Physio ch. 13 cardiac arrhythmias
How is blockage of impulse one of the necessary conditions for a re-entrant signal to develop?
When an impulse is block transmission depolarization waves will occur around the heart in only some directions but not in other directions.
The respiratory type of sinus arrhythmia results mainly from
spillover of signals from the medullary respiratory center into the adjacent vasomotor center during inspiratory and expiratory cycles of respiration. The spillover signals cause alternate increases and decreases in the number of impulses transmitted through the sympathetic and vagus nerves to the heart.
The congenital forms of LQTS are rare disorders caused by mutations in what genes?
sodium or potassium channel genes
ventricular escape
some part of the Purkinje system beyond the block, usually in the distal part of the A-V node becomes the dominant pace maker
How is atrial flutter different than fibrillation?
the electrical signal travels as a single large wave, always in one direction, around and around the atrial muscle mass
If the pathway of cardiac impulse is longer than usual, what happens to the impulse? Happens in what pathological state?
the originally stimulated muscle will no longer be refractory, and the impulse will continue around the circle again and again. State - dilated heart
voltages of the waves in the ECG in ventricular fibrillation are?
usually about 0.5 millivolt when ventricular fibrillation first begins, but they decay rapidly; thus, after 20 to 30 seconds, they are usually only 0.2 to 0.3 millivolt
some abnormalities in conducting system can occasionally cause rapid rhythmical discharge of impulses that spread in all directions throughout the heart...what is said to cause this?
believed to be caused most frequently by re-entrant circus movement feedback pathways that set up local (irritable focus) repeated self-re-excitation. Because of the rapid rhythm in the irritable focus, this focus becomes the pacemaker of the heart.
Because of the rapid rate of fibrillatory impulses in the atria, the ventricle is driven at a fast heart rate, usually at?
between 125 and 150 beats/min.
The major reason that LQTS (long QT syndrome) is of concern is?
delayed repolarization of ventricular muscle increases a person's susceptibility to developing ventricular arrhythmias called torsades de pointes, which literally means "twisting of the points."
How is defibrillation current delivered? effects?
delivered to the heart in biphasic waveforms, alternating the direction of the current pulse through the heart. This form of delivery substantially reduces the energy needed for successful defibrillation, thereby decreasing the risk for burns and cardiac damage.
Mechanism of Lidocaine as an antiarrhythmic drug
depresses the normal increase in sodium permeability of the cardiac muscle membrane during generation of the action potential, thereby often blocking the rhythmical discharge of the focal points
How long does lack of blood flow to brain have to last usually to cause permanent mental impairment or even destruction of brain tissue?
for more than 5 to 8 minutes
Due to the division of impulses many circuitous routes for impulses are made, how does this effect the conductive pathway?
greatly lengthening the conductive pathway, which is one of the conditions that sustains the fibrillation
When the heart contracts ahead of schedule, what happen to ventricles? pulse?
Ventricles will not fill up completely and will lead to weak peripheral pulses
How does cardiopulmonary resuscitation (CPR) work as as an aid to Defibrillation?
- After 1 min of fibrillation the heart is usually too weak to be revived by defibrillation because of the lack of nutrition from coronary blood flow. - by pumping the heart by hand -small quantities of blood are delivered into the aorta, and a renewed coronary blood supply develops. - Then, after a few minutes of hand pumping, electrical defibrillation often becomes possible.
Stokes-Adams syndrome
- Because the brain cannot remain active for more than 4 to 7 seconds without blood supply, most people faint a few seconds after complete block occurs (no blood supply due to delay of ventricular Rythm - after escape beat, recovery from faint will occur) - Off and On AV conduction leads to periodic fainting spells (syncope)
division of impulses
- Important feature of ventricular fibrillation - When a depolarization wave reaches a refractory area in the heart, it travels to both sides around the refractory area. Thus, a single impulse becomes two impulses. Then, when each of these impulses reaches another refractory area, it divides again...causing a chain reaction of new wave fronts
what are "circus movements"? what can they lead to?
- Normal cardiac impulse in the normal heart travel through the extent of the ventricles, it has no place to go because all the ventricular muscle is refractory and cannot conduct the impulse farther. - If this impulse does goes through it can cause ventricular fibrillation ( reentry of cardiac impulse)
Atrial flutter feature in EKG
- P waves are strong because of the contraction of semicoordinated masses of muscle. - QRS-T complex follows an atrial P wave only once for every two beats of the atria, giving a 2:1 rhythm.
paroxysmal tachycardia: what is it? how can it be stopped?
- a period of rapid heartbeats that begins and ends suddenly - can be stopped by eliciting a vagal reflex. (press on the neck in the regions of the carotid sinuses, which may cause enough of a vagal reflex to stop the paroxysm.)
How does ventricular defibrillation work?
- a strong high-voltage electrical current passed through the ventricles for a fraction of a second can stop fibrillation by throwing all the ventricular muscle into refractoriness simultaneously. - all AP stop -heart remains quiescent for 3 to 5 seconds, after which it begins to beat again, usually with the sinus node or some other part of the heart becoming the pacemaker.
Electrical alternans: EKG pattern? results from? what can cause it and why?
- alternating amplitude of the P, QRS and T waves on the ECG (partial intraventricular block every other heartbeat) - can be caused by tachycardia, when the rate of the heart is rapid, it may be impossible for some portions of the Purkinje system to recover from the previous refractory period
due to the reduced atrial contractile function in atrial fibrillation, what can occur? how can this be reduced?
- blood can become stagnate, allowing blood clots to form in the atrial appendage. These blood clots can dislodge and travel to the brain, causing stroke, or to other parts of the body. - blood thinner medications (anticoagulants) to reduce the risk of embolism
ventricular fibrillation: caused by? effect on ventricular muscles? ventricular chambers?
- cardiac impulses stimulating first one portion of the ventricular muscle, then another portion, then another, and eventually feeding back onto itself to re-excite the same ventricular muscle over and over, never stopping. - many small portions of the ventricular muscle will be contracting at the same time, while equally as many other portions will be relaxing. Thus, there is never a coordinated contraction of all the ventricular muscle at once - chambers neither enlarge nor contract but remain in an indeterminate stage
ectopic foci: what are they? caused by?
- cardiac tissue outside the normal cardiac conduction pathway that emit abnormal impulses at odd times during the cardiac rhythm - caused by, local ischemia, pressure on cardiac fibers by calcified plaques, and toxic irritation of the A-V node, Purkinje system, or myocardium
Third degree AV block
- complete heart block - ventricles begin to beat at intrinsic rate = <40 bpm
In Atrial fibrillation, what do P waves look like? and why?
- either no P waves from the atria or only a fine, high-frequency, very low voltage wave record.
Discharge procedure of ventricular defibrillation?
- fibrillation can usually be stopped using 1000 volts of direct current applied for a few thousandths of a second.
How does hypoxia lead to cardiac arrest?
- hypoxia prevents the muscle fibers and conductive fibers from maintaining normal electrolyte concentration differentials across their membranes - their excitability may be so affected that the automatic rhythmicity disappears.
Compensatory pause: what is it? caused by?
- interval between the premature contraction and the next succeeding normal contraction is slightly prolonged Cause - premature contraction originated in the atrium some distance from the sinus node, and the impulse had to travel through a considerable amount of atrial muscle before it discharged the sinus node.
features of Atrial or A-V nodal paroxysmal tachycardia
- referred to as supraventricular tachycardias -usually occur in young, otherwise healthy people, and they generally grow out of it after adolescence. -usually does not cause permanent harm from the attack.
Electroshock Treatment of Atrial Fibrillation
- single electric shock is programmed (or synchronized) to fire only during the QRS complex when the ventricles are refractory to stimulation. - In the sitting of ventricular fibrillation this procedure is called synchronized cardioversion
In Atrial fibrillation what causes Irregularity of Ventricular Rhythm?
- the A-V node will not pass a second impulse for about 0.35 second after a previous one - variable interval of 0 to 0.6 second occurs before one of the irregular atrial fibrillatory impulses happens to arrive at the A-V node. - Thus, the interval between successive ventricular contractions varies from a minimum of about 0.35 second to a maximum of about 0.95 second, causing a very irregular heartbeat.
Carotid sinus syndrome
- the pressure receptors (baroreceptors) in the carotid sinus region of the carotid artery walls are excessively sensitive. - causing intense vagal-acetylcholine effects on the heart, including extreme bradycardia.
In Atrial flutter why do signals reach the A-V node too rapidly for all of them to be passed into the ventricles? How does this effect beating?
- the refractory periods of the A-V node and A-V bundle are too long to pass more than a fraction of the atrial signals. - Therefore, there are usually two to three beats of the atria for every single beat of the ventricles.
In patients with a high risk for ventricular fibrillation, what can be used to help monitor and revert fibrillations?
-a small, battery-powered, implantable cardioverter-defibrillator (ICD) with electrode wires lodged in the right ventricle may be implanted. - detect ventricular fibrillation and revert it by delivering a brief electrical impulse to the heart.
What two changes in cardiac muscles that occur due to rapid stimulation of the heart, both of which predispose to circus movement?
1) the velocity of conduction through the heart muscle decreases 2) the refractory period of the muscle is shortened
Ventricular tachycardia is usually a serious condition for what two reasons?
1) this type of tachycardia usually does not occur unless considerable ischemic damage is present in the ventricles. 2) ventricular tachycardia frequently initiates the lethal condition of ventricular fibrillation because of rapid repeated stimulation of the ventricular muscle
PVCs can cause what specific effects in the ECG
1. QRS prolonged - impulse is conducted mainly through slowly conducting muscle of the ventricles rather than through the Purkinje system. 2. QRS has high voltage - the impulse almost always travels in only one direction, so there is no such neutralization effect, and one entire side or end of the ventricles is depolarized ahead of the other, which causes large electrical potentials 3. T wave inversion - polarity exactly opposite to that of the QRS complex because the slow conduction of the impulse through the cardiac muscle causes the muscle fibers that depolarize first also to repolarize first
second degree heart block: what is it? PR interval?
AV block in which occasional electrical impulses from the SA node fail to be conducted to the ventricles - cause additional P waves PR interval - 0.25 to 0.45 seconds
If the length of the cardiac impulse pathway remains constant but the velocity of conduction becomes decreased, what happens to the impulse? Happens in what pathological state?
An increased interval of time will elapse before the impulse returns to the 12 o'clock position (original start point of pathway). By this time, the originally stimulated muscle might be out of the refractory state, and the impulse can continue around the circle again and again. State - blockage of the Purkinje system, ischemia of the muscle, or high blood potassium levels,
Atrial flutter: rate? Why is blood pumping reduced?
Rate - usually between 200 and 350 beats/min reduced - one side of the atria is contracting while the other side is relaxing
Why isn't atrial fibrillation as lethal as ventricular fibrillation?
Most blood flows between atria and ventricles (with only some being pushes through during atrial contraction) thus there is only reduced efficiency of overall heart pumping.
If AV conduction is blocked, there is a delay before ventricular rhythm kicks in, why?
Overdrive suppression means that ventricular excitability is at first suppressed because the ventricles have been driven by the atria at a rate greater than their natural rate of rhythm.
Second degree Type II block is characterized by? abnormality where?
a fixed number of nonconducted P waves for every QRS complex. For example, a 2:1 block implies that there are two P waves for every QRS complex. abnormality - almost always caused by the bundle of His-Purkinje system
Treatment: acute LQTS? chronic LQTS?
acute - magnesium sulfate chronic - antiarrhythmic medications such as beta-adrenergic blockers or surgical implantation of a cardiac defibrillator
A frequent cause of atrial fibrillation is? how?
atrial enlargement dilated atrial walls provide ideal conditions of a long conductive pathway, as well as slow conduction
If The refractory period of the muscle becomes greatly shortened, what happens to the impulse? Happens in what pathological state?
impulse could also continue around and around the circle (cardiac impulse pathway) state - response to various drugs, such as epinephrine, or after repetitive electrical stimulation.
Some causes of tachycardia include
increased body temperature, dehydration, blood loss anemia, stimulation of the heart by the sympathetic nerves, and toxic conditions of the heart
Intoxication of what drug can lead to ventricular tachycardia? what drugs treat this type of tachycardia?
intoxication - from the heart failure treatment drug digitalis causes irritable foci that lead to ventricular tachycardia. to treat - Antiarrhythmic drugs such as amiodarone or lidocaine
PR interval time in first-degree incomplete heart block
more than 0.20 second (.16 - normal)
More common are the acquired forms of LQTS that are associated with?
plasma electrolyte disturbances, such as hypomagnesemia, hypokalemia, or hypocalcemia, or with the administration of excess amounts of antiarrhythmic drugs such as quinidine or some antibiotics such as fluoroquinolones or erythromycin, which prolong the Q-T interval.
Bigeminy: EKG pattern
premature ventricular contractions (PVCs) alternating with normal contractions
Second degree Type I block is characterized by? abnormality where?
progressive prolongation of the P-R interval until a ventricular beat is dropped and is then followed by resetting of the P-R interval and repeating of the abnormal cycle. abnormality - almost always caused by the A-V node
Actions of Amiodarone as an antiarrhythmic drug
prolonging the action potential and refractory period in cardiac muscle and slowing A-V conduction
PVC: Vector direction? first portion of the heart to become depolarized?
vector - negative end (origin) at the base of the heart and its positive end toward the apex. first portion - is near the base of the ventricles, which therefore is the origin of the ectopic focus.
