Sodium (Na+)
goes into cell and initiates depolarization
Potassium (K+):
flows out of the cell to initiate repolarization
Hypokalemia
Hyperkalemia
Hypokalemia: increased myocardial irritability
Hyperkalemia: decreased automaticity/conduction
Calcium (Ca)
has major role in the depolarization of pacemaker cells (maintain depolarization) and in myocardial contractility
Hypocalcemia
Hypercalcemia
Hypocalcemia: decreased contractility and increased myocardial irritability
Hypercalcemia: increased contractility
Magnesium (Mg)
stabilizes the cell membrane: acts in concert with potassium and opposes the actions of calcium
Hypomagnesemia
Hypermagnesemia
Hypomagnesemia: decreased conduction
Hypermagnesemia: increased myocardial irritability
Absolute Refractory Period
the cardiac muscle cell is completely insensitive to further stimulation
Start of the QRS and ends at middle of T wave
Relative Refractory Period
During the relative refractory period, the muscle cell is more difficult than normal to excite, but it can still be stimulated.
Nervous system controlling heart rate
Two nerves link the cardiovascular center in the medulla oblongata of brain with the SA node of the heart
- Accelerator nerve (sympathetic NS)
- Vagus nerve (parasympathetic NS):
Accelerator nerve (sympathetic NS)
when stimulated, releases neurotransmitter at the SA node to increase heart rate
Vagus nerve (parasympathetic NS)
when stimulated, releases neurotransmitter at the SA node to decrease heart rate
Electrical Conduction System
- Automaticity
- Generates its own electrical impulses without stimulation from nerves
- Unique feature of the heart
- Specialized conduction tissue
- Pacemaker
The Sinoatrial (SA)Node
• The Primary Pacemaker
- Theoretically, any cell can act as a pacemaker.
- Located in the right atrium, near the inlet of the superior vena cava
- Receives blood from the Right Coronary Artery in 50-60% of population
- Fastest pacemaker in the heart
- Inherent rate of 60 to 100 beats per minute(bpm)
Secondary Pacemakers
If the SA becomes damaged or is suppressed
- AV node inherent rate 40-60 bpm (RCA blood flow in 85-90% of population).
- Purkinje fibers 20-40 bpm
ECGs
- The ECG is a graphic representation of the heart’s electrical activity.
- It does not provide information regarding mechanical events.
- Valuable diagnostic tool for identifying cardiac abnormalities.
- For good blood flow the electrical signal must send and the heart must respond
Indications for ECG Monitoring
- Patients at risk for dysrhythmias shall receive continuous ECG monitoring
- Known or suspected cardiac patients
- Suspected Overdose
- Electrical Injuries
- Syncope
- Elderly patients “feeling unwell”
- Issues concerning the Sympathetic System
Voltage
Positive:
Negative:
positive
- Seen as an upward deflection on the ECG tracing.
negative
- Seen as a downward deflection on the ECG tracing.
- Flowing backwards
isoelectric
- No electrical current detected.
- Seen as a straight baseline on the ECG.
ECG Leads
- Bipolar lead*:
- Unipolar lead:
- Bipolar lead*:
o Two electrodes of opposite polarity. - Unipolar lead:
o Single positive electrode and reference point.
Lead Placement
- White (upper right), black (upper left), green (lower right), red (lower left)
- Between ribs, does not conduct well on bone
- Positive always at the bottom
lead triangle
GO LOOK AT IT
- double positive is on the bottom
- top right (patients right) double negative
- top left (patients left) negative and positive
ECG Graph Paper
- Each Little square is 1mm x 1mm 0.04sec
- Each Large square is 5mm x 5mm 0.20 sec
Calibration
- The sensitivity of the 12-lead ECG machine is standardized
- When properly calibrated, a 1-mV electrical signal produces a 10-mm deflection (two large squares) on the ECG tracing
Time Interval
- Denoted by short vertical lines on the ECG graph paper.
- At standard speed, the distance between each short vertical line is 75 mm (3 seconds).
