Electrical Activity of the

Heart

Electrical Activity of the

HeartTopic Number 2Topic Number 2

Introduction

✦ What’s really happening when the heart is stimulated or where does the “electro” in electrocardiography come from?

Under this condition, the heart cell is said to be polarized

Polarization

✦ Two micro-electrodes

✦ One outside the cell, one inside the cell

✦ Difference between the two (for example, -90 mV inside)

✦ The cell is said to be ‘polarized’

Action Potential

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Action Potential

✦ Ion influx

✦ Fast channel for Na

✦ Slow channel for Na and Ca

✦ maintains action potential which increases force of contraction

Action Potential

Skeletal

Cardiac

Ions

Ion Extra- Intra-

Na 140 10

K 4 135

Ca 2 0.1

Inside

Outside

thevirtualheart.org/CAPindex.html

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✦ Absolute refractory period

✦ Phase 2 ✦ Ions

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

✦ Plateau✦ Relative refractory period

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

Refractory Periods✦ Absolute refractory period - Phase 1

✦ Relative refractory period - Phase 2

✦ Effective refractory period - Phase 1 & 2

✦ The effective refractory period is shorter in the atria node and longer in the ventricles.

SA Node Action Potential

✦ No fast sodium channels

✦ Calcium channels

✦ Long-lasting, L-type

✦ Transient, T-type

✦ Potassium channels

✦ “Funny” currents; slow Na channels that initiate spontaneous depolarization

Action Potential✦ Fast and slow

✦ Fast response is more negative to start with

✦ Faster up-slope with greater amplitude, and overshoot

✦ Slow conduction increases rhythm disturbances

❖ Conduction speed varies throughout the heart

❖ Slow - AV node

❖ Fast - Purkinje fibers

Action Potential

✦ ECG records depolarization and repolarization

✦ Atrial depolarization

✦ Ventricular depolarization

✦ Atrial repolarization

✦ Ventricular repolarization

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.

ECG Complexes

ECG Complexes

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

ECG Complexes

ECG Complexes

Action Potential & Mechanical Contraction

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 accurate•take time to calculate•only use with regular rhythms

1 lg sq = 300 bpm

2 lg sq = 150 bpm

3 lg sq = 100 bpm

4 lg sq = 75bpm

5 lg sq = 60 bpm

6 lg sq = 50 bpm

300 divided by the number of large boxes between two R waves

•quick•not too accurate•only use with regular rhythm

10 multiplied by the number of R waves in 6 seconds

•less precise•use with irregular rhythms•very 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 abnormalities

3. ECG does not record all the heart’s electrical activity

Questions

✦ End of chapter, questions 1-6.