ECG's

The waveform and what each part means, the rhythms to recognise, and reading a STEMI by consecutive leads.

ECG's

The ECG is a recording of the electrical activity of the heart. We can analyze the rhythm, assess for conduction issues and determine if there are signs of ischemia.

Normal Electrical Activity

The electrical impulse will start at the Sinoatrial (SA) node(in the right atrium) and travel down the atria, causing them to contract. It will then reach the atrioventricular (AV) node. The AV node holds the electrical signal for a brief moment to allow the atria to fully contract and the ventricles to fill with blood. Then the signal goes down the Bundle of His, to the right and left bundle branches and ultimately the Purkinje fibers, causing the ventricles to contract.

ECG

The ECG waveform goes as follows: P -> Q -> R ->S -T, then it repeats. It follows the conduction pathway of the heart.

• P Wave: represents atrial depolarization or atrial contraction. The P wave originates in the SinoAtrial node, which is why if there are P waves, it is called a "Sinus" rhythm. • QRS Complex: represents ventricular depolarization or contraction. A normal QRS complex is less than 0.12 seconds. A widened QRS can signify significant issues like ventricular tachycardia, worsening hyperkalemia, a Bundle Branch Block, or even as a result of certain medications/toxins. It can also be as a result of having a paced rhythm or from Premature Ventricular Contractions (PVCs) • T Wave: represents ventricular repolarization or the ventricles relaxing and getting ready to contract again. • PR Interval: From the beginning of the P wave to the beginning of the QRS complex. Helps assess how long it takes for the electrical signal to go from the atria to the ventricles. Normal duration is between 0.12 to 0.20 seconds. If it's prolonged, or longer than normal, it can mean that there is an issue with the AV node such as a heart block or ischemia that is affecting the SA or AV node. • RR Interval: The time between the R waves of QRS complexes. Used to note whether the rhythm is regular if each R wave is the same length from each other and can quickly help us determine how fast the heart rate is by the length of the RR Interval. Varying RR intervals suggest an irregular rhythm, such as atrial fibrillation. • QT Interval: From the beginning of the QRS complex to the end of the T wave. It shows how long it takes the ventricles to depolarize and repolarize. If the QT interval is long or becoming longer, we worry that the patient may go into a deadly arrhythmia like Torsades de Pointes. A baseline prolonged QT interval also prompts providers to avoid medications that have been known to cause arrhythmias and may cause providers to assess underlying electrolyte levels like magnesium (low levels may promote Torsades de Pointes). • ST Segment: A key component of the ECG that helps assess for myocardial injury or ischemia. See below for more details.

Rhythms to Recognize

Normal Sinus Rhythm

Ventricular Tachycardia

Ventricular Fibrillation

Atrial Fibrillation

Pulseless Electrical Activity

Sinus Tachycardia

Sinus Bradycardia

First degree AV block

Second degree AV block (Mobitz I or Wenckebach)

Second degree AV block (Mobitz II)

Second degree AV block (2:1 block)

Third degree AV block with junctional escape

Remember that as a New ER nurse you should learn to recognize certain rhythms and signs of ischemia. The job of fully interpreting an ECG falls on the ER provider, and at times they don't even know so cardiology has to get involved, so be patient with yourself.

Interpreting the Basics of an ECG

So how should you go about it? • Is it sinus? o Is there a P wave before every QRS? If there is, then it is sinus. o If not sinus, potential rhythms include Afib, Heart Blocks, Junctional, Aflutter, VT/VFIB • Is it regular? o Is the RR interval the same throughout? Or are they different lengths from each other and therefore irregular? o If not regular, potential rhythms include Afib, Heart Blocks, Sinus Arrhythmia, VT/Vfib • What is the rate? o Normal for adults is 60-100. Is the patient, brady tachy or normal? o You can use the 300 Method or 6 second method to quickly assess heart rate o If low, can include heart blocks and sinus bradycardia o If high, can include SVT, Aflutter, Afib, Sinus Tach, Vtac • Is the QRS normal? Or wide? Or progressive widening? o If wide it can signal worsening Hyperkalemia or OD from a toxin, Bundle Branch Block, Ventricular Tachycardia or Ventricular Fibrillation • Signs of Ischemia? o ST changes (depression or elevation) in consecutive leads? o Inverted T waves? • Do I recognize the rhythm? o You know what normal sinus rhythm looks like, does it look like it? If not, have someone with experience quickly assess it, especially in patients who are ill-appearing. Again, as a new ER nurse you don't have to be a pro, but you need to at least be able to recognize certain rhythms, when ischemia is occurring, and whether there are changes to the ECG so you can quickly get someone with more experience and qualifications (aka your provider) to read the ECG and make a care decision. The more you practice the better you will be. Think of the answer and see what your provider went with and compare your answer and ask questions as to why. That way little by little you will get better. If your organization offers an ECG course please take it!

STEMI and Consecutive Leads

Now let's go over consecutive leads. Consecutive leads just mean part of the ECG that are looking at the same region of the heart.

It's extremely useful to know about consecutive leads because if signs of ischemia are present, you can tell what part of the heart is being affected by which leads have the ischemia in. • I Lateral • II Inferior • III Inferior • aVR • aVL Lateral • aVF Inferior • V1 Septal • V2 Septal • V3 Anterior • V4 Anterior • V5 Lateral • V6 Lateral

• ST segment elevations in leads II, III, and aVF o Indicates inferior MI with an occlusion of the Right Coronary Artery since it typically supplies blood supply to that section of the heart. • ST segment elevations in leads V5, V6, I, and aVL o Indicates a lateral MI with an occlusion to the Left circumflex coronary artery since it commonly supplies that section of the heart. • ST segment elevation in V1-V4 o Indicates a septal anterior MI with an occlusion to the Left anterior descending coronary artery (LAD) It's crucial to know this because when there are changes to consecutive leads, it is more likely that something is going on, and it points directly to what part of the heart the issue is in. If the patient is having a STEMI and goes to the cath lab, the cardiologists will know exactly what vessel they have to go to and put a stent in. Key Point: A quick point on electrolyte disturbances and ECG rhythms. Do not forget to check the levels of electrolytes. If your patient keeps going in and out of a deadly rhythm like ventricular tachycardia, how are the electrolytes? Could it be caused by a high or low potassium level? Or a magnesium disturbance? What about calcium and sodium? The heart loves balance, especially when it comes to electrolytes. So when your patient keeps on having rhythm issues, don't forget to check the electrolytes.

Updated 30 Aug 2026