ECG Interpretation

 

So let's start with the most basic way of interpreting the ECG the first thing you do is you have to check the voltage calibration what's this about this is about the amplification of the image that the ECG machine is going sometimes with arrhythmias we want to enlarge the complexes so we will double the the amplification of the image but usually for almost all routine ECGs the calibration is so-called 10 millimeters that's 10 little boxes and we'll show you examples of that and it's always good to check that to begin with because if they had been using the Machine at a higher amplification that complexes are gonna look bizarre they're gonna be big and so forth and you might make a mistake in reading so first make sure that that the calibration is correct usually the technician takes care of that that but it's there's a little box that shows you the calibration is okay and I'll point that out later the next thing is to determine the rhythm and how do you do that you're looking for P waves followed by qrs's in other words the normal progression remember starting up with the sinus node atrial depolarization P wave QRS ventricular depolarization T wave resetting once you know what the rhythm is in other words is this a normal sinus rhythm or is it not normal sinus rhythm but uh but in arrhythmia and we're gonna have whole lectures about the kinds of abnormal electrical events that can occur when it's not sinus rhythm you then calculate the heart rate which is a there's a normals there.

I'm going to be going over the normals as we go along you then do the timing intervals what's the PR interval from the beginning of the P wave to the beginning of the QRS what's the QRS duration that's the period of ventricular depolarization and then how long does it take for the whole contraction and resetting of the ventricle from the Q wave to the end of the T wave we would then determine the electrical axis in other words what is the main vector force of the electrical depolarization wave what direction is it going in and there's a certain normal area for that and for example if you have abnormalities of a ventricular mass you can get abnormal of vectors abnormal electrical vectors we want to look at the p-wave morphology to see if it's normal we look at its voltage and its shape because certain abnormalities of p-wave morphology can occur with certain diseases.

We want to do the same for the QRS morphology again certain diseases and for example heart blocks things that can lead to pacemakers will change the morphology of the QRS will then look at the ST segment and the T wave morphology ischemia lack of blood flow in the heart that can lead to angina or heart attacks changes the ST segment and T wave morphology and if we're lucky enough to have an earlier baseline EKG we compare it to see hey have there been changes is something going on here that's something acute so let's start here is a normal ECG notice in the upper left corner there is a little green box that's the standard and that standard is if you count the little tiny boxes ten little boxes each one of the big boxes has five smaller boxes so there'll be two of the big boxes which constitute ten small boxes each one of those boxes is one millimeter and corresponds to 1 millivolt of electrical activity so again this is the standard on all ECG machines and if you had set the standard to 1/2 then of course it would be each one of those little boxes would be less voltage if you doubled it they would be more voltage but usually again almost all EKGs use this green box of the 10 standard you'll notice how the leads are placed here on the left-hand side there's lead 1 2 & 3 remember zero Degree + 60 + 120 the next 3 leads AV RL + F AV r + 210 AV el - 30 AV f + 90 and then come the cordial leads the ones that are sticking through the heart like like needles in the sagittal plane v 1 v 2 v 3 4 5 and 6 and this is a normal ECG notice there's a p-wave in front of each QRS the QRS is are nice and narrow there's a nice T wave up right and and not way prolonged after each QRS this is sinus rhythm normal sinus rhythm that that's set off by the sinus node passes normally through the heart there's no evidence here of ischemia or heart attack or or hypertrophy of the heart muscle as I said you'll notice that there's sinus rhythm in the green box you'll see that each QRS is preceded by a P wave the a true the atrium depolarizes before the ventricle all the P waves are followed by a QRS there's no blockage of the beat as it goes down through the heart each QRS is preceded by a P and the peas are all identical they're upright in leads to and AVF and they're nice and narrow they're not prolonged if any of the comments just made the answer was no then you're talking about an arrhythmia and as I said we're going to have whole lectures on the arrhythmias later so right now we're just worrying about the normal also you notice how nicely of the QRS is progress in fact that whole strip along the bottom even though they're different leads it's continuous so you're actually seeing one set of P waves after another in order to obtain the heart rate you count the number of big boxes between two qrs's and you divide by 300 so if there were two big boxes between the two qrs's that would be 2 into 300 or 150 if there were three big boxes in between a QRS that would be a rate of 103 into 300 if there were four boxes in between the two qrs's that would be 75 4 into 375 you can also do it by counting the number of qrs's in 3 seconds remember the ECG is moving at a certain rate you can calculate a number of seconds and then multiplied by 20 but usually what we do is we use the rule of 300 mentioned before it's important to note that the normal heart beat is sixty to a hundred beats per minute so now we have a heart rate by the way the computer is almost always right on the heart rate again let's talk about the intervals so a normal PR interval is 0.2 seconds to 0.20 seconds that's three little boxes two five little boxes alright you remember there's five little boxes within the big bigger box so the normal PR interval from the beginning of the P wave to the beginning of the QRS is somewhere between three and five little boxes 0.122 0.20 seconds the normal QRS interval is less than 0.1 that's two and a half little boxes and the normal QT which is corrected for heart rate in a formula is somewhere between point three and point four six seconds remember the Qt from the beginning of the Q wave to the end of the T wave remember each small box is 0.04 seconds so a large box is five times 0.04 seconds or 0.20 seconds and there are five small boxes in each large box as I've said before again what about the axis well there's a general rule that most med students use and that is if the QRS is upright and leads one and two it's a normal axis you can actually calculate the axis because the axis is perpendicular to any lead where the R and the S or the upstroke and the down stroke are equal in this example lead three you see that pretty much the amount above the line and below the line is about the same so the axis is going to be 90 degrees from lead three so 90 degrees plus 120 lead 3 is 120 remember that's 210 and that would be in other words the axis would be towards AVR or minus 90 from 120 would be 30 that would be somewhere needly near lead 2 which is +60 well you can see how do we tell where is the maximum our wave well the maximum our wave is a round lead to it's there's no up right our wave and AV are so therefore the axis is actually something like a plus 30 so again the rule of thumb is you look to see where the amount of voltage up and down is equal it's 90 to the axis is 90 degrees from that then you look for the lead with the maximum R wave that's the direction because you could go this way on the 90 degrees or you could go that way what tells you which way you go is where's the maximum R wave in this one it's lead to and again normal axis is between minus 30 and plus 90 and if the axis is not between plus minus 30 and plus 90 then it's an axis deviation if it goes more - that's so-called left axis deviation if it goes more plus than 90 it's called right axis deviation and we'll talk about how that's used in reading various electrocardiographic diagnosis so again just to reiterate the axis for the mean frontal plane electrical vector of the heart is near the limb lead with the tallest R wave and perpendicular to the lead where the size of the upward deflection and the downward deflection are equal remember the upward deflection is called an R wave the downward deflection is called an S wave so let's take a look at the P wave itself the normal P wave is going to be three little boxes or less in duration remember that the PR interval that's the duration from the beginning of the P wave to the beginning of the QRS is going to be less than five little boxes but the length of the P wave itself should be only three little boxes and it should be upright in lead one and two and a negative deflection of less than one box wide or one box deep in v1 if the P wave in lead v1 is more negative than one box and wider than one box that suggests that the atrium is the left eight trim is enlarged so-called left atrial enlargement if the box is pointed and higher than two millimeters then and usually wider than two then that defines right atrial enlargement now these numbers are not anatomically perfect the echo and the MRI and so forth would be more perfect but they carry prognostic information they're very important because when they appear it really means that there's quite significant either left atrial dilatation or right atrial dilatation let's look at the R wave now the normal R wave should transit in the precordial lead starting with lead v1 there should be a very small R wave and then it gets a little bigger in v2 and somewhere between v3 and v4 you have a dominant R wave with not much s wave and then it progresses out to v6 with usually the maximum R wave somewhere in v4 five and six with the transition from more negative to positive somewhere around v3 or v for small Q's that is initial downward deflections of less than one little box can occur but that they are never longer than one box if they're wider than one box it suggests that there's been damage to the myocardium and the voltage should be within a normal range also the ST segment should be isoelectric that as it should be flat you can have a little bit of depression but if it's substantially depressed more than a tiny amount it suggests a number of things let's look at what it suggests if there's a sort of curved sagging of the ST segment as in this example that means the patient is usually taking digitalis did you tell us has that affect if there's a ski MIA or lack of blood flow you see a squared off flattening of the ST segment we see that with a positive exercise test and we see that when patients come in and have a so-called non-st elevation myocardial infarct and we'll talk more about those definitions later and then in hypo Kaleigh Mia where the potassium is low you may see a mildly down sloping ST segment the T wave is often flattened and as I mentioned before you may see a little additional wave after the T wave the you wave again here's the normal EKG look at the ST segments here they're all fine they're not depressed they're not elevated they're in exactly the right sequence.