In an ECG (electrocardiogram), "sinus" indicates that the heart's electrical activity is originating from the sinus node (also known as the sinoatrial or SA node). This is the heart's natural pacemaker.
Here's a breakdown:
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Normal Heart Rhythm: A "sinus rhythm" on an ECG signifies a normal and healthy heart rhythm. The SA node is initiating the electrical impulses that cause the heart to contract.
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SA Node Function: The SA node is located in the right atrium of the heart. It spontaneously generates electrical impulses, setting the pace for the entire heart.
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How it Appears on an ECG: An ECG showing sinus rhythm will have specific characteristics, including:
- P waves: Upright P waves preceding each QRS complex, indicating atrial depolarization originating from the SA node. The P wave morphology will be consistent.
- QRS complexes: Normal shaped QRS complexes following each P wave. The QRS complex represents ventricular depolarization (contraction of the ventricles).
- Regular R-R Intervals: Relatively consistent time intervals between each heartbeat (R wave to R wave). While slight variations due to breathing are normal (sinus arrhythmia), large irregularities are not.
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Non-Sinus Rhythms: If the electrical activity originates from somewhere other than the SA node (e.g., the atria, AV node, or ventricles), it is not a sinus rhythm. These other rhythms can indicate underlying heart conditions.
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Examples:
- Sinus Tachycardia: A sinus rhythm with a heart rate greater than 100 beats per minute. Still originating from the SA node, just faster than normal.
- Sinus Bradycardia: A sinus rhythm with a heart rate less than 60 beats per minute. Still originating from the SA node, but slower than normal.
- Sinus Arrhythmia: Slight variations in the heart rate associated with breathing. A normal finding, particularly in young people.
In summary, "sinus" on an ECG report tells you the heart's electrical activity is starting where it should be, in the SA node. It's a key indicator of normal heart function, even if the heart rate itself is faster or slower than average (as in sinus tachycardia or bradycardia).
