Treadmill Exercise ECG interpretation & Axis Determination

 

Discussion on treadmill exercise ECG, also known as treadmill test, TMT and stress ECG. Treadmill Exercise ECG is usually done with a computerized treadmill unit which controls the motor speed of the treadmill as well as monitors the ECG. Please subscribe to this channel for future updates. Click on the subscribe button, press the bell icon after that for all updates. ECG is recorded periodically during the test in addition to documenting any specific events like arrhythmias. Ideally treadmill test is done in a basal state so that the process of digestion of food and consequent increase in cardiac output does not interfere with the assessment. If it is a diagnostic test, the individual should be off medications. But if it is for assessment of effort tolerance while on treatment, it may be done on medications. Though various protocols like Naughton and ramp protocol are in vogue, the popular one is Bruce protocol. There is also a Modified Bruce protocol for those with lower functional capacity or for early post infarction evaluation. Standard Bruce protocol has seven 3 minute stages. In stage I the gradient is 10% and it rises 2% per stage. The starting speed is 1.7 mph and increases in increments of 0.8 to 0.9 mph per stage. In Modified Bruce protocol, stage I has a gradient of zero and stage II a gradient of 5%. Speed is the same in the first 3 stages of Modified Bruce protocol (1.7 mph). Stage 3 of Modified Bruce protocol is equivalent to Stage I of standard Bruce protocol. Further stages are similar to Bruce protocol, though the number of the stage will be higher by a magnitude of 2. Treadmill exercise test ECG series starts with the pretest ECG and recordings in every stage of exercise and recovery phase. Sometimes an ECG during hyperventilation is also recorded before the start of exercise. This pretest ECG of a treadmill exercise test series shows a bit of artifacts, especially in leads II and III. The pretest heart rate is about 100/minute, possibly due to apprehension. ECG recording in stage 1 of Bruce protocol of treadmill exercise test. The heart rate has increased and there are now many artifacts and no significant ST segment shift is evident. Recording at the peak exercise shows significant horizontal ST segment depression in inferior and lateral leads at a fast heart rate. But the significant level of artifacts in the raw rhythm strip make us suspect whether the ST segment depression could be artefactual due computerized averaging in the computer synthesised rhythm. The recording in early phase of recovery at 1 minute, shows very little ST segment depression, making us suspect further whether the earlier recording was really due to myocardial ischemia. But the ST segment is down sloping in inferior leads. ECG at 3 minutes of recovery, shows further worsening of ST segment depression, in inferior and lateral leads, establishing the presence of significant myocardial ischemia. TMT recovery phase ECG at 6 minutes showing the persistence of down sloping ST segment depression. This calls for further evaluation including coronary angiography and revascularization if feasible. Moreover, ECG recording has to be continued till ST segment shift resolves. Another exercise ECG showing deep ST depression in inferior and anterolateral leads, with ST segment elevation in aVR and V1 in stage 1. This test also calls for angiography and revascularization if feasible. Basal metabolic requirement is taken as 1 MET or metabolic equivalent, which is equivalent to 3.5 ml/kg/minute of oxygen consumption. If stage 1 of Bruce protocol is completed, 4.6 METS are achieved. For stage 2 it is 7.0 and 10.1 for stage 3. The treadmill speeds at these stages are 1.7, 2.5 and 3.4 miles per hour, respectively. For stage 4, the speed is 4.2 miles per hour. Most important change noted in treadmill test is ST segment depression as seen in the illustration. If it occurs in stage 1 of standard Bruce protocol, and if it is severe, it is a strongly positive TMT as in this illustration. The lower the workload at which the ST segment depression occurs, the more likely the person is to have severe coronary artery disease. ST segment depression occurring within 5 minutes of Bruce protocol and persisting more than 5 minutes into recovery is highly significant. ST segment depression can be upsloping, horizontal, or down sloping, in the order of increasing significance. An upsloping ST segment depression can be normal if it is a rapid upsloping ST, which is said to represent the atrial repolarization wave or Ta wave, extending into the region of the ST segment with exercise. ST segment depression is measured at 80 ms from the J point at the end of QRS at heart rates below 120/min and at 60 ms at higher heart rates. ST segment elevation has the same significance as ST segment depression in leads not showing an initial Q wave. In those leads with a Q wave, it can be due to dyskinesia or left ventricular aneurysm. It is can also be a feature of viable myocardium in an infarcted territory, with poor collateralization. Hibernating myocardium has been invoked as a reason by Malouf and associates as their patient had resolution of wall motion abnormalities after revascularization. Coronary spasm is another potential cause of ST elevation during treadmill exercise ECG. In one study, 12 patients without history of myocardial infarction or left ventricular aneurysm, had provocation of coronary spasm with ergonovine. Here are the first set of references on ST elevation during exercise ECG. Second set of references are here. 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AXIS Dtermination on ECG:

I hope you enjoy this video on the cardiac axis. The mean QRS axis, also referred to as the cardiac axis, is the net overall direction of electrical activity in the frontal plane. Within a normal heart, it falls between +90 and -30 degrees (left and inferior) as seen here on the hexaxial frontal plane diagram. An axis that falls outside this range is considered abnormal or “deviated”. A QRS axis outside of the normal range can be further divided into left superior axis, often referred to as left axis deviation, right superior axis and right inferior axis, often referred to as right axis deviation. Identifying the cardiac axis involves looking at the main direction (positive or negative) of the QRS complex in particular frontal leads. The main QRS direction, also referred to as the mean QRS, is the difference in the amplitude of the positive and negative deflections. With this QRS complex, the initial deflection is negative (a q wave), the second is positive (an R wave) and the third deflection is negative (an s wave). The size of each negative deflection is represented by a blue line and the size of each positive deflection is represented by a red line. If we compare the sum of the red (+) lines to the sum of the blue (-) lines we clearly see that the red line is much taller than the blue line so the mean QRS is positive. Here is another example. If we compare the sum of the red (+) lines to the sum of the blue (-) lines we see that the blue line is taller so the mean QRS is negative. In this example, the positive and negative deflection are the same amplitude so the difference is zero. This is referred to as equiphasic. To understand the cardiac axis and how to identify it on the ECG, there are a few key concepts to be familiar with. First is the hexaxial frontal plane diagram and second is the hemisphere concept. Here is the hexaxial frontal plane diagram. All 6 frontal lead vectors (I, II, III, aVL, aVR & aVF) are represented on this diagram. Each positive lead vector (represented by a solid line) has a negative counterpart (represented by a dashed line) that is diametrically opposite in direction. The hexaxial frontal plane diagram is the key to understanding the relationships of the various frontal plane leads and determining the QRS axis. How both the standard limb leads (I, II & III) and the augmented leads (aVR, aVL & aVF) are incorporated into the hexaxial frontal plane diagram is covered in a separate video entitled ECG Leads Simplified. On the frontal plane hexaxial diagram, each lead has a corresponding pair of hemispheres, one positive and one negative. The intersection point where the sphere divides into positive and negative is the line on the hexaxial frontal plane diagram that runs perpendicular to the lead axis. This is where the mean QRS vector is zero (or the sum of the positive and negative deflections of the QRS complex is zero). For lead I, shown in grey, this perpendicular line runs directly on the axis for lead aVF. The hemisphere to the left of the perpendicular line is the negative hemisphere which is where the QRS vector would be situated if the mean QRS on the ECG is negative in lead I. The hemisphere to the right of the perpendicular line is the positive hemisphere which is where the mean QRS vector would be situated if the mean QRS on the ECG is positive in lead I. You can apply the hemisphere concept to all of the leads. For lead aVF, the line on the hexaxial frontal plane diagram that runs perpendicular to the lead aVF axis is here. Again, this is where the mean QRS vector for this lead is zero (or the sum of the positive and negative deflections of the QRS complex is zero). Here, in red, is the positive hemisphere for lead aVF which is where the QRS vector would be situated if the mean QRS on the ECG is positive in lead aVF. Here is the negative hemisphere for lead aVF which is where the mean QRS vector would be situated if the mean QRS on the ECG is negative in lead aVF. For lead II, the line on the hexaxial frontal plane diagram that runs perpendicular to the lead II axis is here. Here in green is the positive hemisphere for lead II which is where the QRS vector would be situated if the mean QRS on the ECG is positive in lead II. Here is the negative hemisphere for lead II which is where the mean QRS vector would be situated if the mean QRS on the ECG is negative in lead II. One way to identify the QRS axis on the ECG involves looking at the mean QRS in leads I and aVF because they are perpendicular to each other. This is not the only way but it is one of the easiest and fastest. First, identify if the mean QRS is positive, negative or equiphasic in lead I. With this ECG, the mean QRS is positive in lead I. So the mean QRS vector is situated in the positive hemisphere seen here in blue. Now look at the mean QRS in lead aVF. It is also positive so the mean QRS vector is situated in the positive hemisphere seen here in red. The area where the two hemispheres overlap is where the axis falls. When you combine red and blue you get purple! The QRS axis is between 0 and 90 degrees which is considered normal. The rule is that if the mean QRS in lead I and lead aVF are both positive, you can immediately state that the QRS axis is in the left inferior quadrant (0 to 90 degrees) and therefore normal. Let’s take a look at another ECG. First look at lead I. The mean QRS is negative in lead I. So the mean QRS vector is situated in the negative hemisphere seen here in blue. Now look at lead aVF. The mean QRS is positive in lead aVF. So the mean QRS vector is situated in the positive hemisphere seen here in red. In this case, the red and blue hemispheres overlap between +90 and 180 degrees. So, here you have a right inferior axis or “right axis deviation” which is abnormal. When the mean QRS is negative in lead I and negative in lead aVF, the hemispheres overlap between -90 and 180 degrees. This indicates a right superior axis which is abnormal. With this ECG, the mean QRS is positive in lead I and negative in lead aVF. Here are the corresponding hemispheres. The hemispheres overlap between 0 and -90 degrees. In this scenario, the QRS axis could be normal (from 0 to -30 degrees) or abnormal (from -30 to -90 degrees). In this situation you check lead II. In the ECG, Lead II is negative. So, here are the corresponding hemispheres for the 3 leads. All 3 hemispheres overlap between -30 and -90 degrees which is the area seen in grey. So, here you have “left axis deviation” which is abnormal. The only time you need to check lead II is when the mean QRS is positive in lead I and negative in lead aVF. When lead II is positive, the axis is between 0 and -30 degrees which is normal. When lead II is negative, the axis is between -30 and -90 degrees which is referred to as left axis deviation and is abnormal. What if you have to check lead II and the mean QRS is equiphasic? This means that the QRS axis is right at -30 degrees. If you happen to see an equiphasic QRS in any of the frontal leads, you can quickly determine the QRS axis without having to look at lead I & aVF first. The QRS axis will be perpendicular to the lead with an equiphasic QRS. In this ECG, there is an equiphasic QRS in lead III. So, the QRS axis runs perpendicular to lead III which is the lead axis for +/- aVR. To determine if the QRS is at 30 degrees or -150 degrees, just look at the mean QRS for lead AVR. In this case, lead aVR is negative so the QRS axis is 30 degrees. In summary, one can conclude the following- #1- If lead I and lead aVF are positive, the QRS axis is normal. #2 - If lead I is negative and lead aVF is positive, there is right axis deviation. #3 - If lead I and aVF are both negative, there is a right superior axis. #4 - If lead I is positive and lead aVF is negative, the axis may be normal or abnormal. In this situation, one must look at lead II. If lead II is positive, the axis is normal. If lead II is negative, there is left axis deviation. And if lead II is equiphasic, the QRS axis is -30 degrees. Thank you.