Workshop 3: The Electrocardiogram

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Workshop 3The Electrocardiogram

• Keith Clements

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The Heart

• The heart consists of four chambers, arranged
  into two pairs.
• Blood enters the atria, which contract pushing
  the blood into the ventricles.
• The ventricles then contract forcing the blood
  into the circulatory system.
• The circulation
• The right atrium receives blood from the general
  circulation via the vena cavae.
• The right ventricle sends blood via the pulmonary
  artery to the lungs.
• Oxygenated blood enters the left atrium via the
  pulmonary vein, and the left ventricle sends this
  oxygenated blood into the aorta and out to the
  body.

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The heartbeat

• The cycle of contraction is coordinated by
  electrical signals generated by the sinoatrial
  node.
• The signals generated are conducted throughout
  the heart, causing the atria to contract
  immediately before the ventricles.
• The sinoatrial node is regulated by the autonomic
  nervous system. The sympathetic nervous system
  increases the heart beat and force of
  contraction, the parasympathetic nervous system
  slows the heart beat.
• The sinoatrial node has a natural rhythm of
  over 100 beats per minute (BPM), the average
  resting heart beat is around 70 BPM.

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The ECG
P Q R S T

• Divided into distinct waves, corresponding to
  events in the cardiac cycle.
• The P wave reflects depolarization of the atria,
  the QRS complex reflects depolarization of the
  ventricles. Repolarization of the ventricles
  gives rise to the T wave.

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Influences on the heart beat

• Respiratory Sinus arrhythmia
• The heartbeat typically speeds up and slows down
  in time with breathing. The inhibitory effect of
  the parasympathetic nervous system, via the vagus
  nerve, is reduced during inspiration. RSA can be
  used to estimate the parasympathetic input, or
  vagal tone.
•  
• Task demands
• The hearts function is to ensure bodys tissues
  receive oxygen. Increases in demand trigger a
  corresponding increase in heart rate. Changes in
  heart rate during stress, cognitive demands,
  etc., can sometimes be explained as due to
  changes in motor activity. However some
  situations may produce changes in excess of those
  needed to meet oxygen demand.

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Workshop Three

• We are going to record the ECG during four
  activities
• Four segments
• Sitting down
• Breathing in vs. breathing out
• After mild exercise
• During mental arithmetic

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Analysis

• Measurement boxes 
• CH1 ?T Delta Time, or duration of the selected
  region
• CH1 BPM Beats Per Minute
• CH1 Delta (?) Delta Amplitude, or change in
  amplitude from the start to the end of the
  selected region
• CH1 Max Maximum Amplitude

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What we want

• All segments
• Heart rate in BPM, by dragging from one R-wave to
  the next. Separately for inhalation exhalation
  in seg. 2. At the start and end of seg. 3.
• Segments 1 3
• Q-T interval, ventricular systole, when the
  ventricles are contracting
• End of T wave to subsequent R wave Ventricular
  diastole
• Segment One
• Amplitude (onset-peak) and maximum for R, T
  waves.
• Each measurement obtained from 3 cycles

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Further Reading

• For information on the physiology of the
  cardiovascular system, ECG measurement and some
  of the applications of the ECG see chapters nine
  and ten in
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• Hugdahl, K. (1995). Psychophysiology The
  Mind-Body Perspective. London Harvard University
  Press.