[image: 21837.png] Diagnostic Code Explanations (1) Diagnoses for this EKG: sinus rhythm (SR) Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/

[image: 1989.png] Diagnostic Code Explanations (1) Diagnoses for this EKG: sinus rhythm (SR) Explanation for sinus rhythm (SR) Identifying Sinus Rhythm on a standard 12-lead ECG is the foundational skill of ECG interpretation. It essentially means the heart's electrical impulse is originating correctly from the Sinoatrial (SA) node. To confirm sinus rhythm, you must look for specific signs related to the P wave, the rhythm regularity, and the conduction intervals. 1. The P Wave (The most critical sign) The definitive sign of sinus rhythm is the "P wave axis." Because the SA node is located in the top right of the heart, the electrical current should flow down and to the left. Lead II: The P wave must be upright (positive). This is the most important lead to check. Lead aVR: The P wave must be inverted (negative). If the P wave is upright in aVR, the rhythm is likely not sinus (e.g., it may be a low atrial or junctional rhythm). Leads I and aVF: P waves are typically upright. Consistency: The P waves should all look the same (consistent morphology) within a single lead. 2. The Relationship Between P and QRS The SA node should be driving the ventricles. 1:1 Ratio: Every P wave must be followed by a QRS complex, and every QRS complex must be preceded by a P wave. PR Interval: The time between the start of the P wave and the start of the QRS complex should be constant and within normal limits (0.12 to 0.20 seconds, or 3–5 small squares). 3. Rhythm Regularity Regularity: The distance between R waves (R-R interval) and P waves (P-P interval) should be consistent. Note: Minor variation is normal due to breathing (called respiratory sinus arrhythmia), but the rhythm should look visibly regular to the naked eye. 4. Heart Rate While "Sinus Rhythm" describes the origin of the beat, the rate determines the specific diagnosis: Normal Sinus Rhythm (NSR): Rate between 60 and 100 bpm. Sinus Bradycardia: All sinus criteria met, but rate is < 60 bpm. Sinus Tachycardia: All sinus criteria met, but rate is > 100 bpm. Summary Checklist When looking at a 12-lead ECG, you can confidently state "Sinus Rhythm" if: P waves are upright in Lead II. P waves are inverted in aVR. There is a P wave before every QRS. The rhythm is regular. Read more on LITFL: https://litfl.com/normal-sinus-rhythm-ecg-library/