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Lecture 2

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University of Toronto Scarborough
Biological Sciences
Stephen Reid

1    Lecture 2: The Electrocardiogram (ECG) 1. Uses of the Electrocardiogram An electrocardiogram, or ECG (also, in some places, called an EKG) is a measurement of electrical activity from the heart (that spreads to the surface ofthe body), allowing you to quantify heart rate, as well as see how this activity changes under different conditions over time. Thus, the ECG is a powerful diagnostic tool. The electrocardiogram produces the well-know trace seen on the screen of a heart monitor. There are several important components to the trace produced by an ECG recording: there are generally three deflections above baseline, and there are also segments between deflections - and all of these tell us about heart function. One of the things we can look at with an ECG is the electrical axis of the heart, which we will talk about more in the next lecture. However, in brief, whilst electrical activity occurs throughout the heart in all directions, there is a mean axis in which electrical activity flows, titled at about 60 degrees through the middle of the heart, and how it shifts left or right can tell us about various disease state in the heart. The ECG can also be used to measureheart rate, including analysing eitherbradychardia (a slow heart beat) ortachycardia (an elevated one). Sometimes these terms are rather rigidly defined (the former being less than 60 beats/minute, the lattermore than 100 beats/minute), but they can also be used in the very general sense used here, i.e., a sowing or speeding-up of heart rate in general. We will look also at arrhythmias, and we will see how the heart has a normal rhythm that can be disrupted. This disruption can be either superventricular (above the ventricles) or ventricular - it is the ventricular arrhythmias that are particularly dangerous to the health of the heart. We will look at sequence activation disorders. In other words, an ECG trace can reveal abnormalities in the conduction of the waves of depolarisation through the heart or disruptions in the normal transit of electrical activity in the heart (e.g., abnormalities in movement through the AV node or branch bundles). An ECG can tell us whether the heart has undergone hypertrophy, which is a particular problem because a heart thathas grown too much muscle will have difficulty pumping blood properly. There are changes in the ECG when the coronary circulation is disrupted and the heart becomes ischemic. There are also changes in the ECG if heart tissue dies, or if there is a heart infarction (a heart attack). Drugs such as digitalis can have effects on heart rhythm and rate and this too can be seen on an ECG. Electrolyte imbalances in extracellular fluid and the and 2    blood can also cause changes in the ECG as can infections of the heart such as myocarditis (infection of cardiac muscle) and peritonitis (infection in the peritoneal cavity. 2. ECG Measurements, Limb Leads and Einthoven’s Triangle and Law The ECG is essentially measuring electrical activity on the body surface that originates in the heart. As the heart depolarises, in its normal sequence, the electrical activity spreads throughout the body, and we can detect this using electrodes placed upon the body surface. Many who have gone to a physical will have experienced leads be ing placed upon the chest and arms, and this gives a very accurate ECG reading from multiple angles; however, simpler bipolar limb leads are more common. For simple bipolar ECG limb leads, electrodes are placed on the left and right arms, and the left leg. By convention,lead I goes from the left arm to the right arm,lead II goes from the right arm to the left leg, and lead III goes from the left leg the left arm, forming a triangle around the heart, called Einthoven's Triangle. There are positive and negative sides to each lead, and so the left leg is positive for both leads, the right arm is negative for both, and the left arm has one negative lead (III) and one positive lead (I). This bring us toEinthoven's Law, which states that in the electrocardiogram, in any given instant, the potential in any wave in lead II is equal to the sum of the potentials in leads I and III. We will look at measuring potential differences in the leads, when we cometo measuring the electrical axis of the heart. So we have these three leads in an equilateral triangle around the heart. If we map the courses of 3    the leads over an image of the hear t, we realise that the leads forma star pattern over the heart, with lead I crossing through the center horizontally, and leads II and III going through the centre at opposing 60 degree angles (i.e., lead I is at 0 degrees, lead II is at 60 degrees and lead III at 120 degrees). Movements of electrical axes around these ranges can be used as a diagnostic tool for different diseases. We are going to be looking atbipolar limb leads primarily from the perspective ECG activity measured via lead II. Before we see what the various components in the ECG actually reflect, we will look at see what are actually causing negative or positive deflections in these traces. If we look at traces from all three leads, we see quite similar patterns, with two slightly rounded positive deflections, and a much sharper one in the middle. These are typical ECG patterns, though there are many other lead configurations, and physicians will often have maybe twelve to fifteen different leads, providing many more views of the heart’s electrical activity. If we look at lead I, with a negative side on the right arm and a positive side on the left arm, if a wave of depolarisation heads toward the positive electrode (left arm), then we will get a positive deflection in lead I. if a wave of depolarisation travels away from the left arm, then a negative deflection will appear in lead I. The reverse is ture for waves of repolarisation. Similarly, a wave of depolarization travelling toward the left leg will appear as positive deflections in leads II and III. The maximum possible deflection will occur when the waves (either depolarization or repolarisation) occur exactly parallel to the lead. So as we look at the stages of electrical transmission in the heart, starting from the SA node down through the AV node, up through Purkinje fibers into the ventricles, we can tell which direction current is flowing at any particular moment by seeing whether a positive or negative deflection occurs on any given lead in the ECG trace. 4    3. Components of the ECG In the standard bipolar limb lead configuration (we will focus on lead II), there are three standard deflections. The first, small deflection is called the P-wave, and it is associated with the depolarization of the atria. We'll see that there are three general stages of the P-wave; the first is due to the pacemaker potential, the second is due to the spread of electrical activity through the internodal pathways, and the final phase is due the depolarisation of the muscle tissue within the atria. The next component consistsof three points: a smalldownward deflection called theQ-wave, then a large positive deflection called the R-wave, and then another downward deflection (this time slightly larger) called the S-wave. This combined QRS deflection is what makes up the blip on an ECG that is most familiar to us. It reflects the depolarisation of the ventricles, however, also hidden within this blip is the activity associated with repolarisation of the atria. Given that the ventricular muscle is much more massive than the atrial muscle, it is hard to distinguish the two separate events (i.e., ventricular depolarisation masks atrial repolarisation), and we simply group the two events together as the QRS complex. The final deflection comes a little while after the QRS complex, and is called the T-wave. It represents the repolarisation of the ventricle, and looks like a somewhat larger version of the P-wave. These three deflections will always appear on anECG trace recorded from a bipolar limb lead, no matter which lead you are looking at. Soon, we're going to look at the segments between these components, and they too will be indicative of the various phases of electrical activity in the heart. 5    4. ECG Components and the Stages of Electrical Transmission in the Heart We can relate the various components of the ECG trace, whether the positive or negative deflections, or the segments in between these events, to the stages of depolarisation of the cardiac muscle, and the changes in electrical conductivity in the heart. On an ECG, we see that the P-wave is significantly smaller than the QRS complex; thisis because the electrical activity generated (and detected by the ECG electrodes) is proportional to the amount of muscle tissue there is to depolarise and since the atria hasless muscle than the ventricles, its depolarisation produces much smaller deflections than does the depolarisation of the ventricles. Any disease state that leads to growth of muscle mass in the ventricles will result in a larger QRS complex, and when we look more closely at the electrical axis of the heart we'll see how it too is affected by hypertrophy of the cardiac muscle. The pacemaker potential in the heart is reflected in the ECG trace as the first phase of the P-wave. So the early phase of the P-wave reflects the electrical activity that is travelling, from the pacemaker cells in the SA node, across the body, to be picked u
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