ECG Interpretation Cheat Sheet

WHAT EXACTLY IS AN ECG? An ECG (or EKG) 3 which stands for electrocardiogram 3 looks at an electrical tracing of the cardiac activity within your heart. Changes can indicate structural, mechanical, or electrical issues. The electrical tracing is referred to as a rhythm strip. Depending on the number of electrodes, this gives various different leads or views of the heart. The most common ECG is a 12-lead ECG, which utilizes 10 electrodes to get 12 different views of the heart. However, continuous telemetry monitoring usually utilizes 3-5 electrodes, viewing only a few important leads, with a primary lead (usually Lead II) being continuously monitored. Interpreting a 12-lead ECG is advanced 3 primarily falling on the responsibility of the physician or advanced practice provider (APP). However, interpreting rhythm strips (in a single lead) is super important for every inpatient nurse to know 3 especially those working in the ED, ICU, Telemetry, or Cardiac units. THE RHYTHM STRIP A rhythm strip is made up of 6-seconds, split into two separate 3-second portions (marked by black marks above strip). You will have to analyze and document readings every so often (usually every 8 hours) 3 especially if you are on a telemetry floor. If you work in the ED, you will have to do this initially on all those patients who it is required, writing your interpretation and any abnormalities. In critical situations, you will need to analyze a rhythm directly from the monitor or the defibrillator. The P-QRS-T complex is each heartbeat broken down into an electrical tracing. The Pwave occurs during atrial depolarization, which causes the atria to contract. The QRS complex signifies ventricular depolarization when the ventricles contract. Finally, the Twave is when the ventricles repolarize 3 meaning the ventricular cells are electrically recharging for another contraction. There is no atrial repolarization seen because this is much smaller and is hidden within the QRS complex. Remember the rhythm tracing indicates electrical impulses through the heart cells. Just because the electrical impulse is there does not mean the heart will have the mechanical response (i.e. contraction). This is the case during PEA. While interpreting a rhythm strip, the graph paper boxes are there so you can easily compare and measure various parts of the tracing. The PR interval represents the amount of time it takes for the electrical impulse to go from the SA node in the atria, until it reaches the ventricles. This comes in handy when determining heart blocks, as blocks will slow conduction of the electrical impulse. Although it is called the PR interval, it actually is the length between the beginning of the P-wave and the beginning of the Q wave. The normal PR interval is 120-200 ms, or 3-5 small boxes. PR intervals that are consistent but longer than 200 ms indicates a 1st degree heart block. The QRS complex width represents how long the electrical impulse takes to depolarize the entire left and right ventricles. Normally, the QRS complex is narrow 80-100 ms (2-2.5 small boxes). If this is widened, it indicates some type of bundle branch block 3 which is a delay of the conduction between the ventricles. The QT interval is the length of time it takes the electrical impulse to go from the beginning of the ventricles 3 until the ventricles completely repolarize and are ready for another contraction. This should be between 350-450 ms. If this is elongated 3 this presents an increased risk of various arrhythmias such as Torsades or Vfib, especially if > 500 ms. However, if the heart rate is abnormally slow (bradycardia) or fast (tachycardia), this will not be accurately reflected. Due to this, the QT-c (QT-corrected) is usually used which corrects for the heart rate. The ST-segment indicates the beginning of ventricular recovery. The point between the QRS complex and where the ST-segment begins is called the J-point. The ST-segment can either be normal (at the isoelectric line), elevated, or depressed. The ST segment should be at the isoelectric line at baseline, measured by the TP segment. ST elevation or depression in at least 2 contiguous leads is likely to indicate cardiac ischemia or infarction. ST depression is defined as greater than 0.5mm (1/2 small box) below the isoelectric line. This usually indicates cardiac ischemia 3 meaning there is a lack of perfusion to some area of the heart. It can also indicate digoxin toxicity or electrolyte abnormalities. ST depression can either be upsloping, down-sloping, or horizontal 3 with down-sloping being more specific for myocardial ischemia. ST elevation is defined as greater than 1mm (1 small box) in limb leads and 2mm in precordial leads (V1, V2, V3) above the isoelectric line and indicates myocardial necrosis aka infarction. This classically occurs during a STEMI (ST-segment elevation myocardial infarction). In order to be classified as a STEMI 3 the ST-segment elevation MUST be in 2 contiguous leads. ST-Segment Depression ST-Segment Elevation The T-wave, as described above, represents ventricular repolarization. The ventricular cells are getting ready for another electrical impulse 3 so they can conduct and create another heartbeat. T waves are typically a smooth 3 small boxes) and strange. This is because the T-waves of a PVC usually have an opposite deflection of the QRS 3 if the complex is positive (above the isoelectric line), then the T-wave will likely be inverted (below the isoelectric line). Sometimes PVCs will succeed each other back-to-back. When there are two PVCs in a row 3 this is called a couplet; when there are 3 or more 3 this is called a run. Technically, this is a run of VTACH. Generally, when this occurs there is significant irritability within the heart, and these patients are at high risk to go into sustained VTACH or VFIB. Sometimes PVCs will fall into patterns 3 and there are specific names for these. If every other beat is a PVC 3 this is Bigeminy. If every 3rd beat is a PVC 3 Trigeminy. If every 4th beat is a PVC 3 Quadrigeminy. These patterns don9t necessarily mean much other than are known to occur in a significantly diseased heart, but they are important to note when monitoring the rhythm over time. Premature Atrial Contractions (PACs) maintain the same principal as PVCs, but originate somewhere in the left or right atria. Because of th 3 P-waves are present 3 although this can be hidden in the preceding wave if close enough. The general morphology (shape) of the P-wave wil appear different (flattened, notched, etc), and will interrupt the underlying rhythm. The PR-interval will also usually be different because the originating cell is not within the SA node. PACs can occur from ischemia and irritability, but can often occur from less dangerous causes such as caffeine, stress, alcohol, fatigue, poor sleep, and various medications. Premature Junctional Contractions (PJCs) follow the same principals as above, but they occur within the AV node between the atria and the ventricles. This means P-waves can be present but are inverted, but often are absent if hidden within the QRS complex itself. A SYSTEMATIC APPROACH TO RHYTHM INTERPRETATION There are specific steps to take in order to analyze and correctly interpret a rhythm strip. This becomes especially helpful for those who are new or uncomfortable with analyzing and interpreting rhythm strips. 1. DETERMINE REGULARITY Rhythms are split between regular rhythms and irregular rhythms. Generally, regular rhythms are less ominous but can still be dangerous (i.e. rapid Afib, SVT, aflutter, severe bradycardia, VTACH). Irregular rhythms usually indicate atrial rhythms, ectopy, or heart blocks. For sinus rhythms that appear to be a normal rate 3 you can likely just eyeball the regularity. However, it can be beneficial to measure the distance between the R waves of each QRS complex and map out if the R-to-R interval, and make sure it is generally the same throughout the strip. While doing this, make sure each QRS complex has a P-wave which precedes it. 2. DETERMINE RATE There are ways to calculate the rate, but with current technology this will usually be provided for you. Almost always 3 you can utilize this calculated rate provided somewhere on the rhythm strip. However, rarely the machines can count incorrectly. Rate = ~73 bpm A quick easy way to estimate the rate, especially during a sinus rhythm, is to count the number of QRS complexes in a 6-second strip, and add a zero (essentially multiply by 10). This estimates how many QRS complexes there are in a full minute 3 or rather the heart rate. For regular rates, you can find an R-wave that is directly on a large box line. Look to where the next R wave lies and count with the intervals provided (300, 150, 100, 75, etc). 3. P WAVES AND PR INTERVAL Make sure there is a P-wave before every QRS complex. If one or two beats are missing a P-wave but the rest have them 3 consider ectopy. The P-wave will generally look different in each lead but should always be positively deflected in a nonjunctional rhythm. Next evaluate the PR-interval. Is it a normal length (3-5 small boxes)? Is it consistent with each beat? If it is elongated or inconsistent 3 think 1st, 2nd, or 3rd degree heart blocks. 4. QRS AND QT INTERVAL Make sure they all appear to look the same (same morphology). If one or two complexes looks completely different, consider a PVC or paced beat. Next measure the QRS complex width 3 remember it should be 2-3 small boxes. If it is longer than 3 small boxes 3 consider Bundle Branch Block. The QT-interval can be measured as well, but this should be given to you on an ECG and isn9t always required to be calculated on a rhythm strip. Remember though 3 if the QRS is widened, the QT-interval will be widened as well but will not necessarily predispose the patient to a fatal arrhythmia. 5. ST-Segment and T Wave Look at the J-point. Is there any ST elevation or depression? If so 3 does the elevation or depression last longer than 2 small boxes? Next evaluate the T-wave 3 does it