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ECG .pdf



Original filename: ECG.pdf
Title: CARDIOVASCULAR PRACTICAL TWO
Author: Life Sciences

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CARDIOVASCULAR SYSTEM - THE ELECTROCARDIOGRAM, HEART
RATE AND HEART SOUNDS
By Lisa Chopin & Neville Marsh

Learning objectives











Know that the ECG represents the sum of all of the action potentials in the heart at one time
o Both direction and amplitude of overall electrical activity
Be able to draw Einthoven’s triangle
o Show the heart in the center of the triangle
o Be able to label leads I, II and III
Know the features of a typical ECG trace
o P wave
o QRS complex
o T wave
o Isoelectric line (straight line between waves where current = 0mv)
Know the electrical event that is occurring in the heart during each wave or complex and the
mechanical event that this triggers (eg ventricular or atrial diastole or systole)
Know that sinus arrhythmia is a normal change in heart rhythm during the respiratory cycle
o Due to a change in the parasympathetic and sympathetic input during inspiration
o Heart rate increases on inspiration
o Be able to recognize an ECG trace showing sinus arrhythmia
Be able to measure the average heart rate and the instantaneous heart rate from an ECG
o Know the difference between instantaneous and average heart rate
Know what causes the first and second heart sounds

1. THE ELECTROCARDIOGRAPH (ECG)
Principle: The ECG is a recording of potential changes at the skin surface resulting from depolarisation
and repolarisation of heart muscle; the record itself is the electrocardiogram. Electrocardiography is an
empirical science in that it is based on observation: certain appearances in ECG patterns are known to
represent electrical events within the heart but the derivation of the ECG cannot be reduced to hard
physical and mathematical terms.
Standard limb leads:
40

Lead I – RA (Right arm) – neg, LA (left arm) positive
Measures the potential difference between the electrodes on the right and left arms
Measures electrical changes across the base of the heart
Lead II – RA – neg, LL (left leg) – positive
Measures the long axis of the heart from the base to the apex
Lead III – LA – neg, LL- positive
Measures electrical activity along the left side of the heart
Draw Einthoven’s triangle showing:
- leads I, III and III
- LA, RA and LL
- the position of the heart (in the center of the triangle)
- the placement of the +ve and –ve electrodes for each lead
- the direction that electrons travel through the leads

Procedure:
You will need:


As many subjects as possible

Settle a subject comfortably on the bench and attach electrodes to both wrists and both ankles. Run
Lead I, II and III ECG recording for about a minute and reassure the subject that all is well. If you do not
obtain a clean recording, reposition the electrodes or seek help. Measure the heart rate using the
methods below:

41

While there are many ways of measuring the heart rate, using an ECG is thought to be the most
precise method. There are several ways of measuring the heart rate from an ECG. The trace in the
ECG that we are using is designed to travel 25mm/second. Therefore there is one second between
each successive large mark on the paper (red lines along the top).
http://www.rnceus.com/ekg/ekghowto.html
If we measure the time taken between each R wave we are measuring the instantaneous heart rate.
This can be calculated by counting the number of mm between each QRS complex (or R wave) and
use the following formula:heart rate (bpm) = 25/number of mm counted X 60
or, less accurately heart rate =300/number of large (.2s) squares between the complexes
The average heart rate is calculated over time and accounts for the fact that the instantaneous heart
rate can vary. One cause of this is sinus arrhythmia. If the RR intervals are not regular, it is important
that the average heart rate is measured rather than the instantaneous heart rate. This can be
determine by:

counting the number of R waves in 10s and multiplying by 6 or

counting the number of R waves in 20s and multiplying by 3.
Use your ECG to measure the instantaneous heart rate at several points, showing your
calculations and answers below:

Calculate the average heart rate over 6s showing your answer and calculations below:
42

Is there any difference between the instantaneous and the average heart rate?

If so, why might this occur?
Be able to recognise the waves of a normal ECG recording and what these waves represent
mechanically and electrically.
Wave/complex

Electrical event

Mechanical event

P wave

QRS complex

T wave

Look at the traces from leads I, II and III. How do these traces differ from each other and why?

What determines the height of the different waves and complexes?

Which lead has the largest R wave?

Why is the R wave largest in this lead? Use Einthoven’s triangle to explain this.

In what direction does depolarizationof the ventricles occur?
43

What angle does the heart sit in the chest?
2. DEMONSTRATION OF SINUS ARRHYTHMIA
Heart rate (HR) is influenced by normal respiratory rhythm. In healthy young individuals, heart rate
increases during inspiration and decreases during expiration and this is called sinus arrhythmia. It is a
normal occurrence which is due to fluctuations in parasympathetic input to the heart. During inspiration
stretch receptors in the lungs send inhibitory messages via the vagus to the cardioinhibitory centre in
the medulla oblongata. The lifting of this inhibition, therefore, increases the heart rate.


Record Lead I in the resting subject for ~ I minute noting the point of inspiration. Some level of
sinus arrhythmia should be apparent, ie. the R-R interval will rise and fall in a cyclical fashion.



Repeat whilst the subject breathes as deeply as possible but at normal respiratory rate. Any
sinus arrhythmia should be accentuated.



Repeat while the subject hold their breath for as long as possible

Record maximum/minimum heart beats during the respiratory cycle.

Explain in your own words how sinus arrhythmia occurs.

What happens when you take a really deep breath and hold your breath for as long as you can?

Why?

44

3. DEMONSTRATION OF THE "DIVING BRADYCARDIA REFLEX"
A series of cardiovascular and respiratory adaptations allow aquatic mammals to leave the surface
of the water for long periods of time (Schagatay et al., www.biol.lu.se/zoofysiol/Johan/Abstract_9.html).
This diving response is shared by humans. It serves to limit overall oxygen use which protects the
heart and the brain from a lack of oxygen. Heart rates can be reduced as much as 40-50%. This can
be induced in humans by chilling the face while holding breath. This response is elicited by
stimulation of the parasympathetic nervous system and cold is detected by the ophthalmic branch
of the trigeminal nerve (cranial nerve V).

You will need
ECG
Volunteers
Timer
Ice
Bucket
Water
Thermometer









Instructions


Calculate HR before the start of experiment (control) and then every 10seconds and record
in the table below.





Normal breathing
o

Record Lead II ECG for 1 minute (resting heart rate)

o

Calculate HR every 10s and fill in table

Breath holding
o

Ensure that the subject is relaxed

o

instruct the subject to take a fairly large (but not maximal) inspiration and hold
breath for as long as possible

o

Record lead II for 1 minute (breathing)


Mark beginning and end of the breath-hold



Allow the subject to rest for a minute and then record the heart rate for the control period



Breath holding + cold
o

Fill the tub with water and fill with ice until the temperature measured by the
thermometer reaches 10C

o

The subject holds their breath and submerges their face and forehead into a bucket
of water at 10C



Graph your results using excel on the computer. Give your results to the demonstrators to
45

graph.

time

control

0s

10s

20s

30s

40s

50s

60s

normal breathing
breath holding
breath holding and head
immersion

Which cranial nerve is detecting the cold pack?

Which cranial nerve causes the heart rate to drop?

Is the response more marked when the subject is breathing or breath holding? Why?

Why might we have this diving reflex?

What effect does an ice pack placed on the face have on heart rate?

What effect does holding the breath have on heart rate?

What is bradycardia?

What is tachycardia?

46

4. HEART SOUNDS
Two main heart sounds can be heard during the cardiac cycle:
• the first is a long low-pitched sound which occurs when the atrioventricular valves close
• the second, a short higher pitched sound, is heard when the aortic and pulmonary valves close.
Traditionally, these sounds are likened to the phrase “lubb-dupp” (however, non-English speaking
people know these sounds by other expressions: Germans use “doop-teup” and Indians, ta-rrupp”!)
Using a stethoscope, listen to your partner’s chest by placing the bell of the stethoscope in the fifth
intercostal space on the left side of the chest. You should hear the first heart sound. Move the
stethoscope to the second intercostal space and listen for the second heart sound.
Using the computer simulation, listen to the heart sounds program and compare what you hear with
the real thing.
Explain how the heart sounds are produced and which mechanical events they represent.

Using the diagram below insert lines showing where the normal heart sounds (H1 and
H2) occur. You can work this out by knowing what is occurring mechanically during each
stage of the ECG.

Revision questions:
Draw a standard ECG trace and label
o P wave
o QRS complex
o T wave
o 0mV
o Isoelectric line
 On the graph list the electrical events that are occurring and the mechanical events that
occur as a result of this.


47

What is the natural pacemaker of the heart?

What is the natural pace set by the pacemaker?

What is a normal resting heart rate?

What part of the nervous system increases heart rate?

What part of the nervous system decreases heart rate?

48


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