Laboratory for Physiology ECG = Electrocardiography

Electrocardiogram Similar to EMG in that we will be recording the electrical activity leading to muscle contraction, however this time we are looking at cardiac rather than skeletal muscle contraction Thus ECG is a display of the electrical events of the cardiac cycle The contraction of the chambers of the heart and the opening and closing of the heart valves leading to the pumping of the blood are called the mechanical events of the cardiac cycle Next week we will look at the mechanical events and relate them to ECG but this week we are going to look at the electrical events of the cardiac cycle

Cardiac Muscle Cells Depolarize just like skeletal muscles fibers Thus we record their electrical activity using electrodes in a similar set up as when we did EMG recordings

Heart Blood Flow Artery vs Vein Contraction of the heart muscle Maintains a ONE WAY flow through the use of valves and an orderly contraction Contraction of the heart muscle FIRST: Atria contract This pushes the last bit of blood out of the atria and into the ventricles (both L and R) SECOND: Ventricles contract Pushes blood out into the arteries via either the pulmonary artery or the aorta Artery vs Vein Artery = Away from heart Vein = towards heart

Electrical Field Potentials Created by the waves of depolarization that sweep across the heart. This is what we measure using the ECG leads. EFPs are conducted through the body fluids to the body surface and we can record these field potentials using electrodes placed on the body surface Thus an ECG recording is NOT the depolarization of individual muscles cells but rather is the electrical field potential created by each part of the heart muscle as it depolarizes IMPORTANT: The size of the field potential is determined by the amount of muscle tissue that is being depolarized to create the field potential Thus the greater the mass of muscle tissue being depolarized, the more ions that are moving across the cardiac muscle cell membranes the greater the amplitude of the electric field potential that is created. Larger muscle mass more ions crossing membrane Larger EFP amplitude = =

5 ELECTRICAL FIELD POTENTIALS

Figure 14-21 - Overview (1 of 9)

Recording of ECGs in the laboratory What is a Lead? The particular arrangement of 2 electrodes (+/-) with respect to a 3rd electrode (the ground) Lead I -/- RS + /-LS Lead III Lead II +/+ LH Together these 3 leads form an electrical triangle around the heart which is called Einthoven’s triangle-1901 inventor of the ECG

Recording of ECGs in the laboratory When a wave of depolarization is directed TOWARDS the + pole, it is displayed as an upward deflection in the ECG recording. When a wave is directed TOWARDS the – pole, it is displayed as a downward deflection. We will use three bipolar limb leads, Lead I, II and III

-/- + /-LS -/- + /-LS -/- + /-LS -/- + /-LS Lead I Lead I -/- RS + /-LS -/- RS + /-LS 1 1 2 Lead I -/- RS + /-LS Lead I -/- RS + /-LS 1 1 2 4 2 3 LV 3

Lead I -/- RS + /-LS 1 2 4 3 5 Leads II and III will give similar ECG recordings and you can work through these to understand the waves you get on these leads on your own.

LAB ACTIVITIES PART 1 Record ECG activity using LEAD II PART 2: The data analysis will guide you through the analysis of the ECG recording that you get in this part PART 2: Examination of the effects of deep breathing on the rhythm of the heart.

Respiratory Arrhythmia Deep breathing causes an arrhythmia called respiratory arrhythmia It is due to the variation in autonomic activity during the breathing cycle. The amount of variation depends upon the individual. 2. EXHALE= Sympathetic Parasympathetic INHALE= Sympathetic Parasympathetic HR HR

HINT PART II Subject should breath SLOW and DEEP to get good results. Recorder must pay close attention to hit the F9 key to demarcate inhalation and exhalation or you will not be able to interpret your results and therefore not score well on the hw questions After the deep breathing exercise have the subject exhale as much as possible and hold their breath for a short time to see how HR is affected. Remember the recorder should mark the time when breathing stops and begins again using the F9 flags.

ECG PART 3 Einthoven’s Law You will record from leads I and III and from these recordings you’ll generate the calculated ECG recording for Lead II based on Einthoven’s Law The amplitude of the peaks in Lead I + Lead III= Lead II Equilateral triangle

Enjoy the lab HAVE FUN DO NOT RUSH!!!!! QUESTIONS? Enjoy the lab HAVE FUN DO NOT RUSH!!!!!