodes placed on the body, producing an electrocardiogram (ECG).

• The ECG allows inference of the heart’s mechanical activity.

• Electrical activity varies through the ECG cycle (Fig. 5.2).

• The ECG represents electrical events (depolarization and repolarization), while ventricular systole and diastole represent mechanical events (contraction and relaxation).

• Electrical events happen quickly; mechanical events occur slowly.

• Mechanical events follow the electrical events that initiate them.

• Ventricular repolarization typically begins before the completion of ventricular systole in a resting Lead II ECG.

• ECG changes can diagnose interruptions in electrical signal generation or transmission.

• During exercise, heart position changes, complicating voltage standardization or quantification.

V. Components of the ECG

• The ECG pattern includes an isoelectric line (baseline), a P wave, a QRS complex, and a T wave.

• Intervals and segments are also components of the ECG (Fig. 5.2).

• Isoelectric Line:

  • It's the baseline, indicating periods when no electrical activity is detected by the ECG electrodes.

• Interval:

  • Time measurement including waves and/or complexes.

• Segment:

  • Time measurement not including waves and/or complexes.

VI. ECG Component Details and Lead II Values

• Table 5.1 lists components, what they represent, typical durations, and amplitudes.

• P wave:

  • Represents depolarization of the right and left atria.

  • Duration: 0.07 – 0.18 seconds.

  • Amplitude: < 0.25 millivolts.

• QRS complex:

  • Represents depolarization of the right and left ventricles.

  • Atrial repolarization is masked by the larger QRS complex.

  • Duration: 0.06 – 0.12 seconds.

  • Amplitude: 0.10 – 1.50 millivolts.

• T wave:

  • Represents repolarization of the right and left ventricles.

  • Duration: 0.10 – 0.25 seconds.

  • Amplitude: < 0.5 millivolts.

• P-R interval:

  • Time from onset of atrial depolarization to onset of ventricular depolarization.

  • 0.12-0.20 seconds.

• Q-T interval:

  • Time from the onset of ventricular depolarization to the end of ventricular repolarization, representing the refractory period of the ventricles.

  • 0.32-0.36 seconds.

• R-R interval:

  • Time between two successive ventricular depolarizations.

  • 0.80 seconds.

• P-R segment:

  • Time of impulse conduction from the AV node to the ventricular myocardium.

  • 0.02 – 0.10 seconds.

• S-T segment:

  • Period representing the early part of ventricular repolarization.

  • < 0.20 seconds.

• T-P segment:

  • Time from the end of ventricular repolarization to the onset of atrial depolarization.

  • 0.0 – 0.40 seconds.

• Tabled values are from a typical Lead II setup (wrist and ankle electrode placement) with a subject heart rate of ~75 BPM and are influenced by heart rate and placement.

VII. Leads

• A lead is an arrangement of two electrodes (one positive, one negative) with respect to a third (ground) electrode.

• Electrode positions are standardized for different leads.

• This lesson records from Lead II: positive electrode on the left ankle, negative electrode on the right wrist, and ground electrode on the right ankle.

• The QRS complex is the dominant ECG component in a normal standard lead record.

• In Lead II, Q and S waves are typically small and negative, while the R wave is large and positive (Fig. 5.2).

• Normal and abnormal factors influence QRS complex duration, form, rate, and rhythm.

• Normal factors: body size (BSA), body fat distribution, heart size (ventricular mass), heart position, metabolic rate, etc.

• In a person with a high diaphragm, the heart's apex shifts upward and to the left, altering the "electrical picture" and decreasing R wave positivity with increased S wave negativity.

• Athletes with larger left ventricular mass may show similar Lead II QRS complex changes without cardiac disease.

• An abnormal inverted QRS complex results when Lead II Q, R, and S wave amplitudes are all negative.

• Abnormal factors include hyper- and hypothyroidism, ventricular hypertrophy, morbid obesity, essential hypertension, and other pathologic states.

VIII. Effects of the Resting Respiratory Cycle on Heart Rate

• Minor heart rate increases/decreases during the resting respiratory cycle reflect adjustments by baroreceptor reflexes to intrathoracic pressure changes (Fig. 5.4).

• Inspiration:

  • Inspiratory muscles contract, reducing intrathoracic pressure, causing thoracic veins to expand slightly.

  • Venous pressure, venous return, cardiac output, and systemic arterial blood pressure drop.

  • The carotid sinus reflex causes a momentary increase in heart rate due to reduced baroreceptor firing.

• Expiration:

  • Inspiratory muscles relax, and early resting expiration increases intrathoracic pressure, compressing thoracic veins.

  • Venous pressure and venous return increase momentarily, reflexively increasing heart rate.

  • Increased cardiac output and systemic arterial blood pressure increase carotid baroreceptor firing, causing heart rate to decrease.

IX. Standard ECG Grid and Values

• Average resting heart rate for adults: 60-80 beats/min (70 bpm for males, 75 bpm for females).

• Athletes may have slower heart rates (as low as 50 beats/min) due to larger, more efficient hearts (“left ventricular hypertrophy”).

• Low heart rate and hypertrophy in sedentary individuals may indicate failing hearts.

• ECGs use standardized grids (Fig. 5.4):

  • Smaller squares: 0.04 seconds on the x-axis.

  • Darker vertical lines: 0.2 seconds apart.

  • Lighter horizontal lines: 0.1 mV apart (amplitude).

  • Darker grid lines: 0.5 mV.