EKG Intro (workbook pg 1-7 summary)
Cardiac Conduction Cells
· Automaticity- cell’s ability to generate their own electrical impulse
· Conductivity- able to transmit stimulus from one cell to another
· Excitability- able to respond to a stimulus
· Contractility- able to take electrical impulse and respond mechanically
Cell membrane- semi permeable
· Extracellular ions: Na+, Ca+ (higher concent)
· Intracellular ions: K+, phosphate (PO4-), sulfate - (higher concent)
· Ionic channels: Na (fast), Ca (slow)
Action Potential
· Polarization- ready to react to stimulus, cell membrane charge is constant
· Depolarization- Na+ goes in the cells à action potential à contraction
· Repolarization- Na+ leaves, K+ back into cell à relaxation
Pacemaker cells- have fastest rate of automaticity (self depolarize/ produce action potentials), permeable to Na+ even at rest, slowly become positive until reach threshold à action potential.
Cardiac Conduction Pathway
· Coordinates contraction & relaxation via conduction of electrical impulses
1. SA Node- primary pacemaker (fastest rate of depolarization & repolarization) in the posterior wall of R atrium
· Depolarizes both atria
2. AV Node- receives impulse from SA node and sends it to ventricles (electrically connects atria w/ ventricles)
· Becomes primary pacemaker if SA node is <60.
3. AV junction (AV node + Bundle of His)
· L Bundle Branch- contains anterior and posterior divisions that stimulate LV
· R Bundle Branch- stimulates RV
4. Purkinje System- terminal, simultaneously stimulates ventricles
EKG: P wave, Q RS wave, ST segment, T wave
· Phase 4 (Resting phase)- no electrical activity (closed ionic channels)- Diastole
· Atrial depolarization (atrial systole/contraction pushing blood into ventricles)- P wave
· SA node signal
· Phase O (Ventricular depolarization- contraction of blood filled ventricles)- QRS wave (larger wave 2° ventricular strength)
· Na+ moves into the cell rapidly, Ca+ moves in slowly, & K+ leaves the cell
· -90 à +20millivolts
· AV node & Bundle of His signals to contract
· Phase 1- Na channels partially closed, K continues to leave cell
· Systole, contraction, first (-) deflection on QRS
· Phase 2 (early ventricular repolarization- ventricles begin to chill out)- ST segment
· Plateau of action potential. Ca+ flows inward, K+ flows outward slowly (keeping depolarization)
· Phase 3 (Late ventricular repolarization- ventricles getting ready for complete relaxation or Phase 4)- T wave
§ Ca+ channels close, Na+ & K+ flow out rapidly.
§ U wave- if present associated w/ ↓ K, slow HR, or ventricular hypertrophy.
EKG (Electrocardiogram)
· Can detect rhythm abnormalities, no info on heart’s mechanical activity (BP, CO…), supports a Dx.
· Rhythm Strips (1 lead strip)- identify rhythm and conduction abnormalities, *have no info on possible ischemia, MI, or injury.
· Interval (mm)- distance btw 2 waves or complexes
· Duration (secs)- time for EKG complex to be recorded
· Segment- EKG baseline btw waves (horizontal/isoelectric segments btw waves)
EKG Paper
· Horizontal (25 mm/sec)
· 25mm=1sec, 5mm=0.20 secs, 1mm=0.04 sec
· Vertical (1mm=0.1mv)
· 1mm= 0.1 mv
Waveforms
P wave (atrial depolarization/contraction)- normal: smooth, round, 2.5mm height, & 0.11 sec duration
· (+) in : I, II, aVF, V2-V6
PR segment/interval
· PR segment- isoelectric (normally) line btw the 2 waveforms
· PR interval- from start of P wave and the start of R wave (P wave + PR segment)
o Atrial depolarization (SA node, bundles of His, & Punkinje fibers)
o Normal: 0.12-0.2 sec, shortens as HR increases (atria contract faster)
QRS complex- 0.06-0.10 sec (up to 2.5 mm), not mandatory to either have Q, R or S.
· Q wave- 1st (-) deflection below baseline after P wave
o <0.04 sec, & 1/3 the amplitude (height) of R wave
· R wave- 1st (+) wave above baseline
ST segment- early ventricular repolarization
· Isoelectric line (from end of S wave (J point) until beginning of T wave)
· Absolute refractory period- beginning of QRS to middle of T wave (1/2 of T wave)
· use TU or TP to evaluate ST segment depression (ischemia if >1mm) or elevation
T wave- late ventricular repolarization
· 1st half of Twave is part of absolute refractory (cannot repolarize further)
· Limb lead- 5mm height, precordial leads- 10mm height
· Abnormalities: aVR (not upright), positive or negative in III & V1, tall peaked in hyperkalemia,
o R on T=absolute refractory (leads to a-fib)
QT interval- ventricular depolarization to ventricular repolarization
· Beginning of QRS to the end of T wave, <0.15 sec (<1/2 the length of R to R interval)
· If Q is absent (normal): beginning of R wave to the end of T wave
· Abnormalities: prolonged (longer ventricular repolarization): ventricular arrhythmias, Quinidine, hypocalcemia.
Rate calculation
1. Six Second method
· For regular and irregular rhythms
· Ventricular rate per min: rate/min = (# of R-R intervals in 6 secs) x (10)
o Ex: (7 R-R intervals) x (10) = 70/min
2. 300 Box Counting Method
· For regular rhythms only
· Count the # of 5mm boxes (“large boxes”) divide the # by 300, find R wave that peaks on a line
· Find R wave on heavy line, if not as 300, go on to 150, 100, 75, 60, or even 50
· If <50 or w/ irregular rhythm- use the six second method
Cardiac monitors- have alarms, printouts, sensitivity control to change size of EKG, ;ead selector, telemetry capable
Leads- red is (+), white is (-), green/brown/black is ground.
· 12 Lead EKG (standard EKG)- 6 on limbs, 6 on chest wall.
· Continuous lead monitoring- done w/ 1 or 2 leads, usually lead I, II (detects arrhythmias), III.
· Limb leads:
o Bipolar- I, II, III. Two electrodes used (1 positive, 1 negative).
o Unipolar- 1 (+) electrode, used for 3 augmented leads.
o Augmented Leads- voltage is amplified to match the size of other limb leads.
§ aVR (augmented voltage right)
§ aVL (augmented voltage left)
§ aVF (augmented voltage foot)
§ Chest or precordial leads
o V1-V6: around left side of chest wall
EKG Deflection
§ Isoelectric- no cardiac current, perpendicular to (+) electrode.
§ (+) EKG wave- depolarization towards a (+) electrode.
§ (-) EKG wave- depolarization away from (+) electrode.
§ Biphasic- depolarization towards (+) electrodeand then away (+ & -).
Artifact- man-made signal disturbance that distorts EKG pattern. Due to pt’s movement or tremor, respiration can lead to movement of baseline (move electrodes towards sternum).