allows heart muscle cells to fire spontainiously and independantly
A group of cells in the Right Atrium that depolarizes faster than any other cells in the heart.
Sinoatrial Node (SA Node)
- heart's pacemaker sets HR
Contraction of the atria on an EKG is seen as a...
The wave of depolarization reaches the ventricles through a cluster of cells called:
The Atrioventricular Node
The PR Interval or P-QRS Interval is:
When the depolarization wave travles through the AV Node causing a delay between contractions of the atrium and Ventricles
Displayed on EKG as a flat line after the P wave
Depolarization of the heart
SA Node fires
AV Node depolarizes
-PR interval (flat line)
Bundle of His depolarises
Purkinje fibers depolarize
ventricular cells repolarize
When the ventricles contract (depolarize) it is seen on an EKG as:
When the ventricles repolarize (reset for the next fire) it is seen on an EKG as a:
Absolute Refractory Period
When the Na+ channels are already open and the Na+ is rushing in the channels cannot open again, the cell cannot fire again despite the magnitude of the stimulus (during phase 0)
Relative Refractory Period
When the cell begins to repolarize a strong stimulus may cause the channels to reopen causing a weak fire (during phases 2 & 3)
Rapid uncoordinated contractions
Flutter or Fibrillation of the atria or ventricles.
Sympathetic Nervous System causes:
Increased HR & force of contraction
Constriction of peripheral blood vessels
Decreases perfusion of nonessential organs and tissues
Decreases activity of the GI tract
Dialates the bronchioles
Neuro transmitters of the SNS
Bind to adrenergic receptors
Mimic the effects of epinephrine and norepinephrine by binding to the adrenergic receptors
Bind to the adrenergic receptors not to produce an effect, but to block the neurotransmitters from binding and stop their effects.
Alpha 1 Adrenergic Receptor
Causes vasoconstriction in skin and intestinal tract.
Alpha 2 Adrenergic Receptor
Located on the ends of adrenergic neurons and help regulate the release of norepinephrine
Beta 1 Adrenergic Receptors
Located in the heart
Increase the HR, contraction strength, & speed of depolarization wave.
Beta 2 Adrenergic Receptors
Located in smooth m. surrounding blood vessels surrounding the heart, skeletal m., arteriols, and terminal bronchioles in the lungs.
Parasympathetic Nervous System
Slows the HR
Increases blood flow to intestinal tract
Decreases the diameter of the bronchioles
little effect on peripheral blood flow
Main neuro transmitter of the PNS
Binds w/ choloinergic receptors
Usually found on neurons associated w/ the PNS
Produce PNS effects: slowed HR, increased blood flow to intestines, pin-point pupils
Nicotinic cholinergic receptors
found in both PNS and SNS and produce effects of both
found at neuromuscular junction
Any abnormal pattern of electrical activity in the heart.
Ectopis Focus (foci pl.)
An area of myocardium (heart m.) or conduction system depolarizing out of sequence or faster than the SA node disrupting the normal electrical pattern.
Often areas of damaged cells allowing more Na+ leakage than normal (lower threshold).
Premature Ventricular Contraction (PVC)
a single, large, bizzar wave on an EKG indicating an ectopic focus
Single intermittent PCVs on an EKG may not significantly affect the heart's ability to pump and sometimes occur in animals under general anesthesia
Paroxysm - short series of multiple PCVs
Ventricular flutter - longer series of PCVs
Ventricular Fibrillation - severe conduction disterbance causing ventricles to quiver.
Short series of Premature Ventricular Contractions (PCVs)
Longer series of Premature Ventricular Contractions (PCVs)
Severe conduction disterbance causing the ventricles to quiver
Used to control ectopic foci or reverse conduction abnormalities of the heart
Lidocaine, quinidine, and procainamide work by:
decreasing the rate of Na+ influx into the cell resulting in a slower automaticity and allows the SA node to once again determine the HR
Only avalible in injectible form
Preferred method IV bolus of infusion
Preferred drug for canine PCVs and ventricular arrythmias and animals under anesthesia.
Not good for Atrial arrthmias
Cats more sensitive to side effects.
Slwo Na+ influx into the cell slowing the automaticity
Lidocaine CNS effects
Light Toxicosis - sedation, ataxia, and drowsiness
Larger overdoses - excitement and sezures
Do NOT confuse w/ lidocaine packaged w/ epinephrine
Switch to Mexiletine for long term use
Procainamide & Quinidine
Slows Na+ influx into cell slowing the automaticity and refractory periods
Control both Ventricular Ectopic Foci and atrial flutter or fibrillation (supraventricular tachycardia)
Most commonly administered PO allowing for long term maintenance
Quinidine frequently causes V/D and anorexia.
Procainamide has fewer GI problems
Atrial flutter and fibrillation
Depolarization wave is altered in its movement enough to allow the wave to encounter some of the myocardia that have completed their refractory phase causing them to depolarize again and the wave continues on.