The Electrocardiogram

Electrocardiography

Anatomy of the Heart Pacemaker and Conduction System

Conducting system of the heart and Equipotentials resulting from cardiac depolarization

The typical EKG waveform

Sinoatrial (SA) node in right atrium generates a rhythmical depolarization (firing frequency)
Modified by stimulation from nerves from the brain

Sympathetic - increases firing frequency (tachycardia) in response to:

Exercise
Anxiety, stress
Coffee, medication - catecholamines such as norepinephrine (adrenalin), salbutamol (asthma treatment)

Para-sympathetic (vagus) - slows the firing frequency (bradycardia)

Breathing (sinus arrhythmia) - rate decreases following prolonged expiration (valsalva maneuver)
Standing - reflex tachycardia occurs to correct transient drop in blood pressure
Medication such as digitalis (digoxin)
Carotid Sinus massage & Diving Reflex

Depolarization causes right atrium to contract (P-wave, 80ms)
Stimulates Atrio-ventricular (AV) node, which fires in turn
Passes down right and left bundles in septum (Q-wave, 220ms)
Depolarizes right and left ventricles, which now contract (R-wave, 240-350ms)
Signal voltage 1 to 10 mV, frequency 0.67 to 40-Hz

12-lead EKG

12 leads commonly used clinically

Standard leads (I, II, III)
Augmented leads (_aV_R, _aV_L, _aV_F) in which the signal is compared to the average of two others

These leads effectively measure the EKG potential at 30º angle about the heart. The R waves of the leads whose axes most closely aligned with the axis of the heart are larger in amplitude, providing a means to quickly assess the patient for hypertrophy of the left (hypertension) or right (lung disease) ventricle.

Chest leads (V_1-6)

Normal EKG

Uses of the EKG

The ECG is useful for three basic parameters of clinically interest:

The rhythm and rate of the heart: whether it is regular and normal, faster (tachycardia) or slower (bradycardia) or normal
The axis of the heart, which can reveal any enlargement (hypertrophy) due to, e.g. chronic high blood pressure (hypertension)
The state of the myocardial muscle

Typical EKG changes in Myocardial Infarction (Heart Attack)

ST elevation in leads II & III (infarction), ST depression in I, _AV_L (ischemia)

Disorders of the Heart: watch man having an MI on ER: part I, part II

Ischemic Heart Disease (IHD)

Atherosclerosis of coronary arteries supplying the myocardium (heart muscle)
Caused by smoking, high fat diet, hypertension (high blood pressure), diabetes
EKG shows ST depression
Can be treated by angioplasty, coronary artery bypass graft (CABG)

Angina

Spasm of sclerotic coronary artery
Causes chest pain reversed by nitroglycerine (nitro spray under tongue)
Unstable (recurring) angina indicates impending infarction

Myocardial Infarction (MI, heart attack)

Thrombosis of coronary artery
Causes chest pain which is not reversed by nitro
Diagnose by EKG (ST elevation, T-wave inversion), cardiac enzymes
Complications: arrhythmia, cardiac failure
Treatment: morphine, aspirin, monitor for complications, thrombolysis

Arrhythmia

Irregular heart rhythm, e.g. atrial fibrillation (AF), heart block, ventricular fibrillation (VF), supraventricular tachycardia (SVT)

Treatment

digoxin for AF
atropine, pacemaker for bradycardias
VF (may cause sudden death) - treat quickly with electroshock (defibrillator) and CPR + lignocaine

Cardiac Failure (heart failure)

Reduced cardiac output (pump failure) due to weakened myocardium
May be acute (complicating myocardial infarction) or chronic (chronic IHD)
Kidneys retain sodium and water - results in pulmonary edema (fluid in lungs)
Treated with diuretics (e.g. Lasix) ± dopamine

Examples of some interesting commercial Electrocardiographs

Medisana's® Cardiocheck provides an easy self-test for people with known heart conditions or risk factors, for athletes and for the elderly. Simply put a few drops of electrode solution on each thumb, then gently press on the built-in thumb electrodes. It determines pulse rate (heartbeats per minute), any irregularity in heart rhythm (arrhythmia), and QRS, T-Wave and ST values. Results are shown on the LCD screen. Flashing asterisks provide an early warning if any measured values vary significantly from a Standard Deviation.

Biolog™ 3000i ECG Monitor/Recorder is a simple to operate, lightweight device (9.5 oz /270 grams). It uses direct contact chest electrodes and displays the results on an LCD screen. It also.allows real time telephonic ECG transmission and monitoring of the ECG may be carried out during transport.

Holter Recorders

Record (log) data to a tape or memory for 24 hours to monitor paroxysmal (intermittent short duration) arrhythmias