Electrical Activity of the

Heart

The Body as a Conductor

This is a graphical representation of the geometry and electrical current flow in a model of the human thorax. The model was created from MRI images taken of an actual patient. Shown are segments of the body surface, the heart, and lungs. The colored loops represent the flow of electric current through the thorax for a single instant of time, computed from voltages recorded from the surface of the heart during open chest surgery.

Electrical Currents

This figure illustrates the voltage distribution of a body surface potential map (BSPM) on the right. The torso on the left has been opened to reveal electrical current paths within the torso volume at one instant in time within a heart beat. Here again, red indicates positive voltage, blue indicates negative voltage, and green indicates voltages of approximately zero.

Polarized

Action Potential✦ Two micro-electrodes✦ One outside the cell, one inside the

cell✦ Difference between the two (-90 mV

inside)✦ The cell is said to be ‘polarized’

Depolarized

Action Potential

Skeletal

Cardiac

IonsIon Extra- Intra-

Na 140 10

K 4 135

Ca 2 0.1

Inside

Outside

Action Potential✦ Phase 0

✦ Stimulation of the myocardial cell✦ Influx of sodium✦ The cell becomes depolarize✦ Inside the cell = +20 mV

Action Potential✦ Phase 1

✦ Ions✦ Influx of sodium✦ Efflux of potassium

✦ Partial repolarization✦ Phase 2

✦ Ions✦ Influx of sodium✦ Efflux of potassium ✦ Influx of calcium

✦ Plateau

Action Potential✦ Phase 3

✦ Ions✦ Influx of sodium✦ Efflux of potassium* ✦ Influx of calcium

✦ Repolarization (slower process than depolarization)

✦ Phase 4 ✦ Interval between repolarization to

the next action potential ✦ Pumps restore ionic concentrations

Ion 0 1 2 3 4

Na influx influx influx influx pump

K efflux efflux efflux* pump

Ca influx influx pump

Action Potential & Mechanical Contraction

Action Potential✦ Fast and slow response

✦ Fast response is more negative✦ Greater up-slope, amplitude, and

overshoot✦ Slow conduction increases rhythm

disturbances

Action Potential✦ ECG records depolarization and

repolarization✦ Atrial depolarization✦ Ventricular depolarization

✦ Atrial repolarization✦ Ventricular repolarization

ECG Complexes

ECG Complexes

Note: No atrial ST segment or T wave due to low amplitude

ECG Complexes

ECG Complexes

ECG Paper✦ Small boxes = 1 mm✦ Large boxes = 5 mm

✦ Small boxes = 0.04 seconds✦ Large boxes = 0.20 seconds✦ Paper speed = 25 mm / sec

ECG Paper

ECG Paper

✦ Standardization mark✦ 10 mm vertical deflection = 1 mVolt

ECG Paper

Top: Low amplitude complexes in an obese women with hypothyroidismBottom: High amplitude complexes in a hypertensive man

✦ Standardization marks✦ Double if ECG is too small✦ Half is ECG is too large

ECG Description✦ ECG description

✦ amplitude (voltage)✦ recorded in mm✦ positive or negative or biphasic

✦ width (duration)

ECG Waves✦ P wave

✦ atrial depolarization✦ ≤ 2.5 mm in amplitude✦ < 0.12 sec in width

✦ PR interval (0.12 - 0.20 sec.)✦ time of stimulus through atria and AV

node✦ prolonged interval = first-degree heart block

ECG Waves✦ QRS ✦ Ventricle depolarization

✦ Q wave: when initial deflection is negative✦ R wave: first positive deflection✦ S wave: negative deflection after the R

wave

ECG Waves✦ QRS

✦ May contain R wave only✦ May contain QS wave only✦ Small waves indicated with small letters (q, r, s)✦ Repeated waves are indicated as ‘prime’

ECG Waves✦ QRS

✦ width usually 0.12 second or less

ECG Waves✦ RR interval

✦ interval between two consecutive QRS complexes

ECG Waves✦ J point:

✦ end of QRS wave✦ beginning of ST segment

✦ ST segment✦ beginning of ventricular repolarization✦ normally isoelectric (flat)✦ changes, elevation or depression, may

indicate pathological condition

ECG Waves

ECG Waves✦ T wave

✦ part of ventricular repolarization✦ asymmetrical shape✦ usually not measured✦ normally upright in lead II

ECG Waves✦ QT interval

✦ from beginning of QRS to the end of the T wave

✦ ventricular repolarization✦ length varies with heart rate (table 2.1)

RR (sec)

HR(bpm)

QT (sec)

1.00 60 0.43

0.40 150 0.27

ECG Waves✦ Rate Corrected QT Interval

✦ QTc = QT divided by square root of RR✦ normal is less than or equal to 0.44

sec.

ECG Waves✦ Long QT interval

✦ certain drugs✦ electrolyte distrubances✦ hypothermia✦ ischemia✦ infarction✦ subarachnoid hemorrhage

✦ Short QT interval✦ drugs or hypercalcemia

ECG Waves✦ U Wave

✦ last phase of repolarization✦ small wave after the T wave✦ not always seen✦ significance is not known

Heart Rate Calculation

✦ Count boxes (for regular rhythm HR)✦ Count the number of large boxes

between two consecutive QRS complexes. Divide 300 by that number

✦ 300 ÷ 4 = 75✦ Count the small boxes. Divide 1500 by

that number✦ 1500 ÷ 20 = 75

Heart Rate Calculation

1500 divided by the number of small

boxes between two R waves

most accuratetake time to

calculateonly use with regular

rhythms1 lg sq = 300

bpm2 lg sq = 150

bpm3 lg sq = 100

bpm4 lg sq =

75bpm5 lg sq = 60

bpm6 lg sq = 50

bpm

300 divided by the number of large

boxes between two R waves

quicknot too accurate

only use with regular rhythm

10 multiplied by the number of R waves

in 6 seconds

less preciseuse with irregular

rhythmsvery quick

Heart Rate Calculation

R-R interval is two large squares. The rate is 300/2=150

Heart Rate Calculation

✦ Count the number of cardiac cycles in 6 seconds and multiple this by 10. (Figure 2.15)

Heart Rate Calculation

✦ Count the number of cardiac cycles in 10 seconds and multiple this by 6.

Irregular rhythm with 21 R-R intervals in 10 seconds. The rate is 21x6=126.

The ECG as a Combination of Atrial and Ventricular

Parts✦ Atrial ECG = P wave✦ Ventricular ECG = QRS-T waves✦ Normally, sinus node paces the heart

and P wave precedes QRS✦ P-QRS-T

✦ Sometimes, atria and ventricles paced separately (e.g. complete heart block)

ECG in Perspective

1. ECG recording of electrical activity not the mechanical function

2. ECG does not depict abnormalities3. ECG does not record all the heart’s

electrical activity

Questions

✦ End of chapter, questions 1-6.