Analyse and evaluate the effect of exercise on the ECG trace

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5.3a Analyse and evaluate the effect of exercise on the ECG trace

        The heart is a hollow, muscular organ which through a muscle contraction generates the force to circulate blood throughout the body. Heart contractions result from a series of electrical potential changes, this is known as depolarization waves. The depolarization waves travel through the heart just before to each beat.

(Seeley 2000)

        Two systems control the effects of the contraction on the heart.

  1. The first is the autonomic nervous system, which is situated through the vagus nerve. This nervous system controls the accelerations and decelerations of heart rate.
  2. The second system is the intrinsic conduction system which ensures that heart muscle tissue depolarizes in order, e.g. from atria to ventricles, which therefore results in a coordinated heart beat.

The intrinsic conduction system is composed of several important components,

  • The SA (sinoatrial) node,
  • The AV (atrioventricular) node,
  • The bundle of His,
  • Right and left bundle branches  
  • The Purkinge fibres. (Seeley 2000)

Fig1 – A diagram of the heart. Featuring the main components of heart contraction

adopted from  

In Fig1 it can be seen when the SA node starts the depolarization wave that the components spread the waves from the top, in the atria, down through the ventricles.  In a healthy heart, the electrical signal is immediately followed by muscle contraction.

The heart is made up of many different muscle fibres each arranged a particular way. So that the muscle fibres are aligned together with a space in the middle of them, they are called chambers and this is where blood sits. When the fibres contract, they exert a force on the wall of the chambers and this decreases the space.         The force of the compression on blood puts blood under pressure. This is how blood pressure is developed.  

        

There are four chambers of the heart:

  • Two small atrium chambers
  • Two large ventricular chambers

        The atria receive blood from the body back to the heart and the ventricles pump blood from the heart out to the body. When the heart beats it is known as the heart contraction cycle.  This cycle begins in the atria then spreads to the ventricles.   The two sides of the heart beat simultaneously. The huge number of fibres contracting generates a large electrical field that is easy to measure from the surface of the body and this is known as the Electrocardiogram (ECG). (Seeley 2000)

        Scientists find the best way to study the hearts activity is through the use of surface electrical recording of the electrical field caused by the depolarization waves beginning from the SA node of the heart.

Fig2 - The ECG wave diagram

adopted from (Bennett 1993)

        Fig2 shows the different waves and what they are commonly known as.

A full complex wave is known as the PQRST wave.

        

The first detection of the depolarization wave is the P wave in the atriums. Then wave continues through the cycle.

P Wave

Electrical impulses starting in the SA node trigger atrial depolarization. The normal P wave is about 0.1 seconds long in duration and approximately 2.5mm high. The direction of electrical activity is from SA to AV node. The P wave is a representation of the time it takes for atrial depolarization. It is viewed normally as small and curved with a positive deflection. Seen at it's tallest on lead II. (Rowell and Shepherd 1997)

 

T Wave

Ventricular repolarization which follows ventricular depolarization, is represented by the T wave. Its shape is rounded and taller and wider than the P wave. It is also more sensitive to physiologic and hormonal changes in shape but usually presents as a positive deflection 5 -10mm in height. (Rowell and Shepherd 1997)

 

U Wave

After T wave an ECG can sometimes show a U Wave. It is of the same deflection as T Wave and similar to shape to P Wave. The U Wave is thought to represent late repolarization of the Purkinje fibers in the Ventricles and is more often not shown on a rhythm strip. (Rowell and Shepherd 1997)

The Complex wave

In he ECG trace there is one complex wave which is the QRS. It can be described as when one wave follows another without intervals between them. The QRS complex represents ventricular depolarization. It consists of three waveforms. The normal complex begins with a downward deflection known as the Q wave, followed by an upward deflection called the R wave. The next downward deflection will be the S wave. All ventricular complexes are known as QRS complexes even if every wave is not present in all complexes. The normal QRS is 0.04 to 0.10 seconds. (Rowell and Shepherd 1997)

 

 

PR Interval

This is the length along the baseline from the beginning of the P wave to the beginning of the QRS complex. This Is normally 0.12 to 2.0 seconds in duration.

QT Interval

This is the beginning of the QRS complex to the end of the T wave. In the presence of a U wave the measure should be from the beginning of the QRS complex to the end of the U wave. (Rowell and Shepherd 1997)

ST Interval

This is the length between the end of the S wave of the QRS complex and the beginning of the T wave. It is electrically neutral. (Rowell and Shepherd 1997)

PR Interval

This segment represents the delay in conduction from atrial depolarization to the beginning of ventricular depolarization. It is also electrically neutral. (Rowell and Shepherd 1997)

Fig3 – The PQRST wave and the timings between components

adopted from (Sherwood 1998)

        By counting the number of PQRST complexes per unit time, you can determine the rate of heart contraction, by measuring internal differences in the timing of the ECG. You can also record the timings of the ability of the electrical signal to move throughout the various regions of the heart. Fig4 also shows the average time in seconds for each wave segment of the ECG.

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        The ECG can also be used to assess the elasticity of the vessels that carry blood away from the heart. The Pulse wave velocity is calculated by dividing the distance between the heart and the wrist, by the time difference between ventricular contraction and generation of pulse pressure in the wrist. (Seeley 2000)

When a person undertakes strenuous exercise the muscles require more energy, in the form of ATP. Therefore, aerobic respiration increases and the heart and lungs must deliver more oxygen to the mitochondria of muscles through the blood.  The heart can increase the quantity of blood ...

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