Lecture 2 Notes 2024.docx

Lecture 2: The Electrocardiogram (ECG) 1. Uses of the Electrocardiogram An electrocardiogram, or ECG (also called an EKG) is a measurement of electrical activity from the heart (that spreads to the surface of the body), allowing you to quantify heart rate, as well as see how this activity changes un...

Lecture 2: The Electrocardiogram (ECG) 1. Uses of the Electrocardiogram An electrocardiogram, or ECG (also called an EKG) is a measurement of electrical activity from the heart (that spreads to the surface of the body), allowing you to quantify heart rate, as well as see how this activity changes under different conditions (i.e., during disease) 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. The trace below is the “stereotypical” trace that you see when an ECG is recorded using a standard lead II limb lead configuration (see below). The traces can and will look different depending upon which electrode lead configuration is in use (see below). There are several important components to the trace produced by an ECG recording. These will look different with different lead configurations but for a “standard trace” 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 moves 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 if this axis shifts left or right this can tell us about various disease states in the heart. The ECG can also be used to measure heart rate, including analysing either bradycardia (a slow heart beat) or tachycardia (an elevated one). Sometimes these terms are rather rigidly defined (the former being less than 60 beats/minute, the latter more than 100 beats/minute), but they can also be used in the very general sense used here, i.e., a slowing 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 (increased in muscle mass), which is a particular problem because a heart that has 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 cardiac 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 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, Bipolar 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 undergone to a medical physical exam will have experienced leads being 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 in a non-medical setting. 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 (LA) to the right arm (RA), lead II goes from the right arm (RA) to the left leg (LL), and lead III goes from the left leg (LL) to the left arm (LA), 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 II and III, the right arm is negative for both leads I and III, and the left arm has one negative lead (III) and one positive lead (I). This bring us to Einthoven'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 come to measuring the electrical axis of the heart. So we have these three leads in an equilateral triangle around the heart. If we map the position of the leads over an image of the heart, we realise that the leads form a 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 at bipolar 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 twelve different leads (plus a ground), 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 true 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. 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. 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 consists of three points: a small downward deflection called the Q-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 an ECG trace recorded from a bipolar limb lead, no matter which lead you are looking at. They can look different if we use an augmented unipolar limb lead or a chest lead (see below). 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. 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; this is 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 has less 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 up by the ECG electrodes. As we proceed from the depolarisation of the pacemaker cells through transmission in the internodal pathways in the atria, this conduction through the atria is reflected in the second half of the upward swing in the P-wave. Then, finally, as the atria fully depolarises, the third phase of the P-wave takes place: the deflection turns downward, but as it is still above baseline, it still represents a depolarisation. Once the atria has depolarised, the waves of depolarization converge onto the AV node, which is the only non-pathological place electrical activity can move from the atria to the ventricles. This causes a slight delay between the P-wave and the QRS complex. The flat segment on an ECG trace between the P-wave and the QRS complex reflects the time it takes for electrical activity to conduct through the AV node. Once the electrical activity has passed through the AV node, and gone through the branch bundles and the Purkinje fibers, then the mass depolarisation of the ventricles and the corresponding repolarisation of the atria begin, and this is reflected as the QRS complex. When we look at the electrical axis of the heart, we will look more closely at what exactly the Q, R and S waves refer to, but taken together the QRS complex represents ventricular depolarisation and atrial repolarisation. The T-wave represents, then, ventricular repolarisation. 5. Augmented Unipolar Limb Leads and Unipolar Chest Leads In a clinical setting, there will almost certainly be more leads than merely the standard bipolar limb leads, and there are two other sets of leads commonly used. The first set of leads are known as augmented unipolar limb leads. These leads use exactly the same electrodes as leads I, II and III - thus there are still only three electrodes attached to the body (RA, LA and LL). The difference is that these three augmented unipolar limb leads are all positive, and the combination of the other electrodes forms the reference (i.e., a negative pole) in which to compare the electrical activity in the first (positive) lead to. These leads are named, with “a” standing for “augmented”, aVL (left arm), aVR (right arm), and aVF (left leg). The orientation of these leads is such that the direction of the lead on the left arm goes at a -30 degree angle, the lead on the right arm is on a -120 to -150 degree angle, and the left leg lead is on a +90 degree angle, with reference to bipolar limb lead I as 0 degrees. aVR; right arm (RA) is the positive end while the electrical potential difference between the left arm (LA) and left leg (LL) is the negative end. aVL; left arm (LA) is the positive end while the electrical potential difference between the right arm (RA) and left leg (LL) is the negative end. aVF: the left leg (LL) is the positive end while the electrical potential difference between the right arm (RA) and left arm (LA) is the negative end. If we look at the ECG traces of the augmented limb leads (aVR, aVL and aVF), we can see that they are at once both similar and quite different to the traces of the bipolar limb leads (I, II and II). The trace from aVR is almost a mirror image, simply flipped towards the negative, of the trace from lead II - this is because lead II, at +60 degrees, and aVR at -120 to -150 degrees, are almost opposite each other (180 degrees apart). There are also six unipolar chest leads that can be placed upon the body, and they are referred to as leads 1 through 6 (V1 to V6). They are placed in a horizontal line across the left half of the ribcage, providing a surrounding view of the heart, and allow for a perpendicular image of the electrical activity in the heart. They produce ECG traces that are as different form each other as they are from the more familiar traces of the bipolar limb leads. When you are getting a full diagnostic of heart activity in, for example, a clinical setting, all 12 leads may be put into play, as well as other, less common lead positions - all to achieve a more thorough diagnosis. ECG interpretation is so complex and varied; it is almost a field of medicine unto itself. 6. Segments of the ECG These deflections (P-wave, QRS Complex and T-wave) are important in telling us about cardiac activity, but the segments in between deflections are important also, and there are four that we are primarily concerned with. The first segment is called the R-R distance, the distance between the R-wave of one QRS complex and the next. This distance can be used to calculate heart rate, which is simply 60 divided by the length of time of the R-R distance (so if it is 0.75 seconds, heart rate is 60/0.75 = 80 beats per minute). Normally heart rate is measured over a longer period of time (for example, a minute or 15 seconds), but if you want to know exactly what is happening in the heart in one specific moment of time, you can calculate what is called the instantaneous heart rate by using the R-R distance. The second distance, either called the P-Q distance or the P-R distance, is the time between the start of the P-wave until the start of the QRS complex. The Q-wave and the R-wave are so close together, that you see either name in different sources. The P-Q distance reflects the time of conduction through the AV node. The next segment is the Q-T distance, from the onset of the QRS complex to the end of the T-wave. It represents the time that the ventricles are contracting; that is, it represents the ventricular systole. The next segment is the T-Q distance, which goes from the end of the T-wave to the beginning of the next QRS complex, and it reflects the period in which the heart is relaxing after beating. This relaxation is called ventricular diastole. At the end of the T-wave the ventricles have finished repolarising, and they won't begin to depolarize until the start of the next QRS complex. Through looking at the three deflections and the four segments, we can learn a lot about the heart, and whether it is affected by various disease conditions. 7. Normal Heart Rhythms and Arrhythmias Next, we are going to look at some conditions that will alter the ECG trace, and how to find out if the heart is functioning normally or not. When the heart is functioning properly, and the ECG looks normal, we refer to the rhythm as a normal sinus rhythm. Sinus refers to the sinoatrial (SA) node). Impulse originate from the SA node between 60 and 100 times a minute, the P-waves are upright and of uniform size and contour from beat to beat, and each P-wave is followed by a QRS complex, with a P wave:QRS complex ratio of 1:1. These are all signs of a normal, healthy heart. When we look at regulation of heart rate, we'll see that this rhythm is significantly influenced by nervous input on the pacemaker cells. In a trace showing bradycarda, the trace looks much the same, except the space in between beats are larger; and the reverse is true of a heart with tachycardia. We use the term arrhythmia to refer to irregular or abnormal rhythms. Sinus arrhythmia shows up on an ECG trace as unevenly-spaced QRS complexes. In this case the heart beat is irregular but not necessarily extreme in its irregularity. The P-waves and P-R distances, however, will be the same (because of their atrial origin). Sinus arrhythmia will disappear with exercise, or the holding of one's breath. Another arrhythmia consists of a wandering atrial pacemaker. We know that the SA node is the normal pacemaker site; however, if it stops functioning other areas of the heart can take over. The SA node is the normal pacemaker simply because its rate of depolarisation is the fastest of any region in the heart. The AV node, as the second fastest, will take over if the SA node fails, and other areas can take over from the AV node if necessary. On an ECG trace, if the P-waves are not uniform, and the P-R distance changes, then it suggests that the heart has a wandering pacemaker. Under normal conditions, electrical activity is generated from the atria; however, in some circumstances the pacemaker will be found somewhere in the ventricles. If this happens, then the QRS complex will look abnormally wide on an ECG trace. We can see premature beats and late beats on an ECG trace; a trace may look normal for the most part, but have the occasional wide QRS complex, or a slightly longer gap between beats. Heart block refers to a slow or absent conduction of electrical activity from the atria to the ventricle. It is classified based on severity, form first to third degree. First degree heart block is for the most part normal, with the exception of the P-R distances, which are slightly prolonged (>0.2 sec). This is not generally a dangerous condition, and can occur in highly-trained athletes, or people with enhanced vagal tone (which is nervous control of the pacemaker activity). Second degree heart block is an intermediate phase, where the electrical activity is only intermittently blocked. The trace for the most part looks normal, except that on occasion, there are P-waves which are not followed by QRS complexes, meaning transmission of activity through the AV node is not occurring. The most severe type of heart block is third degree heart block, in which there is no transmission of electrical activity from the atria to the ventricles. Thus, on a trace, there are P-waves, but they are not followed by QRS complexes, and if a QRS complex does directly follow a P-wave it is simply coincidence. The QRS complexes are wide, (and sometimes 'm' shaped if other conditions are also present) - indicative of pacemaker rhythm being generated in the ventricles. QRS complexes generated in the ventricles are known as escape QRS complexes, and when they originate above the bundle of His, the heart will be stable; but below it, the heart will be unstable and in a serious problem.