Automaticity

• the ability of atrial and ventricular muscle cells to shorten their fiber length in response to electrical stimulation, generating sufficient pressure to propel blood forward
• the mechanical activity of the heart

• conduction system of the heart begins here
• located close to the surface of the right atrium near its junction with the superior vena cava
• the heart's primary pacemaker
• has the greatest degree of automaticity

consists of a transitional cell zone, the AV node itself, and the bundle of His

lies just beneath the right atrial endocardium, between the tricuspid valve and the ostium of the coronary sinus

• make up the bundle of His, bundle branches, and terminal Purkinje fibers.
• responsible for the rapid conduction of electrical impulses throughout the ventricles, leading to ventricular depolarization and the subsequent ventricular muscle contraction

deflection representing atrial depolarization

the shape may be a positive, negative, or a biphasic (both) deflection, depending on the lead selected.[image]

measured from the beginning of the P wave to the end of the PR segment.

it represents the time required for atrial depolarization as well as the impulse delay in the AV node and the travel time to the purkinje fibers

[image]

represents ventricular depolarization

the shape depends on the lead selected

[image]

the first negative deflection and is not present in all leads

when present, it is small and represents initial ventricular septal depolarization.

when it is abnormally present in a lead, it represents myocardial necrosis

[image]

the first positive deflection

it may be small, large, or absent, depending on the lead.

[image]

a negative deflection following the R wave

is not present in all leads

[image]

represents the time required for depolarization of both ventricles

it is measured from the beginning of the QRS complex to the J point (the junction where the QRS complex ends and the ST segment begins)

[image]

• normally an isoelectric line and represents early ventricular repolarization.
• its length varies with changes in the heart rate, the administration of medications, and electrolyte disturbances.
• elevation or depression can be caused by myocardial injury, ischemia or infarction, conduction abnormalities, or the administration of meds

[image]

follows the ST segment and represents ventricular repolarization

usually positive, rounded, and slightly asymmetric

if an ectopic stimulus excites the ventricles during this time, it may cause ventricular irritability, lethal dysrhythmias, and possible cardiac arrest in the vulnerable heart

[image]

represents late ventricular repolarization

follows the T wave and may result from slow repolarization of ventricular Purkinje fibers

an abnormal U wave may suggest an electrolyte imbalance (hypokalemia)

[image]

represents the total time required for ventricular depolarization and repolarization.

measured from the beginning of QRS complex to the end of the T wave

[image]

the rhythm originating from the SA note (dominant pacemaker) that meets these ECG criteria

1. Rate: 60-100bmp
2. Rhythm: regular
3. P waves: present, consistent configuration, one P wave before each QRS complex
4. PR interval: 0.12 to 0.20 seconds and constant
5. QRS duration: 0.04 to 0.10 seconds and constant

• variant of normal sinus rhythm (all the characteristics except for its irregularity)
• results from changes in intrathoracic pressure during breathing
• in this context, it does not mean absence of rhythm
• the heart rate increase slightly during inspiration and decreases slightly during exhalation
• irregular rhythm is freq observed in healthy children as well was adults

any disorder of the heartbeat

result from:

• a disturbance in the relationship bt electrical conductivity and the mechanical response of the myocardium
• a disturbance in impulse formation
• a disturbance in impulse conduction
• a combination of these

• early rhythm complexes
• they occur when a cardiac cell or cell group, other than the SA node, becomes irritable and fires an impulse before the next sinus impulse is produced

the sinus node discharge rate is less than 60 beats/min in adults

increases coronary perfusinon time

decreases coronary perfusion pressure

myocardial o2 demand decreases

occurs when atrial tissue becomes irritable

an atrial dysrhythmia

ectopic focus fiires an impuls before the next sinus impulse is due

involves the rapid stimulation of atrial tissue at a rate of 100-280 beats/min in adults and 200-300bpm in children

an atrial dysrhythmia

a term used when the rhythm is intermittent in a SVT

initiated suddenly by a premature complex such as a PAC and terminated suddenly with or w/o intervention

most common dysrhythmia seen in clinical practice

multiple rapid impulses from many atrial foci depolarize the atria in a totally disorganized manner at a rate of 350-600 times per minute

quivers

rapid atrial depolarization occurring at a rate of 250-350 times per minute

the AV node blocks the number of imulses that reach the ventricles as a portective mechanism

the ventricular nodal cells pace the ventricles

rate is usually less than 40bpm

rhythm in a the dying heart, where downward displacement of the pacemaking has occurred

characterized by no palpable pulse and therefore, no perfusion

electrical activity is displayed on the monitor

result from increased irritability of ventricular cells and are seen as early ventricular complexes followed by a pause

frequently occur in repetitive rhythms

"V Tach"

occurs with repetitive firing of an irritable ventricular ectopic focus

usually at a rate of 140 to 180bpm or more

result of electrical chaos in the ventricles

life threatening

impulses from many irritable foci fire in a totally disorganized manner so that ventricular contraction cannot occur

sometimes called ventricular standstill

complete absence of any ventricular rhythm

a conduction delay or block within one of the two main bundles of His branches

usually sinus in origin

may be a temorary or a permanent conduction disorder

life-threatening VT

magnesium sulfate can be used

the physician massages over one carotid artery for a few seconds, observing for a change in cardiac rhythm

causes vagal stimulation, slowing SA and AV nodal conduction

a system that consists of an external battery-operated pulse generator and pacing electrodes (leads)

the wires are attached to the generator on one end and are in contact with the heart on the other end

electrical impulses are emitted from the negative terminal of the generator and flow through the lead wire, and then stimulate the cardiac cells to depolarize