12.5 Lecture Cardiac Arrhythmias and Heart Blocks

• Cardiac arrhythmias stem from issues like abnormal pacemaker rhythms, pacemaker shifts, blocks in the heart's pathways, or spontaneous impulse generation.

Abnormal Sinus Rhythms

• Tachycardia and bradycardia are examples of abnormal sinus rhythms.
• The baroreceptor reflex, which can cause bradycardia, may be triggered during carotid artery surgeries.
• This reflex can be pre-emptively treated with anticholinergics or local anesthetic blocks.

Heart Blocks

• Heart blocks have multiple causes.
• An SA block is rare, resulting in the absence of a P wave and atrial contraction.
• Normally, impulses pass from the atria to the ventricles only through the bundle of His (AV bundle)

AV Blocks

• First-degree AV block: PR interval greater than 0.2 seconds, indicating a conduction delay, but the PR interval remains constant.
• Second-degree AV block: Some impulses don't reach the ventricles, leading to dropped beats.
• Type 1 (Wenckebach/Mobitz I) is marked by progressive PR interval prolongation until a beat is dropped, followed by a reset.
• Type 2 (Mobitz II) has a fixed number of non-conducted P waves for every QRS complex, and the PR interval remains constant.
• Third-degree (complete) AV block: Complete block in the AV bundle, ventricles generate their signal, leading to P waves and QRS complexes being completely disassociated.

Ventricular Escape Beat

• Ventricular escape beat: Total AV block occurs intermittently.
• After AV conduction ceases, Purkinje system takes over pacing after a 5-30 second delay, discharging at 15-40 beats per minute.
• Brain function ceases after 4-7 seconds without blood supply, so fainting often occurs shortly after the block.
• Ventricular standstill can cause death, necessitating a pacemaker.

Premature Contractions

• Premature contractions can result from local ischemia, plaques pressing on cardiac muscles, or toxins like drugs, nicotine, or caffeine.
• In some cases, P waves may be obscured by the QRS complex, depending on the origin.
• PVCs are usually widened due to slow conduction through ventricular muscle.
• Impulses typically travel from one ventricle to the other, creating a large electrical potential.
• T wave polarity is usually opposite the QRS complex in PVCs.
• PVCs can be benign, caused by irritants, or caused by stray impulses or re-entry signals, which can raise spontaneous lethal V-fib risk.
• The Q wave indicates depolarization, while the T wave indicates repolarization.
• The QT interval is the time from the start of depolarization to the start of repolarization.
• QT prolongation increases susceptibility to Torsades de Pointes.
• QT prolongation is commonly caused by electrolyte imbalances (hypomagnesemia, hypokalemia, or hypercalcemia) or antiarrhythmic drug overdose.

Re-entry Arrhythmias

• Re-entry arrhythmia: It occurs when a part of the ventricular muscle isn't refractory.
• Ventricular fibrillation (V-fib) is almost always fatal if not stopped within 1-3 minutes.

Ventricular Fibrillation

• During V-fib, electrical impulses are uncoordinated, with some portions excited while others are repolarizing, so there is no coordination.
• Ventricles neither fill nor contract, so no pumping occurs.
• Conditions that can cause the impulse to continue are if the pathway around the circle is longer than normal, if the velocity of the conduction is slower, or if the refractory period of the muscle is shortened.
• In fibrillation, impulses are blocked in one direction but successful in another.
• Rapid heart stimulation decreases conduction velocity and shortens the refractory period.
• A key feature of V-fib is the division of impulses: depolarization waves divide around refractory areas.
• This creates irregular patterns of patchy areas, with uncoordinated depolarization and repolarization.
• Defibrillation uses a strong, high-voltage electrical current to make all ventricular muscles go into a refractory state simultaneously.
• All activity and action potentials ideally stop for 3-5 seconds, then a node or heart part becomes the pacemaker.
• If defibrillation isn't performed quickly (within one minute), the heart weakens due to lack of coronary blood flow.
• CPR or hand pumping can supply blood to the heart.
• CPR for 1-2 minutes before defibrillation has been found to be effective

Atrial Fibrillation

• Atrial fibrillation (A-Fib) occurs in the atrial muscle.
• A-Fib is frequently caused by atrial enlargement.
• During A-Fib, the atria become ineffective as pumps, decreasing pumping effectiveness by 20-30%.
• EKG: numerous small depolarization waves spread in all directions, often canceling each other out, leading to no P waves or fine high-frequency waves with normal QRS complexes.
• Impulses from the atria arrive at the AV node rapidly and irregularly.
• The AV node only allows one impulse every 0.35-0.95 seconds, resulting in irregular ventricular contraction.
• Treatments include synchronized cardioversion, which can only occur when the ventricle is refractory to stimulation.

Atrial Flutter

• Atrial flutter: electrical activity travels in a single large wave in one direction around the atrial muscle mass.
• The AV node cannot conduct all atrial signals, resulting in 2-3 atrial beats for every ventricular beat.
• Blood pumping is reduced because while one side of the atria is contracting, the other side is relaxing.