Chapter 18: Heart Part 2

Electrocardiogram (ECG)

Introduction to ECG

An electrocardiogram (ECG or EKG) is a diagnostic tool that records the electrical activity of the heart over time using electrodes placed on the skin. It provides a composite recording of all the action potentials produced by myocardial cells.

• Electrodes are placed on the wrists, ankles, and chest.

• ECG is invaluable for diagnosing abnormalities in conduction pathways, myocardial infarction (MI), heart enlargement, and electrolyte or hormone imbalances.

ECG Waves and Intervals

• P wave: Represents atrial depolarization. Initiated by the SA node, it leads to atrial systole (contraction).

• QRS complex: Represents ventricular depolarization. Atrial repolarization occurs simultaneously but is masked by the larger QRS complex. This leads to ventricular systole.

• ST segment: Corresponds to the period of ventricular systole (contraction).

• T wave: Represents ventricular repolarization (relaxation).

Electrical Activity of the Myocardium

The heart's electrical activity follows a precise sequence:

1. Atrial depolarization begins (P wave).

2. Atrial depolarization completes (atria contract).

3. Ventricles begin to depolarize at the apex (QRS complex); atria repolarize (relax).

4. Ventricular depolarization completes (ventricles contract).

5. Ventricles begin to repolarize at the apex (T wave).

6. Ventricular repolarization completes (ventricles relax).

ECG Interpretation: Normal and Abnormal Patterns

Normal ECG

• Sinus rhythm: Normal rhythm originating from the SA node.

Abnormal ECG Patterns

• Bradycardia: Heart rate < 60 bpm (e.g., athletes or SA node block).

• Tachycardia: Heart rate > 100 bpm (e.g., exercise, hyperthyroidism).

• Flutter: Rapid, regular atrial contractions (250–300 bpm).

• Fibrillation: Uncoordinated contractions (up to 400 bpm); includes atrial and ventricular fibrillation ("bag of worms" appearance).

• Arrhythmias: Irregular intervals between PQRST complexes.

• PVC (Premature Ventricular Contraction): Extra systole with inverted or misshapen QRS and T, and absence of a preceding P wave. Often caused by caffeine, nicotine, lack of sleep, or anxiety.

• Heart Block: Impairment of the conduction signal, often at the AV node, leading to arrhythmia and possible loss of pumping action.

ECG Tracing Identification

ECG Pattern	Description
Normal Sinus	Regular PQRST intervals, normal rate
Tachycardia	Rapid, regular rhythm, HR > 100 bpm
Bradycardia	Slow, regular rhythm, HR < 60 bpm
Heart Block	P waves not always followed by QRS, irregular conduction
Premature Ventricular Contraction	Inverted/misshapen QRS, absent P wave before contraction
Ventricular Fibrillation	Chaotic, irregular baseline, no effective pumping
Atrial Flutter	Rapid, regular atrial activity
Myocardial Infarction	ST segment elevation or depression

The Physiology of Heart Contraction

Phases of the Cardiac Cycle (0.8 sec)

1. Ventricular filling and atrial contraction (systole): Blood flows from atria to ventricles; atria contract to complete filling.

2. Isovolumetric contraction and ventricular ejection: Ventricles contract with all valves closed (isovolumetric), then semilunar valves open and blood is ejected.

3. Isovolumetric relaxation: Ventricles relax, all valves closed, pressure drops before next cycle.

Heart Sounds

• First heart sound (S1): "Lubb"; AV valves close, marks beginning of ventricular systole.

• Second heart sound (S2): "Dupp"; semilunar valves close, marks beginning of ventricular diastole.

• Third heart sound (S3): Rarely heard in adults over 30; may indicate pathology if present.

Ventricular Volumes and Cardiac Output

Ventricular Volumes

• End Diastolic Volume (EDV): Volume of blood in each ventricle at the end of relaxation (about 130 mL).

• Stroke Volume (SV): Volume of blood ejected by each ventricle per beat (about 70 mL).

• End Systolic Volume (ESV): Volume of blood remaining in each ventricle after contraction (about 60 mL).

Both ventricles must eject the same amount of blood to maintain circulatory balance.

Cardiac Output (CO)

• Definition: Amount of blood ejected by each ventricle in one minute.

• Formula:

$\text{Cardiac Output (CO)} = \text{Heart Rate (HR)} \times \text{Stroke Volume (SV)}$

• Normal resting CO: 4–6 L/min

• Vigorous exercise: CO can increase to 21 L/min (fit individuals) or up to 35 L/min (elite athletes)

• Endurance training increases CO

Example Comparison

Person	HR (rest)	SV (rest)	CO (rest)	HR (exercise)	SV (exercise)	CO (exercise)
Lance Armstrong (39 yrs)	32 bpm	200 ml/beat	6,400 ml/min (6.4 L/min)	156 bpm	200 ml/beat	31,200 ml/min (31.2 L/min)
My husband (45 yrs)	75 bpm	70 ml/beat	5,250 ml/min (5.2 L/min)	156 bpm	70 ml/beat	10,920 ml/min (10.9 L/min)

Ejection Fraction

• Definition: Percentage of EDV ejected per beat.

• Formula:

$\text{Ejection Fraction (EF)} = \frac{\text{Stroke Volume (SV)}}{\text{End Diastolic Volume (EDV)}} \times 100\%$

• Normal: 50–60%

• Vigorous exercise: up to 90%

• Diseased heart: <50%

Heart Rate and Its Regulation

Heart Rate (HR)

• Pulse: Surge of pressure in an artery with each heartbeat.

• Infants: HR averages 120 bpm or more

• Young adult females: 72–80 bpm

• Young adult males: 64–72 bpm

• Tachycardia: Resting HR > 100 bpm

• Bradycardia: Resting HR < 60 bpm

Chronotropic Agents

• Positive chronotropic agents: Increase HR (e.g., epinephrine)

• Negative chronotropic agents: Decrease HR (e.g., beta-blockers)

Factors Affecting and Regulating Heart Rate

• Cardiac center of the medulla oblongata receives input from:

  • Cerebral cortex and limbic system: Sensory and emotional stimuli

  • Proprioceptors: Receptors in muscles and joints; increase HR before metabolic demand rises

  • Baroreceptors: Pressure receptors in the aortic arch and carotid arteries; decrease in BP signals to increase HR, increase in BP signals to decrease HR

  • Chemoreceptors: Sensitive to pH, CO2, and O2; located in aortic arch and carotids; important in respiratory control; HR increases in response to increased CO2

• Other factors:

  • Hormones (e.g., epinephrine, thyroxin)

  • Drugs (e.g., caffeine, nicotine)

  • Age, gender, physical fitness

  • Electrolytes (e.g., increased K+ slows HR)

Additional info: These notes synthesize and expand upon the provided slides and text, offering definitions, physiological context, and clinical relevance for ECG interpretation and cardiac function.